Magnetic resonance thermometry for image-guided thermal therapy

grantor: University of TorontoThe integration of medical-image guidance in medicine can enable the adaption of novel minimally-invasive surgeries and therapies. Thermal-coagulation therapy is a procedure that will benefit from image guidance and have clinical significance as an alternative to conventional treatments such as resective surgery or radiation therapy. The goal of this thermal therapy is to elevate diseased tissue temperature to induce necrosis while ensuring that the thermal treatment to surrounding healthy tissue is below toxic levels. The spatial-temporal delivery of heat energy requires control which is ideally provided through mapping of the three dimensional distribution of temperature in tissue. This thesis explores the use of real-time magnetic resonance imaging (MRI) for monitoring and guiding thermal-coagulation therapy. Specifically, a method of spatially-resolving the temperature distribution is described that incorporates the temperature-sensitivity of the proton-resonance frequency (PRF). This thermal-imaging method is implemented by measuring temperature-dependent phase shifts from a gradient-echo pulse sequence. Thermal calibration results are presented which show the temperature dependent PRF-shift to be independent of 'ex vivo' tissue type, within an experimental precision of 4%. The PRF-shift method of MR thermometry is shown to be capable of quantitatively accurate temperature measurements. However, it is identified that attention is required to several considerations, such as temperature-induced changes in the volume-magnetic susceptibility and electrical conductivity of tissue. The ability for real-time prediction of the spatial extent of thermal-coagulation necrosis is demonstrated with results obtained in an 'in vivo' canine prostate model. In this experiment, laser heating of the prostate gland was performed simultaneously with MR-thermal mapping. Using histological sections of the prostate gland to indicate the boundary of necrosis, a discriminant analysis revealed the onset of thermal-coagulative necrosis to be characterized by a critical temperature of approximately 51°C, or equivalently with an Arrhenius 't'43 period of approximately 200 minutes. It is concluded that real-time quantitative MR-thermal mapping is a visualization tool that can be used effectively for delivery and control of thermal-coagulation therapy.Ph.D