Towards cone-beam CT thermometry

Temperature monitoring and therefore the final treatment zone achieved during a cone-beam CT (CBCT) guided ablation can prevent overtreatment and undertreatment. A novel method is proposed to detect changes in consecutive CBCT images obtained from projection reconstructions during an ablation procedure. The possibility is explored of using this method to generate thermometry maps from CBCT images, which can be used as an input function for ablation treatment planning. This novel method uses a baseline and an intermittent CBCT scan, which are routinely acquired to confirm the needle position and monitor progress of the ablation. Accurate registration is required and assumed in vitro and ex vivo. A Wronskian change detector algorithm is applied on the compensated images to obtain a difference image between the intermittent and baseline scans. Finally, a thermal map created by applying a calibration determined experimentally is used to obtain the corresponding temperature at each pixel or voxel. We applied Wronskian change detector to detect the difference of two CBCT images, which have low signal to noise ratio, and calibrate Wronskian change model to temperature data using a gel phantom. We tested the temperature mapping with water and gel phantoms as well as pig shoulder. The experimental results show this method can detect temperature change within 5 degrees C for a voxel size of 1mm(3) (within clinical relevancy), and by consequence delineate the ablation zone. The preliminary experimental results show that CBCT thermometry is possible and promising, but may require pre-processing, such as registration for motion compensation between the baseline and intermittent scans. Further, quantitative evaluations have to be conducted for validation prior to clinical assessment and translation. CBCT is a widely available technology that could make thermometry clinically practical as an enabling component of iterative ablation treatment planning.