Non-invasive magnetic resonance thermography during regional hyperthermia

Regional hyperthermia is a non-invasive technique in which cancer tissue is exposed to moderately high temperatures of approximately 43–45°C. The clinical delivery of hyperthermia requires control of the temperatures applied. This is typically done using catheters with temperature probes, which is an interventional procedure. Additionally, a catheter allows temperature monitoring only at discrete positions. These limitations can be overcome by magnetic resonance (MR) thermometry, which allows non-invasive mapping of the entire treatment area during hyperthermia application. Various temperature-sensitive MRI parameters exist and can be exploited for MR temperature mapping. The most popular parameters are proton resonance frequency shift (PRFS) (Δφ corresponding to a frequency shift of 0.011 ppm, i.e. 0.7 Hz per °C at 1.5 Tesla), diffusion coefficient D (ΔD/D = 2–3 % per °C), longitudinal relaxation time T1 ( per °C), and equilibrium magnetisation M0 ( per °C). Additionally, MRI temperature mapping based on temperature-sensitive contrast media is applied. The different techniques of MRI thermometry were developed to serve different purposes. The PRFS method is the most sensitive proton imaging technique. A sensitivity of ±0.5°C is possible in vivo but use of PRFS imaging remains challenging because of a high sensitivity to susceptibility effects, especially when field homogeneity is poor, e.g. on interventional MR scanners or because of distortions caused by an inserted applicator. Diffusion-based MR temperature mapping has an excellent correlation with actual temperatures in tissues. Correct MR temperature measurement without rescaling is achieved using the T1 method, if the scaling factor is known. MR temperature imaging methods using exogenous temperature indicators are chemical shift and 3D phase sensitive imaging. TmDOTMA− appears to be the most promising lanthanide complex because it showed a temperature imaging accuracy of <0.3°C.