Diffusion-weighted imaging in multiple sclerosis

Diffusion-weighted imaging (DWI) provides a unique form of magnetic resonan ce (MR) contrast that enables the diffusional motion of water molecules to be quantitatively measured. As a consequence, DWI provides information abou t the orientation, size and geometry of brain structures. Cellular structur es in the central nervous system restrict water molecular motion, and the a pparent diffusion coefficient (ADC) is reduced compared to diffusion in bul k water. Pathological processes that modify tissue integrity, thus removing some of the "restricting"' barriers, can result: in increased ADC. Prelimi nary studies in multiple sclerosis (MS) using DWI showed that the ADC is hi gher in macroscopic lesions than in the normal appearing white matter (NAWM ). The ADC is also dependent on the direction in which diffusion is measure d, thus making comparison of ADC values meaningless without taking into acc ount the measurement direction. One measurement of diffusion that is indepe ndent of the orientation of structures is provided by measuring the ADC in three orthogonal directions, and then averaging the results to form the mea n diffusivity, (D) over bar. We obtained DW scans from 35 patients with rel apsing-remitting MS and 24 healthy volunteers. (D) over bar was measured in side T2-visible lesions and regions located in different areas of the NAWM. (D) over bar histograms from a large portion of the brain were created. MS lesions had a significantly higher (D) over bar than NAWM, T1-hypointense lesions had the highest diffusion values, consistent with more severe tissu e disruption. (D) over bar was higher in the NAWM from patients than in the white matter from healthy controls. We also found significant differences between (D) over bar histogram-derived measures from patients and controls, confirming the presence of diffuse damage in the brain of patients with MS .