Non-invasive magnetic resonance thermography during regional hyperthermia

Abstract

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.

• regional hyperthermia (RHT)
• magnetic resonance imaging (MRI)
• magnetic resonance thermography
• water proton resonance frequency shift (PRFS)

Acknowledgement

The authors are very thankful for the support of the Berliner Sparkassenstiftung Medizin.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.