High b-value diffusion-weighted MRI of normal brain

Purpose: As MR scanner hardware has improved, allowing for increased gradie nt strengths, we are able to generate higher b values for diffusion-weighte d (DW) imaging. Our purpose was to evaluate the appearance of the normal br ain on DW MR images as the diffusion gradient strength ("b value") is incre ased from 1,000 to 3,000 s/mm(2) Method: Three sets of echo planar images were acquired at 1.5 T in 25 norma l subjects (mean age 61 years) using progressively increasing strengths of a diffusion-sensitizing gradient (corresponding to b values of 0, 1,000, an d 3,000 s/mm(2)). All other imaging parameters remained constant. Qualitati ve assessments of trace images were performed by two neuroradiologists, sup plemented by quantitative measures of MR signal and noise in eight differen t anatomic regions. Results: As gradient strength increased from b = 1,000 to 3,000, both gray and white matter structures diminished in signal as expected based on their relative diffusion coefficients [calculated average apparent diffusion coe fficient (ADC) values: gray matter = 8.5 x 10(-4) mm(2)/s, white matter = 7 .5 x 10(-4) mm(2)/s]. The signal-to-noise ratios for the b = 1,000 images w ere approximately 2.2 times higher than for the b = 3,000 images (p < 0.000 1). As the strength of the diffusion-sensitizing gradient increased, white matter became progressively hyperintense to gray matter. Relative to the th alamus, for example, the average MR signal intensity of white matter struct ures increased by an average of 27.5%, with the densely packed white matter tracts (e.g., middle cerebellar peduncle, tegmentum, and internal capsule) increasing the most. Conclusion: Brain DW images obtained at b = 3,000 appear significantly diff erent from those obtained at b = 1,000, reflecting expected loss of signal from all areas of brain in proportion to their ADC values. Consequently, wh en all other imaging parameters are held constant, b = 3,000 DW images appe ar significantly noisier than b = 1,000 images, and white matter tracts are significantly more hyperintense than gray matter structures.