Magnetic resonance imaging (MRI) is a medical imaging technique primarily used in Radiology to visualize the structure and function of the body. Medical imaging refers to the techniques and processes used to create Images of the human body (or parts thereof for clinical purposes ( Medical procedures seeking to Radiology is the medical specialty directing Medical imaging technologies to diagnose and treat diseases It provides detailed images of the body in any plane. MRI provides much greater contrast between the different soft tissues of the body than does computed tomography (CT), making it especially useful in neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer) imaging. Contrast is the difference in visual properties that makes an object (or its representation in an image distinguishable from other objects and the background Computed tomography (CT is a Medical imaging method employing Tomography. The human musculoskeletal system is the organ system that gives humans the ability to physically move by using the Muscles and Skeletal system. Cardiovascular Magnetic Resonance (CMR sometimes know as cardiac MRI is a medical imaging technology for the non-invasive assessment of the function and structure of the cardiovascular Oncology is the branch of medicine that studies Tumors ( Cancer) and seeks to understand their development diagnosis treatment and prevention Unlike CT it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body. Image talkNew_radiation_symbol_ISO_21482svg for details --> Ionizing radiation In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. The nuclear magnetic moment is the Magnetic moment of an Atomic nucleus and arises from the spin of the Protons and Neutrons It is mainly a magnetic Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny Radiofrequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. This signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body.

Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. In its early years the technique was referred to as nuclear magnetic resonance imaging (NMRI). However, as the word nuclear was associated in the public mind with ionizing radiation exposure it is generally now referred to simply as MRI. Image talkNew_radiation_symbol_ISO_21482svg for details --> Ionizing radiation Scientists still use the term NMRI when discussing non-medical devices operating on the same principles. One of the contributors to modern MRI, Paul Lauterbur, originally named the technique zeugmatography, a Greek term meaning "that which is used for joining". Paul Christian Lauterbur ( May 6, 1929 – March 27, 2007) was an American Chemist who shared the Nobel Prize ^[1] The term referred to the interaction between the static and the gradient magnetic fields necessary to create an image, but this term was not adopted.

Contents 1 How MRI works 1.1 Brief lay explanation of MRI physics 1.2 Physics Principles 1.2.1 Nuclear Magnetism (for more details on this topic see Nuclear magnetic resonance) 1.2.2 Resonance and Relaxation 1.2.3 Imaging 1.2.4 Example of a Pulse Sequence 1.2.5 K-space 1.3 Scanner construction and operation 1.3.1 Magnet 1.3.2 Radio frequency system 1.3.3 Gradients 1.3.4 Image Contrast 1.3.5 Contrast enhancement 2 Applications 2.1 Specialized MRI scans 2.1.1 Diffusion MRI 2.1.2 Magnetic resonance angiography 2.1.3 Magnetic resonance spectroscopy 2.1.4 Functional MRI 2.1.5 Interventional MRI 2.1.6 Radiation therapy simulation 2.1.7 Current density imaging 2.1.8 Magnetic resonance guided focused ultrasound 2.1.9 Multinuclear imaging 2.1.10 Magnetic Resonance Spectroscopy 2.1.11 Susceptibility Weighted Imaging (SWI) 2.1.12 Experimental MRI techniques 2.2 Portable instruments 2.3 MRI vs CT 2.4 Economics of MRI 3 Safety 3.1 Implants and foreign bodies 3.2 Projectile or missile effect 3.3 Radio frequency energy 3.4 Peripheral nerve stimulation 3.5 Acoustic Noise 3.6 Cryogens 3.7 Contrast agents 3.8 Pregnancy 3.9 Claustrophobia and discomfort 3.10 Guidance 3.11 The European Physical Agents Directive 4 2003 Nobel Prize 4.1 Controversy 5 See also 6 Footnotes 7 References 8 External links

How MRI works

Brief lay explanation of MRI physics

When a person is in the scanner, the hydrogen nuclei (i. e. , protons) found in abundance in the human body in water molecules, align with the strong magnetic field. A radio wave at just the right frequency for the protons to absorb energy pushes some of the protons out of alignment. The protons then snap back to alignment, producing a detectable rotating magnetic field as they do so. Since protons in different tissues of the body (e. g. , fat v. muscle) realign at different speeds, the different structures of the body can be revealed.

Gradient fields in the three dimensions allow the scanner to work only with protons from a "slice" at a time, allowing the creation of a whole volume that can be looked at in three dimensions.

Contrast agents may be injected intravenously to show enhancement of blood vessels, tumors or inflammation. Contrast medium Radiocontrast agents (also simply contrast agents or contrast materials) are compounds used to improve the visibility of internal bodily structures Intravenous therapy or IV therapy is the giving of Liquid substances directly into a Vein. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body Inflammation ( Latin, inflamatio, to set on fire is the complex biological response of vascular tissues to harmful stimuli such as Pathogens Unlike CT scanning MRI uses no ionizing radiation and is generally a very safe procedure. Patients with some metal implants and cardiac pacemakers are prevented from having an MRI due to effects of the powerful magnetic field and powerful radio waves.

MRI is used to image every part of the body, but is particularly useful in neurological conditions, disorders of the muscles and joints, for evaluating tumors and showing abnormalities in the heart and blood vessels.

Physics Principles

Nuclear Magnetism (for more details on this topic see Nuclear magnetic resonance)

Subatomic particles such as protons have the quantum mechanical property of spin. The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Quantum mechanics, spin is a fundamental property of atomic nuclei, Hadrons and Elementary particles For particles with non-zero spin Certain nuclei such as ¹H (protons), ²H, ³He, ²³Na or ³¹P, have a non–zero spin and therefore a magnetic moment. In Physics, Astronomy, Chemistry, and Electrical engineering, the term magnetic moment of a system (such as a loop of Electric current In the case of the so-called spin-1/2 nuclei, such as ¹H, there are two spin states, sometimes referred to as "up" and "down". Nuclei such as ¹²C have no unpaired neutrons or protons, and no net spin: however the isotope ¹³C (referred to in this context as "carbon 13") does.

When these spins are placed in a strong external magnetic field they precess around an axis along the direction of the field. Precession refers to a change in the direction of the axis of a rotating object Protons align in two energy eigenstates (Zeeman effect) one low-energy, and one high-energy, which are separated by a quantum of energy. The Zeeman effect (ˈzeɪmɑːn is the splitting of a Spectral line into several components in the presence of a static Magnetic field.

Resonance and Relaxation

In the static magnetic fields commonly used in MRI, the energy difference between the nuclear spin states corresponds to electromagnetic radiation at radio frequency (rf) wavelengths. Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz Resonant absorption of energy by the protons due to an external oscillating magnetic field will occur at the Larmor frequency for the particular nucleus. In Physics, Larmor precession (named after Joseph Larmor) refers to the Precession of the Magnetic moments of Electrons atomic

The net magnetization vector has two components. The longitudinal magnetization is due to an excess of protons in the lower energy state. This gives a net polarization parallel to the external field. The transverse magnetization is due to coherences forming between the two proton energy states. In Physics, atomic coherence is the induced coherence between levels of a multi-level Atomic system sometimes observed when it interacts with This gives a net polarization perpendicular to the external field in the transverse plane. The recovery of longitudinal magnetization is called T1 relaxation and the loss of phase coherence in the transverse plane is called T2 relaxation. In nuclear magnetic resonance (NMR spectroscopy and magnetic resonance imaging (MRI the term relaxation describes several processes by which nuclear Magnetization T1 is thus associated with the enthalpy of the spin system while T2 is associated with its entropy. In Thermodynamics and molecular chemistry, the enthalpy (denoted as H, h, or rarely as χ) is a quotient or description of In Thermodynamics (a branch of Physics) entropy, symbolized by S, is a measure of the unavailability of a system ’s Energy

When the radio frequency pulse is turned off, the transverse vector component produces an oscillating magnetic field which induces a small current in the receiver coil. This signal is called the free induction decay (FID). In Fourier Transform NMR, a free induction decay (FID is the observable NMR signal generated by non-equilibrium nuclear spin magnetisation precessing about the In an idealized nuclear magnetic resonance experiment, the FID decays with a time constant T2, but in practical MRI small differences in the static magnetic field at different spatial locations cause the Larmor frequency to vary across the body creating destructive interference which shortens the FID. The time constant for the observed decay of the FID is called the T2* ("T 2 star") relaxation time, and is always shorter than T2.

In MRI, the static magnetic field is caused to vary across the body (a field gradient), so that different spatial locations become associated with different precession frequencies. Usually these field gradients are pulsed, and it is the almost infinite variety of rf and gradient pulse sequences that gives MRI its versatility. Application of field gradient destroys the FID signal, but this can be recovered and measured by a refocusing gradient (to create a so-called "gradient echo" which imparts a T2*-weighting to the signal), or by a radio frequency pulse (to create a so-called "spin-echo" which imparts a T2-weighting to the signal). The whole process can be repeated when T1-relaxation is complete and the thermal equilibrium of the spins is restored, although usually MRI pulse sequences are repeated much faster than this, imparting an additional T1-weighting to the signal.

Typically in soft tissues T1 is around 1 second while T2 and T2* are a few tens of milliseconds, but these values vary widely between different tissues (and different external magnetic fields), giving MRI its tremendous soft tissue contrast.

Imaging

A number of schemes have been devised for combining field gradients and radiofrequency excitation to create an image. One involves 2D or 3D reconstruction from projections, much as in Computed Tomography. Computed tomography (CT is a Medical imaging method employing Tomography. Others involve building the image point-by-point or line-by-line. One even uses gradients in the rf field rather than the static field. Although each of these schemes is occasionally used in specialist applications, the majority of MR Images today are created either by the Two-Dimensional Fourier Transform (2DFT) technique with slice selection, or by the Three-Dimensional Fourier Transform (3DFT) technique. Another name for 2DFT is spin-warp. What follows here is a description of the 2DFT technique with slice selection.

Slice selection is achieved by applying a magnetic gradient in addition to the external magnetic field during the radio frequency pulse. Only one plane within the object will have protons that are on–resonance and contribute to the signal.

A real image can be considered as being composed of a number of spatial frequencies at different orientations. A two–dimensional Fourier transformation of a real image will express these waves as a matrix of spatial frequencies known as k–space. This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and In Mathematics, Physics, and Engineering, spatial frequency is a characteristic of any structure that is periodic across position in space k -space is a formalism widely used in Magnetic resonance imaging independently introduced in 1983 by Ljunggren and Twieg Low spatial frequencies are represented at the center of k–space and high spatial frequencies at the periphery. Frequency and phase encoding are used to measure the amplitudes of a range of spatial frequencies within the object being imaged.

The frequency encoding gradient is applied during readout of the signal and is orthogonal to the slice selection gradient. During application of the gradient the frequency differences in the readout direction progressively change. At the midpoint of the readout these differences are small and the low spatial frequencies in the image are sampled filling the center of k-space. Higher spatial frequencies will be sampled towards the beginning and end of the readout filling the periphery of k-space.

Phase encoding is applied in the remaining orthogonal plane and uses the same principle of sampling the object for different spatial frequencies. However, it is applied for a brief period before the readout and the strength of the gradient is changed incrementally between each radio frequency pulse. For each phase encoding step a line of k–space is filled.

Either a spin echo or a gradient echo can be used to refocus the magnetisation.

The 3DFT technique is rather similar except that there is no slice selection and phase-encoding is performed two separate directions.

Another scheme which is sometimes used, especially in brain scanning or where images are needed very rapidly, is called echo-planar imaging (EPI): in this case each rf excitation is followed by a whole train of gradient echoes with different spatial encoding.

Example of a Pulse Sequence

In the timing diagram, the horizontal axis represents time. The vertical axis represents: (top row) amplitude of radiofrequency pulses; (middle rows) amplitudes of the three orthogonal magnetic field gradient pulses; and (bottom row) receiver analog-to-digital converter (ADC). Radiofrequencies are transmitted at the Larmor frequency of the nuclide to be imaged: for example for ¹H in a magnetic field of 1T, a frequency of 42578100 Hz would be employed. The three field gradients are labeled G_X (typically corresponding to a patient's Left-to-Right direction and colored red in diagram), G_Y (typically corresponding to a patient's Front-to-Back direction and colored green in diagram), and G_Z (typically corresponding to a patient's Head-to-Toe direction and colored blue in diagram). Where negative-going gradient pulses are shown, they represent reversal of the gradient direction, i. e. Right-to-Left, Back-to-Front or Toe-to-Head. For human scanning gradient strengths of 0. 001-0. 01 T. m^-1 are employed: higher gradient strengths permit better resolution and faster imaging. The pulse sequence shown here would produce a transverse (axial) image.

The first part of the pulse sequence, SS, achieves Slice Selection. A shaped pulse (shown here with a sinc modulation) causes a 90° (π/2 radian) nutation of longitudinal nuclear magnetization within a slab, or slice, creating transverse magnetization. Sinc or SINC may mean Sinc function, a mathematical function Site of Importance for Nature Conservation, a designation used The second part of the pulse sequence, PE, imparts a phase shift upon the slice-selected nuclear magnetization, varying with its location in the Y direction. The third part of the pulse sequence, another Slice Selection (of the same slice) uses another shaped pulse to cause a 180° (π radian) rotation of transverse nuclear magnetization within the slice. This transverse magnetisation refocuses to form a spin echo at a time TE. During the spin echo, a frequency-encoding (FE) or readout gradient is applied, making the resonant frequency of the nuclear magnetization vary with its location in the X direction. The signal is sampled n_FE times by the ADC during this period, as represented by the vertical lines. Typically n_FE of between 128 and 512 samples are taken.

The longitudinal relaxation is then allowed to recover somewhat and after a time TR the whole sequence is repeated n_PE times, but with the phase-encoding gradient incremented (indicated by the horizontal hatching in the green gradient block). Typically n_PE of between 128 and 512 repetitions are made.

The negative-going lobes in G_X and G_Z are imposed to ensure that, at time TE (the spin echo maximum), phase only encodes spatial location in the Y direction.

Typically TE is between 5msec and 100msec, while TR is between 100msec and 2000msec.

After the two-dimensional matrix (typical dimension between 128x128 and 512x512) has been acquired, producing the so-called K-space data, a two-dimensional Fourier transform is performed to provide the familiar MR image.

K-space

See main article K-space

In 1983 Ljunggren^[2] and Tweig^[3] independently introduced the k-space formalism, a technique that proved invaluable in unifying different MR imaging techniques. k -space is a formalism widely used in Magnetic resonance imaging independently introduced in 1983 by Ljunggren and Twieg They showed that the demodulated MR signal S(t) generated by freely precessing nuclear spins in the presence of a linear magnetic field gradient G equals the Fourier transform of the effective spin density i. e.

where:

In other words, as time progresses the signal traces out a trajectory in k-space with the velocity vector of the trajectory proportional to the vector of the applied magnetic field gradient. By the term effective spin density we mean the true spin density corrected for the effects of T₁ preparation, T₂ decay, dephasing due to field inhomogeneity, flow, diffusion, etc. and any other phenomena that affect that amount of transverse magnetization available to induce signal in the RF probe.

From the basic k-space formula, it follows immediately that we reconstruct an image simply by taking the inverse Fourier transform of the sampled data viz. In Mathematics, Fourier inversion recovers a function from its Fourier transform.

Using the k-space formalism, a number of seemingly complex ideas became simple. For example, it becomes very easy to understand the role of phase encoding (the so-called spin-warp method). In a standard spin echo or gradient echo scan, where the readout (or view) gradient is constant (e. g. G_x), a single line of k-space is scanned per RF excitation. When the phase encoding gradient is zero, the line scanned is the k_x axis. When a non-zero phase-encoding pulse is added in between the RF excitation and the commencement of the readout gradient, this line moves up or down in k-space i. e. we scan the line k_y=constant.

The k-space formalism also makes it very easy to compare different scanning techniques. In single-shot EPI, all of k-space is scanned in a single shot, following either a sinusoidal or zig-zag trajectory. Since alternating lines of k-space are scanned in opposite directions, this must be taken into account in the reconstruction. Multi-shot EPI and fast spin echo techniques acquire only part of k-space per excitation. In each shot, a different interleaved segment is acquired, and the shots are repeated until k-space is sufficiently well-covered. Since the data at the center of k-space represent lower spatial frequencies than the data at the edges of k-space, the T_E value for the center of k-space determines the image's T₂ contrast.

The importance of the center of k-space in determining image contrast can be exploited in more advanced imaging techniques. One such technique is spiral acquisition - a rotating magnetic field gradient is applied, causing the trajectory in k-space to trace out spiral out from the center to the edge. Due to T₂ and T₂ * decay the signal is greatest at the start of the acquisition, hence acquiring the center of k-space first improves contrast to noise ratio (CNR) when compared to conventional zig-zag acquisitions, especially in the presence of rapid movement.

Since and are conjugate variables (with respect to the Fourier transform) we can use the Nyquist theorem to show that the step in k-space determines the field of view of the image (maximum frequency that is correctly sampled) and the maximum value of k sampled determines the resolution i. The Nyquist–Shannon sampling theorem is a fundamental result in the field of Information theory, in particular Telecommunications and Signal processing e. .

(these relationships apply to each axis [X, Y, and Z] independently).

Scanner construction and operation

The three systems described above form the major components of an MRI scanner: a static magnetic field, an RF transmitter and receiver, and three orthogonal, controllable magnetic gradients.

Magnet

The magnet is the largest and most expensive component of the scanner, and the remainder of the scanner is built around it. Just as important as the strength of the main magnet is its precision. The straightness of the magnetic lines within the center (or, as it is technically known, the iso-center) of the magnet needs to be nearly perfect. This is known as homogeneity. Fluctuations (non-homogeneities in the field strength) within the scan region should be less than three parts per million (3 ppm). Three types of magnets have been used:

• Permanent magnet: Conventional magnets made from ferromagnetic materials (e. g. , steel) can be used to provide the static magnetic field. A permanent magnet that is powerful enough to be used in an MRI will be extremely large and bulky; they can weigh over 100 tonnes. But permanent magnet MRIs are very inexpensive to maintain; this cannot be said of the other types of MRI magnets. But there are significant drawbacks to using permanent magnets. They are only capable of achieving relatively weak field strengths compared to other MRI magnets (usually less than 0. 4 T), and they are of limited precision and stability. The tesla (symbol T) is the SI derived unit of Magnetic field B (which is also known as "magnetic flux density" and "magnetic Permanent magnets also present special safety issues; since their magnetic fields cannot be "turned off," ferromagnetic objects are virtually impossible to remove from them once they come into direct contact. Permanent magnets also require special care when they are being brought to their site of installation.
• Resistive electromagnet: A solenoid wound from copper wire is an alternative to a permanent magnet. A solenoid is a three-dimensional Coil. In Physics, the term solenoid refers to a loop of wire often wrapped around a Metallic core which An advantage is low initial cost, but field strength and stability are limited. The electromagnet requires considerable electrical energy during operation which can make it expensive to operate. This design is essentially obsolete.
• Superconducting electromagnet: When a niobium-titanium alloy is cooled by liquid helium to 4K (−269°C, −452°F) it becomes a superconductor, losing resistance to flow of electrical current. A superconducting magnet is an Electromagnet that is built using superconducting coils Niobium (naɪˈoʊbiəm or columbium (/kəˈlʌmbiəm/ is a Chemical element that has the symbol Nb and Atomic number 41 Titanium (taɪˈteɪniəm is a Chemical element with the symbol Ti and Atomic number 22 Helium exists in Liquid form only at very low Temperatures The Boiling point and critical point depend on the Isotope Superconductivity is a phenomenon occurring in certain Materials generally at very low Temperatures characterized by exactly zero electrical resistance An electromagnet constructed with superconductors can have extremely high field strengths, with very high stability. The construction of such magnets is extremely costly, and the cryogenic helium is expensive and difficult to handle. However, despite its cost, helium cooled superconducting magnets are the most common type found in MRI scanners today.

Most superconducting magnets have their coils of superconductive wire immersed in liquid helium, inside a vessel called a cryostat. A Cryostat (cryo=cold and stat=stable is a vessel similar in construction to a Vacuum flask, or Dewar used to maintain cold Cryogenic temperatures Despite thermal insulation, ambient heat causes the helium to slowly boil off. Such magnets, therefore, require regular topping-up with liquid helium. Generally a cryocooler, also known as a coldhead, is used to recondense some helium vapor back into the liquid helium bath. Cryocoolers are the devices used to reach cryogenic temperatures Several manufacturers now offer 'cryogenless' scanners, where instead of being immersed in liquid helium the magnet wire is cooled directly by a cryocooler.

Magnets are available in a variety of shapes. However, permanent magnets are most frequently 'C' shaped, and superconducting magnets most frequently cylindrical. However, C-shaped superconducting magnets and box-shaped permanent magnets have also been used.

Magnetic field strength is an important factor in determining image quality. Higher magnetic fields increase signal-to-noise ratio, permitting higher resolution or faster scanning. Signal-to-noise ratio (often abbreviated SNR or S/N) is an Electrical engineering concept also used in other fields (such as scientific Measurements However, higher field strengths require more costly magnets with higher maintenance costs, and have increased safety concerns. 1. 0 - 1. 5T field strengths are a good compromise between cost and performance for general medical use. However, for certain specialist uses (e. g. , brain imaging), field strengths up to 3. 0 T may be desirable.

Radio frequency system

The radio frequency (RF) transmission system consists of a RF synthesizer, power amplifier and transmitting coil. This is usually built into the body of the scanner. The power of the transmitter is variable, but high-end scanners may have a peak output power of up to 35 kW, and be capable of sustaining average power of 1 kW. The receiver consists of the coil, pre-amplifier and signal processing system. While it is possible to scan using the integrated coil for transmitting and receiving, if a small region is being imaged then better image quality is obtained by using a close-fitting smaller coil. A variety of coils are available which fit around parts of the body, e. g. , the head, knee, wrist, or internally, e. g. , the rectum.

A recent development in MRI technology has been the development of sophisticated multi-element phased array coils which are capable of acquiring multiple channels of data in parallel. This article is about general theory and electromagnetic phased array This 'parallel imaging' technique uses unique acquisition schemes that allow for accelerated imaging, by replacing some of the spatial coding originating from the magnetic gradients with the spatial sensitivity of the different coil elements. However the increased acceleration also reduces the signal-to-noise ratio and can create residual artifacts in the image reconstruction. Two frequently used parallel acquisition and reconstruction schemes are SENSE^[4] and GRAPPA^[5]. A detailed review of parallel imaging techniques can be found here: ^[6]

Gradients

Gradient coils are used to spatially encode the positions of protons by varying the magnetic field linearly across the imaging volume. The Larmor frequency will then vary as a function of position in the x, y and z-axes.

Gradient coils are usually resistive electromagnets powered by sophisticated amplifiers which permit rapid and precise adjustments to their field strength and direction. Typical gradient systems are capable of producing gradients from 20 mT/m to 100 mT/m (i. e. in a 1. 5 T magnet, when a maximal z-axis gradient is applied the field strength may be 1. 45 T at one end of a 1 m long bore, and 1. 55 T at the other). It is the magnetic gradients that determine the plane of imaging - because the orthogonal gradients can be combined freely, any plane can be selected for imaging.

Scan speed is dependent on performance of the gradient system. Stronger gradients allow for faster imaging, or for higher resolution; similarly, gradients systems capable of faster switching can also permit faster scanning. However, gradient performance is limited by safety concerns over nerve stimulation.

Image Contrast

In order to understand MRI contrast, it is important to have some understanding of the time constants involved in relaxation processes that establish equilibrium following RF excitation. In Physics and Engineering, the time constant usually denoted by the Greek letter \tau, (tau characterizes the Frequency As the high-energy nuclei relax and realign they emit energy at rates which are recorded to provide information about the material they are in. The realignment of nuclear spins with the magnetic field is termed longitudinal relaxation and the time required for a certain percentage of the tissue's nuclei to realign is termed "Time 1" or T1, which is typically about 1 second at 1. Spin-lattice relaxation time, known as T1, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. 5 tesla main field strength. T2-weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse; the transverse relaxation time is termed "Time 2" or T2, typically < 100 ms for tissue at 1. Spin-spin relaxation time, known as T2, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. A transverse wave is a moving Wave that consists of oscillations occurring perpendicular to the direction of energy transfer Spin-spin relaxation time, known as T2, is a time constant in Nuclear Magnetic Resonance and Magnetic Resonance Imaging. A millisecond (from Milli- and Second; abbreviation ms is one thousandth of a Second. 5 tesla main field strength. A subtle but important variant of the T2 technique is called T2* imaging. T2 imaging employs a spin echo technique, in which spins are refocused to compensate for local magnetic field inhomogeneities. In Nuclear magnetic resonance, spin echo refers to the refocusing of precessing nuclear spin magnetisation by a 180° pulse of resonant radiofrequency T2* imaging is performed without refocusing. This sacrifices some image integrity (resolution) but provides additional sensitivity to relaxation processes that cause incoherence of transverse magnetization. Image resolution describes the detail an Image holds The term applies equally to Digital images film images and other types of images Applications of T2* imaging include functional MRI (fMRI) or evaluation of baseline vascular perfusion (e. Functional MRI or functional Magnetic Resonance Imaging (fMRI is a type of specialized MRI scan In Physiology, perfusion is the process of nutritive delivery of Arterial Blood to a Capillary bed in the Biological tissue. g. cerebral blood flow (CBF)) and cerebral blood volume (CBV) using injected agents; in these cases, there is an inherent trade-off between image quality and detection sensitivity. Cerebral blood flow, or CBF, is the blood supply to the Brain in a given time Because T2*-weighted sequences are sensitive to magnetic inhomogeneity (as can be caused by deposition of iron-containing blood-degradation products), such sequences are utilized to detect subtle areas of recent or chronic intra cranial hemorrhage ("Heme sequence"). Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26

Image contrast is created by using a selection of image acquisition parameters that weights signal by T1, T2 or T2*, or no relaxation time ("proton-density images"). In the brain, T1-weighting causes the nerve connections of white matter to appear white, and the congregations of neurons of gray matter to appear gray, while cerebrospinal fluid appears dark. White matter is one of the three main solid components of the Central nervous system. Cerebrospinal fluid ( CSF) Liquor cerebrospinalis, is a clear Bodily fluid that occupies the Subarachnoid space and the Ventricular system The contrast of "white matter," "gray matter'" and "cerebrospinal fluid" is reversed using T2 or T2* imaging, whereas proton-weighted imaging provides little contrast in normal subjects. Additionally, functional information (CBF, CBV, blood oxygenation) can be encoded within T1, T2, or T2*. Pulse oximetry is a non-invasive method allowing the monitoring of the Oxygenation of a patient's Hemoglobin.

Contrast enhancement

Both T1-weighted and T2-weighted images are acquired for most medical examinations; However they do not always adequately show the anatomy or pathology. Anatomy (from the Greek anatomia, from ana separate apart from and temnein, to cut up cut open is a branch of Biology that is the consideration The first option is to use a more sophisticated image acquisition technique such as fat suppression or chemical-shift imaging. ^[7] The other is to administer a contrast agent to delineate areas of interest. About CONTRAST CONTRAST is a Multidisciplinary alliance bringing together key skills and expertise to generate new knowledge on biological environmental

A contrast agent may be as simple as water, taken orally, for imaging the stomach and small bowel although substances with specific magnetic properties may be used. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Most commonly, a paramagnetic contrast agent (usually a gadolinium compound^[8][9]) is given. Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field Gadolinium (ˌgædəˈlɪniəm is a Chemical element that has the symbol Gd and Atomic number 64 Gadolinium-enhanced tissues and fluids appear extremely bright on T1-weighted images. This provides high sensitivity for detection of vascular tissues (e. g. tumors) and permits assessment of brain perfusion (e. g. in stroke). There have been concerns raised recently regarding the toxicity of gadolinium-based contrast agents and their impact on persons with impaired kidney function. Special actions may be taken, such as hemodialysis following a contrast MRI scan for renally-impaired patients. In Medicine, hemodialysis (also haemodialysis) is a method for removing waste products such as Potassium and Urea, as well as free water Renal failure or kidney

More recently, superparamagnetic contrast agents (e. Superparamagnetism is a form of Magnetism. A superparamagnetic material is composed of small Ferromagnetic clusters (e g. iron oxide nanoparticles^[10][11]) have become available. Altogether there are sixteen known Iron Oxides and oxyhydroxides In Nanotechnology, a particle is defined as a small object that behaves as a whole unit in terms of its transport and properties These agents appear very dark on T2*-weighted images and may be used for liver imaging - normal liver tissue retains the agent, but abnormal areas (e. The liver is a vital organ in the human body and is present in Vertebrates and some other animals g. scars, tumors) do not. They can also be taken orally, to improve visualization of the gastrointestinal tract, and to prevent water in the gastrointestinal tract from obscuring other organs (e. g. pancreas). The pancreas is a Gland organ in the digestive and Endocrine system of Vertebrates.

Diamagnetic agents such as barium sulfate have been studied for potential use in the gastrointestinal tract, but are less frequently used. Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied Magnetic field, thus causing a repulsive effect Barium sulfate is a white crystalline solid with the chemical formula BaSO4

Applications

In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue. A brain tumor is any intracranial Tumor created by abnormal and uncontrolled cell division, normally either in the Brain itself ( Neurons One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range. Compare this to CT scans and traditional X-rays which involve doses of ionizing radiation and may increase the risk of malignancy, especially in a fetus. Computed tomography (CT is a Medical imaging method employing Tomography. For medical radiography see Radiology Radiography is the use of X-rays to view unseen or hard-to-image objects Image talkNew_radiation_symbol_ISO_21482svg for details --> Ionizing radiation Cancer (medical term Malignant Neoplasm) is a class of Diseases in which a group of cells display uncontrolled

While CT provides good spatial resolution (the ability to distinguish two structures an arbitrarily small distance from each other as separate), MRI provides comparable resolution with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues). Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera Contrast resolution is referred to as the ability of an imaging modality such as Magnetic resonance imaging or Fluoroscopy to distinguish between various contrasts The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.

For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, a sequence will take on the property of T2-weighting. On a T2-weighted scan, fat-, water- and fluid-containing tissues are bright (most modern T2 sequences are actually fast T2 sequences). Damaged tissue tends to develop edema, which makes a T2-weighted sequence sensitive for pathology, and generally able to distinguish pathologic tissue from normal tissue. Oedema (or Edema in American English formerly known as dropsy or hydropsy, is the increase of Interstitial fluid in any organ &mdash swelling With the addition of an additional radio frequency pulse and additional manipulation of the magnetic gradients, a T2-weighted sequence can be converted to a FLAIR sequence, in which free water is now dark, but edematous tissues remain bright. Fluid Attenuated Inversion Recovery (FLAIR is a Pulse sequence used in Magnetic Resonance Imaging which was invented by Dr Graeme Bydder. This sequence in particular is currently the most sensitive way to evaluate the brain for demyelinating diseases, such as multiple sclerosis. Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the

The typical MRI examination consists of 5-20 sequences, each of which are chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician. A physician, medical practitioner or medical doctor who practices Medicine, and is concerned with maintaining or restoring human Health

Specialized MRI scans

Diffusion MRI

Main article: Diffusion MRI

Diffusion MRI measures the diffusion of water molecules in biological tissues. Diffusion MRI is a Magnetic resonance imaging (MRI method that produces In vivo images of biological tissues weighted with the local microstructural Diffusion MRI is a Magnetic resonance imaging (MRI method that produces In vivo images of biological tissues weighted with the local microstructural Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement ^[12] In an isotropic medium (inside a glass of water for example) water molecules naturally move randomly according to Brownian motion. Isotropy is uniformity in all directions Precise definitions depend on the subject area This article is about the physical phenomenon for the stochastic process see Wiener process. In biological tissues however, the diffusion may be anisotropic. Anisotropy (pronounced with stress on the third syllable ˌænaɪˈsɒtrəpi is the property of being directionally dependent as opposed to Isotropy, which means homogeneity For example a molecule inside the axon of a neuron has a low probability of crossing the myelin membrane. An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell Myelin is an electrically-insulating Dielectric Phospholipid layer that surrounds only the Axons of many Neurons It is an outgrowth Therefore the molecule will move principally along the axis of the neural fiber. If we know that molecules in a particular voxel diffuse principally in one direction we can make the assumption that the majority of the fibers in this area are going parallel to that direction. A voxel (a Portmanteau of the words Volumetric and Pixel) is a volume element representing a value on a Regular grid in

The recent development of diffusion tensor imaging (DTI) enables diffusion to be measured in multiple directions and the fractional anisotropy in each direction to be calculated for each voxel. Diffusion MRI is a Magnetic resonance imaging (MRI method that produces In vivo images of biological tissues weighted with the local microstructural This enables researchers to make brain maps of fiber directions to examine the connectivity of different regions in the brain (using tractography) or to examine areas of neural degeneration and demyelination in diseases like Multiple Sclerosis. In Neuroscience, tractography is a procedure to demonstrate the Neural tracts It utilizes special techniques of Magnetic resonance imaging (MRI and computer-based

Another application of diffusion MRI is diffusion-weighted imaging (DWI). Diffusion MRI is a Magnetic resonance imaging (MRI method that produces In vivo images of biological tissues weighted with the local microstructural Following an ischemic stroke, DWI is highly sensitive to the changes occurring in the lesion. A stroke is the rapidly developing loss of brain functions due to a disturbance in the blood vessels supplying blood to the brain ^[13] It is speculated that increases in restriction (barriers) to water diffusion, as a result of cytotoxic edema (cellular swelling), is responsible for the increase in signal on a DWI scan. The DWI enhancement appears within 5-10 minutes of the onset of stroke symptoms (as compared with computed tomography, which often does not detect changes of acute infarct for up to 4-6 hours) and remains for up to two weeks. Computed tomography (CT is a Medical imaging method employing Tomography. Coupled with imaging of cerebral perfusion, researchers can highlight regions of "perfusion/diffusion mismatch" that may indicate regions capable of salvage by reperfusion therapy.

Like many other specialized applications, this technique is usually coupled with a fast image acquisition sequence, such as echo planar imaging sequence.

Magnetic resonance angiography

Magnetic resonance angiography (MRA) is used to generate pictures of the arteries in order to evaluate them for stenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture). Angiography or arteriography is a Medical imaging technique in which an X-ray image is taken to visualize the inside or lumen, of blood vessels A stenosis ( Plural: stenoses; from Ancient Greek στένωσις "narrowing" is an abnormal narrowing in a Blood vessel An aneurysm (or aneurism) is a localized blood-filled dilation (balloon-like bulge of a blood vessel caused by disease or weakening of the vessel wall MRA is often used to evaluate the arteries of the neck and brain, the thoracic and abdominal aorta, the renal arteries, and the legs (called a "run-off"). A variety of techniques can be used to generate the pictures, such as administration of a paramagnetic contrast agent (gadolinium) or using a technique known as "flow-related enhancement" (e. Gadolinium (ˌgædəˈlɪniəm is a Chemical element that has the symbol Gd and Atomic number 64 g. 2D and 3D time-of-flight sequences), where most of the signal on an image is due to blood which has recently moved into that plane, see also FLASH MRI. FLASH MRI ( Fast Low Angle Shot Magnetic Resonance Imaging) is a basic measuring principle for rapid MRI invented in 1985 by Jens Frahm and Axel Haase (German Magnetic resonance venography (MRV) is a similar procedure that is used to image veins. In this method the tissue is now excited inferiorly while signal is gathered in the plane immediately superior to the excitation plane, and thus imaging the venous blood which has recently moved from the excited plane.

Magnetic resonance spectroscopy

In vivo ('in the living organism') magnetic resonance spectroscopy (MRS), also known as MRSI (MRS imaging) and volume selective NMR spectroscopy, is a technique which combines the spatially-addressable nature of MRI with the spectroscopically-rich information obtainable from NMR. In vivo (that is 'in the living organism' magnetic resonance spectroscopy (MRS is a specialised technique associated with magnetic resonance imaging (MRI That is to say, MRI allows one to study a particular region within an organism or sample, but gives relatively little information about the chemical or physical nature of that region (its chief value is in being able to distinguish the properties of that region, how much fat or water is present, relative to those of surrounding regions). MR spectroscopy, however, provides a wealth of information about other biological chemicals ('metabolites') within that region, as would an NMR spectrum of that region. In vivo (that is 'in the living organism' magnetic resonance spectroscopy (MRS is a specialised technique associated with magnetic resonance imaging (MRI

Functional MRI

Main article: Functional magnetic resonance imaging

Functional MRI (fMRI) measures signal changes in the brain that are due to changing neural activity. Functional MRI or functional Magnetic Resonance Imaging (fMRI is a type of specialized MRI scan The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information The brain is scanned at low resolution but at a rapid rate (typically once every 2-3 seconds). Increases in neural activity cause changes in the MR signal via T2* changes^[14]; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Functional MRI or functional Magnetic Resonance Imaging (fMRI is a type of specialized MRI scan Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body Hemoglobin ( also spelled haemoglobin and abbreviated Hb or Hgb) is the Iron -containing Oxygen -transport Metalloprotein Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.

While BOLD signal is the most common method employed for neuroscience studies in human subjects, the flexible nature of MR imaging provides means to sensitize the signal to other aspects of the blood supply. Alternative techniques employ arterial spin labeling (ASL) or weight the MRI signal by cerebral blood flow (CBF) and cerebral blood volume (CBV). The CBV method requires injection of a class of MRI contrast agents that are now in human clinical trials. Because this method has been shown to be far more sensitive than the BOLD technique in preclinical studies, it may potentially expand the role of fMRI in clinical applications. The CBF method provides more quantitative information than the BOLD signal, albeit at a significant loss of detection sensitivity.

Here is a video compiled of MRI scans showing two arachnoid cysts: http://www.youtube.com/watch?v=PF_mDsdxSsg

Interventional MRI

Main article: Interventional MRI

The lack of harmful effects on the patient and the operator make MRI well-suited for "interventional radiology", where the images produced by a MRI scanner are used to guide minimally-invasive procedures. Interventional magnetic resonance imaging, also Interventional MRI or IMRI, is the use of Magnetic resonance imaging (MRI to do Interventional radiology Of course, such procedures must be done without any ferromagnetic instruments.

A specialized growing subset of interventional MRI is that of intraoperative MRI in which the MRI is used in the surgical process. Some specialized MRI systems have been developed that allow imaging concurrent with the surgical procedure. More typical, however, is that the surgical procedure is temporarily interrupted so that MR images can be acquired to verify the success of the procedure or guide subsequent surgical work.

Radiation therapy simulation

Because of MRI's superior imaging of soft tissues, it is now being utilized to specifically locate tumors within the body in preparation for radiation therapy treatments. For therapy simulation, a patient is placed in specific, reproducible, body position and scanned. The MRI system then computes the precise location, shape and orientation of the tumor mass, correcting for any spatial distortion inherent in the system. The patient is then marked or tattooed with points which, when combined with the specific body position, will permit precise triangulation for radiation therapy.

Current density imaging

Current density imaging (CDI) endeavors to use the phase information from images to reconstruct current densities within a subject. Current density imaging (CDI is an extension of Magnetic resonance imaging (MRI developed at the University of Toronto. Current density imaging works because electrical currents generate magnetic fields, which in turn affect the phase of the magnetic dipoles during an imaging sequence. To date no successful CDI has been performed using biological currents, but several studies have been published which involve applied currents through a pair of electrodes.

Magnetic resonance guided focused ultrasound

In MRgFUS therapy, ultrasound beams are focused on a tissue - guided and controlled using MR thermal imaging - and due to the significant energy deposition at the focus, temperature within the tissue rises to more than 65°C, completely destroying it. HIFU ( high intensity focused ultrasound) (sometimes FUS or HIFUS) is a highly precise medical procedure using high-intensity focused Ultrasound The Celsius Temperature scale was previously known as the centigrade scale. This technology can achieve precise "ablation" of diseased tissue. Ablation is defined as the removal of material from the surface of an object by Vaporization, Chipping, or other erosive processes MR imaging provides a three-dimensional view of the target tissue, allowing for precise focusing of ultrasound energy. The MR imaging provides quantitative, real-time, thermal images of the treated area. This allows the physician to ensure that the temperature generated during each cycle of ultrasound energy is sufficient to cause thermal ablation within the desired tissue and if not, to adapt the parameters to ensure effective treatment.

Multinuclear imaging

Hydrogen is the most frequently imaged nucleus in MRI because it is present in biological tissues in great abundance. However, any nucleus which has a net nuclear spin could potentially be imaged with MRI. Such nuclei include helium-3, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Fluorine, fluorum meaning "to flow" is the Chemical element with the symbol F and Atomic number 9 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 Phosphorus, (ˈfɒsfərəs is the Chemical element that has the symbol P and Atomic number 15 Xenon (ˈzɛnɒn or) is a Chemical element represented by the symbol Xe. ²³Na and ³¹P are naturally abundant in the body, so can be imaged directly. Gaseous isotopes such as ³He or ¹²⁹Xe must be hyperpolarized and then inhaled as their nuclear density is too low to yield a useful signal under normal conditions. Hyperpolarization is the nuclear spin polarization of a material far beyond Thermal equilibrium conditions ¹⁷O, ¹³C and ¹⁹F can be administered in sufficient quantities in liquid form (e. g. ¹⁷O-water, ¹³C-glucose solutions or perfluorocarbons) that hyperpolarization is not a necessity. Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology.

Multinuclear imaging is primarily a research technique at present. However, potential applications include functional imaging and imaging of organs poorly seen on ¹H MRI (e. g. lungs and bones) or as alternative contrast agents. Inhaled hyperpolarized ³He can be used to image the distribution of air spaces within the lungs. Injectable solutions containing ¹³C or stabilized bubbles of hyperpolarized ¹²⁹Xe have been studied as contrast agents for angiography and perfusion imaging. ³¹P can potentially provide information on bone density and structure, as well as functional imaging of the brain.

Magnetic Resonance Spectroscopy

Magnetic resonance spectroscopy is used to measure the levels of different metabolites in body tissues. In vivo (that is 'in the living organism' magnetic resonance spectroscopy (MRS is a specialised technique associated with magnetic resonance imaging (MRI The MR signal produces a spectrum of resonances that correspond to different molecular arrangements of the isotope being "excited". This signature is used to diagnose certain metabolic disorders, especially those affecting the brain,^[15] as well as to provide information on tumor metabolism. Metabolism is the set of Chemical reactions that occur in living Organisms in order to maintain Life. ^[16]

Susceptibility Weighted Imaging (SWI)

Susceptibility Weighted Imaging (also known as SWI), is a new type of contrast in MRI different from spin density, T1, or T2 imaging. This method exploits the susceptibility differences between tissues and uses a fully velocity compensated, three dimensional, rf spoiled, high-resolution, 3D gradient echo scan. This special data acquisition and image processing produces an enhanced contrast magnitude image very sensitive to venous blood, hemorrhage and iron storage. It is used to enhance the detection and diagnosis of tumors, vascular and neurovascular diseases (stroke and hemorrhage, multiple sclerosis, Alzheimer's), and also detects traumatic brain injuries that may not be diagnosed using other methods. ^[17]

Experimental MRI techniques

Currently there is active research in several new MRI technologies like magnetization transfer MRI (MT-MRI), diffusion tensor MRI (DT-MRI), Susceptibility Weighted Imaging MRI (SWI), and proton MR spectroscopy, plus recent research in to Dendrimer-enhanced MRI as a diagnostic and prognostic biomarker of sepsis-induced acute renal failure and additional developments based on SWI as an imaging biomarker for tumors, neurological and neurovascular diseases. Magnetization transfer (MT refers to the transfer of longitudinal magnetization from free water protons to hydration water protons in NMR and MRI. Diffusion MRI is a Magnetic resonance imaging (MRI method that produces In vivo images of biological tissues weighted with the local microstructural Acute renal failure ( ARF) also known as acute kidney failure or acute kidney injury, is a rapid loss of Renal function due to damage to the

With between 72% and 90% accuracy where chance would achieve 0. 8%,^[18] fMRI techniques can decide which of a set of known images the subject is viewing. ^[19]

Portable instruments

Portable magnetic resonance instruments are available for use in education and field research. Using the principles of Earth's field NMR, they have no powerful polarizing magnet, so that such instruments can be small and relatively inexpensive. Nuclear magnetic resonance (NMR in the Geomagnetic field is conventionally referred to as Earth's field NMR (EFNMR. Some can be used for both EFNMR spectroscopy and MRI imaging^[20]. The low strength of the Earth's field results in poor signal to noise ratios, requiring relatively long scan times to capture spectroscopic data or build up MRI images.

MRI vs CT

A computed tomography (CT) scanner uses X-rays, a type of ionizing radiation, to acquire its images, making it a good tool for examining tissue composed of elements of a higher atomic number than the tissue surrounding them, such as bone and calcifications (calcium based) within the body (carbon based flesh), or of structures (vessels, bowel). Computed tomography (CT is a Medical imaging method employing Tomography. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. Image talkNew_radiation_symbol_ISO_21482svg for details --> Ionizing radiation MRI, on the other hand, uses non-ionizing radio frequency (RF) signals to acquire its images and is best suited for non-calcified tissue, though MR images can also be acquired from bones and teeth^[21] as well as fossils^[22]. Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz

CT may be enhanced by use of contrast agents containing elements of a higher atomic number than the surrounding flesh such as iodine or barium. RadiocontrastIn a medical setting a contrast medium is any Substance that is used to enhance the visibility of structures or fluids within the body Iodine (ˈaɪədaɪn ˈaɪədɪn or /ˈaɪədiːn/ from ιώδης iodes "violet" is a Chemical element that has the symbol I and Atomic Barium (ˈbɛəriəm is a Chemical element. It has the symbol Ba, and Atomic number 56 Contrast agents for MRI are those which have paramagnetic properties, e. Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field g. gadolinium and manganese. Gadolinium (ˌgædəˈlɪniəm is a Chemical element that has the symbol Gd and Atomic number 64 Manganese (ˈmæŋgəniːz is a Chemical element, designated by the symbol Mn.

Both CT and MRI scanners can generate multiple two-dimensional cross-sections (slices) of tissue and three-dimensional reconstructions. Unlike CT, which uses only X-ray attenuation to generate image contrast, MRI has a long list of properties that may be used to generate image contrast. By variation of scanning parameters, tissue contrast can be altered and enhanced in various ways to detect different features. (See Application below. )

MRI can generate cross-sectional images in any plane (including oblique planes). In the past, CT was limited to acquiring images in the axial (or near axial) plane. The scans used to be called Computed Axial Tomography scans (CAT scans). However, the development of multi-detector CT scanners with near-isotropic resolution, allows the CT scanner to produce data that can be retrospectively reconstructed in any plane with minimal loss of image quality. Isotropy is uniformity in all directions Precise definitions depend on the subject area

For purposes of tumor detection and identification in the brain, MRI is generally superior. ^[23][24][25] However, in the case of solid tumors of the abdomen and chest, CT is often preferred due to less motion artifact. However, CT usually is more widely available, faster, much less expensive, and may be less likely to require the person to be sedated or anesthetized.

MRI is also best suited for cases when a patient is to undergo the exam several times successively in the short term, because, unlike CT, it does not expose the patient to the hazards of ionizing radiation.

Economics of MRI

MRI equipment is expensive. New 1. 5 tesla scanners often cost between $1,000,000 USD and $1,500,000 USD. New 3. 0 tesla scanners often cost between $2,000,000 and $2,300,000 USD. Construction of MRI suites can cost $500,000 USD.

For over a dozen years, MRI scanners have been significant sources of revenue for healthcare providers in the US. This is because of favorable reimbursement rates from insurers, both private and federal government programs. Insurance reimbursement has historically been provided in two components, technical for the actual performance of the MRI scan and professional for the radiologist's review of the images and/or data.

In the US, the 2007 Deficit Reduction Act (DRA) significantly reduced reimbursement rates paid by federal insurance programs for the technical component of many scans, shifting the economic landscape. Many private insurers have followed suit.

Currently, in the US, there is increasing interest in reducing the costs associated with MRI services and simultaneously improving the ability to effectively and efficiently provide MRI examination services to larger numbers of patients with the same equipment.

Safety

Implants and foreign bodies

Pacemakers are generally considered an absolute contraindication towards MRI scanning, though highly specialized protocols have been developed to permit scanning of select pacing devices. For other uses see Pacemaker (disambiguation A pacemaker (or artificial pacemaker, so as not to be confused with the heart's natural pacemaker In Medicine, a contraindication (pronounced as contra-indication is a condition or factor that increases the Risks involved in using a particular drug, Several cases of arrhythmia or death have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. Dysrhythmia redirects here For the American band see Dysrhythmia (band. Notably, the Medtronic company has received FDA approval for the first-ever clinical trial for a MR-Conditional pacemaker device. Other electronic implants have varying contraindications, depending upon scanner technology, and implant properties, scanning protocols and anatomy being imaged.

Though pacemakers receive significant attention, it should also be noted that many other forms of medical or biostimulation implants may be contraindicated for MRI scans. These may include vagus nerve stimulators, implantable cardioverter-defibrillators, loop recorders, insulin pumps, cochlear implants, deep brain stimulators, and many others. An implantable cardioverter-defibrillator ( ICD) is a small battery -powered electrical impulse generator which is implanted in patients who are at risk of Sudden A cochlear implant (CI is a surgically implanted electronic device that provides a sense of Sound to a person who is profoundly deaf or severely hard of hearing Medical device patients should always present complete information (manufacturer, model, serial number and date of implantation) about all implants to both the referring physician and to the radiologist or technologist before entering the room for the MRI scan.

While these implants pose a current problem, scientist and manufacturers are working on improved designs which will further minimize the risks that MRI scans pose to medical device operations. One such development in the works is a nano-coating for implants intended to screen them from the radio frequency waves, helping to make MRI exams available to patients currently prohibited from receiving them. The current article for this is from New Scientist.

Ferromagnetic foreign bodies (e. Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it g. shell fragments), or metallic implants (e. A shell is a payload-carrying Projectile, which as opposed to shot, contains an explosive or other filling though modern usage includes large solid projectiles g. surgical prostheses, aneurysm clips) are also potential risks, and safety aspects need to be considered on an individual basis. In Medicine, a prosthesis (plural prostheses) is an Artificial extension that replaces a missing Body part. An aneurysm (or aneurism) is a localized blood-filled dilation (balloon-like bulge of a blood vessel caused by disease or weakening of the vessel wall Interaction of the magnetic and radio frequency fields with such objects can lead to: trauma due to movement of the object in the magnetic field, thermal injury from radio-frequency induction heating of the object, or failure of an implanted device. Induction heating is the process of Heating an electrically conducting object (usually a Metal) by Electromagnetic induction, where Eddy currents These issues are especially problematic when dealing with the eye. Most MRI centers require an orbital x-ray to be performed on anyone suspected of having metal fragments in their eyes, something not uncommon in metalworking. Orbital x-ray or orbital radiography is a specialized form of Radiography used to study the orbit of the eye Metalworking is craft and practice of working with Metals to create individual parts assemblies or large scale structures

Because of its non-ferromagnetic nature and poor electrical conductivity, titanium and its alloys are useful for long term implants and surgical instruments intended for use in image-guided surgery. Ferromagnetism is the basic mechanism by which certain materials (such as Iron) form Permanent magnets and/or exhibit strong interactions with Magnets it Titanium (taɪˈteɪniəm is a Chemical element with the symbol Ti and Atomic number 22 Image-guided surgery is the general term used for any surgical procedure where the Surgeon uses indirect visualization to operate i In particular, not only is titanium safe from movement from the magnetic field, but artifacts around the implant are less frequent and less severe than with more ferromagnetic materials e. g. stainless steel. Artifacts from metal frequently appear as regions of empty space around the implant - frequently called 'black-hole artifact' e. g. a 3mm titanium alloy coronary stent may appear as a 5mm diameter region of empty space on MRI, whereas around a stainless steel stent, the artifact may extend for 10-20 mm or more.

In 2006, a new classification system for implants and ancillary clinical devices has been developed by ASTM International and is now the standard supported by the US Food and Drug Administration:

MR-Safe: The device or implant is completely non-magnetic, non-electrically conductive, and non-RF reactive, eliminating all of the primary potential threats during an MRI procedure.

MR-Conditional: A device or implant that may contain magnetic, electrically conductive or RF-reactive components that is safe for operations in proximity to the MRI, provided the conditions for safe operation are defined and observed (such as 'tested safe to 1. 5 teslas' or 'safe in magnetic fields below 500 gauss in strength').

MR-Unsafe: Nearly self-explanatory, this category is reserved for objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room.

In the case of pacemakers, the risk is thought to be primarily RF induction in the pacing electrodes/wires causing inappropriate pacing of the heart, rather than the magnetic field affecting the pacemaker itself. Much research and development is being undertaken, and many tools are being developed in order to predict the effects of the RF fields inside the body.

Patients that have been prescribed MRI exams who are concerned about safety may be interested in the 10 Questions To Ask Your MRI Provider.

MRI providers who wish to measure the degree to which they have effectively addressed the safety issues for patients and staff may be interested in the MRI Suite Safety Calculator provided through a radiology website.

Projectile or missile effect

As a result of the very high strength of the magnetic field needed to produce scans (frequently up to 60,000 times the earth's own magnetic field effects), there are several incidental safety issues addressed in MRI facilities. Missile-effect accidents, where ferromagnetic objects are attracted to the center of the magnet, have resulted in injury and death. ^[26] A video simulation of a fatal projectile effect accident illustrates the extreme power that contemporary MRI equipment can exert on ferromagnetic objects.

In order to help reduce the risks of projectile accidents, ferrous objects and devices are typically prohibited in proximity to the MRI scanner, with non-ferromagnetic versions of many tools and devices typically retained by the scanning facility. Patients undergoing MRI examinations are required to remove all metallic objects, often by changing into a gown or scrubs. Scrubs are the shirts and trousers or gowns worn by Nurses surgeons and other Operating room personnel when "scrubbing in" for Surgery

New ferromagnetic-only detection devices are proving highly effective in supplementing conventional screening techniques in many leading hospitals and imaging centers and are now recommended by the American College of Radiology's Guidance Document for Safe MR Practices: 2007 and the Joint Commission's Sentinel Event Alert #38.

The magnetic field and the associated risk of missile-effect accidents remains a permanent hazard — as superconductive MRI magnets retain their magnetic field, even in the event of a power outage. Superconductivity is a phenomenon occurring in certain Materials generally at very low Temperatures characterized by exactly zero electrical resistance

Radio frequency energy

A powerful radio transmitter is needed for excitation of proton spins. This can heat the body to the point of risk of hyperthermia in patients, particularly in obese patients or those with thermoregulation disorders. Hyperthermia, in its advanced state referred to as heat stroke or sunstroke, is an acute condition which occurs when the Body produces or absorbs more Several countries have issued restrictions on the maximum specific absorption rate that a scanner may produce. Specific absorption rate (SAR is a measure of the rate at which Radio frequency (RF energy is absorbed by the body when exposed to radio-frequency electromagnetic field

Peripheral nerve stimulation

The rapid switching on and off of the magnetic field gradients is capable of causing nerve stimulation. Volunteers report a twitching sensation when exposed to rapidly switched fields, particularly in their extremities. The reason the peripheral nerves are stimulated is that the changing field increases with distance from the center of the gradient coils (which more or less coincides with the center of the magnet). Note however that when imaging the head, the heart is far off-center and induction of even a tiny current into the heart must be avoided at all costs. Although PNR was not a problem for the slow, weak gradients used in the early days of MRI, the strong, rapidly-switched gradients used in techniques such as EPI, fMRI, diffusion MRI, etc. are indeed capable of inducing PNR. American and European regulatory agencies insist that manufacturers stay below specified dB/dt limits (dB/dt is the change in field per unit time) or else prove that no PNR is induced for any imaging sequence. As a result of dB/dt limitation, commercial MRI systems cannot use the full rated power of their gradient amplifiers.

Acoustic Noise

Rapidly switched magnetic gradients interact with the main magnetic field to cause minute expansions and contractions of the coil itself, resulting in loud noises and vibrations. This is most marked with high-field machines and rapid-imaging techniques in which sound intensity can reach 130 dB (equivalent to a jet engine at take-off).

Appropriate use of ear protection is essential for anyone inside the MRI scanner room during the examination.

Cryogens

As described above in 'Scanner Construction And Operation', many MRI scanners rely on cryogenic liquids to enable superconducting capabilities of the electromagnetic coils within. Though the cryogenic liquids most frequently used are non-toxic, their physical properties present specific hazards.

An emergency shut-down of a superconducting electromagnet, an operation known as "quenching", involves the rapid boiling of liquid helium from the device. A superconducting magnet is an Electromagnet that is built using superconducting coils Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical If the rapidly expanding helium cannot be dissipated through an external vent, sometimes referred to as 'quench pipe', it may be released into the scanner room where it may cause displacement of the oxygen and present a risk of asphyxiation.

Liquid helium, the most commonly used cryogen in MRI, undergoes near explosive expansion as it changes from liquid to a gaseous state. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Rooms built in support of superconducting MRI equipment should be equipped with pressure relief mechanisms and an exhaust fan, in addition to the required quench pipe.

Since a quench results in rapid loss of all cryogens in the magnet, recommissioning the magnet is extremely expensive and time-consuming. Cryogenics is often used incorrectly to refer to Cryonics, cryopreserving humans or animals Spontaneous quenches are uncommon, but may also be triggered by equipment malfunction, improper cryogen fill technique, contaminates inside the cryostat, or extreme magnetic or vibrational disturbances.

Contrast agents

The most frequently used intravenous contrast agents are based on chelates of gadolinium. Chelation is the binding or complexation of a bi- or multidentate Ligand. Gadolinium (ˌgædəˈlɪniəm is a Chemical element that has the symbol Gd and Atomic number 64 In general, these agents have proved safer than the iodinated contrast agents used in X-ray radiography or CT. Anaphylactoid reactions are rare occurring in approx 0. 03-0. 1%. ^[27] Of particular interest is the lower incidence of nephrotoxicity, compared with iodinated agents, when given at usual doses—this has made contrast-enhanced MRI scanning an option for patients with renal impairment, who would otherwise not be able to undergo contrast-enhanced CT. ^[28]

Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis there is a risk of a rare but serious illness, nephrogenic systemic fibrosis, that may be linked to the use of certain gadolinium-containing agents: the most frequently linked is gadodiamide, but other agents have been linked too. Nephrogenic systemic fibrosis ( NSF) or Nephrogenic fibrosing dermopathy is a rare and serious Syndrome that involves Fibrosis of skin joints ^[29] Although a causal link has not been definitively established, current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential, and that dialysis should be performed as soon as possible after the scan is complete, in order to remove the agent from the body promptly. The United States of America —commonly referred to as the In Medicine, dialysis (from Greek "dialusis" meaning dissolution "dia" meaning through and "lusis" meaning loosening is primarily ^[30] In Europe where more gadolinium-containing agents are available, a classification of agents according to potential risks has been released. ^[31][32]

Pregnancy

No harmful effects of MRI on the fetus have been demonstrated. In particular, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. Image talkNew_radiation_symbol_ISO_21482svg for details --> Ionizing radiation However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as organogenesis takes place during this period. In animal development, organogenesis is the process by which the Ectoderm, Endoderm, and Mesoderm develop into the Internal organs The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects—particularly to heating and to noise. However, one additional concern is the use of contrast agents; gadolinium compounds are known to cross the placenta and enter the fetal bloodstream, and it is recommended that their use be avoided. Gadolinium (ˌgædəˈlɪniəm is a Chemical element that has the symbol Gd and Atomic number 64

Despite these concerns, MRI is rapidly growing in importance as a way of diagnosing and monitoring congenital defects of the fetus because it can provide more diagnostic information than ultrasound and it lacks the ionizing radiation of CT. A congenital disorder is a disease or disorder that is present at birth Not to be confused with Supersonic. Ultrasound is cyclic Sound pressure with a Frequency greater than the upper MRI without contrast is the imaging mode of choice for pre-surgical, in-utero diagnosis and evaluation of fetal tumors, primarily teratomas, facilitating open fetal surgery, other fetal interventions, and planning for procedures (such as the EXIT procedure) to safely deliver and treat babies whose defects would otherwise be fatal. A teratoma is a type of neoplasm. The word teratoma comes from Greek and means roughly "monstrous tumor" Open fetal surgery is an invasive form of Fetal intervention in the treatment of Birth defects where the pregnant Uterus is opened up for direct surgery Fetal intervention involves In utero surgical treatment of a Fetus. The EXIT procedure, or ex utero intrapartum treatment procedure, is a specialized surgical delivery procedure used to deliver babies who have airway compression due to Bronchopulmonary

Claustrophobia and discomfort

Due to the construction of MRI scanners, they are potentially unpleasant to lie in. The part of the body being imaged needs to lie at the center of the magnet (which is often a long, narrow tube). Because scan times may be long (perhaps one hour), people with even mild claustrophobia are often unable to tolerate an MRI scan without management. Claustrophobia (from Greek κλειστο closed is the fear of enclosed spaces

Management may include:

• Advance preparation
  • visiting the scanner to see the room and practice lying on the table
  • visualization techniques
  • chemical sedation
  • general anesthesia
• Coping while inside the scanner
  • holding a "panic button"
  • listening to music on headphones or watching a movie with a Head-mounted display while in the machine
• Modified scanner designs
  • upright MRIs
  • open-bore design scanners. In modern medical practice general anaesthesia ( AmE: anesthesia) is a state of total unconsciousness resulting from General anaesthetic drugs A head-mounted display or Helmet mounted display, both abbreviated 'HMD' is a display device worn on the head or as part of a helmet that has a small display optic
  • New Scan Rooms with lighting, sound and images on the wall. Some rooms come with images on the walls or ceiling.

Though open MRIs have increased in popularity as of late, they produce inferior scan quality because they operate at lower magnetic fields than closed MRIs. However, commercial 1 Tesla open MRIs have recently become available, providing much better image quality than previous lower field strength open models.

[1] Video of Open MRI

For babies and children, chemical sedation or general anesthesia are the norm. These MRI subjects are too young to be instructed to hold still during the scanning session. Obese patients and pregnant women may find the MRI machine to be a tight fit, and some claustrophobics may find the experience intolerable without sedation. Pregnant women may also have difficulty lying on their backs for an hour or more without moving.

The noise associated with the operation of an MRI scanner (audible noise associated with the gradient pulses applied to the subject) can also exacerbate the discomfort associated with the procedure.

Guidance

Safety issues, including the potential for biostimulation device interference, movement of ferromagnetic bodies, and incidental localized heating, have been addressed in the American College of Radiology's White Paper on MR Safety which was originally published in 2002 and expanded in 2004. The American College of Radiology ( ACR) founded in 1923 is a non-profit professional medical organization composed of diagnostic radiologists radiation oncologists interventional The ACR White Paper on MR Safety has been rewritten and was released early in 2007 under the new title ACR Guidance Document for Safe MR Practices.
In February of 2008, the Joint Commission, a US healthcare accrediting organization, issued a Sentinel Event Alert #38, their highest patient safety advisory, on MRI safety issues. The Joint Commission is a private sector United States -based non-profit organization.

The European Physical Agents Directive

The European Physical Agents (Electromagnetic Fields) Directive is European legislation that has been adopted in European legislature. By 2008 each individual state within the European Union must include this directive in its own law.

The directive applies to occupational exposure to electromagnetic fields (not medical exposure) and was intended to limit workers’ acute exposure to strong electromagnetic fields, as may be found near electricity substations, radio or television transmitters or industrial equipment. However, the regulations impact significantly on MRI, with separate sections of the regulations limiting exposure to static magnetic fields, changing magnetic fields and radio frequency energy. Field strength limits are given which may not be exceeded for any period of time. An employer may commit a criminal offense by allowing a worker to exceed an exposure limit if that is how the Directive is implemented in a particular Member State.

The Directive is based on the international consensus of established effects of exposure to electromagnetic fields, and in particular the advice of the European Commissions's advisor, the International Commission on Non-Ionizing Radiation Protection (ICNIRP). The aims of the Directive, and the ICNIRP guidelines upon which it is based, are to prevent exposure to potentially harmful fields. The actual limits in the Directive are very similar to the limits advised by the Institute of Electrical and Electronics Engineers, with the exception of the frequencies produced by the gradient coils, where the IEEE limits are significantly higher.

Many Member States of the EU already have either specific EMF regulations or (as in the UK) a general requirement under workplace health and safety legislation to protect workers against electromagnetic fields. In almost all cases the existing regulations are aligned with the ICNIRP limits so that the Directive should, in theory, have little impact on any employer already meeting their legal responsibilities.

The introduction of the Directive has brought to light an existing potential issue with occupational exposures to MRI fields. There are at present very few data on the number or types of MRI practice that might lead to exposures in excess of the levels of the Directive^[33][34]. There is a justifiable concern amongst MRI practitioners that if the Directive were to be enforced more vigorously than existing legislation, the use of MRI might be restricted, or working practices of MRI personnel might have to change.

In the initial draft a limit of static field strength to 2 T was given. This has since been removed from the regulations, and whilst it is unlikely to be restored as it was without a strong justification, some restriction on static fields may be reintroduced after the matter has been considered more fully by ICNIRP. The effect of such a limit might be to restrict the installation, operation and maintenance of MRI scanners with magnets of 2 T and stronger. As the increase in field strength has been instrumental in developing higher resolution and higher performance scanners, this would be a significant step back. This is why it is unlikely to happen without strong justification.

Individual government agencies and the European Commission have now formed a working group to examine the implications on MRI and to try to address the issue of occupational exposures to electromagnetic fields from MRI.

2003 Nobel Prize

Reflecting the fundamental importance and applicability of MRI in the medical field, Paul Lauterbur of the University of Illinois at Urbana-Champaign and Sir Peter Mansfield of the University of Nottingham were awarded the 2003 Nobel Prize in Physiology or Medicine for their "discoveries concerning magnetic resonance imaging". Paul Christian Lauterbur ( May 6, 1929 – March 27, 2007) was an American Chemist who shared the Nobel Prize This article is about the flagship campus For other uses and locations of University of Illinois, see University of Illinois (disambiguation The University of Sir Peter Mansfield, FRS, (born 9 October 1933) is a British Physicist who was awarded the 2003 Nobel Prize in Physiology or The University of Nottingham is a Public, Co-educational institution of Higher learning in the city of Nottingham, England. Year 2003 ( MMIII) was a Common year starting on Wednesday of the Gregorian calendar. The Nobel Prize in Physiology or Medicine (Nobelpriset i fysiologi eller medicin is awarded once a year by the Swedish Karolinska Institute. The Nobel Prize committee acknowledged Lauterbur's insight of using magnetic field gradients to introduce spatial localization, a discovery that allowed rapid acquisition of 2D images. Sir Peter Mansfield was credited with introducing the mathematical formalism and developing techniques for efficient gradient utilization and fast imaging.

Controversy

The 2003 Nobel Prize in Medicine award was vigorously protested by Raymond Vahan Damadian, who claimed that he was the inventor of MRI, and that Lauterbur and Mansfield had merely refined the technology. Raymond Vahan Damadian (born March 16 1936) is an American practitioner of Magnetic resonance imaging. An ad hoc group, called "The Friends of Raymond Damadian", took out full-page advertisements in the New York Times and The Washington Post entitled "The Shameful Wrong That Must Be Righted", demanding that he be awarded at least a share of the Nobel Prize. Ad hoc is a Latin phrase which means "for this [ Purpose ]" The Washington Post is the largest and most circulated Newspaper in Washington D ^[35] The Nobel Assembly at Karolinska Institutet, which picks the winner in medicine, refused, as is their custom, to comment on Damadian's claims or change the awardees. Karolinska Institutet (often translated from Swedish into English as the Karolinska Institute, and in older texts often as the Royal Caroline

In a letter to Physics Today, Herman Carr pointed out his own early use of field gradients for one-dimensional MR imaging. Physics Today magazine created in 1948 is the membership journal of The American Institute of Physics. Herman Y Carr ( November 28, 1924 - April 9, 2008) was an American physicist and pioneer of Magnetic resonance imaging. ^[36] The contribution by John Mallard and colleagues at the University of Aberdeen, who developed the spin-warp technology, as well as producing the first clinically useful images in patients, is also often overlooked. John Mallard OBE FRSE was Professor of Medical Physics at the University of Aberdeen The University of Aberdeen is an Ancient university founded in 1495, in Old Aberdeen, Scotland. ^{[37] [38] [39]}

See also

• Earth's field NMR (EFNMR)
• Electron-spin resonance (spin physics)
• High intensity focused ultrasound
• History of brain imaging
• InVesalius
• Medical imaging
• Neuroimaging software
• Nephrogenic fibrosing dermopathy
• Nobel Prize controversies
• Nuclear magnetic resonance (NMR)
• Pneumoencephalogram
• Relaxation
• Robinson oscillator
• Rabi cycle
• EMMI European Master in Molecular Imaging

Footnotes

References

• James Mattson and Merrill Simon. The Pioneers of NMR and Magnetic Resonance in Medicine: The Story of MRI. Jericho & New York: Bar-Ilan University Press, 1996. ISBN 0961924314.
• E. M. Haacke, R. W. Brown, M. L. Thompson, R. Venkatesan. Magnetic Resonance Imaging: Physical Principles and Sequence Design, John Wiley, 1999. ISBN 0471351288.

External links

• A Guided Tour of MRI: An introduction for laypeople National High Magnetic Field Laboratory
• Joseph P. Hornak, Ph. D. The Basics of MRI. Underlying physics and technical aspects.
• Video: What to Expect During Your MRI Exam from the Institute for Magnetic Resonance Safety, Education, and Research (IMRSER)
• 3D Animation Movie about MRI Exam
• Interactive Flash Animation on MRI - Online Magnetic Resonance Imaging physics and technique course
• International Society for Magnetic Resonance in Medicine
• Article on helium scarcity and potential effects on NMR and MRI communities

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