Drobnjak Ivana, Zhang Hui, Ianuş Andrada, Kaden Enrico, Alexander Daniel C
Department of Computer Science and Centre for Medical Image Computing, University College London (UCL), London, UK.
Magn Reson Med. 2016 Feb;75(2):688-700. doi: 10.1002/mrm.25631. Epub 2015 Mar 25.
To identify optimal pulsed gradient spin-echo (PGSE) and oscillating gradient spin-echo (OGSE) sequence settings for maximizing sensitivity to axon diameter in idealized and practical conditions.
Simulations on a simple two-compartment white matter model (with nonpermeable cylinders) are used to investigate a wide space of clinically plausible PGSE and OGSE sequence parameters with trapezoidal diffusion gradient waveforms. Signal sensitivity is measured as a derivative of the signal with respect to axon diameter. Models of parallel and dispersed fibers are investigated separately to represent idealized and practical conditions.
Simulations show that, for the simple case of gradients perfectly perpendicular to straight parallel fibers, PGSE always gives maximum sensitivity. However, in real-world scenarios where fibers have unknown and dispersed orientation, low-frequency OGSE provides higher sensitivity. Maximum sensitivity results show that on current clinical scanners (Gmax = 60 mT/m, signal to noise ratio (SNR) = 20) axon diameters below 6 µm are indistinguishable from zero. Scanners with stronger gradient systems such as the Massachusetts General Hospital (MGH) Connectom scanner (Gmax = 300 mT/m) can extend this sensitivity limit down to 2-3 µm, probing a much greater proportion of the underlying axon diameter distribution.
Low-frequency OGSE provides additional sensitivity to PGSE in practical situations. OGSE is particularly advantageous for systems with high performance gradients.
确定最佳的脉冲梯度自旋回波（PGSE）和振荡梯度自旋回波（OGSE）序列设置，以在理想化和实际条件下最大化对轴突直径的敏感度。
在一个简单的两室白质模型（带有不可渗透圆柱体）上进行模拟，以研究具有梯形扩散梯度波形的一系列临床上合理的PGSE和OGSE序列参数。信号敏感度通过信号相对于轴突直径的导数来衡量。分别研究平行纤维和分散纤维模型，以代表理想化和实际条件。
模拟表明，对于梯度与直平行纤维完全垂直的简单情况，PGSE始终具有最大敏感度。然而，在纤维方向未知且分散的实际场景中，低频OGSE具有更高的敏感度。最大敏感度结果表明，在当前临床扫描仪上（最大梯度强度Gmax = 60 mT/m，信噪比SNR = 20），直径小于6 µm的轴突与零无法区分。拥有更强梯度系统的扫描仪，如麻省总医院（MGH）连接体扫描仪（Gmax = 300 mT/m），可将此敏感度极限扩展至2 - 3 µm，探测到更大比例的潜在轴突直径分布。
在实际情况下，低频OGSE比PGSE具有更高的敏感度。OGSE对于具有高性能梯度的系统尤其有利。
