Example of Iodine based contrast in Cerebral Angiography

Digital Subtraction Angiography (DSA) is a type of Fluoroscopy technique used in interventional radiology to clearly visualize blood vessels in a bony or dense soft tissue environment. Fluoroscopy is an imaging technique commonly used by Physicians to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope Images are produced using contrast medium by subtracting a 'pre-contrast image' or the mask from later images, once the contrast medium has been introduced into a structure. Hence the term 'digital subtraction angiography'.

Applications

DSA, is primarily used to image blood vessels. It is useful in the diagnosis and treatment of:

• Arterial and venous occlusions, including carotid artery stenosis, pulmonary embolisms and acute limb ischaemia. Carotid stenosis is a narrowing of the lumen of the Carotid artery, usually caused by Atherosclerosis. Pulmonary embolism (PE is a blockage of the Pulmonary artery or one of its branches usually occurring when a venous Thrombus (blood clot from a vein
• Arterial stenosis, which is particularly useful for potential renal donors in detecting renal artery stenosis. Renal artery stenosis is the narrowing of the Renal artery, most often caused by Atherosclerosis or Fibromuscular dysplasia.
• Cerebral aneurysms and AVMs.

DSA and Fluoroscopy

In traditional angiography images are acquired by exposing an area of interest with time-controlled x-rays while injecting contrast medium into the blood vessels. 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 The image obtained would also include all overlying structure besides the blood vessels in this area. This is useful for determining anatomical position and variations but unhelpful for visualising blood vessels accurately.

In order to remove these distracting structures to see the vessels better, first a mask image is acquired. The mask image is simply an image of the same area before the contrast is administered. The radiological equipment used to capture this is usually an image intensifier, which will then keep producing images of the same area at a set rate (1 - 6 frames per second), taking all subsequent images away from the original 'mask' image. A night vision device (NVD is an Optical instrument that allows images to be produced in levels of light approaching total darkness The radiologist controls how much contrast media is injected and for how long. Smaller structures require less contrast to fill the vessel than others. Images produced appear with a very pale grey background, which produces a high contrast to the blood vessels, which appear a very dark grey.

The images are all produced in real time by the computer, as the contrast is injected into the blood vessels.

The future

DSA is being used less and less routinely in imaging departments. It is being taken over by Computed Tomography Angiography, which can produce 3D images through a test which is less invasive and stressful for the patient. CTA or Computed tomography angiography is an examination that uses X-rays to visualize blood flow in arterial and venous vessels throughout the body

See also

• Intravenous digital subtraction angiography
• Angiography
• Contrast Medium
• Image intensifier
• Computed Tomography Angiography
• Peripheral artery occlusive disease

External links

• http://www.droid.cuhk.edu.hk/web/service/angio/dsa.htm