MR Thermometry - an Overview of the Proton Resonance Frequency, T1, T2, and Diffusion methods

Abstract The growing adoption of non- and minimally-invasive thermal therapies has amplified the need for precise, real-time temperature monitoring within the body. Ideal thermometry techniques must be non-invasive and capable of delivering high spatial (millimeter-scale) and temporal (second-scale) resolution across treatment-relevant volumes (hundreds of millimeters). Magnetic Resonance Imaging (MRI) is currently the only imaging modality which fulfills these requirements, which has made it the standard approach to monitor thermal therapies. MRI offers multiple temperature-sensitive parameters that have been exploited to non-invasively map temperature changes in vivo. This review provides a brief overview of MRI principles, including signal generation and image formation, followed by an in-depth discussion of four widely used MR thermometry methods: the Proton Resonance Frequency (PRF) shift method, T1-based, T2-based, and diffusion-based techniques. Among these, the PRF method stands as the current gold standard due to its simplicity, working with any gradient echo pulse sequence, ease of implementation, and high accuracy and precision in all water-based tissues. T1- and T2-based methods, while less commonly employed, offer valuable alternatives in tissues where PRF does not work, such as in fat and bone. Diffusion-based thermometry, despite challenges such as motion sensitivity, shows promise in specialized applications such as measuring temperatures in cerebrospinal fluid and ocular applications. Together, these methods underscore MRI's versatility and power in thermal therapy monitoring.