Purpose
Oscillating gradient spin‐echo (OGSE) diffusion MRI provides information about the microstructure of biological tissues by means of the frequency dependence of the apparent diffusion coefficient (ADC). ADC dependence on OGSE frequency has been explored in numerous rodent studies, but applications in the human brain have been limited and have suffered from low contrast between different frequencies, long scan times, and a limited exploration of the nature of the ADC dependence on frequency.
Theory and Methods
Multiple frequency OGSE acquisitions were acquired in healthy subjects at 7T to explore the power‐law frequency dependence of ADC, the “diffusion dispersion.” Furthermore, a method for optimizing the estimation of the ADC difference between different OGSE frequencies was developed, which enabled the design of a highly efficient protocol for mapping diffusion dispersion.
Results
For the first time, evidence of a linear dependence of ADC on the square root of frequency in healthy human white matter was obtained. Using the optimized protocol, high‐quality, full‐brain maps of apparent diffusion dispersion rate were also demonstrated at an isotropic resolution of 2 mm in a scan time of 6 min.
Conclusions
This work sheds light on the nature of diffusion dispersion in the healthy human brain and introduces full‐brain diffusion dispersion mapping at clinically relevant scan times. These advances may lead to new biomarkers of pathology or improved microstructural modeling.
Half-Fourier acquisition single-shot turbo spin-echo diffusion-weighted (DW-HASTE) is a clinically established magnetic resonance imaging tool for the detection of small lesions, particularly cholesteatoma. However, in the standard approach, half of the available signal is sacrificed through displacing one echo parity out of the acquisition window to fulfil the Carr-Purcell-Meiboom-Gill condition. We present a selective parity approach to tackle this problem. The proposed method features a near full sensitivity, a low specific absorption rate for long echo trains, and about two-fold increase in signal-to-noise ratio, compared to the manufacturer's method under the same conditions.
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