MRA of the Neck (Carotids)
Basics: What is an MRA of the Neck (Carotids)?
MRA stands for “magnetic resonance angiography”. In short, MRA of the Neck is a quick (20 minutes), extremely accurate noninvasive test (other than an IV), to get a close look at the arteries in your neck. It is designed to look at the main arterial trunk carrying oxygenated blood from your heart (aortic arch), to the 3 main vessels that supply blood to the neck, head and arms (great vessels), and the four main arteries supplying blood to your head: These are the left and right carotid arteries, and the left and right vertebral arteries. The carotid and vertebral arteries are usually the focus of this exam, as they are the ones most commonly affected with atherosclerosis. MRA of the Neck (Carotids) extremely good at finding narrowed or blocked arteries, as well as a tear in one of your arteries, which may lead to a stroke.
Do you have to put an IV in to inject dye?
We provide the most accurate MRA of the neck of anywhere in the entire USA. This is because: 1) Our Magnet provides the most powerful magnetic field, (3 Tesla), 2) we have a special neck vascular coil, which provides the highest resolution images, and 3) we purposely set up the test to take the finest detailed images. It takes longer, and more skill, but we believe it is worth it, 4) we cover a larger area of the neck and are able to clearly visualize more distal vessels that other centers are not able to do, 5) We are the only center in the NW that routinely can not just see the aortic arch very clearly, but also see the entirety of the subclavian arteries, and often even proximal axillary arteries, and finally 6) We run a new type of sequence called time resolved angiography, so that we can actually watch the blood flow through the arteries and veins “real time”. This allows us to discover important information about blocked blood vessels, and how areas with poor blood flow get help from other blood vessels (“collateral blood flow”).
MRA of the carotid arteries has become the mainstay for evaluating for vascular disease of the vessels of the neck (arteries). The images are so good, there is no longer any need for carotid angiography except for unusual pathologies, and unless the neurointerventionalist is actually going to go in and fix something. If someone tells you that you need an (invasive) elective diagnostic angiogram just to look at the vessels, I would not accept this at face value. I would challenge the physician, and I would get a second opinion. The obvious exception for this is for a patient that is having something acute going on (acute stroke symptoms, or hemorrhage), particularly when there is a significant chance that during the angiogram, a neurointervention (thrombolysis, stenting, embolization) will be performed. We perform very few cerebral angiograms (less than 15%) on any of our patients, unless we are actually going to go up and repair something. The most common reasons why MRA images of the carotid arteries are obtained is to look for carotid narrowing/stenoses, a tear in an artery, (called a “dissection”), blocked/occluded arteries, as well as other more unusual problems.
An MRA Carotid study takes approximately 20 minutes to complete and does require intravenous injection of gadolinium. Obtaining a very good study requires great skill and care.
Some of the more common reasons why many MRA Neck studies obtained at imaging centers and hospitals are not very good include:
- The MRI machine is not powerful enough (1.5 Tesla or less), to get clear enough images, especially where blood vessels take turns, or where there are flow abnormalities.
The clarity/resolution at the aortic arch is not very good, and many carotid studies on less powerful magnets (1.5 Tesla or less) over estimate disease/narrowing at the base of the vertebral arteries. This can happen at 3T as well, but it is easier to overcome. We have spent a tremendous amount of time adjusting our sequences, which has virtually eliminated these problems at our facility.
- Less than perfect technique by the MRI technologist, (patient positioning, timing of the contrast/dye injection), is accepted at the particular hospital or center where the images are acquired. This has become a common reality, as technologists are asked to do more and more complex studies in an increasingly limited amount of time. There are a number of subtle things that a skilled technologist can do to improve the patient’s ability to cooperate during the exam to limit movement, swallowing etc, but they must be given the time and support/infrastructure to provide high quality.
- The machine is set up to obtain an average quality study, rather than an exceptional one. It takes more time per patient to collect higher resolution images. That costs hospitals and centers money. We take the extra time for the additional resolution.
- The studies are allowed to be read by radiologists who are not fellowship trained in neuroradiology. These studies are only read at our facility by extremely experienced neuroradiologists well trained in reading them.
LIMITATIONS OF GETTING A GREAT MRA OF THE NECK STUDY:
If the patient can’t lie still, or has excessive motion of the upper chest/thorax during breathing, the study may not be the best. In addition, if the patient is large (more than 300 lbs) the study can be limited. Despite these issues, we usually are still able to obtain a very diagnostic study.
Answers to commonly asked questions about MRA of the Carotids:
Q: How does one decide whether to get an MRA of the neck, versus CTA of the neck, versus ultrasound of the neck?
A: If the patient is asymptomatic, ultrasound is an excellent relatively inexpensive initial screening tool. However if there are carotid bruits, or stroke like symptoms, or recent stroke with suspicion of carotid or vertebral artery disease/ posterior circulation questions, then I would recommend MRA of the neck. If there are carotid bruits suggesting stenosis, an ultrasound will only evaluate the distal common carotids above the clavicle, and the proximal and mid cervical internal carotid arteries. Ultrasound tells you very little about the vertebral arteries other than antegrade (forward) or retrograde(backward) flow. In addition, an MRA evaluates the great vessels from the aortic arch all the way up into the head. Tandem lesions, collateral vessels, and visualization of the vertebral arteries, are excellent reasons to have a low threshold to get an MRA instead of an ultrasound. In addition, we are currently the only center in the northwest routinely doing “CINE”(Movies) of the MRAs of the neck and head (“time resolved angiography”), so like an angiogram, we can actually watch blood flow from the arch to the head and back during the arterial, capillary, and venous phases.