Magnetic Resonance Pulse Sequences

CONVENTIONAL SPIN ECHO IMAGING The most common pulse sequence traditionally used for cardiac imaging is a conventional spin echo sequence, gated to the cardiac cycle. In this technique, the TR is equal to the duration of the RR interval, and the minimum TE time is used (10 to 20 ms). Since the T1 time of the myocardium is on the order of 800 ms, only moderate T1 weighting is achieved using this method. These images are commonly referred to as black blood images, since radio frequency saturation bands are placed above and below the imaging planes, causing blood to lose MR signal and become dark ( Fig 57-12). Since the TR

depends on the RR interval, imaging times are typically long (e.g., 6 to 8 min). Image quality is only moderate, and highly dependent on consistent ECG gating and lack of patient motion during the relatively long acquisition times. However, the contrast difference between the heart, the epicardial fat, and the ventricular cavities is relatively good.

FIG. 57-12. Magnetic resonance imaging of aortic dissection. Axial T1-weighted image shows the site of prior repair of the ascending aorta, following dissection. There is residual soft tissue thickening around the ascending aorta. A residual dissection remains in the descending aorta. The patient had a history of Marfan disease.

T2 weighting for spin echo images is achieved by gating at several multiples of the RR interval, together with a TE time of 80 to 100 ms. Imaging times are quite long if conventional spin echo techniques are used. The image quality of T2-weighted images is lower than that of T1 images, because of lower signal-to-noise ratios typically associated with T2 images. Also, artifacts due to respiratory and cardiac motion may be pronounced because of long imaging times. T2-weighted images are usually not necessary for anatomic evaluation of the heart but are more helpful for characterization of cardiac or paracardiac masses ( Fig 57-.l.lb). In these cases, the mass frequently has somewhat restricted motion relative to the myocardium, so that image quality is often adequate. Fast spin echo or turbo spin echo T2-weighted pulse sequences are available on most new MR sequences. These methods reduce imaging time for T2-weighted images by a factor of 8 to 16. Because of the high signal intensity of fat on fast spin echo images, an additional radio frequency pulse is often added to cause fat to be very dark. This in turn helps to highlight nonfatty structures.

CINE IMAGING Cine imaging for cardiac MRI consists of a motion picture loop of various phases of the cardiac cycle ( Fig ... 57.-13). To generate cine images, images must be gated to the cardiac cycle. In cine acquisitions, the imaging pulse is triggered, or gated, to begin based on the R wave of the electrocardiogram. Segmented k-space pulse sequences are very rapid, cardiac gated pulse sequences in which a complete cine sequence of the heart is acquired within a breath hold of 12 to 18 heartbeats. This consists of a fast gradient echo acquisition with relatively short TR (e.g., 5 to 10 ms) combined with very short TE (approximately 2 ms), and a flip angle of 10° to 15°. The TR and TE are usually the minimum values achievable by the MR gradient system, so that the system software may automatically set these parameters. The above acquisition parameters result in saturation of background, stationary tissues, while flowing blood has a higher and brighter signal. Breath-hold cine imaging of the heart can be performed in any anatomic plane, e.g., parallel to the long axis and short axis of the heart.

FIG. 57-13. Magnetic resonance imaging of mitral valve dysfunction. Images during systole and diastole show a dark regurgitant jet extending from the valve plane to the left atrium during systole, compatible with mitral valve regurgitation. (Reprinted from Bluemke and Boxerman,19 with permission.)

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