Comparison of thermal damage calculated using magnetic resonancethermometry, with magnetic resonance imaging post-treatment and histology,after interstitial microwave thermal therapy of rabbit brain

Abstract

Clinical application of high-temperature thermal therapy as a treatment forsolid tumours requires an accurate and close to real-time method forassessing tissue damage. Imaging methods that detect structural changesduring heating may underestimate the extent of thermal damage. This is dueto the occurrence of delayed damage manifested at tissue locations exposedto temperatures lower than those required to cause immediate structuralchanges. An alternative approach is to measure temperature and thencalculate the expected damage based on the temperature history at eachtissue location. Magnetic resonance (MR) imaging methods now allowtemperature maps of the target and surrounding tissues to be generated inalmost real-time. The aim of this work was to evaluate whether thermaldamage zones calculated on the basis of MR thermometry maps measured duringheating correspond to actual tissue damage as measured after treatment byhistological methods and MR imaging.Four male rabbits were treated with high-temperature thermal therapydelivered in the brain by a single microwave antenna operating at 915 MHz.MR scanning was performed before, during and after treatment in a 1.5 Twhole-body scanner. Temperature maps were produced using the protonresonance frequency (PRF) shift method of MR thermometry. In addition,conventional T1-weighted and T2-weighted spin-echo images wereacquired after treatment. Thermal damage zones corresponding to cell death,microvascular blood flow stasis and protein coagulation were calculatedusing an Arrhenius analysis of the MR temperature/time course data. Thecalculated zones were compared with the lesions seen on histopathologicalexamination of the brains which were removed within 6-8 h of treatment.The results showed that calculated damage zones based on MR thermometryagreed well with areas of damage as assessed using histology after heatingwas completed. The data suggest that real-time calculations of finalexpected thermal damage based on an Arrhenius analysis of MR temperaturedata may provide a useful method of real-time monitoring of thermal therapywhen combined with conventional T2-weighted images taken aftertreatment.