Angiography and DSA

Principle

Angiography or arteriography is a medical imaging technique in which an X-ray picture is taken to visualize the inner volume of blood filled structures, including arteries, veins and the heart chambers. The X-ray film or image of the blood vessels is called an angiograph, or more commonly, an angiogram.

Angiograms require the insertion of a catheter into a peripheral artery, e.g. the femoral artery.

As blood has the same radiodensity (see CT scan (dual energy)) as the surrounding tissues, a radiocontrast agent (which absorbs X-rays) is added to the blood to make angiographic visualization possible. The image shows shadows of the inside of the vascular structures carrying blood with the radiocontrast agent. The tissue of the vessels or heart chambers themselves remain largely to totally invisible on the X-Ray image.

The images may be taken as either still images, displayed on a fluoroscope (see Fluoroscopy and Fluorescence) or film, useful for mapping an area. Alternatively, they may be motion images, usually taken at 30 frames/s, which also show the speed of blood (i.e. the speed of radiocontrast).

Digital Subtraction Angiography (DSA) is the procedure to visualize blood vessels with contrast medium in a bony environment by subtracting the pre-contrast image (the mask) from the image with contrast medium (see More Info).

Intravenous DSA (IV-DSA) compares an X-ray image before and after radiopaque iodine based dye has been injected intravenously. (Radiopacity is the ability of electromagnetic radiation to pass through a particular material.) 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.

Application

With a catheter (in groin or forearm) the radiocontrast agent is administrated at the desired area. X-ray images of the transient radiocontrast distribution visualize of the inner size of the arteries. Presence or absence of atherosclerosis or atheroma within the walls of the arteries cannot be clearly determined. The most common angiogram is that of the coronary arteries.

Angiography is also commonly performed to identify vessel narrowing in patients with retinal vascular disorders, such as diabetic retinopathy and macular degeneration.

Other common clinical applications consider the cerebrum, extremities, liver, kidneys, lungs and lymph system.

Fig. 1 Coronary angiogram with a stenosis in the left branch.

DSA is useful in diagnosing arterial occlusion, including carotid artery stenosis and pulmonary artery thrombosis, and in detecting renal vascular disease. After contrast material is injected into an artery or vein, fluoroscopic images are produced.

IV-DSA can be used for studying the vessels of the brain and heart, detecting carotid artery obstruction and lesions, and mapping cerebral blood flow. IV-DSA has also been useful in assessing patients prior to surgery and after coronary artery bypass surgery and some transplant operations. However, IV-DSA is unsuitable for patients with diabetes or renal insufficiency (due to the high dose of dye).

More Info

The subtraction technique

In traditional angiography, we acquire images of blood vessels on films by exposing the area of interest with time-controlled X-ray energy while injecting contrast medium into the blood vessels. The images thus obtained would also record other structure besides blood vessels as the X-ray beam passes through the body. In order to remove these distracting structures to see the vessels better, we need to acquire a mask image (image of the same area without contrast administration) for subtraction. So, using manual darkroom technique, clear pictures of blood vessels are obtained by taking away the overlying background.

In DSA, the images are acquired in digital format through the computer. Then all images would be recorded into the computer and subtracted automatically. As a result, a near-instantaneous film shows the blood vessels alone.