Microwave-induced thermoacoustic imaging (TAI) is a hybrid imaging technique that combines electromagnetic radiation and ultrasonic waves to achieve high imaging contrast and submillimeter spatial resolution. These characteristics make TAI a good candidate to detect material anomalies that change the material electric properties without a noticeable variation in material density. Conventional pulsed TAI systems work by sending a single short pulse to the imaged target and then detecting the generated pressure signal; therefore, a very high peak power microwave pulse or data averaging is needed to produce images with a high signal-to-noise ratio (SNR). In this paper, we propose to enhance the SNR of pulsed TAI systems by using non-coherent pulse compression. In this approach, a predefined pulse coded signal is used to illuminate the imaged sample and the received pressure signal is cross correlated with a template that is related to the power profile of the excitation signal. The proposed approach can be easily deployed to pulsed TAI systems without the need for major system modifications to the RF source because it only requires a timing circuit to control the triggering time of the RF pulses. In this paper, we demonstrate experimentally that the proposed approach highly improves the SNR of TAI signals and images and can be used to reduce the acquisition time by lowering the number of data averaging or reduce the required peak power from RF sources.
Microwave induced thermoacoustic imaging (TAI) is a hybrid imaging technique combining microwaves and ultrasound waves to achieve both superior spatial resolution and high image contrast. Here, we present results from a hybrid finite element model and an experimental setup using a microwave peak power of less than 5 kW (average power of only 4.5 W), significantly less than for comparable imaging performance in previous works. Microwave pulses with a duration less than 1 µs are used to excite ultrasound waves with a frequency higher than 1 MHz. Experimental measurements show agreement with simulation results using hybrid finite element modeling capturing microwave heating and acoustic wave propagation. Simulations suggest targets with a conductivity of approximately 0.9 S/m yield the strongest thermoacoustic signatures. Both B-mode images and time-reversal based reconstructed images are obtained and clearly demonstrate the enhanced contrast and high resolution by exploiting the dielectric absorption properties of microwaves and the sub-millimeter resolution of ultrasound. The use of a time reversal algorithm on recorded data demonstrates the effectiveness of TAI for biomedical applications. Standing wave patterns are identified in targets and their relation to the target characteristics and their effect on the resulted images are investigated. The novelty of this work is in lowering the microwave average power while still being able to detect induced acoustic signals, along with developing a numerical model to provide an insight into the imaging process and analyze anomalies in image reconstruction.
Microwave-Induced Thermoacoustic Tomography (MI-TAT) is a noninvasive hybrid modality which improves contrast by using thermoelastic wave generation induced by microwave absorption. Ultrasonography is widely used in medical practice as a low-cost alternative and supplement to magnetic resonance imaging (MRI). Although ultrasonography has relatively high image resolution (depending on the ultrasonic wavelength at diagnostic frequencies), it suffers from low image contrast of soft tissues. In this work samples are irradiated with sub-microsecond electromagnetic pulses inducing acoustic waves in the sample that are then detected with an unfocused transducer. The advantage of this hybrid modality is the ability to take advantage of the microwave absorption coefficients which provide high contrast in samples. This in combination with the superior spatial resolution of ultrasound waves is important to providing a low-cost effective imaging technique. Here we propose to use this hybrid imaging technique to image composite materials to further investigate the NDE applications for MI-TAT.
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