Mercredi Morgan, Vincent Trevor J, Bidinosti Christopher P, Martin Melanie
Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada.
Department of Physics, University of Winnipeg, Winnipeg, Manitoba, R3B 2E9, Canada.
MAGMA. 2017 Feb;30(1):1-14. doi: 10.1007/s10334-016-0575-y. Epub 2016 Jul 13.
Current magnetic resonance imaging (MRI) axon diameter measurements rely on the pulsed gradient spin-echo sequence, which is unable to provide diffusion times short enough to measure small axon diameters. This study combines the AxCaliber axon diameter fitting method with data generated from Monte Carlo simulations of oscillating gradient spin-echo sequences (OGSE) to infer micron-sized axon diameters, in order to determine the feasibility of using MRI to infer smaller axon diameters in brain tissue.
Monte Carlo computer simulation data were synthesized from tissue geometries of cylinders of different diameters using a range of gradient frequencies in the cosine OGSE sequence . Data were fitted to the AxCaliber method modified to allow the new pulse sequence. Intra- and extra-axonal water were studied separately and together.
The simulations revealed the extra-axonal model to be problematic. Rather than change the model, we found that restricting the range of gradient frequencies such that the measured apparent diffusion coefficient was constant over that range resulted in more accurate fitted diameters. Thus a careful selection of frequency ranges is needed for the AxCaliber method to correctly model extra-axonal water, or adaptations to the method are needed. This restriction helped reduce the necessary gradient strengths for measurements that could be performed with parameters feasible for a Bruker BG6 gradient set. For these experiments, the simulations inferred diameters as small as 0.5 μm on square-packed and randomly packed cylinders. The accuracy of the inferred diameters was found to be dependent on the signal-to-noise ratio (SNR), with smaller diameters more affected by noise, although all diameter distributions were distinguishable from one another for all SNRs tested.
The results of this study indicate the feasibility of using MRI with OGSE on preclinical scanners to infer small axon diameters.
当前磁共振成像（MRI）轴突直径测量依赖于脉冲梯度自旋回波序列，该序列无法提供足够短的扩散时间来测量小轴突直径。本研究将AxCaliber轴突直径拟合方法与振荡梯度自旋回波序列（OGSE）的蒙特卡罗模拟生成的数据相结合，以推断微米级轴突直径，从而确定使用MRI推断脑组织中较小轴突直径的可行性。
使用余弦OGSE序列中的一系列梯度频率，从不同直径圆柱体的组织几何结构合成蒙特卡罗计算机模拟数据。将数据拟合到经修改以允许新脉冲序列的AxCaliber方法。分别和共同研究轴突内和轴突外的水。
模拟显示轴突外模型存在问题。我们发现，与其改变模型，不如限制梯度频率范围，以使测量的表观扩散系数在该范围内保持恒定，这样能得到更准确的拟合直径。因此，AxCaliber方法需要仔细选择频率范围以正确模拟轴突外的水，或者需要对该方法进行调整。这种限制有助于降低使用布鲁克BG6梯度集可行参数进行测量所需的梯度强度。对于这些实验，模拟推断在方形堆积和随机堆积的圆柱体上直径小至0.5μm。发现推断直径的准确性取决于信噪比（SNR），较小的直径受噪声影响更大，尽管在所有测试的SNR下所有直径分布彼此都可区分。
本研究结果表明在临床前扫描仪上使用带有OGSE的MRI推断小轴突直径的可行性。
