目的:通过应用信噪比有效的RF切片编码和螺旋读出,证明T2加权(单回波)自旋回波(SE)成像具有接近最佳的采集效率。
方法:二次相位(频率扫描)激励RF脉冲代替了T2加权SE序列中的常规激励,以激励厚的平板,该平板在内部由沿切片方向的可变相位进行空间编码。在多遍中获取了以每个期望切片位置为中心的高度重叠的平板,使得整个成像体积在任何给定的通过中被连续的平板激发。90°激励后,每个平板用传统的180°RF重新聚焦以产生SE信号,接着是螺旋进出读数。噪声不敏感的重建消除了空间频域中的二次相位,产生所需的切片分辨率和改善的信噪比。
结果:增加RF频率扫描(因此,激励宽度)允许在多遍上对每个切片进行更频繁的编码,提高最终图像信噪比,直到与它们的中心到中心间距相比,在过大的板宽度处产生串扰。具有优化的板宽度,所提出的技术使用所有通道来获取每个规定的切片,与常规SE或2D-turbo-自旋回波(TSE)扫描相比具有显著改善的SNR。定量信噪比测量表明信噪比与3D-TSE相似,但是放射科医生评分更喜欢3D-TSE,主要是因为螺旋相关的伪影,也可能是因为3D-TSE中的正则化重建。
结论:使用信噪比有效的切片激励方案和螺旋读出有助于消除传统T2加权成像中的信噪比和时间效率低下,产生与TR或通过次数无关的SNR。
OBJECTIVE: To demonstrate T2 -weighted (single-echo) spin-echo (SE) imaging with near-optimal acquisition efficiency by applying SNR-efficient RF slice encoding and spiral readout.
METHODS: A quadratic-phase (frequency swept) excitation RF pulse replaced the conventional excitation in T2 -weighted SE sequence to excite a thick slab that is internally spatially encoded by a variable phase along the slice direction. Highly overlapping slabs centered at every desired slice location were acquired in multiple passes, such that the entire imaging volume was excited by contiguous slabs in any given pass. Following 90° excitation, each slab was refocused with a conventional 180° RF to produce a SE signal, followed by a spiral in-out readout. A noise-insensitive reconstruction removed the quadratic phase in the spatial frequency domain, yielding desired slice resolution and improved SNR.
RESULTS: Increasing the RF frequency sweep (hence, excitation width) allowed more frequent encoding of each slice over the multiple passes, improving final image SNR, until crosstalk ensued at excessive slab widths compared to their center-to-center spacing. With an optimized slab width, the proposed technique used all passes to acquire every prescribed slice, with substantially improved SNR over conventional SE or 2D-turbo-spin-echo (TSE) scans. Quantitative SNR measurements indicated similar SNR as 3D-TSE, but radiologist scoring favored 3D-TSE, mainly because of spiral-related artifacts and possibly because of regularized reconstructions in 3D-TSE.
CONCLUSIONS: Using SNR-efficient slice excitation scheme and spiral readout helped eliminate SNR and temporal inefficiencies in conventional T2 -weighted imaging, yielding SNR independent of TR or number of passes.