GPP05 Presentation Time: 11:06 AM: Visualize the Target, Adjust Your Crosshairs: EM-Tracked MR-US Live Fusion and Needle Location in Gyn Brachytherapy.

The disease control and toxicity benefits of adding interstitial needles to cervix intracavitary implants are well established, as are the advantages of MRI for tumor visualization. Yet widespread adoption of these advanced techniques remains elusive, limited by access to frequent MR imaging and the technical challenges of precisely placing needles into the tumor. Ultrasound (US) remains the most accessible form of live imaging but the poorer image quality limits clear visualization of the tumor area and the inserted interstitial needles. We describe a novel system which combines a freehand, stepper-less transrectal ultrasound probe with an electro-magnetic (EM) tracker to continuously fuse a pre-acquired MR, offering a reconstructed MR image to the corresponding live ultrasound image. Inserted needles can be easily visualized using an EM tracked stylet/mandrin, placing a solid circle on the live ultrasound image where the needle is located. The clinical ultrasound system, the BK Spekto with a biplanar side-fire 9048 US probe, was instrumented with a Northern Digital Inc. EM tracker, part of the 3D Guidance Trakstar system. Software was developed using the 3D Slicer toolkit to enable live and continuous fusion of a pre-acquired MR. Contours generated on the MR can be imported and displayed on the live US image. An additional EM tracker can be placed inside a needle to visualize its location on the live US image and removed for treatment. The system was assessed on a Viomerse Gyn phantom before being deployed in clinical implants (Fig 1A). Initial registration between the live US image and the MR is accomplished by placing the freehand transrectal US probe within the patient to a known location, such as the top of the vaginal canal/cervix area. The corresponding MR/US fusion is then locked, any movement of the freehand probe will update both the US and MR images correspondingly. Fine tuning of the registration can be done in all six degrees of freedom as necessary to accommodate any shifts and/or deformations during the procedures. Needles can easily be identified on the live US when a stylet equipped with a miniature EM tracker is inserted: a yellow circle appears on the live US at the intersection on the imaging plane where the needle is expected (Fig 1B). In case of multiple needles, the trajectory of each needle can be digitally saved within the EM frame of reference, providing a colored mark where each needle was placed on the live US. HR-CTV contours can be overlaid on the live US to help highlight the area of interest (Fig 1C). A novel system has been developed and clinically tested to enhance the capabilities of US imaging for gyn brachytherapy procedures by incorporating clarifying MR information and easy needle recognition. These features can help guide the practitioner during the procedure to a geometrically more robust implant with actionable feedback. [ABSTRACT FROM AUTHOR]