SU‐G‐IeP1‐01: A Novel MRI Post‐Processing Algorithm for Visualization of the Prostate LDR Brachytherapy Seeds and Calcifications Based On B0 Field Inhomogeneity Correction and Hough Transform

Abstract

Purpose:This study aims at developing an MRI‐only workflow for post‐implant dosimetry of the prostate LDR brachytherapy seeds. The specific goal here is to develop a post‐processing algorithm to produce positive contrast for the seeds and prostatic calcifications and differentiate between them on MR images.Methods:An agar‐based phantom incorporating four dummy seeds (I‐125) and five calcifications of different sizes (from sheep cortical bone) was constructed. Seeds were placed arbitrarily in the coronal plane. The phantom was scanned with 3T Philips Achieva MR scanner using an 8‐channel head coil array. Multi‐echo turbo spin echo (ME‐TSE) and multi‐echo gradient recalled echo (ME‐GRE) sequences were acquired. Due to minimal susceptibility artifacts around seeds, ME‐GRE sequence (flip angle=15; TR/TE=20/2.3/2.3; resolution=0.7×0.7×2mm3) was further processed.The induced field inhomogeneity due to the presence of titaniumencapsulated seeds was corrected using a B0 field map. B0 map was calculated using the ME‐GRE sequence by calculating the phase difference at two different echo times. Initially, the product of the first echo and B0 map was calculated. The features corresponding to the seeds were then extracted in three steps: 1) the edge pixels were isolated using “Prewitt” operator; 2) the Hough transform was employed to detect ellipses approximately matching the dimensions of the seeds and 3) at the position and orientation of the detected ellipses an ellipse was drawn on the B0‐corrected image.Results:The proposed B0‐correction process produced positive contrast for the seeds and calcifications. The Hough transform based on Prewitt edge operator successfully identified all the seeds according to their ellipsoidal shape and dimensions in the edge image.Conclusion:The proposed post‐processing algorithm successfully visualized the seeds and calcifications with positive contrast and differentiates between them according to their shapes. Further assessments on more realistic phantoms and patient study are required to validate the outcome.