Implementation of Anatomical Navigators for Real Time Motion Correction in Diffusion Tensor Imaging

Thesis: Implementation of Anatomical Navigators for Real Time Motion Correction in Diffusion Tensor Imaging Author‟s Name: Alkathafi A. Alhamud Date: 10-02-2012 Diffusion Tensor Imaging (DTI) is used to detect microstructural changes in the human brain. Although echo planar imaging (EPI) has been implemented in diffusion pulse sequences to minimize the effect of subject motion, motion artifacts may reappear in the diffusion volume images when scanning the brain repeatedly with different diffusion directions and strengths. Both retrospective and prospective motion correction approaches have been implemented in DTI to correct for misalignments in diffusion volume images. Most of these methods rely on the contrast differences between the diffusion weighted images and a target image, which renders the registration for motion correction inaccurate, especially in acquisitions with high diffusion weightings. In order to develop a motion correction method that is independent of the diffusion contrast, a separate motion tracking technique has been introduced using a volumetric 3D-EPI navigator. This technique performs prospective motion correction in diffusion weighted images without having to reacquire volumes during which motion occurred, unless motion exceeded some pre-set thresholds. The additional scan time for the navigator and feedback is only 526 ms per diffusion volume, which takes 9500 ms to acquire. The navigated diffusion sequence (vNav) was validated using a water phantom, and in vivo in sixteen children (aged 5-6 years) and 6 adults. A multiecho MPRAGE sequence was also acquired in paediatric and adult subjects. FreeSurfer was used to automatically extract volumes of interest (VOI‟s). The mean and the histogram-derived measurements of the FA and MD for the whole brain (WBH) and for different VOI‟s were analysed. Our results show that adding the navigator does not alter the DTI data. In the paediatric and adult scans acquired using the standard diffusion sequence, subject head motion caused significant changes in all the DTI measures, except FA of the whole brain white matter in children. Retrospective motion correction did not recover diffusion measures but instead resulted in decreased FA and generated spurious fiber tracts in the corpus callosum. The DTI measures are recovered substantially in the prospective motion corrected data acquired using the navigated sequence.