Center for Magnetic Resonance Research, University of Illinois Chicago, Chicago, Illinois, USA.
Department of Biomedical Engineering, University of Illinois Chicago, Chicago, Illinois, USA.
NMR Biomed. 2024 Dec;37(12):e5261. doi: 10.1002/nbm.5261. Epub 2024 Sep 22.
Conventional diffusion-weighted imaging (DWI) sequences employing a spin echo or stimulated echo sensitize diffusion with a specific b-value at a fixed diffusion direction and diffusion time (Δ). To compute apparent diffusion coefficient (ADC) and other diffusion parameters, the sequence needs to be repeated multiple times by varying the b-value and/or gradient direction. In this study, we developed a single-shot multi-b-value (SSMb) diffusion MRI technique, which combines a spin echo and a train of stimulated echoes produced with variable flip angles. The method involves a pair of 90° radio frequency (RF) pulses that straddle a diffusion gradient lobe (G), to rephase the magnetization in the transverse plane, producing a diffusion-weighted spin echo acquired by the first echo-planar imaging (EPI) readout train. The magnetization stored along the longitudinal axis is successively re-excited by a series of n variable-flip-angle pulses, each followed by a diffusion gradient lobe G and a subsequent EPI readout train to sample n stimulated-echo signals. As such, (n + 1) diffusion-weighted images, each with a distinct b-value, are acquired in a single shot. The SSMb sequence was demonstrated on a diffusion phantom and healthy human brain to produce diffusion-weighted images, which were quantitative analyzed using a mono-exponential model. In the phantom experiment, SSMb provided similar ADC values to those from a commercial spin-echo EPI (SE-EPI) sequence (r = 0.999). In the human brain experiment, SSMb enabled a fourfold scan time reduction and yielded slightly lower ADC values (0.83 ± 0.26 μm/ms) than SE-EPI (0.88 ± 0.29 μm/ms) in all voxels excluding cerebrospinal fluid, likely due to the influence of varying diffusion times. The feasibility of using SSMb to acquire multiple images in a single shot for intravoxel incoherent motion (IVIM) analysis was also demonstrated. In conclusion, despite a relatively low signal-to-noise ratio, the proposed SSMb technique can substantially increase the data acquisition efficiency in DWI studies.
传统的扩散加权成像（DWI）序列采用自旋回波或激发回波，在固定的扩散方向和扩散时间（Δ）下用特定的 b 值对扩散进行敏化。为了计算表观扩散系数（ADC）和其他扩散参数，需要通过改变 b 值和/或梯度方向来多次重复序列。在本研究中，我们开发了一种单次多 b 值（SSMb）扩散 MRI 技术，该技术结合了自旋回波和具有可变翻转角的一系列激发回波。该方法涉及一对跨越扩散梯度叶（G）的 90°射频（RF）脉冲，用于重新相位横向平面中的磁化，通过第一个回波平面成像（EPI）读出序列采集扩散加权自旋回波。沿纵轴存储的磁化被一系列 n 个可变翻转角脉冲相继重新激发，每个脉冲后面跟着一个扩散梯度叶 G 和随后的 EPI 读出序列，以采集 n 个激发回波信号。因此，在单次激发中采集了(n+1)个具有不同 b 值的扩散加权图像。在扩散体模和健康人脑上进行了 SSMb 序列演示，以产生扩散加权图像，并使用单指数模型对其进行定量分析。在体模实验中，SSMb 提供的 ADC 值与商业自旋回波 EPI（SE-EPI）序列（r=0.999）相似。在人脑实验中，SSMb 使扫描时间减少了四倍，并在所有除了脑脊液的体素中产生了略低的 ADC 值（0.83±0.26 μm/ms），低于 SE-EPI（0.88±0.29 μm/ms），这可能是由于扩散时间不同的影响。还证明了使用 SSMb 在单次激发中采集多个图像用于体素内不相干运动（IVIM）分析的可行性。总之，尽管信噪比相对较低，但所提出的 SSMb 技术可以显著提高 DWI 研究中的数据采集效率。
