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Pushing the boundaries of in-vivo human diffusion MRI using Romer-EPTI: initial results on the Connectome 2.0 scanner1Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 2Department of Radiology, Harvard Medical School, Boston, MA, United States, 3Institute of Medical Physics and Radiation Protection, TH Mittelhessen University of Applied Science, Giessen, Germany, 4Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States
Synopsis
Keywords: Diffusion Acquisition, Data Acquisition, Diffusion Acquisition
Motivation: The progress in in-vivo high-resolution and high-b-value dMRI has advanced the investigation of structural connectivity and tissue microstructure.
Goal(s): We aim to further push the spatial resolution and b-value of in-vivo diffusion MRI.
Approach: A high-SNR(√25x), distortion-free, motion-robust Romer-EPTI technique that addresses the challenges in dMRI acquisition with further enhanced capabilities by taking advantage of the cutting-edge Connectome 2.0 scanner.
Results: We demonstrate motion-robust distortion-free in-vivo mesoscale dMRI (~485um-istropic) on clinical 3T&7T scanners, and further improvements in SNR on Connectome2.0 scanner enabling high-SNR, high b-value dMRI and a spatial resolution up to 400um-iso.
Impact: Romer-EPTI achieves high-SNR motion-robust distortion-free in-vivo mesoscale dMRI (~485 um) with exceptional quality on clinical 3T&7T scanners. Its capabilities were further enhanced on the Connectome 2.0 scanner, enabling high-SNR high b-value dMRI and a spatial resolution up to 400um-iso.
Introduction
The advancement in in-vivo dMRI has provided exciting insights into the structural connectivity and tissue microstructure of the human brain. Notably, high-spatial-resolution dMRI techniques1-7 have enabled the investigation of brain’s fine-scale structures by imaging crucial but small fibers1 and the cytoarchitecture of the thin cortical layers7. Furthermore, the use of stronger diffusion gradients8-13 has become available due to hardware advances14-16, combined with advanced diffusion models17-22, providing more precise information about tissue microstructure. However, further advancing dMRI in these directions faces several major technical challenges, which become increasingly pronounced as we seek higher resolution or diffusion gradients: i) reduced SNR as voxel-size is diminished and/or b-values are raised; ii) severe image degradation (distortion and T2*-blurring); iii) motion sensitivity.Here, we address these challenges of in-vivo dMRI by maximizing the capabilities of our recently-developed Romer-EPTI23-25 technique in conjunction with the newly-built Connectome 2.0 scanner (C2.0-scanner)26. Romer-EPTI is a novel dMRI technique that provides significantly higher SNR efficiency (e.g.,~√25x) with high motion robustness. Moreover, it provides distortion-free images with minimized blurring and slab-boundary artifacts. Romer-EPTI was able to achieve, for the first time, in-vivo dMRI at mesoscale ~500um resolution at both 3T and 7T23,25, and high-SNR ultra-high-b-value and time-dependent dMRI24. In this work, we first further characterize Romer-EPTI’s performance on clinically-available scanners, and demonstrate its high motion robustness and image-quality at mesoscale resolution up to 485-um-iso (~0.1 mm3). Then, to further enhance Romer-EPTI’s capabilities, we pursue its development on the ultra-high-gradient-strength C2.0-scanner, equipped with Gmax=500mT/m and maximal slew-rate=600T/m/s—about 5x gradient performance over modern MRI systems26. We demonstrated the improved SNR of Romer-EPTI on C2.0 derived from the shorter echo-spacing and therefore stronger spatiotemporal correlation. We then showed that Romer provides significant SNR gain compared to EPI on C2.0-scanner and can enable spatial resolution up to 400um-iso (0.064mm3, 2x-smaller voxel than 500um-iso).
Methods
Romer-EPTI integrates the efficient ky-t EPTI27-30 encoding with a rotating-view thick-slice super-resolution acquisition25,31-34 that encodes the kx-slice dimensions (Fig.1). It resolves high-SNR distortion-free high-isotropic-resolution volumes from multiple encoded volumes with different slice orientations using a motion-aware reconstruction25. The significant SNR gain of Romer-EPTI (e.g.,√25x) comes from: i) EPTI encoding that enables continuous readout with minimal dead-time, minimal TE, and optimal readout length (~1.3xSNR), and ii) short-TR thick-slice acquisition combined with SMS (e.g., additional √14.6xSNR). Romer-EPTI’s motion robustness stems from: i) its motion-aware reconstruction which models the motion between different Romer-encoded volumes; ii) the distortion-free EPTI readout which eliminates geometric inconsistency between volumes caused by motion-induced field changes. This avoids blurring/artifacts on the reconstructed high-resolution images, a problem that plagued conventional EPI (Fig.1c,~400% blurring in EPI @500μm even with 4x in-plane-acceleration). In addition, EPTI images are free from T2*-decay blurring, eliminating additional blurring seen in EPI (e.g., 45% @500um). Slab-boundary artifacts are minimized in Romer with modeled slice profiles.We validate the approach in high-quality mesoscale in-vivo dMRI data acquired on a Siemens 3T-Prisma and 7T-Terra as well as high-b-value and ultra-high-resolution data on the C2.0-scanner (MAGNETOM Connectom.X, Siemens Healthineers) with a custom-built 72-channel head-coil. The C2.0-scanner’s gradient performance provided 2x shorter echo-spacings, strengthening the spatiotemporal correlation within EPTI readout. This, in turn, improves reconstruction conditioning and further improves SNR (in addition to that from minimized-TE and optimal readout-length).
Results
Fig.2A demonstrates Romer-EPTI provides distortion-free images at 500um compared to the severe distortions of EPI (even when using a high in-plane-acceleration of 4). Fig.2B highlights the high motion robustness of Romer-EPTI, as it successfully recovers detailed structures through motion-aware reconstruction. Romer-EPTI images from both 3T&7T clinical scanners reveal fine-scale anatomical structures in exceptional quality at 500um-iso and 485um-iso resolutions (Fig.3), characterized by their distortion-free, high SNR, and minimal motion and slab-boundary artifacts.Moving to C2.0-scanner, >2x shorter echo-spacings can be achieved, resulting in >40% g-factor reduction and SNR boost for EPTI when using high accelerations compared to 3T-Prisma. In Fig.5A, SNR was improved for EPI at b=5000s/mm2 and 1.2-mm resolution on C2.0 thanks to shortened TEs (36ms vs. 88ms). Using Romer, additional significant SNR gain was achieved from the SNR-efficient acquisition. Moreover, an initial experiment combining Romer-acquisition and conventional EPI achieves 400um-iso resolution. While Romer offers high SNR, image blurring results from the EPI readout, which will be fixed by EPTI in ongoing work.
Conclusion
We demonstrated Romer-EPTI’s ability to provide high-SNR, distortion-free, motion-robust in-vivo dMRI at mesoscale resolution on clinical scanners, and further enhanced its performance by taking advantage of the C2.0-scanner to achieve high-resolution high-b-value, and mesoscale dMRI at 400um-iso.Acknowledgements
This work was supported by the NIH BRAIN Initiative (U24NS129893), NIH (K99AG083056, U01EB026996, R01-EB019437, P41EB030006), and the instrumentation Grants (S10OD023637).References
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