Department of Biomedical Imaging and Radiological Sciences, National Yang-Ming University, Taipei, Taiwan.
Neuroimage. 2010 Jun;51(2):743-51. doi: 10.1016/j.neuroimage.2010.02.041. Epub 2010 Feb 24.
An essential step for fibre-tracking is the accurate estimation of neuronal fibre orientations within each imaging voxel, and a number of methods have been proposed to reconstruct the orientation distribution function based on sampling three-dimensional q-space. In the q-space formalism, very short (infinitesimal) gradient pulses are the basic requirement to obtain the true spin displacement probability density function. On current clinical MR systems however, the diffusion gradient pulse duration (delta) is inevitably finite due to the limit on the achievable gradient intensity. The failure to satisfy the short gradient pulse (SGP) requirement has been a recurrent criticism for fibre orientation estimation based on the q-space approach. In this study, the influence of a finite delta on the DW signal measured as a function of gradient direction is described theoretically and demonstrated through simulations and experimental models. Our results suggest that the current practice of using long delta for DW imaging on human clinical MR scanners, which is enforced by hardware limitations, might in fact be beneficial for estimating fibre orientations. For a given b-value, the prolongation of delta is advantageous for estimating fibre orientations for two reasons: first, it leads to a boost in DW signal in the transverse plane of the fibre. Second, it stretches out the shape of the measured diffusion profile, which improves the contrast between DW orientations. This is especially beneficial for resolving crossing fibres, as this contrast is essential to discriminate between different fibre directions.
纤维追踪的一个重要步骤是在每个成像体素内准确估计神经元纤维的方向，已经提出了许多方法来基于对三维 q 空间的采样来重建方向分布函数。在 q 空间形式中，非常短（无穷小）的梯度脉冲是获得真实自旋位移概率密度函数的基本要求。然而，在当前的临床磁共振系统中，由于可实现的梯度强度的限制，扩散梯度脉冲持续时间（delta）不可避免地是有限的。未能满足短梯度脉冲（SGP）要求一直是基于 q 空间方法的纤维方向估计的反复批评。在这项研究中，理论上描述了有限 delta 对作为梯度方向函数测量的 DW 信号的影响，并通过模拟和实验模型进行了演示。我们的结果表明，当前在人类临床磁共振扫描仪上进行 DW 成像时使用长 delta 的做法实际上可能有利于纤维方向的估计，这是由硬件限制强制实施的。对于给定的 b 值，delta 的延长有利于纤维方向的估计，原因有二：首先，它导致纤维横向平面中的 DW 信号增强。其次，它拉伸了测量扩散分布的形状，这提高了 DW 方向之间的对比度。这对于解析交叉纤维特别有益，因为这种对比度对于区分不同的纤维方向至关重要。
