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Incorporating T2-orientational dependence into diffusion-T2 correlation experiments using a tiltable coil

Chantal M.W. Tax¹, Elena Kleban¹, Muhamed Barakovic^1,2,3, Maxime Chamberland¹, and Derek K. Jones¹
¹CUBRIC, Cardiff University, Cardiff, United Kingdom, ²Signal Processing Laboratory 5, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland, ³Translational Imaging in Neurology Basel, Department of Biomedical Engineering, University Hospital Basel, Basel, Switzerland

Synopsis

The anisotropy of white matter is reflected in various white matter contrasts. Transverse relaxation rates can be probed as a function of fibre-orientation with respect to the main magnetic field, while diffusion properties are probed as a function of fibre-orientation with respect to the gradient field. While the latter is easy to obtain in the same head position, the former involves reorientation of the subject’s head inside the scanner. In this work we deployed a tiltable RF-coil to study R2 anisotropy of the brain white matter in diffusion-T2 correlation experiments.

Introduction

It is well established that in myelinated white matter T2* depends on the orientation of the fibre with respect to B0 due to microscopic susceptibility effects^1–5. Orientation-dependence of T2 was also reported recently^6-8 and could potentially characterise microscopic-susceptibility more reliably⁷ and reflect effects related to axon diameter^8,9. Experiments designed to probe relaxation-anisotropy commonly involve reorienting the head inside the scanner, and are thus challenged by unintended SNR variations across orientations caused by differences in proximities to the receiver coil, and by increased susceptibility to motion and artefacts due to patient-discomfort.

In this work, we re-purpose a tiltable RF coil (originally designed for patient comfort) to investigate T2-orientational dependence within the context of a diffusion-T2 correlation experiment. The coil can be tilted around the left-right axis by 0˚, 9˚and 18˚ to B0, which: 1) minimises patient-discomfort and thus improves reliability; 2) offers a new degree of freedom as tilting around the left-right axis is otherwise difficult to achieve; 3) fixes the coil-to-brain distance across orientations and thus reduces SNR variations; and 4) increases the reproducibility of the experiment. Instead of studying global variations across the whole brain volume, we adopt an along-tract profiling tractometry approach to assess spatial variations into more detail.

Methods

Acquisition
Two healthy volunteers were scanned on a 3T 300mT/m Connectom scanner equipped with a modified 20-channel head/neck tiltable coil (Siemens Healthcare, Erlangen, Germany) in the default (0˚) and tilted (18˚) position (Fig. 1ab). Diffusion-T2 correlation data were acquired with different TE and b-values (Fig. 1c). Additional b0-images were acquired in the halfway-tilted (9˚) position (not shown). One subject had a second scan in the default position to examine test-retest variability.

Processing
The diffusion-T2 data were preprocessed^10–12 and spatial correspondence between the tilted and default position was obtained in two ways:
1) Nonlinear registration¹³ to the halfway-tilted (9˚) space; and
2) A tractometry (along-tract profiling) approach in each orientations’ native space: multi-shell multi-tissue tractography¹⁴ on the TE=54ms data was performed for each coil orientation, and 29 bundles were extracted¹⁵ and segmented (20 segments per bundle)¹⁶. For segments that visually had minimal fanning, voxels with a single-fibre-population were identified¹⁷ and their average orientation within a segment computed after removing directional outliers¹⁸.

Estimation
SNR estimates were obtained from the background of the TE=54ms b=0s/mm² images for both the 0˚ and 18˚ coil-orientations¹⁹. The voxel-wise T2 was estimated from the b0-images using a nonlinear least-squares trust-region-reflective algorithm in Matlab.
Residual bootstrapping (100 permutations) was performed to test if the difference in T2 due to coil-orientation was larger than the estimation-uncertainty.

Results

1) Registration to halfway-tilted space
The estimated noise standard deviation was similar for the two coil-orientations, and the signal (and thus SNR) varied within a similar range as could be expected from test-retest scans in the same orientation (Fig.2). Globally the images are aligned but some local misalignments can still be observed.
Fig.3a highlights differences in estimated T2 for different coil-orientations; globally the difference is larger than would be expected from test-retest variability. Fig. 3b highlights challenges with voxel-wise comparison; the inherent scan-rescan variability already induces significant T2-differences and more power is needed, which could be achieved with a tractometry approach.

2) Tractometry in native space
Fig.4 shows how T2 changes as a function of the fibre-orientation w.r.t. the main magnetic field in single-fibre-population voxels.
Tractometry of T2 in the CST reveals a significant increase in T2 (p numerically indistinguishable from 0) in a segment that experiences change in orientation w.r.t. B0 from ~45˚ in the default position to ~30˚ in the tilted position (Fig.5). This is in the regime where the derivative of the relaxation rate as a function of angle is the largest (Fig.4). In the callosal midbody the angle remains relatively unchanged and so does T2.

Discussion

Including a tiltable coil into the experimental set-up for diffusion-T2 correlation measurements paves the way for a more reliable assessment of orientational T2 dependence. Microstructural origins of the T2 difference could include differences in pathway properties (e.g. axon diameter) or susceptibility effects. T2 orientational-dependence would furthermore impact analyses frameworks that assume constant T2 along pathways²⁰. Voxel-wise comparison of T2 from different head-orientations remains challenging due to complications in experiment setup, imperfect correction for geometric distortions, and intrinsic scan-variability. Using a tractometry framework, we found significant regional changes in T2 upon tilting of the head. In future work, the diffusion-T2 correlation experiments can be used to study compartmental T2 orientation-dependence.

Acknowledgements

CMWT is supported by a Rubicon grant (680-50-1527) from the Netherlands Organisation for Scientific Research (NWO) and a Sir Henry Wellcome Fellowship (215944/Z/19/Z). DKJ and MC are supported by a Wellcome Trust Investigator Award (096646/Z/11/Z) and a Wellcome Trust Strategic Award (104943/Z/14/Z). We are grateful to Fabrizio Fasano, Peter Gall, and Matschl Volker from Siemens Healthcare GmbH for their support.

References

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Figures

Fig.1: a) The coil in default (0˚) and tilted (18˚) position. b) b=0s/mm² images at different TE. c) Acquisition parameters for the diffusion-correlation experiment.

Fig.2: b=0s/mm² image for the default (0˚) and tilted (18˚) orientation registered to the halfway-tilted (9˚) space, and their difference (tilted—default).

Fig.3: a) Estimated T2 for the default (0˚) and tilted (18˚) orientation registered to the halfway-tilted (9˚) space, and their difference. Red arrows indicate regions of visible difference. b) p-value upon a paired t-test, the p-value is scaled to indicate significance when Bonferroni correcting for multiple comparisons (>50000 voxels).

Fig.4 T2 as a function of the fibre-angle θ with the main magnetic field, in the default and tilted position. Each point represents a single-fibre-population voxel and is colour-coded according to its orientation (red, blue, and green correspond to the left-right, superior-inferior and anterior-posterior axis, respectively). The black line represents a model-fit of R2 as a function of θ as described in previous literature and references therein^1,4,7.

Fig.5: a) Tractometry results in the corticospinal tract (CST, left) for segments that visually had minimal fanning. The graphs show the estimated T2 (mean and standard deviation) for the default (blue), default-retest (green), and tilted (red) configuration, and the mean angle of single-fibre-populations in the segment w.r.t. B0. The highlighted segment has a significant change in T2. The segments are visualised in the brain image with different colours, and the single-fiber-populations are highlighted. b) As a), but for the genu of the corpus callosum (CC).

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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