Friday, June 17, 202212:30 PM - 1:30 PM PHSOR01 Presentation Time: 12:30 PM: Characterization of a Novel Multi-Modality Imaging Phantom for Ultrasound Interstitial Needle Guidance for High Dose Rate Gynecological Brachytherapy

Abstract

<h3>Purpose</h3> To design, manufacture, and validate a realistic female pelvic phantom for multi-modality imaging on CT, MRI, and transrectal ultrasound (TRUS) to guide and track interstitial needles during an HDR gynecological procedure using a commercial needle tracking system. <h3>Materials and Methods</h3> The pelvic phantom was designed using 3D Slicer and Autodesk Fusion360 to model an average uterus from a 50 patient data study¹, a vaginal canal from KleenSpec speculum dimensions, and a rectum to accommodate a BK 8848 endocavity biplane US probe. A 1.5 cm x 3.0 cm x 1.25 cm (WxLxH) half ellipsoid CTV_HR was also designed. Modeled anatomy was Boolean subtracted from rectangular solids to create a negative space mold. An Ultimaker S3 printer was used to print the 3D molds using polylactic acid. Smooth-On Silicone 20T was used as a casting agent for the uterus, cervix, vaginal canal, and rectum based on their similar mechanical properties. A 3D printed box was constructed to house the designed anatomy to provide structural integrity and allow the insertion of a speculum, tandem and ovoid applicator, needles, and TRUS probe. Agarose gelatin with glycerol and cellulose was used as a background medium to surround and support the pelvic anatomy and to allow for US propagation. After manufacturing, the phantom was imaged with CT to identify potential imperfections that might impact its visualization on US images. Following initial CT validation, the tracking system with the endorectal probe was used to image the phantom. US images from a 180 degree sweep were acquired and a 3D US volume was generated that allowed uterus contouring to aid in real-time insertion of 2 needles (Needle1 in CTV_HR, Needle2 in uterus). After insertion, the phantom was imaged with a Siemens Biograph mCT using a slice thickness of 0.6 mm and with a Siemens Skyra 3T MRI using the institutional HDR brachytherapy MR imaging protocol with T1 and T2 weighted sequences. The CTV_HR and uterus were contoured on CT and MR images and their dimensions were checked against the CAD model. <h3>Results</h3> Fig. 1a shows the CAD model for the anatomy (uterus dimensions of 7.87 cm x 4.45 cm, rectum diameter of 2 cm), the encasing box, and the casting mold used to fabricate the phantom. The volumes of the CTV_HR and uterus reconstructed from the CAD model were 3 cm³ and 76.8 cm³, respectively. Figs. 1b-c show sagittal CT and US images of the phantom. The measurements of uterus dimensions (7.87 cm x 4.4 cm) and rectum diameter (2 cm) on the sagittal CT image demonstrated a high degree of fidelity with the CAD model. The uterus volume (77.1 cm³) reconstructed from CT images was also very similar to that from the CAD model. Fig. 1d shows the phantom setup for US imaging using the needle tracking system and BK 8848 endorectal probe. Representative sagittal images through the phantom at the location of the 2 implanted needles are shown in Figs. 1e-h (CT, T1w MRI, T2w MRI, and US, respectively). The overall volumes CTV_HR (3.2 cm³) and uterus (82.9 cm³) reconstructed from the T2w MR images also agreed with those from the CAD model. Fig. 1f shows axial and sagittal images of the 3D US volume from the tracking software (uterus contour in green). The generated volume allowed for intraoperative needle insertion guidance. <h3>Conclusions</h3> The manufactured pelvic phantom allows for accurate visualization with multiple imaging modalities and is conducive to applicator and needle insertion. The phantom is low cost and can be reproducibly manufactured with the 3D molding process. Our initial experiments demonstrate the ability to integrate the phantom with the commercial tracking system for future needle tracking validation studies. 1. Campelo, et al. (2020). Brachytherapy, 19(6), 767-776