First-pass Coronary MR Angiography Using a Spiral-Ring Trajectory

Purpose: 2D multislice interleaved spiral imaging [1] for coronary magnetic resonance angiography (MRA) has been shown to be capable of imaging multiple slices with submm in-plane resolution and high temporal resolution within a breath-hold. However, an important issue with this sequence is blood-lesion contrast. In this work, we developed a spiral-ring [2] version of the sequence, which is aimed for first-pass contrast-enhanced coronary MRA for potentially better blood-muscle and blood-lesion contrast. Methods: Spiral-Ring: The spiral-ring trajectory [2] is generated by segmenting a relatively long 2D interleaved spiral trajectory. By dividing each of Nintlv interleaves into Nseg segments (Fig. 1), a total of Nrdout = Nintlv × Nseg readouts with 1/Nseg of the original readout duration are made to have the same k-space coverage as the original spiral trajectory. Considering that each set of segments (e.g., a set of Nintlv 1 st segments) collects full-FOV information for a specific spatial frequency band, the spiral-ring trajectory better captures the transient contrast generated by magnetization preparation and a contrast agent compared to a conventional 2D spiral sequence. Due to the phase discontinuities at the boundaries of segments, the trajectory requires slice-by-slice shimming and multifrequency reconstruction [3] to reduce off-resonance effects. Pulse Sequence: During each heartbeat of a breath-hold, one of Nrdout readouts for all the slices is acquired sequentially [1]. A saturation pulse is applied right before the acquisition of each slice to selectively saturate a slice that is Nshift acquisitions after. This saturation scheme allows enough recovery of contrast-enhanced blood for blood-muscle and blood-lesion contrast, without introducing an explicit delay time between the preparation and the acquisition [4]. A total of Nrdout heartbeats is needed to collect all the k-space data, but the first set of segments that collects the innermost k-space data is acquired redundantly. In Fig. 2., e.g., each set of segments needs to collect Nintlv = 4 interleaves for the full-FOV coverage, but the set of innermost (1) segments is acquired more times (8 vs. 4) and combined with the same sets of outer segments to provide five view-shared time-resolved datasets [5]. Imaging Parameters: Phantom and in vivo studies were performed on a GE Excite 1.5 T scanner with an 8-channel cardiac coil. The RTHawk real-time system (HeartVista, Inc) [6] was used for fluoroscopic triggering for bolus detection [5] as well as for the prospective shim correction, pulse generation, and multifrequency reconstruction. Informed written consent approved by our IRB was obtained prior to scanning. The spiral-ring was formed from a variable-density spiral [7] (28/22 cm FOV at k-space origin/edge, respectively) with Nintlv/Nseg = 4/4 and acquired with an SPGR sequence to provide in-plane resolution = 1 mm, slice thickness = 5 mm, readout duration = 8 ms, TR (temporal resolution for each slice) = 26 ms, and flip angle = 60°. The total scan time was 20 heartbeats as illustrated in Fig. 2. A total of 20 slices were acquired with Nshift = 2, which provides a 2⋅TR = 52 ms effective delay time for each saturation pulse.