The abdominal aorta and its branches are best examined with the patient placed in the supine position. Intravenous access is obtained using a 20- or 22-gauge angiocatheter. Special care has to be taken to make sure the IV access is patent with no kinks in the tubing or occlusion of the IV line.5
Contrast can be injected manually or by using an automated MR-compatible injector. The decision whether to use a multiarray surface coil or a body coil is dependent on the extent of the coverage desired, the size of the patient, and hardware limitations, if any. In general, if a larger field-of-view is desired, it is preferable to use the body coil (Figure 1).5 If smaller coverage is needed and an improved signal-to-noise ratio is desirable, then a multiarray surface coil is preferred.
Whenever possible, the patient's arms should be placed above the head or on top of the chest. When using a noncentric or sequentially ordered k-space acquisition, the center of the k-space is filled approximately three- or four-eighths into the acquisition. This is important to understand because the center of the k-space is primarily responsible for the contrast information and the periphery of the k-space is responsible for spatial resolution in the final image.
Accurate timing of the peak of arterial enhancement to the center of the k-space is essential to optimizing arterial enhancement and minimizing venous contamination.6 To accurately time the peak of contrast enhancement in the abdominal vessels to the center of the k-space, the time of arrival of the contrast in the abdominal aorta must be determined first. This is accomplished by using a small test bolus. Typically, 1 to 2 cc of gadolinium-based contrast agent is injected and images are obtained in rapid succession (about one image per second) using a turbo-flash sequence. The number of images can then be counted to see when the contrast peaks in the vessel of interest so that scan delay can be calculated.
Evaluation for renovascular hypertension requires additional images obtained through the kidneys and the adrenal glands to exclude any adrenal or renal pathology, which could also be the cause of hypertension (Figure 2). When the abdominal aorta is being evaluated for aneurysm, additional images are obtained to depict the outer wall of the aorta and give a true estimate of the aneurysm size.
The size of the aneurysm may be underestimated on a CEMRA, as it is primarily a luminogram similar to the conventional angiogram. As the background is being suppressed on a CEMRA to depict the vessels accurately, it may be difficult to visualize the outer wall of the aneurysm. This becomes a major problem, especially when patients are being considered for endovascular stent graft placement, which requires outer-to-outer wall measurements for placement of a stent graft. The solution is to use a T1-weighted sequence or a true fast imaging with steady-state precession (FISP) sequence (Figure 3).
In general, MRA images of the abdominal aorta are obtained in end inspiration. When evaluating patients with suspected mesenteric ischemia, the images should be obtained in end expiration to minimize the impression of the arcuate ligament on the celiac axis.
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