Complementary Sequences

A PC-MR scan can be employed to determine the direction of portal venous blood flow. A single 510 mm thick 2D phase contrast image is acquired in an axial or oblique plan, perpendicular to the portal vein. Typical imaging parameters are: 28 ms/6 ms TR/TE/Flip = 28/6/45° and VENC = 40 cm/s. On 2D phase contrast velocity map images, background tissues are gray, while blood flow is shown as either bright vixels or black pixels, depending on their direction of flow. By convention, flow in the superior-to-inferior (S/I), right-to-left (R/L), and anterior-to-posterior (A/P) directions is bright, whereas flow in the opposite direction is displayed as dark on velocity-encoded 2D phase contrast images. Through plane flow can similarly be mapped on oblique acquisitions. In order to interpret flow data correctly, the orthogonal plane coming closest to the scan obliquity needs to be determined. Alternatively, if the portal vein is more vertical than horizontal, a straight axial 2D phase contrast image can be acquired and the flow direction compared to the aorta and inferior vena.

For patients with limited breath-holding capabilities who could not suspend breathing during the portal venous phase, axial 2D gradient echo images can be acquired post-gadolinium during either short periods of apnea (5s) or quiet respiration. Paramagnetic contrast within the vascular system enhances time-of-flight image quality allowing use of relatively thick, 5-8 mm slices. For non-breath-held scans a sufficient number of averages, in conjunction with respiratory ordered phase encoding, will usually result in diagnostic image quality [5].

Patients with suspected parenchymal pathology benefit from T1- and T2-weighted spin echo imaging prior to contrast injection. These images can be used as a guide to ensure inclusion of all pathology in 3D contrast MRA data sets. For patients with suspected biliary obstruction or pancreatitis, a HASTE or single shot fast spin echo MRCP-type sequence in coronal or coronal oblique planes is also useful and can generally be performed in a single breath-hold or during quiet respiration.

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