Aidin Arbabi^{1} and Corey A. Baron^{1}

Oscillating gradient spin-echo (OGSE) diffusion MRI allows measurement of the frequency dependence of the apparent diffusion coefficient (ADC), which gives insight into tissue microstructure. OGSE has been utilized in numerous animal models, but its application in the in vivo human brain is challenging. Further, a parameterization that allows visualization of maps of the frequency dependence of ADC throughout the human brain has thus-far not been demonstrated. In this work, we report on an efficient method to generate maps of diffusion dispersion (DD), which characterizes the frequency dependence of the ADC, and demonstrate full-brain DD mapping in vivo at 7T.

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Figure 1: Example ADC and DD maps acquired at b = 565 s/mm^{2}, from top to bottom: PGSE ADC (diffusion time=55 ms), 30Hz OGSE ADC, 60Hz OGSE ADC, 30 Hz DD, and 60Hz DD. Uniform maps are observed throughout the brain tissue. CSF was masked in DD maps by setting to zero all voxels with PGSE ADC> 1.2 mm^{2}/s.

Figure 2: Comparison of ADC and DD maps on two sample slices for the frequency of 60Hz acquired at b=565 s/mm^{2}: (left) PGSE ADC, (middle) 60Hz OGSE ADC, and (right) 60Hz DD. Red rectangles depict areas affected by Gibbs ringing on ADC maps.

Figure 3: Mean ADC change and diffusion dispersion (DD) measured within example regions-of-interest (ROI) on two different slices for 30Hz and 60Hz: (A) ROI placements on 60Hz DD map, (B) ADC change with frequency, (C) ADC change with the square root of frequency, with solid lines indicating linear regression, and (D) Mean DD for 30Hz and 60Hz, with dashed lines indicating the mean DD for each ROI estimated from the linear regression in (C). As expected, ADC increases linearly with the square root of frequency for all ROIs. Error bars indicate the standard deviation of voxel values within each ROI.