Ultrasonic Thermometry using Coded Excitations

Background: Focused Ultrasound (FUS) is a promising noninvasive therapeutic modality. It can target precise regions within the tissue and produce beneficial effects at its focal zone such as ablation, hyperthermia, and enhanced drug uptake. The use of FUS may induce temperature elevation in the target and therefore, continuous thermal monitoring is essential. MR-based thermometry is the current thermal guidance method used in FUS-based procedures. Nonetheless, MRI is expensive and lacks accessibility. Ultrasound (US), on the other hand, is inexpensive and has high availability which makes it a potentially attractive monitoring tool. US-based thermometry techniques, however, are hampered by attenuation at deep tissue penetration and by high impedance changes, i.e. near bones. Here, it is hypothesized that this can be circumvented by using Coded Excitations (CE).

Methods: Pre-heated water chamber phantoms were positioned in a water tank with a FUS transducer. The focal zone of the FUS was positioned within the phantom. CE pulses were transmitted and detected by an embedded hydrophone. Scanning was conducted every second while the phantom cooled down (50oC – 30oC). The temperature was continuously registered with thermocouples. The CE echoes were analyzed and the changes in the speed of sound (SoS) were retrieved. The SOS changes were then related to temperature.

Results: The approximated SoS was successfully recovered with high correlation in relation to the expected SoS.

Conclusion: Ultrasonic Coded Excitations can potentially be a method for performing temperature monitoring.