Radiology, Stanford University, Stanford, California, USA.
Bioengineering, Stanford University, Stanford, California, USA.
Magn Reson Med. 2024 Aug;92(2):519-531. doi: 10.1002/mrm.30102. Epub 2024 Apr 16.
Diffusion-weighted (DW) imaging provides a useful clinical contrast, but is susceptible to motion-induced dephasing caused by the application of strong diffusion gradients. Phase navigators are commonly used to resolve shot-to-shot motion-induced phase in multishot reconstructions, but poor phase estimates result in signal dropout and Apparent Diffusion Coefficient (ADC) overestimation. These artifacts are prominent in the abdomen, a region prone to involuntary cardiac and respiratory motion. To improve the robustness of DW imaging in the abdomen, region-based shot rejection schemes that selectively weight regions where the shot-to-shot phase is poorly estimated were evaluated.
Spatially varying weights for each shot, reflecting both the accuracy of the estimated phase and the degree of subvoxel dephasing, were estimated from the phase navigator magnitude images. The weighting was integrated into a multishot reconstruction using different formulations and phase navigator resolutions and tested with different phase navigator resolutions in multishot DW-echo Planar Imaging acquisitions of the liver and pancreas, using conventional monopolar and velocity-compensated diffusion encoding. Reconstructed images and ADC estimates were compared qualitatively.
The proposed region-based shot rejection reduces banding and signal dropout artifacts caused by physiological motion in the liver and pancreas. Shot rejection allows conventional monopolar diffusion encoding to achieve median ADCs in the pancreas comparable to motion-compensated diffusion encoding, albeit with a greater spread of ADCs.
Region-based shot rejection is a linear reconstruction that improves the motion robustness of multi-shot DWI and requires no sequence modifications.
扩散加权（DW）成像提供了一种有用的临床对比，但容易受到强扩散梯度应用引起的运动相位去相位的影响。相位导航仪常用于解决多shot 重建中shot-to-shot 运动引起的相位问题，但较差的相位估计会导致信号丢失和表观扩散系数（ADC）高估。这些伪影在腹部很明显，腹部容易发生无意识的心脏和呼吸运动。为了提高腹部 DW 成像的稳健性，评估了基于区域的射束拒绝方案，该方案选择性地对 shot-to-shot 相位估计不佳的区域进行加权。
从相位导航仪幅度图像中估计每个射束的空间变化权重，反映估计相位的准确性和亚像素去相位的程度。使用不同的公式和相位导航仪分辨率将加权集成到多shot 重建中，并在使用不同相位导航仪分辨率的情况下，对肝脏和胰腺的多shot DW-echo 平面成像采集进行测试，使用传统的单极和速度补偿扩散编码。对重建图像和 ADC 估计进行定性比较。
所提出的基于区域的射束拒绝减少了肝脏和胰腺中生理运动引起的带纹和信号丢失伪影。射束拒绝允许传统的单极扩散编码在胰腺中实现与运动补偿扩散编码相当的 ADC 中位数，尽管 ADC 的分布范围更大。
基于区域的射束拒绝是一种线性重建，可以提高多 shot DWI 的运动稳健性，并且不需要序列修改。
