Vascular Application Tips:

Three dimensional, contrast-enhanced magnetic resonance angiography (CE-MRA) is increasingly being used for the evaluation of patients with atherosclerotic peripheral vascular disease. This technique does not rely on blood motion like time-of-flight (TOF) techniques, but on the T1 shortening of gadolinium chelate contrast agents on blood. T1-weighted sequences can then be used to image blood directly without flow effects and the acquired MRA images are comparable to those of conventional angiography.

Because many patients with atherosclerotic peripheral vascular disease often have multiple areas of synchronous disease, it is important to image an extensive region of arterial anatomy from the abdominal aorta to the feet. 3D CE-MRA, until recently, was limited to imaging of a single region of the body, i.e. a single "station," corresponding to the field of view of the MR scanner, typically 40 to 50 cm. This small field of view is not sufficient for evaluating patients with peripheral vascular disease where the distance from upper abdominal aorta to feet is typically more than 100 cm. However, by imaging rapidly and moving the patient through the isocenter of the scanner as the gadolinium contrast passes down the legs, it is possible to image the arteries of the entire lower half of the body using multiple overlapping fields of view or “stations”. This technique is known as bolus-chase MRA. Typically, the first station includes the abdomen and pelvis; the second station includes the thigh; and the third station includes the knee and calf. On occasion a fourth station for the distal calf and feet is desired for patients with limb-threatening ischemia or nonhealing ulcerations.

The peripheral coil and the spine elements (SP1-SP6) are fixed in position on the table. The phased array coil is positioned adjacent to the peripheral coil, over the abdomen.

These protocols cover approximately 900 and 1200mm respectively. The “short” protocol moves the table 300mm between each station and incorporates 100mm of overlap between stations. The “tall” protocol moves the table 400mm between each station and incorporates 50mm of overlap between stations. In general, even patients of average height are better imaged with the “tall” protocol. A small error margin is incorporated in the proximal overlap section at the top of the FOV.

However, by measuring up from the distal point of the FOV (typically midfoot – ankle which allows more distal coverage in the distal overlap section) a distance of 900 or 1200mm the coverage can be estimated. In order to center the patient, simply measure up 750mm from the distal point of the FOV for the “short” protocol or 1000mm from the distal point of the FOV for the “tall” protocol. This will ensure that the last station includes the feet (or whatever you choose to be the distal point).

When planning the 3D slab in each station, the minimum slab thickness should be used (angle the slab parallel to the expected course of the vessels). Ideally you want the acquisition time of the first station to be approximately 10 seconds and the acquisition time of the second station to be approximately 14 seconds. This should allow 1.5mm effective thickness. The short acquisition times allow you to chase the bolus down the legs with minimized venous contamination. Note that the table move usually requires about 5 seconds.

For the third station, again the slab should be minimized, but the desired effective thickness is about 1-1.2mm. In this station elliptical, centric k-space filling is used. This allows for a longer acquisition (needed for the higher resolution) without significant venous contamination in patients with relatively normal perfusion. Two acquisitions are performed in order to capture the arterial phase in patients with significantly delayed distal transit time.

After scout images are obtained, the timing exam is performed at the level of the femoral heads (2 ml of contrast are used). Then, a 3-plane scout is obtained at each of the 3 table positions/stations. At each of the 3 locations, in turn, a pre-contrast acquisition is planned (slab sized and positioned, coils selected, etc.) and run. Then, during contrast injection, the 3 pre-contrast scans are re-run sequentially. A 2-phase injection seems to work well; 20 ml of the contrast mixture at 2 ml/sec, followed by 10 ml of the mixture at 1 ml/sec and then a 20 ml flush at 2 ml/sec.

The timing run is typically performed at the level of the femoral heads in order to assess the proximal transit time. Since this value represents the time to the distal point of the first station, you can use the time to peak as the scan delay, even with a short first station acquisition time.

Online subtraction and MIP images should be used to generate quick angiographic images for review before the patient leaves.

Because of variability in lower extremity circulation the quality of the images in the third station (calf and pedal vessels) is often suboptimal. Supplemental imaging of this station can be performed with time-of-flight or low-dose contrast-enhanced time-resolved imaging prior to the bolus chase. (refer to Time Resolved MRA)