The B‐matrix must be rotated when correcting for subject motion in DTI data

Abstract

To estimate diffusion tensor MRI (DTI) measures, such as fractional anisotropy and fiber orientation, reliably, a large number of diffusion-encoded images is needed, preferably cardiac gated to reduce pulsation artifacts. However, the concomitant longer acquisition times increase the chances of subject motion adversely affecting the estimation of these measures. While correcting for motion artifacts improves the accuracy of DTI, an often overlooked step in realigning the images is to reorient the B-matrix so that orientational information is correctly preserved. To the best of our knowledge, most research groups and software packages currently omit this reorientation step. Given the recent explosion of DTI applications including, for example, neurosurgical planning (in which errors can have drastic consequences), it is important to investigate the impact of neglecting to perform the B-matrix reorientation. In this work, a systematic study to investigate the effect of neglecting to reorient the B-matrix on DTI data during motion correction is presented. The consequences for diffusion fiber tractography are also discussed. Reliable calculation of diffusion measures, such as fractional anisotropy (FA) and fiber orientation, is important for quantitative diffusion tensor MRI (DTI) analyses, and has been studied extensively with respect to image noise (1-5), eddy current induced distortion artifacts, (6-15), and data reproducibility (16-18). Previous research has also shown that the choice of gradient sampling scheme can affect the precision of the FA and mean diffusivity (MD), suggesting that a larger number of gradient directions (i.e., > 20-30) is required to estimate these diffusion measures robustly (19)(20)(21)(22)(23)(24)(25)(26). However, longer acquisition times increase the chances of subject motion, potentially nullifying the benefit of optimizing the gradient acquisition scheme.When correcting for subject motion, an important step in realigning the diffusion weighted (DW) images is to reorient the corresponding B-matrix so that orientational information is correctly preserved (6). This reorientation step is often neglected, however, when estimating the diffusion tensor and its contribution to the accuracy and precision of DTI measures has not yet been investigated systematically.Given the recent proliferation of clinical DTI applications, it is important to understand the consequences of neglecting to reorient the B-matrix during motion correction, especially for neurosurgical planning, in which tractography is being introduced as a tool for assessing white matter (WM) architectural configurations (27)(28)(29). In this work, we systematically investigated the effect of omitting the B-matrix reorientation on the estimation of diffusion measures, such as the FA, MD, and first eigenvector (FE), during motion correction. The adverse consequences for diffusion fiber tractography (both deterministic and probabilistic approaches) are also demonstrated. METHODS Definitions and Notationst with a constant b-value (...

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