Segmented K-space cine breath-hold cardiovascular MR imaging: Part 1. Principles and technique.

S everal well-established MR techmques are routinely used for evaluation of the heart and great vessels. Tl-weighted spin-echo images gated to the cardiac cycle provide black-blood images of the heart and vessels. Spin-echo images provide good anatomic detail, but imaging times are relatively long (approximately 5-7 mm, depending on the heart rate). In addition, the images are static: only a single cardiac phase is typically acquired during this interval to avoid excessively long imaging times. Cine image acquisition often provides essential information about the cardiac chambers and flowing blood-information complementat)’ to the anatomic information provided on Tl-weighted spin-echo images. A cine sequence of the heart consists of multiple static images that are rapidly displayed in a continuous loop. Most often. gradient-recalled echo pulse sequences are used to provide brightblood cine images of the heart and vessels. Standard gradient-recalled echo sequences require 3-5 mm to produce a cine acquisition of one to four image slices using either retrospective on prospective cardiac gating [1]. Segmented K-space cardiac sequences were described by Atkinson and Edelman [2] and allow 12-18 phases of the cardiac cycle (referred to later in this article as movie frames of a cine sequence) to be acquired in a single breath-hold using prospective cardiac gating. Segmented K-space acquisitions have been used extensively in research protocols-for example. in conjunction with MR tagging techniques [3]although they have been less frequently used in Fig. 1.-Diagram summarizes segmented K-space gradient-recalled echo acquisition in a multiphase, single-slice mode. Each R-R interval is divided into multiple cardiac phases (cine frames). Cine frame has relatively short duration, so that only portion, or segment, of K-space can be acquired. Multiple RF pulses can be applied during this segment, each corresponding to line of K-space. Number of views per segment (NVS) is number of RF pulses that are applied during each segment, shown as NVS = 8 in this diagram. For TR of 10 msec, duration of each cine frame is 10 msec x 8 = 80 msec; hence, multiple cine frames can be acquired during typical R-R interval (which lasts approximately 1000 msec(.Two segments are shown here, one obtained during systole and one obtained during diastole. Note that segments of K-space are usually not contiguous (like contiguous segments shown here) but are interleaved to minimize image artifacts. To complete 128 phase-encoding steps requires 128/8 = 16 R-R intervals.