Predicting pubic arch interference using multiplanar transrectal ultrasound (TRUS) for prostate brachytherapy

Prostate brachytherapy is a treatment procedure for localized prostate cancer. It involves placing needles and subsequently radioactive seeds under ultrasound guidance into predetermined targets within the prostate. As the pubic arch can be a barrier to successful placement of the needles, preoperative assessment requires visualization of the pubic arch with respect to the prostate. Current CT- based techniques to assess pubic arch interference (PAI) are expensive and time-consuming. This paper describes a new technique using transrectal ultrasound that enables the visualization of the pubic arch bone and the prostate gland simultaneously. The technique involves speckle suppression in the pubic arch ultrasound image and contrast enhancement of the pubic bones using sticks algorithm. This step is followed by noise filtering using percentile thresholding and curve fitting. The detected arch is superimposed on the transverse cross-sectional image of the prostate at its largest position predicted by the algorithm was compared with the 'true' pubic arch position determined at surgery by placing needles into multiple coordinates corresponding and adjacent to the predicted arch position. The accuracy of the algorithm in detecting the pubic arch was tested on 50 patients. Of 1030 points tested, the algorithm prediction was correct at 932 points. The mean Type II error, i.e., the algorithm predicted soft tissue while bone was encountered during needle insertion, was 2.9 percent, which corresponds to less than 1 out of 22 test points along the predicted pubic arch. The accuracy of our algorithm is good and the errors are within clinically-acceptable limits.