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Abstract:
Purpose: To introduce a fast and stable diffusion imaging sequence by incorporating quadratic phase saturation pulses (QPP), and a reduced field-of-view (FOV) to reduce readout time and thereby minimize susceptibility effects in single-shot EPI diffusion imaging of the cervical spinal cord.
Subjects and Methods: Diffusion-weighted (DW) images (b=750
s/mm2, 6 DW directions) of the healthy spinal cord were acquired in axial orientation on a 3T Philips Achieva MR system using a single-shot spin-echo EPI sequence (slice thickness = 5 mm, NSA = 6/12 for b=0/b=750, half-scan = 0.6, fold-over-direction = LR). Different resolutions were evaluated for susceptibility effects and signal-to-noise-ratio (SNR). Matrix sizes (Mx) were varied between 96x24 and 144x36 on a rectangular FOV of 130x32.5 mm2.
The reduced number of phase encoding lines enabled a reduction in EPI-readout time from 59.6 ms to 16.3 ms for a matrix size of 128x32. To suppress fold-over, QPP were designed as described in [1] and were applied on both sides of the spinal cord. Phantom and in vivo optimization resulted in an effective saturation profile (\verb=~=5\% residual
fold-over-signal) for a pulse-duration of 6 ms with a suitable bandwidth, and a flip angle of 93o.
Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) images were calculated. Fiber-tracking was also applied on one dataset with 22 slices acquired in 16 min (slice thickness = 3.5 mm).
Results: The obtained DW and FA images contain neither visible susceptibility distortions nor fold-over in the spinal cord (Fig. 1). Even for Mx = 144x32 SNR is high enough for grey and white matter to be clearly distinguished. Fig.2 shows an FA image with main
eigenvectors overlaid (directions are color coded).
The mean FA in the gray/white matter was 0.3/0.7, with a mean ADC in the spinal cord of 1.1*10-3 mm2/s.
Fiber tracking (Fig.3) clearly visualizes the dominant cranio-caudal orientation of the longitudinal white matter tracts.
Conclusion: The feasibility of using a reduced FOV combined
with QPP-pulses to reduce the readout time for diffusion weighted single-shot EPI imaging of the cervical spinal cord was demonstrated. The virtually artefact-free images, and the short imaging time make this method promising for clinical diffusion imaging of the spinal cord.
