Zhan Wang, Yang Yihong
Neuroimaging Research Branch, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, MD 21224, USA.
J Magn Reson. 2006 Dec;183(2):193-202. doi: 10.1016/j.jmr.2006.08.005. Epub 2006 Sep 11.
The q-space imaging techniques and high angular resolution diffusion (HARD) imaging have shown promise to identify intravoxel multiple fibers. The measured orientation distribution function (ODF) and apparent diffusion coefficient (ADC) profiles can be used to identify the orientations of the actual intravoxel fibers. The present study aims to examine the accuracy of these profile-based orientation methods by comparing the angular deviations between the estimated local maxima of the profiles and the real fiber orientation for a fiber crossing simulated with various intersection angles under different b values in diffusion-weighted MRI experiments. Both noisy and noise-free environments were investigated. The diffusion spectrum imaging (DSI), q-ball imaging (QBI), and HARD techniques were used to generate ODF and ADC profiles. To provide a better comparison between ODF and ADC techniques, the phase-corrected angular deviations were also presented for the ADC method based on a circular spectrum mapping method. The results indicate that systematic angular deviations exist between the actual fiber orientations and the corresponding local maxima of either the ADC or ODF profiles. All methods are apt to underestimation of acute intersection and overestimation of obtuse intersection angle. For a typical slow-exchange fiber crossing, the ODF methods have a non-deviation zone around the 90 degrees intersection. Before the phase-correction, the deviation of ADC profiles approaches a peak at the 90 degrees intersection, while after the correction the ADC deviations are significantly reduced. When the b factor is larger than 1000 s/mm2, the ODF methods have smaller angular deviations than the ADC methods for the intersections close to 90 degrees . QBI method demonstrates a slight yet consistent advantage over the DSI method under the same conditions. In the noisy environment, the mean value of the deviation angles shows a high consistency with the corresponding deviation in the nose-free condition.
q空间成像技术和高角分辨率扩散（HARD）成像已显示出识别体素内多纤维的潜力。测量得到的取向分布函数（ODF）和表观扩散系数（ADC）剖面图可用于识别实际体素内纤维的取向。本研究旨在通过比较在扩散加权MRI实验中不同b值下，用各种相交角度模拟的纤维交叉处，剖面图估计局部最大值与真实纤维取向之间的角度偏差，来检验这些基于剖面图的取向方法的准确性。研究了有噪声和无噪声两种环境。使用扩散谱成像（DSI）、q球成像（QBI）和HARD技术生成ODF和ADC剖面图。为了在ODF和ADC技术之间进行更好的比较，还基于圆谱映射方法给出了ADC方法的相位校正角度偏差。结果表明，实际纤维取向与ADC或ODF剖面图相应局部最大值之间存在系统性角度偏差。所有方法都倾向于低估锐角交叉，高估钝角交叉角度。对于典型的慢交换纤维交叉，ODF方法在90度交叉处有一个无偏差区。在相位校正之前，ADC剖面图的偏差在90度交叉处接近峰值，而校正后ADC偏差显著减小。当b因子大于1000 s/mm2时，对于接近90度的交叉，ODF方法的角度偏差比ADC方法小。在相同条件下，QBI方法相对于DSI方法表现出轻微但一致的优势。在有噪声环境中，偏差角的平均值与无噪声条件下的相应偏差具有高度一致性。
