Use of A-scan for penetration control during dual-frequency ultrasound thermal therapy of superficial tissues overlaying bone and lung

An ultrasonic system capable of Lateral Power Conformability (LPC), Penetration Depth Control (PDC), and the ability to deliver hyperthermia concomitantly with external beam radiation is being developed. PDC is achieved by simultaneously insonating with beams of low (1 MHz) and high (5 MHz) frequency. This paper presents a sono-thermal numerical evaluation of the impact of PDC on thermal dose in the treatment of chest wall volumes. The main goal is to assess the potential advantages of impedance-mismatched interface depth-mapping, using therapy transducers in A-scan mode, to select optimal relative output intensities of the beams as a function of bone and lung depths. Simulation results for a representative chest wall anatomy showed that there exist a strong relationship between optimal relative output intensities and bone/lung depth for maximum thermal dose and minimum muscle-bone interface temperature. Consequently, interface depth-mapping prior to a dual-frequency ultrasound hyperthermia treatment would provide patient-specific data useful for selecting PDC parameters that maximize thermal dose and minimize bone heating.