Carotid Artery Disease Diagnosis
Imaging procedures are often used to diagnose carotid artery disease. In these tests, the radiologist primarily wants to determine what is going on at the carotid bifurcation (where the carotid arteries divide in the neck).
Ultrasound to Diagnose Carotid Artery Disease
The carotid arteries are small and focused and located not too far under the skin. In some cases, the bifurcation is located at the angle of the jaw and be almost impossible to image, but in most cases it is perfectly accessible. Plaque that is narrowing the artery is especially visible if it is new and uncalcified, but as the clot grows, it begins to degenerate. More often than not, parts of the clot begin to harden and calcify (i.e., hard calcium mineral is deposited). Calcium is visible on x-rays, as are the calcifications in the carotid bifurcations on x-rays of the neck. In ultrasound, however, sound waves often bounce right back when they hit these hard calcium deposits. Such calcified plaque makes it often quite difficult to see the bifurcation well enough for diagnostic purposes.
For a more in depth discussion of Doppler ultrasound, please go to ultrasound. Imaging modalities visualize flowing blood differently. In x-ray, it is not visible at all. In CT scan, the slightly increased density within large arterial structures (e.g., the aorta) as compared to the veins, in which the blood is flowing much more slowly, is apparent. In MRI scan, vessels with flowing blood have such a unique appearance that a whole new modality was created, magnetic resonance angiography or MRA, a form of blood vessel imaging.
With ultrasound you can almost "see" flowing blood. The big veins in the belly look as black as pure water in a dark basin—slowly flowing blood, no turmoil, no turbulence. The walls contract and relax in a slow rhythm. It is completely different in the arteries, through which the blood is pumped at high speeds.
For diagnostic purposes the radioloist must determing if there is a narrowing, and then just how much narrowing is present. This is where the Doppler ultrasound comes in.
The radiologist directs a fine sound wave toward the carotid bifurcation, aimed right in the middle of the vessel where the red blood cells are flowing. Flowing cells cause a change in what the receiver sees in the reflected beam. The machine analyzes the difference and calculates the velocity of the flowing blood. The Doppler signal, also, gives information about the degree of turbulence in the flowing blood, which is a distinctive sign of significant narrowing or stenosis.
Now we are able to direct finely focused sound waves into a vessel and determine how fast the red cells are moving during the entire cardiac cycle. Stenosis is measured by measuring the velocity of red cells going into the bifurcation and the velocity of red cells coming out, just like radar traps can sample the speeds of the cars coming by. We do that in two places, once up and other down from the critical bifurcation.
So now we have a test that shows us what is going on with the bifurcation and shows the percent of stenosis with good reliability. Who needs any other tests? Ultrasound is very dependent on the experience of the technologist. Also, patients with very poor heart strength experience slow blood flow and are therefore not good ultrasound subjects.
Ultrasound is now used almost universally as the first and most often, only imaging test, assuming that there are no discrepancies between the patient's clinical history, the physical findings, and the ultrasound results. In borderline situations, more information is needed for diagnosis.
MRA and CTA to Diagnose Carotid Artery Disease
Both MRA (magnetic resonance angiogram) and CTA (computed tomography angiogram) are used to examine the blood flow in vascular, mostly arterial structures such as the carotid arteries. In magnetic resonance imaging (MRI), the arteries appear as flow void, which present as black holes on cross sectional scans. If the plane of the image is redirected from cross-sectional to say frontal (down the front), all these black holes would line up into a black tube. Using various electronic tools to sharpen the image, the radiologist produces a magnetic resonance angiogram (MRA).
For the CT angiogram (CTA) dye is injected intravenously to light up the carotid arteries. The radiologist waits for the dye to circulate, then gets multiple slices (images) when the dye is most concentrated in the carotids. The computer takes the data and reconstructs it into a frontal or side plane so that the vessels are visible. As with MRA, the pictures are often quite good. With both modalities, radiologists often prefer to study the myriad of cross-sectional images than count too heavily on a computer generated "reconstruction."
Carotid Arteriography to Diagnose Carotid Artery Disease
Carotid arteriography involves taking pictures after dye has been injected into the arteries, which produces some risk. A radiologist will place a small tube (catheter) in an artery, usually in the groin, but occasionally elsewhere such as in the underarm. That tube will slide inside the artery to the spot where the radiologist will inject an iodine-based dye. In some cases, the catheter is advanced into the main artery in the chest—the aorta. This test is called thoracic aortagram. The dye fills in the various parts of the thoracic aorta, as well as all its branches, among which are the two carotid arteries (usually the one on the left comes off the aorta directly, the one on the right usually comes from the innominate artery that arises from the aorta). Digital subtraction is a special technique to take away bone and other confusing shadows so the dye in the vessels is even more visible.
In other circumstances, the catheters are inserted directly into the carotid arteries. Carotid arteriography does increase the risk to the patient. The pictures are great, as good as they can be, but the test is usually used for difficult cases when the rest of the information doesn't come together. Carotid arteriography is a very safe test with complication rates under 1% in most institutions. However, even when performed by an experienced radiologist with experienced technologists, it carries some significant risk.