ISMRM 24th Annual Meeting & Exhibition • 07-13 May 2016 • Singapore

Focused Discussion Session: Frontiers of Diffusion

Wednesday, May 11, 2016
Summit 2
16:00 - 18:00
Moderators: Alexander Leemans, Dmitry Novikov

False positive bundles in tractography - Permission Withheld
Maxime Descoteaux1, Jasmeen Sidhu1, Eleftherios Garyfallidis1, Jean-Christophe Houde1, Peter Neher2, Bram Stieltjes3, and Klaus H. Maier-Hein2
1Computer Science, Université de Sherbrooke, Sherbrooke, QC, Canada, 2German Cancer Research Center, Heindeberg, Germany, 3Basel University, Basel University Hospital, Switzerland
This work provides novel insights in false positive bundles produced by tractography using a highly realistic diffusion MRI phantom with known underlying white matter ground truth anatomy. This MRI phantom was used in the ISMRM 2015 Tractography Challenge. We show that regardless of the tractography pipeline used, many invalid bundles with dense and meaningful structures are found in the tractograms.

Mapping the brain’s “Sheet Probability Index” (SPI) with diffusion MRI: Sheet happens?!
Chantal Tax1,2, Tom Dela Haije3, Andrea Fuster3, Carl-Fredrik Westin2, Max A. Viergever1, Luc Florack3, and Alexander Leemans1
1Image Sciences Institute, University Medical Center Utrecht, Utrecht, Netherlands, 2Department of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, United States, 3Mathematics and Computer Science, Eindhoven University of Technology, Eindhoven, Netherlands
The prevalence of sheet structure in the brain has been a debated issue since its proposal. This structure can be analyzed by means of the Lie bracket, which can be derived from diffusion MRI (dMRI) data. Due to the occurrence of noise, however, it is difficult to quantify to what degree the local structure effectively resembles a sheet. In this work, we propose a new and robust local measure based on the Lie bracket that can be interpreted as the sheet probability index (SPI).

To be Dispersed or Not to be Dispersed: A Study Using HCP Data
Aurobrata Ghosh1, Daniel C Alexander1, and Hui Zhang1
1Centre for Medical Image Computing, University College London, London, United Kingdom
We conduct model comparison experiments on the widely available HCP dataset to assess the importance of fibre-dispersion when modelling the brain’s tissue-microstructure from diffusion MRI (dMRI). Although many fibre dispersion configurations have been identified in the brain, most dMRI methods only model parallel or crossing fibres. To highlight the importance of dispersion, we design k-fold cross-validation experiments, on two HCP subjects, and compare ten compartment-based models using three metrics. We find that up to 50% of the brain-voxels, including white matter regions, support dispersion models over crossing models. Hence we conclude that it is important to model dispersion in dMRI.

Challenges in solving the two-compartment free-water diffusion MRI model
Ørjan Bergmann1,2, Carl-Fredrik Westin1, and Ofer Pasternak1
1Dept of Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, United States, 2Norwegian Competency Center for MS, Haukeland University Hospital, Bergen, Norway
In this work we explore the solution space of the two-compartment free-water problem under different noise levels. Based on the shape of the solution space we show that solving this model in an intuitive and straightforward manner may result in solutions which are sensitive to noise, and that are biased towards neglecting the free-water component. Although multi-shell techniques improve the situation we show that more advanced methods are required to further stabilize the solution.

Quantification of demyelination and remyelination with diffusion MRI: specific in vivo White Matter Tract Integrity metrics agree with electron microscopy-derived features
Ileana O Jelescu1, Magdalena Zurek1, Kerryanne V Winters1, Jelle Veraart1, Anjali Rajaratnam1, Nathanael S Kim1, James S Babb1, Timothy M Shepherd1, Dmitry S Novikov1, Sungheon G Kim1, and Els Fieremans1
1Center for Biomedical Imaging, Radiology, New York University School of Medicine, New York, NY, United States
White Matter Tract Integrity (WMTI) metrics derived from diffusion data provide a compartment-specific characterization of white matter. Here, we evaluated the specificity of the axonal water fraction (AWF) and extra-axonal radial diffusivity (De,-) by assessing their correlations to metrics derived from electron microscopy (EM), in the splenium of control, cuprizone-treated and recovering mice. As the model predicted, the WMTI-derived AWF correlated very strongly with the EM-derived AWF, but not with the g-ratio, while De,- correlated with the g-ratio, but not with the EM-derived AWF. WMTI parameters are therefore promising biomarkers for specific biophysical aspects of white matter pathology in vivo.

Exploring fibre orientation dispersion in the corpus callosum: Comparison of Diffusion MRI, Polarized Light Imaging and Histology
Jeroen Mollink1,2, Michiel Kleinnijenhuis1, Stamatios N Sotiropoulos1, Michiel Cottaar1, Anne-Marie van Cappellen van Walsum2, Menuka Pallebage Gamarallage3, Olaf Ansorge3, Saad Jbabdi1, and Karla L Miller1
1FMRIB centre, University of Oxford, Oxford, United Kingdom, 2Donders Institute for Brain, Cognition and Behaviour, Department of Anatomy, Radboud University Medical Centre, Nijmegen, Netherlands,3Department of Neuropathology, University of Oxford, Oxford, United Kingdom
In this study we explored fibre orientation dispersion in the corpus callosum using diffusion-weighted MRI, Polarized Light Imaging and Histology. Microscopic fibre orientations were derived from Polarized Light Imaging and histological myelin and glial cell staining, with the aim of understanding the microstructural features that correlate with the diffusion signal.

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