Digital subtraction angiography
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|Digital subtraction angiography|
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. 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'.
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. 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. 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.
Intravenous digital subtraction angiography
Intravenous digital subtraction angiography (IV-DSA) is a form of angiography which was first developed in the late 1970s.
IV-DSA uses a computer technique which compares an x-ray image of a region of the body before and after radiopaque iodine based dye has been injected intravenously into the body. Tissues and blood vessels on the first image are digitally subtracted from the second image, leaving a clear picture of the artery which can then be studied independently and in isolation from the rest of the body.
Some studies have indicated that IV-DSA is not suitable for patients with diabetes or renal insufficiency because the dye load is significantly higher than is used in arteriography. However, IV-DSA has been used successfully to study the vessels of the brain and heart and has helped detect carotid artery obstruction and to map patterns of cerebral blood flow. It also helps detect and diagnose lesions in the carotid arteries which is a potential cause of strokes.
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.
- Arterial stenosis, which is particularly useful for potential renal donors in detecting renal artery stenosis. DSA is the gold standard investigation for renal artery stenosis. 
- Cerebral aneurysms and arterio-venous malformations (AVM).
DSA is being used less and less routinely in imaging departments. It is being replaced by computed tomography angiography (CTA), which can produce 3D images through a test which is less invasive and stressful for the patient. It is still considered the gold standard for arterial imaging 
- Contrast medium
- Image intensifier
- Computed Tomography Angiography (CTA)
- Peripheral artery occlusive disease
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- Signs of a stroke
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