Digital subtraction angiography

Digital Subtraction Angiography (DSA) is a 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'.

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.
 * Arterial stenosis, which is particularly useful for potential renal donors in detecting renal artery stenosis.
 * Cerebral aneurysm's and AVM's.

DSA and Fluoroscopy
In traditional angiography (sometimes called fluoroscopy), images are acquired by exposing an area of interst 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 vessles 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 usallly 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 vessle than others. Images produced appear with a very pale grey background, which produces a high contrast to the blood vessles, 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 vessles.

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.