MRA Technique

General techniques for MRA in the abdomen include lo-calizer images, a fast T2 scan, a dose-timing acquisition, and breath-hold three-dimensional gradient recalled echo images acquired before, during, and following the intravenous administration of gadolinium contrast agent. In addition, delayed, post-contrast T1-weighted gradient echo imaging with fat suppression is helpful. Additional optional scans include two-dimensional cardiac-gated phase contrast images, as well as three-dimensional phase contrast imaging.

Localizer images are typically obtained using a three-plane localizer acquisition, therefore allowing the visualization of the aorta along its long axis, as well the position of the kidneys posteriorly and the mesenteric circulation anteriorly. Subsequently, a fast, breath-hold T2-weighted image is very useful to evaluate masses incidentally found on the MRA portion of the exam. A fat-suppressed single-shot fast spin echo image is highly desirable (HASTE, single-shot fast spin echo). Using these techniques, T2-weighted images can be obtained throughout the abdomen in a single breath-hold, therefore speeding the examination. The heavily T2-weighted single-shot methods are sensitive to fluid, however, it should be cautioned that subtle differences in T2 may be masked due to magnetization transfer effects occurring within the fast spin echo sequence. Therefore, these techniques are most useful for confirming cystic fluid collections, rather than complete characterization of all abdominal masses.

Subsequently, the arrival time of contrast agent in the region-of-interest must be determined. Several options for determining the arrival time of contrast media are available. These include first, a dose-timing scan, in which a small test dose of contrast agent (1-2 cc) is administered, and serial gradient recalled images are obtained at 1-2 second intervals. The arrival time of contrast can be determined by measuring a region of interest in the serial images [1]. Second, fluoroscopically-triggered MRA, in which serial gradient recalled images are obtained and rapidly reconstructed so that the individual performing the exam can detect the arrival of contrast agent within the region-of-interest [2]. Third, automatically triggered techniques, where a sampling volume is placed over the aorta, and the MRI scanner detects the arrival of contrast agent within the sampling volume automatically, triggering the three-dimensional acquisition [3].

For the contrast-enhanced scan, meticulous attention to detail is important to ensure diagnostic studies. Typically, a three-dimensional T1-weighted spoiled-gradient recalled echo acquisition is acquired. Fast gradients are important in order to ensure that a short repetition time (TR) and echo time (TE) are achieved, therefore maximizing spatial resolution per unit time. Images are acquired prior to contrast agent in order to verify that the location of the three-dimensional volume covers the anatomic region-of-interest. In addition, the pre-contrast image set is used as a mask, if mask subtraction is necessary. An image volume is then acquired during the arterial phase of contrast enhancement, followed by an additional volume during the venous phase.

Following the three-dimensional volume acquisition, delayed post-contrast T1-weighted gradient echo imaging with fat suppression is performed. These images are very useful for determining the true size of aortic aneurysms, evaluating venous contrast enhancement patterns for parenchymal lesions that are detected on the three-dimensional contrast MRA, as well for evaluating venous abnormalities including venous thrombosis. These images are typically obtained using an interleaved gradient echo technique in order to optimize signal-to-noise ratio and contrast resolution.

Finally, optional supportive sequences include two-dimensional phase contrast methods for measuring the direction of blood flow, and the volumetric flow rate in selected vessels of interest. Typically, a segmented k-space gradient recalled echo acquisition is used, and is acquired during a single breath-hold. Accurate flow measurements may be obtained if high spatial resolution studies are acquired to limit partial volume errors associated with flow measurement. Three-dimensional phase contrast acquisitions are also helpful, especially in the renal arteries, where the presence of signal void at, or distal to, a stenosis implies a hemodynamically significant stenosis.

The dose of gadolinium contrast agent required for abdominal vascular imaging is dependent on the resolution that is acquired during the examination. Many studies have been performed demonstrating that a single dose (20 cc) is satisfactory for aortic and renal artery imaging [1]. For high-resolution examinations, it may be necessary to increase the dose to 30, or even 40 cc, gadolinium contrast agent. The rationale for the increased dose is that it is necessary to provide consistent arterial enhancement throughout the acquisition technique. Typically, the gadolinium contrast is injected at a rate of 3 cc per second. The contrast administration is followed immediately by a saline flush of at least 20 cc in order to clear the line and flush the contrast agent through the venous system. It should be noted that a saline flush as large as possible should be used, because large saline flushes have been shown to provide greater arterial signal.

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