Thermo-acoustic ultrasound monitoring of implanted conductors : applications in MRI and breast-conserving surgery

A number of medical sensing and imaging technologies interrogate the body with electromagnetic fields to acquire useful information. For patients who have implanted medical devices with electrically conductive components, interactions of applied electromagnetic fields with these implants can cause significant local amplification of electromagnetic power absorption in the tissue around the implants. Thermo-acoustic ultrasound (TAUS) is an emerging sensing technique in which the rapid deposition of radiofrequency (RF) or microwave energy generates acoustic signals via thermoelastic expansion. Because power absorption from the RF/microwave fields that are applied for TAUS excitations is amplified near implanted conductors, TAUS can be used to characterize the interactions of RF/microwave fields with conductive implants in the body and to monitor the tissue near implants in which power absorption is amplified. This dissertation examines two separate applications of TAUS monitoring of implanted conductors. The first application is the use of TAUS to assess whether patients with implanted medical devices can safely undergo magnetic resonance imaging (MRI) scans. Interactions between the high-power RF fields used in MRI and implanted medical devices with long conductive leads, such as pacemakers and neurostimulators, can induce the formation of dangerous RF power absorption hotspots that cause excessive tissue heating at the tips of device leads. An inability to determine the severity of these hotspots creates uncertainty regarding whether MRI scans will cause harmful RF-induced tissue heating at device lead tips. This uncertainty prevents many patients who could be scanned without issue from receiving MRI scans. Here, a TAUS system is proposed for use in an MRI scanner to determine whether scanning is safe on a case-by-case basis. To assess RF safety in MRI, the proposed system would apply TAUS excitations using the RF transmit coils of an MRI scanner and detect the resulting ultrasonic signals excited from device lead tips. In the work presented here, simulations of the TAUS acoustic fields generated from device lead tips show how the observed TAUS signals depend on the lead geometry, the acoustic frequencies used for acquisitions, and the positions of acoustic receivers used to detect the signals. Lead tip TAUS signals are detected in an MRI setting, with TAUS excitations performed using the MRI body coil and signal detection performed using an ultrasonic transducer. Then, these TAUS signals are used to provide accurate estimates of the temperature at a lead tip in a case with significant RF-induced lead tip heating. The second application is the use of TAUS to provide surgical guidance during lumpectomy, also known as breast-conserving surgery. Lumpectomy is performed to remove early-stage cancerous tumors from the breast. Due in part to the limitations of current surgical guidance techniques, about 24% of lumpectomies fail to fully excise tumors, resulting in many reoperations. Here, a TAUS system is proposed for surgical guidance for lumpectomy. This system would apply microwave excitations to the breast to excite TAUS signals from a metallic breast biopsy marker placed at the tumor site. Detection of these TAUS signals by multiple ultrasonic transducers on the breast would allow for localization of the marker position through acoustic trilateration. The proposed TAUS-based method would perform constant localization of the tumor site with a hands-free setup, providing key advantages over current guidance techniques to help improve the lumpectomy success rate. The work shown here demonstrates TAUS signal excitation and detection from a biopsy marker. The TAUS acoustic fields generated from different markers are also determined through simulation. This initial work suggests that a full TAUS-based localization system for surgical guidance is feasible

Keywords: Electrical Engineering.
Advisor: Pauly John (John M.) 
Nishimura Dwight George 
Scott Greig Cameron