Improved gradient waveforms for oscillating gradient spin-echo (OGSE) diffusion tensor imaging
Improved gradient waveforms for oscillating gradient spin-echo (OGSE) diffusion tensor imaging
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DOI：
10.1002/nbm.4434
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发表时间：
2020-10-29
影响因子：
2.9
通讯作者：
Pruessmann, Klaas P.
中科院分区：
文献类型：
--
作者：
Hennel, Franciszek;Michael, Eric Seth;Pruessmann, Klaas P.
扩散张量对频率的依赖关系在组织微结构研究中具有重要意义，因为它揭示了细胞膜对水分子布朗运动的限制。振荡梯度自旋回波(OGSE)序列可以以梯度形状对这种相关性进行采样，其零阶矩的功率谱聚焦于目标频率。为了保持总的频谱功率(即b值)，振荡梯度幅度必须随着频率的平方而增大。因此，OGSE在临床MRI扫描仪上的应用仅限于较低的频率，在有用的回波时间内很难获得较窄的梯度矩频带。具体地说，通常使用的由半个周期隔开的单周期梯形余弦脉冲对产生远离目标频率的显著旁瓣。为了减轻这种影响，提出了改进的OGSE波形，其将两个梯度脉冲之间的间隔减小到重新聚焦RF脉冲所需的最小持续时间。此外，建议稍微偏离波形的周期性，以便允许对梯形波形的所有波瓣使用梯度系统的最大转换速率，同时保持有利的频谱特性，这不是当前使用的OGSE序列的情况。数值计算验证了这些变化，表明两种修正都显著地缩小了梯度矩功率谱，并增加了其主瓣对b值的贡献，从而提高了测量的特异度。通过使用高性能梯度插入物，在30-75赫兹范围内对人体白质进行扩散张量测量，b值接近1000 S/mm(2)，证明了新形状的实用性。然而，改进应该提高所有梯度系统的OGSE实验的采样精度。
The dependence of the diffusion tensor on frequency is of great interest in studies of tissue microstructure because it reveals restrictions to the Brownian motion of water molecules caused by cell membranes. Oscillating gradient spin-echo (OGSE) sequences can sample this dependence with gradient shapes for which the power spectrum of the zeroth moment is focused at a target frequency. In order to maintain the total spectral power (ie the b-value), oscillating gradient amplitudes must grow with the frequency squared. For this reason, OGSE applications on clinical MRI scanners are limited to low frequencies, for which it is difficult to obtain a narrow frequency bandwidth of the gradient moment in a useful echo time. In particular, the commonly used pair of single-period trapezoidal-cosine pulses separated by a half-period produces significant side lobes away from the target frequency. To mitigate this effect, improved OGSE waveforms are proposed, which reduce the gap between the two gradient pulses to the minimum duration required for the refocusing RF pulse. Additionally, a slight deviation from the periodicity of the waveforms is proposed in order to permit using the maximum slew rate of the gradient system for all lobes of trapezoidal waveforms while maintaining advantageous spectral properties, which is not the case for the currently used OGSE sequences. Numerical calculations validate these changes, showing that both modifications significantly narrow the gradient moment power spectrum and increase the contribution of its main lobe to the b-value, thus improving the specificity of the measurement. The utility of the new shapes is demonstrated by diffusion tensor measurements of human white matter in vivo over the range of 30-75 Hz with a b-value of nearly 1000 s/mm(2), using a high-performance gradient insert. However, the improvement should increase the sampling precision of OGSE experiments for all gradient systems.