Self-supervised arbitrary-scale super-angular resolution diffusion MRI reconstruction.

Background: Diffusion magnetic resonance imaging (dMRI) is currently the unique noninvasive imaging technique to investigate the microstructure of in vivo tissues. To fully explore the complex tissue microstructure at sub-voxel scale, diffusion weighted (DW) images along many diffusion gradient directions are usually acquired, this is undoubtedly time consuming and inhibits their clinical applications. How to estimate the tissue microstructure only from DW images acquired with few diffusion directions remains a challenge.

Purpose: To address this challenge, we propose a self-supervised arbitrary scale super-angular resolution diffusion MRI reconstruction network (SARDI-nn), which can generate DW images along any directions from few acquisitions, allowing to overcome the limits of diffusion direction number on exploring the tissue microstructure.

Methods: SARDI-nn is mainly composed of a diffusion direction-specific DW image feature extraction (DWFE) module and a physics-driven implicit expression and reconstruction (IRR) module. During training, dual downsampling operations are implemented. The first downsampling is used to produce the low-angular resolution (LAR) DW images; the second downsampling is for constructing input and learning target of SARDI-nn. The input LAR DW images pass through a DWFE module (composed of several residual blocks) to extract the feature representations of DW images along input directions, and then these features and the difference between the any querying diffusion direction and the input directions are input into a IRR module to derive the implicit representation and DW image along this query direction. Finally, based on the principle of dMRI, an adaptive weighting method is used to refine the DW image quality. During testing, given any diffusion directions, we can simply infer the corresponding DW images along these directions, accordingly, SARDI-nn can realize arbitrary scale angular super resolution. To test the effectiveness of the proposed method, we compare it with several existing methods in terms of peak signal to noise ratio (PSNR), structural similarity index measure (SSIM), and root mean square error (RMSE) of DW image and microstructure metrics derived from diffusion kurtosis imaging (DKI) and neurite orientation dispersion and density imaging (NODDI) models at different upsampling scales on Human Connectome Project (HCP) and several in-house datasets.

Results: The comparison results demonstrate that our method achieves almost the best performance at all scales, with SSIM of reconstructed DW images improved by 10.04% at the upscale of 3 and 5.9% at the upscale of 15. Regarding the microstructures derived from DKI and NODDI models, when the upscale is not larger than 6, our method outperforms the best supervised learning method. In addition, the test results on external datasets show the well generality of our method.

Conclusions: SARDI-nn is currently the only method that can reconstruct high-angular resolution DW images with any upscales, which allows the variation of both input diffusion direction number and upscales, therefore, it can be easily extended to any unseen test datasets, not requiring to retrain the model. SARDI-nn provides a promising means for exploring the tissue microstructures from DW images along few diffusion gradient directions.