Remote Embedded Target Imaging

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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 [1-4].

We also show fully reconstructed images of targets embedded in dispersive media, with mm-scale resolution. Here, we further demonstrate that the air-sample interface presents a trade-off with the advantage of improved resolution, as the change in wave velocity at the interface creates a strong focusing effect alongside the attenuation, resulting in axial resolutions more than 10x smaller than that predicted by the traditional speed/bandwidth limit. Applications that can utilize this system – such as the imaging of objects buried underground are currently being investigated.

Another 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 [5].

Collaborators

Prof. Pierre Khuri-Yakub, Electrical Engineering, Stanford University
Prof. José R. Dinneny, Biology, Stanford University
Dr. David Ehrhardt, Carnegie Institution for Science

Funding Sources

ARPA-E
ONR

Block diagram of our non-contact TA setup.

Wave propagation for an embedded target inside (a) layered air-agarose background with interference and diffraction effects from the acoustic impedance interface, (b) homogeneous agarose background. This demonstrates the focusing effect that occurs due to refraction at the air-agarose interface, resulting in improved resolution.

(a) Cross-sectional view of imaging setup for CMUT centered at 910 kHz, (b) image reconstructed from measurement data using a custom piecewise SAR algorithm developed for reconstructing images in layered media.

[1] A. Singhvi, K.C. Boyle, M. Fallahpour, B. T. Khuri-Yakub, and A. Arbabian, “A Microwave-Induced Thermoacoustic Imaging System with Non-Contact Ultrasound Detection,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control

[2] 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.

[3] K. C. Boyle, H. Nan, N. Apte, M. S. Aliroteh, A. Bhuyan, A. Nikoozadeh, B. T. Khuri-Yakub, and A. Arbabian, “Non-Contact Thermoacoustic Imaging of Tissue with Airborne Ultrasound Detection,” IEEE Ultrasonics Symposium, Taipei, Taiwan, Oct. 21-24, 2015.

[4] 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.

[5] 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.