Malmberg Michael A, Odéen Henrik, Kim Seong-Eun, Parker Dennis L
Department of Biomedical Engineering, University of Utah, Salt Lake City, Utah, USA.
Department of Radiology and Imaging Sciences, University of Utah, Salt Lake City, Utah, USA.
Magn Reson Med. 2025 Sep;94(3):949-964. doi: 10.1002/mrm.30521. Epub 2025 Apr 3.
To demonstrate in theory and practice that concomitant gradient fields (CCGF) can produce substantial imaging artifacts in scans utilizing radial-based trajectories and to provide strategies to mitigate these effects.
A framework was developed to relate concomitant gradient phase to local point-spread-function distortion, which was used to evaluate the effects of trajectory choice and imaging parameters on imaging artifacts. Gradient waveforms for realistic imaging scenarios were simulated and used to determine the effect of CCGF. Phantom and in vivo experiments were performed at 3 T to validate theoretical predictions.
CCGF-induced artifacts are shown to be produced in part by increased variation in concomitant gradient phase across view angles. This is shown to increase with increasing gradient strength and contrast index in radial-based trajectories. Phase variation across view angles and the associated artifacts are shown to be effectively diminished via azimuthal rotation down the echo train in the helical EPI and helical stack-of-stars trajectories introduced in this work.
Concomitant gradient fields are found to produce non-negligible imaging artifacts in long readout radial-based trajectories due to variations in the associated phase accrual across view angles. Azimuthal rotation of the readout direction down the echo train, as implemented in the helical EPI and helical stack-of-stars trajectories, is shown in simulations, phantoms, and in vivo to mitigate these effects. Substantial improvement is seen in cases of nonaxial imaging with multiple contrasts, quickly varying B inhomogeneity, and/or high gradient amplitudes.
从理论和实践上证明伴随梯度场（CCGF）在利用基于径向轨迹的扫描中会产生大量成像伪影，并提供减轻这些影响的策略。
建立了一个将伴随梯度相位与局部点扩散函数失真相关联的框架，用于评估轨迹选择和成像参数对成像伪影的影响。模拟了实际成像场景的梯度波形，并用于确定CCGF的影响。在3T下进行了体模和体内实验，以验证理论预测。
结果表明，CCGF诱导的伪影部分是由视角上伴随梯度相位的变化增加所产生的。在基于径向的轨迹中，这种变化随着梯度强度和对比度指数的增加而增加。通过在本工作中引入的螺旋回波平面成像（EPI）和螺旋星状堆叠轨迹中沿回波链进行方位角旋转，视角上的相位变化和相关伪影被有效减少。
由于视角上相关相位累积的变化，发现伴随梯度场在长读出的基于径向的轨迹中会产生不可忽略的成像伪影。如在螺旋EPI和螺旋星状堆叠轨迹中所实现的，沿回波链对读出方向进行方位角旋转，在模拟、体模和体内实验中均显示可减轻这些影响。在多对比度非轴向成像、快速变化的B不均匀性和/或高梯度幅度的情况下，可以看到显著改善。
