Microstructural changes in ischemic cortical gray matter predicted by a model of diffusion‐weighted MRI

Abstract

Purpose: To understand the diffusion attenuated MR signal from normal and ischemic brain tissue in order to extract structural and physiological information using mathematical modeling, taking into account the transverse relaxation rates in gray matter. Materials and Methods:We fit our diffusion model to the diffusion-weighted MR signal obtained from cortical gray matter in healthy subjects. Our model includes variable volume fractions, intracellular restriction effects, and exchange between compartments in addition to individual diffusion coefficients and transverse relaxation rates for each compartment. A global optimum was found from a wide range of parameter permutations using cluster computing. We also present simulations of cell swelling and changes of exchange rate and intracellular diffusion as possible cellular mechanisms in ischemia. Results:Our model estimates an extracellular volume fraction of 0.19 in accordance with the accepted value from histology. The absolute apparent diffusion coefficient obtained from the model was similar to that of experiments. The model and the experimental results indicate significant differences in diffusion and transverse relaxation between the tissue compartments and slow water exchange. Our model reproduces the signal changes observed in ischemia via physiologically credible mechanisms. Conclusion:Our modeling suggests that transverse relaxation has a profound influence on the diffusion attenuated MR signal. Our simulations indicate cell swelling as the primary cause of the diffusion changes seen in the acute phase of brain ischemia.

Biexponential Signal Attenuation Analysis of Diffusion-weighted Imaging of Breast

“…7,15,17 The decrease in ADC mainly results from increased cellularity, but some tumors show other changes, such as ‰uid viscosity and cell permeability, as well. [23][24][25][26][27][28][29][30] In this respect, biexponential signal analysis will provide more precise information about tissue character. Thus, we performed DWI with high b-values up to 3500 s/mm 2 and examined the character of biexponential signal attenuation using Equation [3], in which fast and slow components are postulated.…”

“…However, the estimated ratios of f 1 :f 2 (9:1¿7:3) are clearly diŠerent from the actual component fractions (1:9¿3:7) determined with other physical methods. After Niendorf'sˆndings, many researchers have investigated this discrepancy and indicated in‰uence from permeability or restriction of cell membrane [23][24][25] and diŠerence of relaxation time of intra-and extracellular space 26,27 as well as other factors. [28][29][30] However, the detailed cause of biexponential signal attenuation remains unclear.…”

How Does Water Diffusion in Human White Matter Change Following Ischemic Stroke?

“…During an application of MPG in a DWI sequence, transverse magnetization, M, can be considered to decline with time, t, by diŠusion as: dM/dt ＝-b(dapp/D)M. 54 Therefore, the transverse magnetizations in the 3 compartments (ME in the extracellular space, MC in the cylinder, and MS in the sphere) can be obtained by solving simultaneous diŠerential equations:…”

An Explanation of Signal Changes in DW-fMRI: Monte Carlo Simulation Study of Restricted Diffusion of Water Molecules Using 3D and Two-Compartment Cortical Cell Models

“…In every simulation, we set 2r to 10 μm to consider the size of the cell and 2l to 11.262 μm to set the volume ratio of the intracellular and the extracellular regions to 70% [14], [15]. Moreover, we assumed that a cell with a length of 10 μm on each side swelled by 1-10% in length, while 2l is fixed to 11.262 μm.…”

“…In the simulations, water molecules were reflected at the surface of the outer cube. We also set D int to 1.0 × 10 −3 mm 2 /s and D ext to 3.0 × 10 −3 mm 2 /s [14], [15]. The remaining parameters were Z = 10 6 , Δt = 100 μs, and M = 10.…”