MRA Carotid Flow Quantification is a technique in which MRI scanners use a specific sequence to help determine if the blood flow is going in the correct direction. It may sound silly, but on some occasions this does not always happen. Typically a carotid MRA, which stands for magnetic resonance angiography, a study of the blood vessels, requires a nominal amount of MRI contrast administered to the veins, while a timed MRA sequence captures the contrast in the carotid neck vessels.
The arterial and venous blood vessels will fill with contrast, but this will not actually tell the reader the determination of the flow direction. On rare occasions, a patient may have a condition called sub-clavian steal syndrome (SSS, for short.) This condition can be discovered through a procedure called MRA carotid flow quantification of the carotid vessels. This is not a procedure that is always done with a carotid MRA study, so it needs to be specifically requested by the referring physician, the radiologist or vascular specialist. Additionally, not all MRI locations are familiar with performing flow quantification on the blood vessels, but this is something easily taught.
Additionally, one should have flow quantification MRI software available to load the images, calculate blood flow and determine the flow directions, flow speeds and more.
Some of the patients present with vertigo (dizziness), positional vertigo (turning of the head), tinnitus (ringing in the ears), numbness of the upper left arm and sometimes occurs more often in patients who have had a CABG (coronary artery bypass graft), or open heart surgical procedure. These episodes may occur (under exertion or not) when the sub-clavian artery “steals” blood flow from the left vertebral artery that feeds the base of the brain.
Phase contrast imaging is usually performed after the MR Angio study above is finished.
All of the arterial blood vessels fill up with an injection, so the blood flow speed and direction cannot be determined like they can using phase contrast flow quantification imaging. The carotid arteries can be clearly defined and a cross-section of the bilateral vessels is selected for phase contrast imaging. Phase contrast imaging works because the hydrogen protons line up North and South and are based on the Flow Quantification flow direction and Venc (Velocity Encoding) used at cc/cm (cc’s per centimeter).
The cross section usually includes both carotids and both vertebral arteries such as in the image below. With the correct VENC and Flow Quantification direction setup, and an accurate slice prescription, the direction of the blood flow can be determined. In the image below, a red and green ROI (region of interest) is placed on each of the carotid arteries, and underneath them inside the blue and yellow ROI’s are the vertebral arteries.
A magnitude image is above with ROI (region of interest) on each carotid and vertebral vessel. All vessels should show blood flow going to the top of the head to feed the brain, but in this example, one of the blood vessels, the blue ROI, shows blood flowing the opposite direction.
Below, the phase image showing the ROI and measurement of each vessel.
The chart below corresponds in color to each blood vessel that shows the direction of blood flow. It seems the blue vertebral artery, shows a reverse flow, showing sub-clavian steal syndrome in the right vertebral artery.
MRA carotid flow quantification showing reverse flow in the blue graph line, which correlates with the right vertebral artery in the cross-sectional image. One might summarize that the flow is a result of right sub-clavian steal syndrome.