Denis Le Bihan

Water diffusion compartmentation and anisotropy at high b values in the human brain

Biexponential diffusion decay is demonstrated in the human brain in vivo using b factors up to 4000 sec mm−2. Fitting of the signal decay data yields values for the slow and fast diffusion components and volume fractions in agreement with previous studies in rat and human brain. In addition, differences in the fitted parameters are demonstrated in the white and gray matter and diffusion anisotropy is demonstrated in both the slow and fast diffusing components. Apparent anisotropy in the component fractions is discussed in terms of directionally dependent exchange rates between the compartments. The lack of a relationship between the estimated contribution to the signal of the fast and slow components and echo time appears to rule out T2 differences in the observed water compartments. Values obtained for the fast diffusion coefficient, including differences between white and gray matter and the degree of anisotropy are compatible with the predictions of extracellular diffusion of water based on tortuosity models and the diffusion of tetramethylammonium ions in rat brain. Magn Reson Med 44:852–859, 2000.

Diffusion and perfusion magnetic resonance imaging in brain tumors

The article reviews recent progress made in the field of diffusion and perfusion magnetic resonance (MR) imaging and presents possible applications in brain tumors. Diffusion, a new parameter, provides useful data to assess tissue structure and function. Perfusion MR imaging gives results that are somewhat similar to those obtained with classic non-MR imaging methods, but it offers several potential advantages.

Assessment of Multiexponential Diffusion Features as MRI Cancer Therapy Response Metrics

The aim of this study was to empirically test the effect of chemotherapy‐induced tissue changes in a glioma model as measured by several diffusion indices calculated from nonmonoexponential formalisms over a wide range of b‐values. We also compared these results to the conventional two‐point apparent diffusion coefficient calculation using nominal b‐values. Diffusion‐weighted imaging was performed over an extended range of b‐values (120–4000 sec/mm2) on intracerebral rat 9L gliomas before and after a single dose of 1,3‐bis(2‐chloroethyl)‐1‐nitrosourea. Diffusion indices from three formalisms of diffusion‐weighted signal decay [(a) two‐point analytical calculation using either low or high b‐values, (b) a stretched exponential formalism, and (c) a biexponential fit] were tested for responsiveness to therapy‐induced differences between control and treated groups. Diffusion indices sensitive to “fast diffusion” produced the largest response to treatment, which resulted in significant differences between groups. These trends were not observed for “slow diffusion” indices. Although the highest rate of response was observed from the biexponential formalism, this was not found to be significantly different from the conventional monoexponential apparent diffusion coefficient method. In conclusion, parameters from the more complicated nonmonoexponential formalisms did not provide additional sensitivity to treatment response in this glioma model beyond that observed from the two‐point conventional monoexponential apparent diffusion coefficient method. Magn Reson Med, 2010.

Imaging focal reperfusion injury following global ischemia with diffusion-weighted magnetic resonance imaging and 1H-Magnetic Resonance Spectroscopy

The purpose of the study was to determine whether diffusion-weighted magnetic resonance imaging (DWI) could identify focal lesions that develop in ischemia-sensitive cerebral tissues during reperfusion following global brain ischemia. Localized 1H-Magnetic Resonance Spectroscopy (1H-MRS) measurements were also obtained to determine whether abnormal spectroscopic markers were associated with focal lesions and to define time correlations between DWI and metabolic changes. Brain diffusion-weighted magnetic resonance imaging measurements were made in a cat model of repetitive global cerebral ischemia and reperfusion. Five animals were exposed to three episodes of 10 min vascular occlusions at hourly intervals. Three animals were evaluated as controls. DWI, T2WI, and 1H-MRS data were acquired for up to 12 h. Transient focal DWI hyperintensity was detected in the hippocampus, basal ganglia, and cortical watershed areas. These focal abnormalities usually appeared during the final reperfusion and eventually spread to encompass all of the gray matter. Spectroscopic measurements demonstrated the expected elevation of the lactate signal intensity during vessel occlusion, which returned to normal during early reperfusion. A subsequent rise in the lactate signal occurred approximately 3-4 h after the beginning of the third reperfusion. This late lactate elevation occurred after focal hyperintensities were identified by DWI. No significant signal changes were seen in spectroscopic metabolites other than lactate. The study illustrates that DWI and 1H-MRS are sensitive to focal cerebral lesions that occur during reperfusion following global cerebral ischemia.