Contrast Enhanced MRA

The acquisition of high quality images of the mesenteric vessels became feasible with the introduction of CE MRA [18, 26, 27]. Moreover, the technique permits the visualization of very small vessels that are not discernible using non-enhanced imaging techniques.

Imaging in the coronal plane permits evaluation of the aorta, splanchnic arteries and portal vein in one examination. A partition thickness of 3-5 mm is acceptable if zero padding is available for interpolation. In the absence of an interpolation algorithm, the slice thickness should be less than 3 mm. To evaluate stenotic disease of the celiac trunk or the proximal mesenteric arteries, imaging should be performed in the sagittal plane. Aliasing is not as severe for acquisitions in the sagittal plane, so it is possible to utilize a rectangular field-of-view with a high spatial resolution acquisition matrix (e.g. 512 x 256). If a slower MR system is used, it is advantageous to acquire images in the sagittal plane so that fewer sections or partitions are required to cover the aorta, celiac artery (CA), superior mesenteric artery (SMA), and inferior mesenteric artery (IMA). Imaging parameters should be adjusted to allow for image acquisition during breath-hold. Axial imaging may be useful if the primary goal is evaluation of the hepatic arteries, hepatic parenchyma or portal vein. However, one difficulty with the axial orientation is aliasing in the slice direction, which tends to be severe with the extremely short radio frequency (RF) pulses used in 3D CE MRA. To minimize aliasing, a coil should be used in which the cranial-dimension is only slightly larger than the caudal-dimension of the imaging volume. Fat saturation or chemically selective fat inversion pulses should also be considered. These will help minimize unwanted signal from pericardial and abdominal fat wrapping onto the image volume.

3D CE MRA datasets should be acquired before, during and after completion of intravenous contrast agent administration. Pre-contrast images should be checked to ensure that the imaging volume is positioned correctly. These images can also be used subsequently for digital subtraction to improve image contrast. Accurate timing of the contrast agent bolus is essential for arterial phase acquisitions. This can be achieved with automatic triggering (SmartPrep or Care Bolus), fluoroscopic triggering (Bolus Track) or by means of a test bolus to the mid-abdominal aorta. After arterial phase imaging, acquisition of a delayed image dataset is useful to show the portal-venous and hepatic venous anatomy. Arterial phase 3D CE MRA is best evaluated by first acquiring multiple overlapping maximum intensity projection (MIP) reconstructions in the coronal plane. Thereafter, reformations and subvolume MIP reconstructions can be prepared in perpendicular planes through each major abdominal aortic branch vessel, including the celiac trunk, and the SMA and IMA. It is also useful to assess the iliac arteries, especially the internal iliac arteries, as they may represent an important collateral pathway in patients with chronic mesenteric ischemia.

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