Fast bound pool fraction imaging of the in vivo rat brain: Association with myelin content and validation in the C6 glioma model

Cross-relaxation imaging (CRI) is a quantitative magnetic resonance technique that measures the kinetic parameters of magnetization transfer between protons bound to water and protons bound to macromolecules. In this study, in vivo, four-parameter CRI of normal rat brains (N= 5) at 3.0 T was first directly compared to histology. The bound pool fraction, f, was strongly associated with myelin density (Pearson's r= 0.99, p<. 0.001). The correlation persisted in separate analyses of gray matter (GM; r= 0.89, p= 0.046) and white matter (WM; r= 0.97, p= 0.029). Subsequently, a new time-efficient approach for solely capturing the whole-brain parametric map of f was proposed, validated with histology, and used to estimate myelin density. Since the described approach for the rapid acquisition of f applied constraints to other CRI parameters, a theoretical analysis of error was performed. Estimates of f in normal and pathologic tissue were expected to have <. 10% error. A comparison of values for f obtained from the traditional four-parameter fit of CRI data versus the proposed rapid acquisition of f was within this expected margin for in vivo rat brain gliomas (N= 4; mean. ±. SE; 3.9. ±. 0.2% vs. 4.0. ±. 0.2%, respectively). In both whole-brain f maps and myelin density maps, replacement of normal GM and WM by proliferating and invading tumor cells could be readily identified. The rapid, whole-brain acquisition of the bound pool fraction may provide a reliable method for detection of glioma invasion in both GM and WM during animal and human imaging.