Quantification of edematic effects in prostate brachytherapy interventions

We present a quantitative model to analyze the detrimental effects of edema on the quality of prostate brachytherapy implants. We account for both tissue expansion and implant migration by mapping intra-operative ultrasound and post-implant CT. We pre-process the ultrasound with a phase congruency filter, and map it to the volume CT using a B-spline deformable mutual information similarity metric. To test the method, we implanted a standard training phantom with 48 seeds, imaged the phantom with ultrasound and CT and registered the two for ground truth. Edema was simulated by distorting the CT volume by known transformations. The objective was to match the distorted implant to the intra-operative ultrasound. Performance was measured relative to ground truth. We successfully mapped 100% of deformed seeds to ground truth under edematic expansion up to 40% of volume growth. Seed matching performance was 98% with random seed migration of 3mm superimposed on 10% edematic volume growth. This method promises to be clinically applicable as the first quantitative analysis tool to measure edematic implant deformations occurring between the operating room and post-operative CT imaging.