Near‐contiguous spin echo imaging using matched‐phase RF and its application in velocity‐selective arterial spin labeling

Crosstalk between slices in multislice imaging is caused by an imperfect RF slice profile (1–4). No selective RF pulse has a perfectly rectangular profile in practice, and therefore partially excites the adjacent slices in contiguous multislice imaging. Unless a very long TR (>4–5 × T1) is used, the perturbed magnetization in the adjacent slices will not fully recover to its equilibrium value before its own RF excitation. This pre-perturbation introduces errors into quantitative imaging such as measurements of T1 or T2 (5–8), alters the contrast between tissues in qualitative imaging which confounds clinical diagnosis (9–11), and reduces overall signal-to-noise ratio (SNR) (12). One simple method to avoid the pre-perturbation is to place a larger gap between slices (4) although this involves a risk of missing lesions in the acquired images. Another simple method is to use a longer TR to allow for a near-full relaxation of the perturbed magnetization. However, a long TR increases total scan time and a shorter TR is required in applications such as T1-weighted imaging. Based on the similar idea of increasing time for relaxation, the order of slice acquisition can be interleaved (2,3,13,14), but the time for relaxation still may not be long enough to achieve full relaxation when a short TR is used. While crosstalk may be significantly reduced in gradient echo sequences using these methods, it is a more severe problem in spin-echo sequences because the profile of the 180° refocusing RF pulse or the 90°–180° combination is worse than that of the 90° RF pulse alone due to the peak RF limits. Spin-echo imaging is beneficial in many applications for its advantage of reducing susceptibility artifacts both those arising from anatomy (temporal/frontal regions) and from pathology/foreign bodies (blood products, clips, etc). The typical goal of RF pulse design is to produce a sharp pulse profile in a minimum amount of time. The tradeoff is limited by the time-bandwidth (TBW) product, specific absorption rate, and peak B1 of the RF pulse. In spin-echo imaging, matched-phase pulses can be used to improve RF excitation performance (15). Unlike conventional RF pulses, the individual 90° and 180° pulses in matched-phase RF pulses can have non-linear phase. However, these phases are designed to be matched to each other such that a linear phase is obtained at the end of the pulse pair. This creates more flexibility in RF pulse design, which can be utilized to sharpen the slice profile (or increase TBW product), reduce the echo time (TE), or lower peak B1 for a given pulse duration. In addition to improving transverse slice profile, we demonstrate that the profile of longitudinal magnetization can be sharpened using matched-phase RF pulses, and therefore inter-slice crosstalk is minimized. We simulate and measure slice profiles using the matched-phase RF and sinc-based RF pulses and show that matched-phase RF pulses achieve a sharper longitudinal slice profile than sinc-based RF pulses, allowing for a smaller slice spacing. We then apply matched-phase RF pulses to cerebral blood flow (CBF) measurements using velocity-selective arterial spin labeling (VSASL) (16). Because of the typical brain motion during image acquisition, higher interslice crosstalk increases a temporal variation in the ASL signal, leading to reduced ASL signal stability. We demonstrate that near-contiguous multislice imaging can be achieved using matched-phase RF pulses in ASL perfusion imaging without reducing stability of ASL signal.