Tissue characterization using R dispersion imaging at low locking fields.

Measurements of the variations of spin-locking relaxation rates (R) with locking field amplitude allow the derivation of quantitative parameters that describe different dynamic processes, such as slow molecular motions, chemical exchange and diffusion. In some samples, changes in R values between locking frequency 0 and 200 Hz may be dominated mainly by diffusion of water in intrinsic field gradients, while those at higher locking fields are due to exchange processes. The exchange and diffusion effects act independently of each other, as confirmed by simulation and experimentally. In tissues, the relevant intrinsic field gradients may arise from the magnetic inhomogeneities caused by microvascular blood so that R dispersion over weak locking field amplitudes (≤ 200 Hz) is affected by changes in capillary density and geometry. Here we first review the theoretical and experimental background to the interpretation of R dispersions caused by intrinsic magnetic susceptibility variations within the tissue. We then provide new empirical results of R dispersion imaging of the human brain and skeletal muscle at low locking field amplitudes for the first time and identify potential applications of R dispersion imaging in clinical studies.