P3C-10 Towards a Reflex Transmission Method For Ultrasound Thermometry

One of the main difficulties for safely and efficiently achieving tumor ablation with focused ultrasound is the ability to monitor the temperature in the target region. Currently only magnetic resonance thermometry methods can provide post- treatment measurement of temperature rise. Based on the desire to develop a cheaper, portable thermometry method, a number of ultrasound techniques have demonstrated promising results but have either been difficult to achieve in vivo or are limited to a through-transmission configuration. We are investigating the use of reflex transmission, backward projection and tomographic reconstruction methods for interrogating a heated region in a pulse-echo ultrasound configuration. The approach is based on the strong temperature dependence of sound speed of tissue and water, which can be recovered from the phase angle of the sound wave. The pulse- echo configuration is achieved via the premise of reflex transmission, which relies on the diffusive tissue structure to scatter the sound field back through the heated region to the diagnostic transducer. An experimental and numerical simulation feasibility study was conducted in water with a 1 MHz focused transducer and a 3 mm diameter (-6 dB) thermal plume with a 7 - 10 deg C peak temperature differential as a steady- state heat source. The sound fields from an unheated reference wave and a wave which has propagated through the heated region were measured separately with a 0.2 mm diameter needle hydrophone and subsequently projected back from the receiver to the thermal plume using a numerical backward projection method. Tomographic reconstruction of the difference in phase angle between the two projected fields was successful in identifying the region and spatial distribution of heating. Results from a simulation using the experimental parameters were similarly successful in demonstrating a change in the phase angle due to the presence of the thermal plume and thus detecting the plume´s loc- ation and spatial temperature distribution. This approach shows promise as an ultrasound pulse-echo technique for not only noninvasively detecting the region of heating but also measuring temperature rise.