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Parallelized Patch2Self & other denoising methods on 7T diffusion weighted imaging: comparisons of quality and effects on tractometry
Paul B Camacho1, Shreyas Fadnavis2, Aaron T Anderson1,3, Eleftherios Garyfallidis4, Bruce Damon3,5, Tracey M Wzsalek1,3, and Brad P Sutton1,3,6
1Beckman Institute for Advanced Science & Technology, University of Illinois at Urbana Champaign, Urbana, IL, United States, 2Johnson & Johnson, Cambridge, MA, United States, 3Carle-Illinois Advanced Imaging Center, Carle Health, Urbana, IL, United States, 4Intelligent Systems Engineering, Indiana University Bloomington, Bloomington, IN, United States, 5Stephens Family Clinical Research Institute, Carle Health, Urbana, IL, United States, 6Bioengineering, University of Illinois at Urbana Champaign, Urbana, IL, United States

Synopsis

Keywords: Data Processing, Data Processing, Denoising, Diffusion Preprocessing

Motivation: Performance of denoising methods and effects on downstream analyses for diffusion-weighted imaging at 7 Tesla are understudied.

Goal(s): Determine which denoising methods provide better performance and whether any skew tractometry outcomes.

Approach: Preliminary data acquired using two different diffusion sequences - 30 or 64 directions/shell multi-shell - were separately denoised using either Patch2Self, oversampled local-PCA, non-local means, or Marchenko-Pastur PCA before QSIPrep pre-processing and DSI Studio AutoTrack GQI.

Results: Contrast-to-noise ratios were best for Patch2Self and oversampled local-PCA, agreeing with visual assessment. Fractional anisotropy distributions were higher and mean diffusivity lower in several major bundles for non-local means than Patch2Self, especially with lower angular resolution.

Impact: The computationally-efficient parallel Patch2Self improves 7T diffusion data quality and produces lower fractional anisotropy values in tractometry of common bundles for biomarker searches than non-local means. Denoising methods should be considered in literature comparisons and image preprocessing in clinical trials.

Introduction

White matter microstructure abnormalities - attributed to inflammation, edema, demyelination, axonal degeneration, etc.- are quantified by a growing number of metrics in the search for robust biomarkers. The variety of options available for preprocessing and analyses of MRI data are a major issue in reproducible neuroscience 1,2; especially important as we address artifacts in ultra-high field 7 Tesla MRI. In this preliminary work, we explore the differences in data quality metrics and tractometry results between four common denoising methods when applied to 7 Tesla diffusion MRI:
  • Non-local means (NLMEANS) 3, 4: similarity-weighted means of all voxels to each target voxel.
  • Overcomplete local PCA (LPCA) 5,6: applying principal component analysis (PCA) around each voxel, thresholding the eigenvalues based on estimating the original local noise variance, and performing PCA reconstruction.
  • Marchenko-Pastur PCA (MP-PCA) 7,8: removing components classified as noise automatically based on the Marcenko-Pastur distribution.
  • Parallelization-accelerated version of Patch2Self 9 (https://github.com/ShreyasFadnavis/p2s_parallel): a self-supervised learning method that uses the entire volume to learn a full-rank locally linear denoiser.

Methods

The data included here are a subset of the Champaign-Urbana Population Study (https://cupopulationstudy.illinois.edu/) - an observational longitudinal study recruited based on the criteria in Table 1 - with 10 participants each from an early protocol and from the first cohort with the updated protocol. Preprocessing begins with DICOM to Brain Imaging Data Structure (BIDS) format NIFTI conversion using HeuDiConv 13 and MP2RAGE UNI images denoising using the LN_MP2RAGE_DNOISE tool from LAYNII 14,15, followed by preprocessing using QSIPrep 0.19.1 16-26. For more details of the pipeline, see the section corresponding to workflows in QSIPrep’s documentation (https://qsiprep.readthedocs.io/en/latest/preprocessing.html). Diffusion orientation distribution functions (ODFs) were reconstructed using generalized q-sampling imaging 27 with a ratio of mean diffusion distance of 1.250000 in DSI Studio). Automatic Tractography was run in DSI Studio and bundle shape statistics were calculated 28. The following quality control metrics were compared across denoising methods: contrast to noise ratio (CNR), framewise displacement 29, and neighbor correlation 30. Code for performing these analyses is available at https://tinyurl.com/2n68fymc.

Results

From visual comparison, Patch2Self outperformed NLMEANS and MP-PCA in both the 64d and 30d data (see Figure 2). We note that the residuals from NLMEANS and LPCA appear to include more visually identifiable structure than those of Patch2Self, especially in the 30 directions per shell data. For the 64d data, the mean contrast-to-noise ratio in the high b-value shell was highest for both Patch2Self and LPCA; producing better neighbor correlations than NLMEANS or MP-PCA (see Figure 3) . The distribution of mean fractional anisotropy for the corpus callosum body was highest for data denoised with NLMEANS, while Patch2Self showed the lowest distribution for both acquisitions (see Figure 4). Mean diffusivity showed a higher distribution for Patch2Self than LPCA or NLMEANS. This general pattern - though varying in magnitude - appears to be common across other tracts, suggesting a skew that is stronger in lower angular resolution data. Mean length shows some differences in distributions, with more pronounced differences in the 64d data.

Discussion

Quality control metrics suggest that Patch2Self performs on par with the less computationally efficient LPCA in 7T data with 64 directions per shell, especially in the higher b = 2000s/mm2 shell. Considering the 6- to 7- fold increase in speed achieved with Patch2Self Parallel (versus the original Patch2Self as shown in Figure 1), the data quality improvement in less than three minutes of real time is an appreciable benefit for large 7T diffusion datasets.

As biomarker discovery continues to incorporate the more spatially informative tract profiles from tractometry, our findings show the visible effects of denoising method choice on both the shape and diffusion metric measures of key white matter bundles. Note that not all tracts are identified for all denoised data by DSI Studio AutoTrack, precluding paired differences testing in this preliminary analysis. Future work with the full CUPS dataset and 7T DWI from other sites will determine which tracts are robustly identified, include paired differences statistics, and investigate how age and site affect the magnitude of differences in data quality, motion estimates, and downstream tractometry outcomes produced by using the denoising methods compared here.

Conclusion

Compared to MP-PCA and NLMEANS, there are significant benefits for using Patch2Self Parallel for denoising in both 30 direction and 64 direction multi-shell 7T diffusion-weighted imaging data. Similar results can be achieved with LPCA, though the computational efficiency of Patch2Self Parallel is an important consideration for large datasets. Our results suggest that choice of denoising methods has a significant impact on tractometry outcomes at 7T and should be considered in as tractometry is incorporated in biomarker research.

Acknowledgements

We extend our gratitude to all participants in the Champaign-Urbana Population Study, the research team, and technicians at the Carle-Illinois Advanced Imaging Center. This research was supported in part by the Illinois Computes project which is supported by the University of Illinois Urbana-Champaign and the University of Illinois System.

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Figures

Table 1: Data collection information: inclusion and exclusion criteria for the CUPS study and MRI acquisition description. MRI data was collected using a Siemens 7T Terra system (Siemens Healthineers AG, Erlangen, Germany) at the Carle-Illinois Advanced Imaging Center, including magnetization prepared 2 rapid acquisition gradient echoes (MP2RAGE) 10 and diffusion-weighted imaging (DWI) collected with the CMRR multiband sequence 11,12 - with 30 directions (30d) or 64 directions (64d) per b-shell in the updated protocol - and accompanying field maps.

Figure 1: Comparison of run-times for Patch2Self and Patch2Self Parallel. Run-time values for Patch2Self - as implemented in DIPY - are plotted as large dots. Parallelized Patch2Self run-times are plotted as lines with increasing values for the Python joblib parameter n_jobs (number of parallel jobs). The data plotted above the dashed line represent denoising the 64d (128 volumes denoised) data, while the group below the dashed line represent the 30d (60 volumes denoised) data. Performance was profiled on a compute node of our cluster with memory = 192 gigabytes and a 24 threads.

Figure 2: Comparison of axial slices from raw and denoised gradient direction sensitized volumes of 7 Tesla diffusion data with 30 (top) or 64 (bottom) directions per non-zero b-shell. The 30d data contained only one b=0 shell, while five b=0 shells were interleaved evenly in the acquisition for the 64d data. NLMEANS: Non-local means, LPCA: over-sampled local principal component analysis, MP-PCA: Marchenko Pastur PCA.

Figure 3: Comparison of quality control metrics: neighbor correlation after denoising, angular contrast-to-noise ratio in the b = 1000 s/mm2 and b = 2000 s/mm2 shells, max and mean motion estimated by framewise displacement in data with 30 directions per shell (a, b, c, e, f respectively) and 64 directions per shell (g,h,i,j,k respectively).

Figure 4: Distributions of values or fractional anisotropy (FA), mean diffusivity (MD), and mean length in millimeters for the corpus callosum body, corpus callosum forceps major, left (L) and right (R) corticospinal tract obtained with each denoising method (indicated by line color) in 30 directions per shell (a, c, e, g, i, k, m, o, q, s, u, w) and 64 directions per shell (b, d, f, h, j, l, n, p, r, t, v) data. Note the trend that the distribution is lower FA for Patch2Self than NLMEANS, especially in 30d. MD distributions were generally higher for Patch2Self than NLMEANS or LPCA.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
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DOI: https://doi.org/10.58530/2024/3013