Monitoring of interstitial laser thermotherapy with heat-sensitive colour subtraction magnetic resonance imaging: Calibration with absolute tissue temperature and correlation with predicted lesion size

Magnetic resonance imaging (MR) is the most sensitive modality for monitori ng interstitial thermotherapy (ILT). A real-time pulse sequence that assign s a colour spectrum to grey-scale changes could potentially increase the ac curacy of MR-guided thermal surgery. This study aimed to calibrate this seq uence with tissue temperature and then to determine whether it could be use d to predict accurately the extent of tissue necrosis produced during the f ormation of a thermal lesion. Porcine livers were studied within a 0.5T Interventional MR Unit. A Nd:YAG laser fibre (lambda=1064 nm) with a diffuser tip was placed within the live r parenchyma adjacent to an MR compatible thermocouple. A template sagittal MR image containing the fibre tip was obtained. A 3 cm region of interest (ROI) was centred on the fibre. Thermal lesions were produced (5 W for 20 m in) with real-time subtraction MR monitoring with colour overlay throughout (acquisition time 4 s). At 60 s intervals the pixel intensity value, tempe rature and colour at the laser tip were noted. Twenty burns were produced. Pixel intensity measurements were expressed as percentages of mean pixel in tensity within the ROI to standardise measurements. Using the colour repres enting the temperature above which tissue necrosis would be expected to occ ur, predicted maximum lesion size was measured from the images and compared with histological assessment. There was a linear relationship between temperature and percentage pixel ch ange (r(2) = 0.84). Six discrete colours were determined, all significantly different from each other in terms of mean percentage pixel change (p<0.01 ) and mean temperature (p<0.01 except between orange and yellow, p = 0.037) . Green had a mean temperature of 55.6 (+/- 5)degrees C, and thus predicted necrosis. Image-predicted maximum lesion size correlated closely with hist ology (r(2)=0.93). The colour changes produced by this unique pulse sequence have been calibra ted with tissue temperature in vitro. The green colour represents the tempe rature above which necrosis would be expected to occur and can be used to a ccurately predict lesion size. This will potentially allow greater accuracy and safety for MR monitoring of ILT in vivo.