<h3>Purpose</h3> Varian Medical Systems (VMS) has recently released the Universal Interstitial Cylinder (UIC) for use with Ir-192 HDR brachytherapy. We commissioned one cylinder with a plastic Universal Cervix Probe (UPC) and plastic interstitial needles using CT and MR images. A phantom specifically designed to avoid the introduction of water or water-like materials into the needle guide tracks was used and various marker strands were investigated to accurately reconstruct source positions. <h3>Materials and Methods</h3> CT and MR scan were taken for the UIC set: one cylinder (30 mm diameter) with plastic UCP (80 mm, 30 degrees) and eight interstitial needles (channel #2-#9). To compare three different methods for channel identification, CT images (Siemens Biograph mCT, Siemens AG) with slice thickness of 0.6 mm were taken with either no radio opaque markers, VMS-numbered markers, or nylon coated stainless steel leader wires ("fish wire") which aid in channel identification and image registration. The MR images were taken on a 3T Magnetom Skyra Siemens MRI scanner using 3D T1 and 2D T2 weighted clinical HDR gynecological MRI protocols. An HDR MRI Lumen Marker (C4 Imaging, LLC) was placed in the channel #9 to check for any applicator rotation. The needle guide track openings at the distal end of the UIC cylinder causes liquid to be introduced into the tracks which results in unintended MR and CT signal. A gynecological phantom was constructed by placing the applicator set into three different phantom materials: wet towels, gelatin, or ground beef. Image quality of the CT and MRI scans for these phantoms were compared for treatment planning. The applicator dimensions were verified on CT and kV planar images. Dimensions from the vendor solid applicator models, which are based on the computer-aided design model files of the specific applicator were compared to those on CT and kV images. <h3>Results</h3> The phantoms made of wet towels or gelatin allowed significant amounts of liquid to enter the needle guide tracks from the holes in the distal end of the cylinder. This can mislead as to the actual source positions in the applicator. Specifically, it resulted in channels with unintended MR and CT signal (Figure 1a). The ground beef phantom was developed to limit this introduction of liquid into the cylinder (Figure 1b). CT scans with no markers displayed substantially wider needle guide tracks than the plastic needles (Figure 1b). CT scans with VMS-markers showed significant artifact (Figure 1c). CT scans with fish wire provided the best visibility of the needles within the needle guide tracks (Figure 1d). This scan along with the solid applicator model was also used to confirm distance from the outer wall to the inside of the needle guide track (8 mm) and to the center of the needle (7 mm) and the 30 mm diameter of the cylinder (Figure 1d). The use of an MR marker allowed that needle (channel #9) to be identified in the T1w MR scan (Figure 1b). The dimensions of the applicators could be validated on CT (Figure 1d) and kV planar images (Figure 1e). <h3>Conclusions</h3> The UIC applicator was commissioned for CT and MR image based treatment planning for HDR brachytherapy. Planar kV images verified dimensions of applicator against vendor specs. Accurate source position reconstruction was feasible using CT scan with 0.6 mm slice thickness and radio-opaque markers. The use of a ground beef phantom rather than a liquid based phantom minimized unintended MR and CT signal by limiting introduction of fluid into the needle guide tracks.