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

Abstract

Diffusion tensor imaging (DTI) noninvasively depicts white matter connectivity in regions where the Gaussian model of diffusion is valid, but yields inaccurate results 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 results in longer acquisition times and the latter results in lower signal-to-noise ratio (SNR). In this project, we developed specialized 7 Tesla acquisition methods utilizing novel radiofrequency pulses, 8-channel parallel imaging EPI, and high-order shimming with a phase-sensitive multichannel B 0 field map reconstruction. These methods were applied in initial healthy adult volunteer studies which demonstrated the feasibility of performing 7T QBI. Preliminary comparisons of 3T with 7T within supratentorial crossing white matter tracts document a 79.5% SNR increase for b=3000 s/mm 2 (p=0.0001), and a 38.6% SNR increase for b=6000 s/mm 2 (p=0.015). Using spherical harmonic reconstruction of the q-ball orientation distribution function at b=3000 s/mm 2 , 7T QBI allowed accurate visualization of crossing fiber tracts with fewer diffusion-encoding acquisitions than at 3T. The improvement of 7T QBI at b factors as high as 6000 s/mm 2 resulted in better angular resolution than 3T for depicting fibers crossing at shallow angles. Although the increased susceptibility effects at 7T caused problematic distortions near brain-air interfaces at the skull base and posterior fossa, these initial 7T QBI studies demonstrated excellent quality in much of the supratentorial brain with significant improvements as compared to 3T acquisitions in the same individuals.

Diffusion Tensor MR Imaging and Fiber Tractography: Theoretic Underpinnings

“…These problems have led to the introduction of more advanced methods such as high angular resolution diffusion imaging [118][119][120] and diffusion spectrum imaging, 121 which promise to overcome many of the inadequacies of the tensor model. These newer methods take better advantage of ongoing technical developments, including the synergistic combination of ultrahigh field diffusion 122 with highly accelerated parallel imaging, [123][124][125] and lead to new scientific and clinical applications such as probabilistic tractography [126][127][128] and whole-brain connectivity networks. 129 Hence, these newer techniques that transcend the diffusion tensor are likely to be adopted in clinical neuroradiology during the years to come.…”

Diffusion Tensor MR Imaging and Fiber Tractography: Technical Considerations

“…They are a more natural fit than DTI for the emerging fields of probabilistic tractography [73][74][75] and whole-brain connectivity analysis. 76 These newer methods also take better advantage of ongoing MR imaging hardware developments, including the synergistic combination of 7T diffusion 17 with highly accelerated parallel imaging, [18][19][20] which should lead to new scientific and clinical applications in neuroradiology.…”

“…16 Parallel imaging is even more helpful for ameliorating the stronger EPI susceptibility artifacts that occur at 3T 14 and is absolutely essential at 7T, 17 thereby permitting high-field and ultra-high field DWI with superior image quality (Fig 5). Conversely, higher field strength enables greater acceleration factors for parallel imaging because the shorter radio-frequency wavelengths, in conjunction with larger numbers of phasedarray receiver elements, improve the ability to reconstruct images with fewer phase-encoding steps without incurring an unacceptable loss of SNR.…”

Variability of Homotopic and Heterotopic Callosal Connectivity in Partial Agenesis of the Corpus Callosum: A 3T Diffusion Tensor Imaging and Q-Ball Tractography Study

“…More subtle patterns of callosal connectivity could be missed at this level of spatial resolution. Conducting QBI tractography at 7T to further boost the signal-to-noise ratio 35 to improve spatial resolution might increase the sensitivity of this investigation. Second, the small cohort of subjects with partial agenesis precluded any generalization about patterns of callosal connectivity or any analysis of the relationship between aberrant connectivity and the neurocognitive profile of these individuals.…”