MR thermometry in fat-containing tissues for MR-guided High Intensity Focused Ultrasound therapy

The aim of the research was to improve and apply MR thermometry techniques in fat -containing tissues. These thermometry methods were researched with the aim of applying them for MR-HIFU treatment of breast and liver tumors. The first study demonstrated the sensitivity of proton resonance frequency shift (PRFS)-based thermometry to heat-induced magnetic susceptibility changes and proposed a correction method. HIFU sonications of breast fat resulted in a magnetic field disturbance which completely disappeared after cooling. To evaluate the correction procedure, an interface of tissue-mimicking ethylene glycol gel and fat was sonicated. The PRFS-thermometry errors in the gel were greatly reduced by using the correction procedure. In the second study, the effect of the aqueous and fatty tissue magnetic susceptibility distribution on absolute and relative temperature measurements as obtained directly from the water/fat frequency difference was investigated. Absolute thermometry was investigated using spherical phantoms filled with pork and margarine. To evaluate relative fat referencing, multi-gradient echo scans were acquired before and after heating pork tissue by HIFU. The sphere experiment showed susceptibility-related errors of 8.4 °C and 0.2 °C for pork and margarine, respectively. For relative fat referencing, fat showed pronounced phase changes of opposite polarity to aqueous tissue. Variations in the observed frequency difference between water and fat are largely due to variations in the water/fat spatial distribution. This effect may lead to considerable errors in absolute MR thermometry. Additionally, fat referencing may exacerbate rather than correct for PRFS-temperature measurement errors. During MR-HIFU therapy, ultrasound absorption in the near field represents a safety risk and limits efficient energy deposition. The feasibility of using T2 mapping to monitor the temperature change in subcutaneous adipose tissue was investigated. The T2 temperature dependence and reversibility was determined for adipose porcine samples. The accuracy was evaluated by comparing T2-based temperature measurements with probe readings in an ex vivo HIFU experiment. T2 changed linearly and reversibly with temperature with a coefficient of 5.2±0.1 ms/°C. For the ex vivo experiment, the difference between the T2-based temperature change and probe was