ISMRM 21st Annual Meeting & Exhibition 20-26 April 2013 Salt Lake City, Utah, USA

Diffusion Biophysics & Modeling
Wednesday 24 April 2013
Room 355 BC  13:30 - 15:30 Moderators: Evren Ozarslan, Dmitriy A. Yablonskiy

13:30 0489.   Observation of Muscle Fiber Diameter Increase with Exercise Using Time-Dependent Diffusion
Els Fieremans1, Gregory Lemberskiy2, Jens H. Jensen3, and Dmitry S. Novikov2
1Center for Biomedical Imaging, Department of Radiology, New York University, New York, NY, United States, 2Center for Biomedical Imaging, Department of Radiology, NYU School of Medicine, New York, NY, United States, 3Center for Biomedical Imaging, Department of Radiology and Radiological Science, Medical University of South Carolina, Charleston, SC, United States

The random permeable barriers model (RPBM) employs the time-dependence of the diffusion coefficient for quantifying cell size and membrane permeability. As an in vivo validation of the RPBM, we performed time-dependent diffusion measurements in the calf muscle of a healthy volunteer over the course of a weight lifting program. The RPBM yields realistic values for muscle fiber diameter and permeability. Additionally as expected, a significant increase in diameter of the gastrocnemius medialis is observed with training. This work demonstrates the feasibility of the RPBM method in quantifying muscle fiber diameter and permeability, and its sensitivity to microstructural changes.

13:42 0490.   Effects of Hypotonic Stress and Ouabain on Apparent Diffusion Coefficient at Cellular and Tissue Levels.
Ileana Ozana Jelescu1, Luisa Ciobanu1, Françoise Geffroy1, and Denis Le Bihan1
1NeuroSpin, Gif-sur-Yvette, Essonne, France

The mechanism causing apparent diffusion coefficient (ADC) decrease in brain tissue with ischemia is not yet clearly established. We evaluated ADC evolution at 17.2T, in isolated Aplysia californica neurons and within “tissue” (ganglia), following hypotonic shock or exposure to ouabain. Both types of stress caused an increase in ADC in single cells, and an overall decrease in ADC in the ganglia. Cell swelling was readily measurable with hypotonicity, but less obvious with ouabain. These results do not favor the extension of intracellular space as the origin of the observed ADC decrease at tissue level.

13:54 0491.   Viable and Fixed White Matter: DTI and Microstructural Comparisons at Physiological Temperature
Simon Richardson1,2, Bernard M. Siow1,2, Eleftheria Panagiotaki3, Torben Schneider4, Mark F. Lythgoe1, and Daniel C. Alexander5
1Division of Medicine and Institute of Child Health, UCL Centre for Advanced Biomedical Imaging, University College London, London, United Kingdom, 2Centre for Medical Image Computing, University College London, London, United Kingdom, 3Dept of Medical Phys and Bioengineering, Centre for Medical Image Computing, University College London, London, United Kingdom, 4NMR Research Unit, Queen Square MS Centre, Department of Neuroinflammation, UCL Institute of Neurology, London, United Kingdom, 5Department of Computer Science, Centre for Medical Image Computing, University College London, London, United Kingdom

We compare fixed and viable isolated tissue (VIT) in identical conditions at physiological temperature. We acquired DTI data sets with various acquisition parameters and a rich multi-b-value diffusion weighted MR (DW-MR) dataset for microstructural tissue model fitting. DTI data demonstrated a significant increase in radial diffusivity (RD) in fixed samples in comparison to VIT. Model fitting demonstrated that similar models best explain data from both samples. We conclude from model ranking stability that fixed tissue is a reasonable model for in-vivo, although significant differences in fitted model parameters suggest that water in individual compartments within the tissues behaves quite differently.

14:06 0492.   Reduced Diffusion Encoding for Accurately Estimating Axonal Injury, Demyelination, and Inflammation in Mouse Optic Nerve
Chia-Wen Chiang1, Yong Wang2, Anne H. Cross3,4, and Sheng-Kwei Song2,4
1Chemistry, Washington University in Saint Louis, Saint Louis, MO, United States, 2Radiology, Washington University School of Medicine, Saint Louis, MO, United States, 3Neurology, Washington University in St. Louis, Saint Louis, MO, United States, 4The Hope Center for Neurological Disorders, Washington University School of Medicine, Saint Louis, MO, United States

Diffusion basis spectrum imaging (DBSI) has been demonstrated to accurately detect and quantify multiple diffusion components resulting from crossing fibers, axonal injury, demyelination, and increased cellularity and water content associated with inflammation in both ex vivo phantom and in vivo animal studies. However, the employed 99-direction diffusion-encoding scheme requires a relatively long scanning time significantly hampering the application to examine living mouse optic nerves or spinal cord white matter tracts. In this study, we proposed a simplified 29-direction diffusion encoding-scheme to greatly reduce the scanning time for in vivo spinal cord and optic nerve studies while preserving the accuracy of DBSI computation. Successful validation of 29-direction scheme will facilitate the potential applications both in clinic and animal study using DBSI.

14:18 0493.   
Accurate Estimation of Intra-Axonal Water Diffusion Requires Proper Modeling of Surrounding Cellularitylower case Greek mulower case Greek mu
Yong Wang1 and Sheng-Kwei Song1,2
1Radiology, Washington University in St. Louis, Saint Louis, MO, United States, 2The Hope Center for Neurological disorders, Washington University in St. Louis, Saint Louis, MO, United States

Diffusion MRI has been proposed to measure the intra-axonal water diffusion to more accurately reflect axonal integrity. Reasonable intra-axonal water fraction and axial diffusivity have been reported in normal healthy brains, suggesting potential application to patients with central nervous system (CNS) diseases. However, the confounding effect of cellularity surrounding the axons has not been adequately modeled in most of those methods. In this study, Monte Carlo simulation was employed to investigate the effect of varied cellularity surrounding the axons on the intra-axonal water diffusion measurement. Preliminary data suggested that proper modeling of cellularity component is critical to accurately estimate intra-axonal water diffusion, especially for CNS lesions with prominent cell infiltration.

14:30 0494.   New Insights Into lower case Greek gamma-Stretched Exponential Anomalous-Diffusion Imaging Experiments
Marco Cavalieri1, Marco Palombo1,2, Alessandro Gozzi3, Andrea Gabrielli4, Angelo Bifone3, and Silvia Capuani1,2
1Physics Department, Sapienza University, Rome, Rome, Italy, 2CNR IPCF UOS Roma, Physics Department, Sapienza University, Rome, Rome, Italy, 3Istituto Italiano di Tecnologia, Center for Nanotechnology Innovation, IIT@NEST, Pisa, Pisa, Italy, 4CNR-ISC Roma, Sapienza University, Rome, Rome, Italy

The aim of the study was to overcome the “first order approximation” in the stretched-exponential lower case Greek gamma-imaging method, to obtain Anomalous Diffusion lower case Greek gamma values in the intrinsic lower case Greek gamma reference frame. To this end, we examined a fixed mouse brain at 7T, by performing conventional DTI and lower case Greek gamma-imaging experiments, and assessing T2*, DTI parameters, DTI main directions, stretched-exponential lower case Greek gamma-imaging parameters, lower case Greek gamma-imaging main directions in various anatomical regions of mouse brain. We show that the lower case Greek gamma reference frame is not coincident with the conventional diffusion reference frame. Moreover, our results suggest that lower case Greek gamma-imaging may provide information on tissue microstructure beyond and above DTI.

14:42 0495.   Singular Behavior of Time-Dependent Diffusion in a Fiber Bundle Geometry Due to a Disordered Packing
Lauren Burcaw1, Els Fieremans2, and Dmitry S. Novikov1
1NYUMC, New York, NY, United States, 2New York University, New York, NY, United States

We demonstrate that disorder in the packing geometry of a fiber bundle, such as an axonal tract, is crucial for the time-dependent diffusion. Using fiber phantom measurements and Monte Carlo simulations, we uncover a logarithmic singular behavior at long times, which makes the time dependence of diffusion in the extra-axonal space more pronounced than that coming from water confined inside axons. This singularity translates into linear-in-frequency dependence of OGSE diffusion coefficient at small frequencies, which again dominates over that from confined spaces. As a result, incorporating disorder in axonal packing is crucial for modeling and characterization of axonal tracts.

14:54 0496.   dPFG MRI Assessment of Axonal Beading in an Injury Model
Michal E. Komlosh1,2, Dan Benjamini1,3, Matthew D. Budde4, Lynne A. Holtzclaw5, Martin J. Lizak6, Ferenc Horkay1, Uri Nevo3, and Peter J. Basser1
1NICHD/PPITS/STBB, NIH, Bethesda, MD, United States, 2CNRM, USUHS, Bethesda, MD, United States, 3Department of Biomedical Engineering, The Iby and Aladar Fleischman Faculty of Engineering, Tel-Aviv University, Tel Aviv, Israel, 4Department of Neurosurgery, Medical College of Wisconsin, Milwaukee, WI, United States, 5NICHD/SCBS, NIH, Bethesda, MD, United States,6NINDS/MIF, NIH, Bethesda, MD, United States

dPFG MRI was used to characterize the microstructure of an injury model using rat sciatic nerve. Three samples were used in this study, one injured nerve and two controls. A theoretical model was used to fit the resulting dPFG MRI data in order to detect alterations in tissue microstructure.

15:06 0497.   Protocol Optimization of the Double Pulsed Field Gradient (D-PFG) Based Filter-Exchange Imaging (FEXI) Sequence Enables Comparative Studies of the Diffusional Apparent Exchange Rate (AXR) at Reduced Scan Times and Smaller Group Sizes.
Björn Lampinen1, Filip Szczepankiewicz1, Danielle van Westen2, Pia Sundgren2, Freddy Ståhlberg1,2, Jimmy Lätt3, and Markus Nilsson4
1Department of Medical Radiation Physics, Lund University, Lund, Sweden, 2Department of Diagnostic Radiology, Lund University, Lund, Sweden, 3Center for Medical Imaging and Physiology, Lund University Hospital, Lund, Sweden, 4Lund University Bioimaging Center, Lund University, Lund, Sweden

Filter-exchange imaging (FEXI) is based on a double pulsed field gradient (d-PFG) sequence, and provides a fast, non-invasive method for mapping water exchange expressed in its parameter apparent exchange rate (AXR). We used an analytical technique based on the Cramer-Rao Lower Bound to optimize the acquisition protocol. A new protocol is presented which reduces the coefficient of variance (CV) by 30% for measured AXR. With this optimized protocol, comparative studies searching for alterations in the AXR of magnitude three times larger than the inter-subject standard deviation can be performed using as few as four individuals per group with scan time below 15 minutes. This will enable future investigations of AXR as a biomarker for disease and treatment effects.

15:18 0498.   Application of Diffusional Kurtosis to Modeling of the Cerebral Microenvironment
Edward S. Hui1,2, Ali Tabesh1,2, Joseph A. Helpern1,2, and Jens H. Jensen1,2
1Center for Biomedical Imaging, Medical University of South Carolina, Charleston, South Carolina, United States, 2Dept of Radiology and Radiological Science, Medical University of South Carolina, Charleston, South Carolina, United States

Diffusion MRI (dMRI) has often been augmented with tissue-specific modeling in order to explicitly relate dMRI data to microstructural properties such as the sizes, orientations, volume fractions, and diffusivities of prescribed cellular compartments. One approach to tissue modeling is to exploit the close link between cytoarhitecture and the non-Gaussanity of water diffusion, which may be obtained with the dMRI technique known as diffusional kurtosis imaging (DKI). In this work, we propose a method, cerebral microenvironment modeling, which generalize the white matter model by Fieremans et al so that specific microstructural properties of the entire brain may be obtained with DKI.