Diffusion imaging and bound pool fraction (BPF) mapping are two quantitative magnetic resonance imaging techniques that measure microstructural features of the white matter of the brain. Diffusion imaging provides a quantitative measure of the diffusivity of water in tissue. BPF mapping is a quantitative magnetization transfer (qMT) technique that estimates the proportion of exchanging protons bound to macromolecules, such as those found in myelin, and is thus a more direct measure of myelin content than diffusion. In this work, we combined BPF estimates of macromolecular content with measurements of diffusivity within human white matter tracts. Within the white matter, the correlation between BPFs and diffusivity measures such as fractional anisotropy and radial diffusivity was modest, suggesting that diffusion tensor imaging and bound pool fractions are complementary techniques. We found that several major tracts have high BPF, suggesting a higher density of myelin in these tracts. We interpret these results in the context of a quantitative tissue model.
Bibliographical noteFunding Information:
This work was supported by the NIH ( P41 RR09784 , EB006471 , EY15000 ), and GE Healthcare. We thank Roland Bammer for the diffusion imaging pulse sequence, Jason Yeatman for his analysis code, and Ewart Thomas for helpful comments on the analysis methods. We also thank Christian Beaulieu and two anonymous reviewers for helpful comments on an early draft of this manuscript.
- Bound pool fractions
- Diffusion tensor imaging
- Magnetic resonance imaging
- Quantitative magnetization transfer
- White matter