The design and development of a contrast detail phantom for the evaluation of trans-rectal ultrasound systems used in prostate brachytherapy

During prostate brachytherapy the dose delivered to the prostate gland, and the optimal locations of the radioactive seeds is informed by the volume of the gland. The volume is determined using planimetery software that utilises images of the prostate with the border defined by the oncologist, acquired at 5 mm intervals along the length of the prostate. Delineation of the prostate from the surrounding tissue in trans-rectal ultrasound (TRUS) guided prostate brachytherapy is vital to plan and perform the procedure, and the relative soft tissue contrast will have an impact on successful delineation. In order to evaluate the ultrasound system’s contrast-detectability, a novel phantom was designed and developed that included 2, 4, 6, 8 and 10 mm diameter spherical targets of a range of clinically relevant contrasts, embedded in background material. IEC agar tissue mimicking material (TMM) was used and was optimised to achieve the clinically relevant contrasts specific to the prostate brachytherapy procedure, −4 dB −5 dB and −6 dB from a background TMM. Images of these contrast detail targets at each size were acquired across the available ultrasound system settings, including the clinical preset used during the procedure on both the linear and tightly curved transducer arrays. A Matlab program was developed to objectively measure the contrast-detectability of the images in terms of the lesion-signal-to-noise ratio over a range of depths within the phantom for each contrast level. The phantom was used to perform evaluation of the contrast-detectability of TRUS systems, and determine optimal system settings to enhance contrast and facilitate prostate delineation.