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Non-Contact Thermoacoustic Imaging

Non-contact imaging and detection in lossy media is an important goal in security screening as well as in the medical imaging and non-destructive testing fields. Standoff operation allows a significant increase in flexibility and portability of an imaging system, especially when detecting potentially hazardous materials embedded or hidden in dispersive packaging such as soil, water, or tissue. Highly dispersive and lossy media present a challenge to traditional radio frequency (RF) and optical imaging systems. A fundamental trade-off between penetration depth and resolution limits performance, and further complications arise from large interface losses, reflections, and two-way attenuation. Typical solutions to these problems require large, expensive, and high power systems, or ionizing radiation that is unfit for many applications.

We have developed a hybrid microwave-ultrasound system for non-contact detection in lossy media based on the thermoacoustic (TA) effect that is capable of distinguishing between a dispersive package that is loaded with an embedded target and a package that is unloaded. Pulsed microwave excitation generates absorption and heating contrast based on the dielectric properties of the target and surrounding media. Small local expansions are then caused by the TA response to the differential heating. These expansions generate an ultrasound pressure signal that travels away from the target interface and out of the surrounding media to a detector at a standoff. The system detects these minute ultrasound signals with highly sensitive airborne capacitive micromachined ultrasonic transducers (CMUTs) to overcome the large acoustic transmission loss at the air/medium interface. Large-scale measurements demonstrate the reliability of this detection method [1-2].

One application of our system of non-contact TA detection that we have explored is the tracking of medical interventional devices for ablation procedures. It is important to know the location of these devices in the body both while they are maneuvered to the location of ablation and during the ablation procedure in order to avoid damage to healthy tissues. Traditionally, this tracking is performed with CT/MRI scanning, or even by contact-based ultrasound imaging. Our non-contact TA detection system can reduce the cost and bulkiness of tracking the location of these devices. As a first proof of concept of our approach, we have demonstrated the tracking of a coaxial probe in an agar phantom via triangulation using time-of-flight information [3].

Block diagram of our non-contact TA setup.

Demonstration of the ability of our non-contact TA setup to detect lossy targets, with the receiver operating characteristic (ROC) curve showing effective binary classification by our baseline control processing algorithm.

Conceptual schematic of system for tracking interventional probes through thermoacoustic detection.

Triangulation of device location accounting for refraction.

[1] H. Nan, K. C. Boyle, N. Apte, M. S. Aliroteh, A. Bhuyan, A. Nikoozadeh, B. Khuri-Yakub, and A. Arbabian, "Non-Contact Thermoacoustic Detection of Embedded Targets Using Airborne-Capacitive Micromachined Ultrasonic Transducers," Appl. Phys. Lett., vol. 106, no. 8, 084101, Feb. 2015.

[2] K. C. Boyle, H. Nan, A. Arbabian, and B. T. Khuri-Yakub, "Noncontact thermoacoustic detection of targets embedded in dispersive media," Proc. SPIE Sec. Def., Edinburgh, 2016.

[3] G. Alexopoulos, K. C. Boyle, N. Dolatsha, H. Han, B. Khuri-Yakub, and A. Arbabian, "Standoff Tracking of Medical Interventional Devices using Non-Contact Microwave Thermoacoustic Detection," Proc. IEEE Int. Microw. Symp., San Francisco, CA, 2016.