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README.rst

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*Eddymotion*
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*NiFreeze*
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============
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Estimating head-motion and deformations derived from eddy-currents in diffusion MRI data.
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An open-source framework for volume-to-volume motion estimation in d/fMRI and PET,
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and Eddy-current-derived distortion estimation in dMRI.
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.. image:: https://zenodo.org/badge/DOI/10.5281/zenodo.4680599.svg
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:target: https://doi.org/10.5281/zenodo.4680599
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:target: https://github.com/nipreps/nifreeze/actions/workflows/pythonpackage.yml
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:alt: Python package
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Retrospective estimation of head-motion between diffusion-weighted images (DWI) acquired within
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diffusion MRI (dMRI) experiments renders exceptionally challenging1 for datasets including
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high-diffusivity (or “high b”) images.
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These “high b” (b > 1000s/mm2) DWIs enable higher angular resolution, as compared to more traditional
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diffusion tensor imaging (DTI) schemes.
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UNDISTORT [#r1]_ (Using NonDistorted Images to Simulate a Template Of the Registration Target)
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was the earliest method addressing this issue, by simulating a target DW image without motion
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or distortion from a DTI (b=1000s/mm2) scan of the same subject.
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Later, Andersson and Sotiropoulos [#r2]_ proposed a similar approach (widely available within the
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FSL ``eddy`` tool), by predicting the target DW image to be registered from the remainder of the
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dMRI dataset and modeled with a Gaussian process.
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Besides the need for less data, ``eddy`` has the advantage of implicitly modeling distortions due
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to Eddy currents.
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More recently, Cieslak et al. [#r3]_ integrated both approaches in *SHORELine*, by
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(i) setting up a leave-one-out prediction framework as in eddy; and
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(ii) replacing eddy’s general-purpose Gaussian process prediction with the SHORE [#r4]_ diffusion model.
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*Eddymotion* is an open implementation of eddy-current and head-motion correction that builds upon
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the work of ``eddy`` and *SHORELine*, while generalizing these methods to multiple acquisition schemes
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(single-shell, multi-shell, and diffusion spectrum imaging) using diffusion models available with DIPY [#r5]_.
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Diffusion and functional MRI (d/fMRI) generally employ echo-planar imaging (EPI) for fast
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whole-brain acquisition.
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Despite the rapid collection of volumes, typical repetition times are long enough for head motion
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to occur, which has been proven detrimental to both diffusion [1]_ and functional [2]_ MRI.
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In the case of dMRI, additional volume-wise, low-order spatial distortions are caused by
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eddy currents (EC), which appear as a result of quickly switching diffusion gradients.
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Unaccounted for EC distortion can result in incorrect local model fitting and poor downstream
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tractography results [3]_, [4]_.
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*FSL*'s ``eddy`` [5]_ is the most popular tool for EC distortion correction, and
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implements a leave-one-volume-out approach to estimate EC distortions.
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However, *FSL* has commercial restrictions that hinder application within open-source initiatives
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such as *NiPreps* [6]_.
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In addition, *FSL*'s development model discourages the implementation of alternative data-modeling
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approaches to broaden the scope of application (e.g., modalities beyond dMRI).
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*NiFreeze* is an open-source implementation of ``eddy``'s approach to estimate artifacts
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that permits alternative models that apply to, for instance, head motion estimation in fMRI
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and positron-emission tomography (PET) data.
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.. BEGIN FLOWCHART
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.. image:: https://raw.githubusercontent.com/nipreps/nifreeze/507fc9bab86696d5330fd6a86c3870968243aea8/docs/_static/nifreeze-flowchart.svg
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.. image:: https://raw.githubusercontent.com/nipreps/nifreeze/9588b4d0e410cc648f73f5581eb8feb38baf6e2b/docs/_static/nifreeze-flowchart.svg
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:alt: The nifreeze flowchart
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.. END FLOWCHART
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.. [#r1] S. Ben-Amitay et al., Motion correction and registration of high b-value diffusion weighted images, Magnetic
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Resonance in Medicine 67:1694–1702 (2012)
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.. [#r2] J. L. R. Andersson. et al., An integrated approach to correction for off-resonance effects and subject movement
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in diffusion MR imaging, NeuroImage 125 (2016) 1063–1078
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.. [#r3] M. Cieslak et al., QSIPrep: An integrative platform for preprocessing and reconstructing diffusion MRI data.
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Nature Methods, 18(7), 775–778 (2021)
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.. [#r4] E. Ozarslan et al., Simple Harmonic Oscillator Based Reconstruction and Estimation for Three-Dimensional Q-Space
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MRI. in Proc. Intl. Soc. Mag. Reson. Med. vol. 17 1396 (2009)
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.. [#r5] E. Garyfallidis et al., Dipy, a library for the analysis of diffusion MRI data. Front. Neuroinformatics 8, 8
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(2014)
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.. [1] Yendiki et al. (2014) *Spurious group differences due to head motion in a diffusion MRI study*.
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NeuroImage **88**:79-90.
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.. [2] Power et al. (2012) *Spurious but systematic correlations in functional connectivity MRI
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networks arise from subject motion*. NeuroImage **59**:2142-2154.
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.. [3] Zhuang et al. (2006) *Correction of eddy-current distortions in diffusion tensor images using
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the known directions and strengths of diffusion gradients*. J Magn Reson Imaging **24**:1188-1193.
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.. [4] Andersson et al. (2012) *A comprehensive Gaussian Process framework for correcting distortions
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and movements in difussion images*. In: 20th SMRT & 21st ISMRM, Melbourne, Australia.
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.. [5] Andersson & Sotiropoulos (2015) *Non-parametric representation and prediction of single- and
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multi-shell diffusion-weighted MRI data using Gaussian processes*. NeuroImage **122**:166-176.
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.. [6] Esteban (2025) *Standardized preprocessing in neuroimaging: enhancing reliability and reproducibility*.
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In: Whelan, R., & Lemaître, H. (eds.) *Methods for Analyzing Large Neuroimaging Datasets. Neuromethods*,
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vol. **218**, pp. 153-179. Humana, New York, NY.
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doi:`10.1007/978-1-0716-4260-3_8 <https://doi.org/10.1007/978-1-0716-4260-3_8>`__.

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