Non-invasive MR thermography using the water proton chemical shift.

Among various proton magnetic resonance (MR) parameters, such as longitudinal relaxation time, transverse relaxation time, diffusion coefficient and chemical shift, the chemical shift of water protons is recognized as the most reliable indicator of temperature. The chemical shift is the only frequency-based parameter and is independent of the other parameters, which are measured based on the intensity of the MR signal. In this paper, the basic principle and the recent progress in imaging temperature by spectroscopic techniques using the water proton chemical shift are discussed. The advantages of spectroscopic imaging over phase mapping for measuring temperature are that the former can distinguish water resonance from other resonances, and that another resonance can be used as an internal reference to reduce the effects of external magnetic field instability, tissue susceptibility and inter-scan tissue movement or deformation. Methods utilizing various magnetic resonance spectroscopy (MRS) techniques, such as single voxel spectroscopy, conventional magnetic resonance spectroscopic imaging (MRSI), echo planar spectroscopic imaging (EPSI) and line scan echo planar spectroscopic imaging (LSEPSI) are discussed.