Spatial alignment of structural images on single-shot and
readout-segmented (RESOLVE) diffusion images in the optic nerve for
tractography template generation
Abstract
Purpose Successful utilization of anatomical templates in the
evaluation of diffusion-weighted neuroimaging-studies requires accurate
registration of intra-individual datasets. We investigated the
feasibility of structural MRI image registration onto single-shot and
read-out segmented echo-planar diffusion-weighted imaging variants for
use in tractographic samplings of the visual system, in particular the
optic nerve. Methods Slab volumes of the optic nerve pathway
from thirteen volunteers were acquired and preprocessed. Three
neuroradiologists marked landmarks (ROIs, regions of interest) on two
diffusion-weighted and one structural dataset. Structural ROIs were
respectively registered (6/12 degrees of freedom, DOF) onto single-shot
(ss-EPI) and readout-segmented (rs-EPI) volumes. All six ROI/FOD (fibre
orientation distribution) combinations underwent a targeted tractography
task (MRtrix3: iFOD2). Results Inter-rater reliability for
ROI-placement was highest in VIBE images (0.66 to 0.95, mean 0.85) and
lower in both ss-EPI (mean 0.77) and rs-EPI (0.46 to 0.84, mean 0.64).
Sufficient FOD generation in the optic nerve was successful in 12/26 of
all cases for ss-EPI volumes (46.2% of cases) and in 18/26 for rs-EPI
volumes (69.2%). Spatial shift of VIBE-drawn ROI-coordinates after
6-DOF registration was highest for ss-EPI-targets (medians: 1.15 to 1.4
mm; rs-EPI 0.82 to 0.93 mm), whereas 12-DOF registration caused less
spatial shifts (ss-EPI: 0.64 to 1.03 mm; rs-EPI: 0.58 mm to 0.79 mm).
Tractography results revealed no significant differences between ss-EPI
and rs-EPI sequences on cases with mutual generation of sufficient FODs
(n=10). Conclusion Structurally-placed ROIs (highest
inter-rater reliability) with 6-DOF registration onto rs-EPI targets
(highest FOD-generation rate) seems most suitable for white matter
template generation.