Development and initial evaluation of 7-T q -ball imaging of the human brain

• Published in: Magnetic resonance imaging Vol. 26; no. 2; pp. 171 - 180
• Main Authors: Mukherjee, Pratik, Hess, Christopher P, Xu, Duan, Han, Eric T, Kelley, Douglas A, Vigneron, Daniel B
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier Inc 01-02-2008
• Subjects: Adult; Anisotropy; Brain Mapping - methods; Diffusion Magnetic Resonance Imaging - methods; Diffusion tensor imaging (DTI); Fiber tractography; High angular resolution diffusion imaging (HARDI); High-field MR imaging; Humans; Image Processing, Computer-Assisted - methods; Male; Nerve Fibers, Myelinated - ultrastructure; q-ball imaging (QBI); Radiology; White matter; High angular resolution diffusion imaging (HARDI); Fiber tractography; Diffusion tensor imaging (DTI); White matter; q-ball imaging (QBI); High-field MR imaging
• ISSN: 0730-725X; 1873-5894
• DOI: 10.1016/j.mri.2007.05.011

Abstract Diffusion tensor imaging (DTI) noninvasively depicts white matter connectivity in regions where the Gaussian model of diffusion is valid but yields inaccurate results in those where diffusion has a more complex distribution, such as fiber crossings. q -ball imaging (QBI) overcomes this limitation of DTI by more fully characterizing the angular dependence of intravoxel diffusion with larger numbers of diffusion-encoding directional measurements at higher diffusion-weighting factors ( b values). However, the former technique results in longer acquisition times and the latter technique results in a lower signal-to-noise ratio (SNR). In this project, we developed specialized 7-T acquisition methods utilizing novel radiofrequency pulses, eight-channel parallel imaging EPI and high-order shimming with a phase-sensitive multichannel B0 field map reconstruction. These methods were applied in initial healthy adult volunteer studies, which demonstrated the feasibility of performing 7-T QBI. Preliminary comparisons of 3 T with 7 T within supratentorial crossing white matter tracts documented a 79.5% SNR increase for b =3000 s/mm2 ( P =.0001) and a 38.6% SNR increase for b =6000 s/mm2 ( P =.015). With spherical harmonic reconstruction of the q -ball orientation distribution function at b =3000 s/mm2 , 7-T QBI allowed for accurate visualization of crossing fiber tracts with fewer diffusion-encoding acquisitions as compared with 3-T QBI. The improvement of 7-T QBI at b factors as high as 6000 s/mm2 resulted in better angular resolution as compared with 3-T QBI for depicting fibers crossing at shallow angles. Although the increased susceptibility effects at 7 T caused problematic distortions near brain–air interfaces at the skull base and posterior fossa, these initial 7-T QBI studies demonstrated excellent quality in much of the supratentorial brain, with significant improvements as compared with 3-T acquisitions in the same individuals.