Search

Microwave-induced thermoacoustic imaging (MTAI) is a promising alternative for breast tumor detection due to its deep imaging depth, high resolution, and minimal biological hazards. However, due to the bulky size and complicated system configuration of conventional benchtop MTAI, it is limited to imaging various anatomical sites and its application in different clinical scenarios. In this study, a handheld MTAI system equipped with a compact impedance matching microwave-sono and an ergonomically designed probe was presented and evaluated. The probe integrates a flexible coaxial cable for microwave delivery, a miniaturized microwave antenna, a linear transducer array, and wedge-shaped polystyrene blocks for efficient acoustic coupling, achieving microwave illumination and ultrasonic detection coaxially, and enabling high signal-to-noise ratio (SNR). Phantom experiments demonstrated that the maximum imaging depth is 5 cm (SNR = 8 dB), and the lateral and axial resolutions are 1.5 mm and 0.9 mm, respectively. Finally, three healthy female volunteers of different ages were subjected to breast thermoacoustic tomography and ultrasound imaging. The results showed that the h-MTAI data are correlated with the data of ultrasound imaging, indicating the safety and effectiveness of the system. Thus, the proposed h-MTAI system might contribute to breast tumor screening.

Microwave-induced thermoacoustic tomography (MITAI) has found diversified applications in biomedical related disciplines. This work reports a novel MITAI modality to homogenize the microwave power deposition in the sample and improve the image quality for small animal imaging via scanning orthogonal polarization (SOP) excitation. Although circular polarization excitation has been reported to enhance MITAI, we perform some simulation works using realistic breast phantoms to prove that the homogeneity of microwave power distribution in the phantoms using orthogonal polarization excitation is comparable to that using circular polarization excitation. Compressive sensing is also applied to enhance the time efficiency of the system. Experiments applying a mouse and a frog sample are conducted. The results indicate that the SOP excitation mechanism can completely reveal the structure of the small animals due to improved power homogeneity. The proposed MITAI-SOP technique is superior to circularly polarized antenna (CPA) based modality since the former avoids the deficiency of CPA in bandwidth, power capacity and efficiency. This work presents a practical and easy-to-implement paradigm for high-quality imaging of small animals and big biological samples using MITAI.

Noninvasive thermal therapies for the treatment of breast cancer depend on accurate monitoring of tissue temperature to optimize treatment and ensure safety. This work describes a real-time system for 3-D thermoacoustic imaging and thermometry (TAI-TAT) for tracking temperature in tissue samples during heating. The study combines a 2.7-GHz microwave pulse generator with a custom 1.5-D 0.6 MHz ultrasound array for generating and detecting TA signals. The system is tested and validated on slabs of biological tissue and saline gel during heating. Calibration curves for relating the TA signal to temperature were calculated in saline gel (3.40%/°C), muscle (1.73%/°C), and fat (1.15%/°C), respectively. The calibrations were used to produce real-time, volumetric temperature maps at ~3-s intervals with a spatial resolution of approximately 3 mm. TAT temperature changes within a region of interest were compared to adjacent thermocouples with a mean error of 17.3%, 13.2%, and 20.4% for muscle, gel, and fat, respectively. The TAT algorithm was also able to simultaneously track temperatures in different tissues. With further development, noninvasive TAI-TAT may prove to be a valuable method for accurate and real-time feedback during breast cancer ablation therapy.

Focused microwave breast hyperthermia (FMBH) represents a newly emerging technique endowed with advantages of high accuracy and low side effect for treating breast tumors. Practical application of the FMBH approach requires monitoring of microwave power deposition in the breast. Microwave-induced thermoacoustic imaging (MITAI) is naturally feasible for such power deposition monitoring task. This work conducts a computational study to evaluate feasibility of the novel FMBH-MITAI modality using realistic breast phantoms. Basic configuration and rationale of both FMBH and MITAI are introduced. Compressive sensing (CS) technique has to be applied in MITAI for sparse acoustic measurement in the FMBH-MITAI modality. Procedure of the computational study consists of microwave simulation, thermoacoustic numerical simulation, CS imaging, and performing the iterative optimization. Simulated results show that CS based MITAI is able to serve as a reliable monitoring mechanism to efficiently guide the iterative optimization toward the best obtainable focusing condition in most of the simulated scenarios. Finally obtained thermoacoustic images agree well with simulated power deposition distribution well. This work offers valuable performance evaluation and is of significant meaning for potential clinical applications of the FMBH-MITAI modality.

Microwave-induced thermoacoustic imaging (MITAI) has found diversified applications in biomedical related disciplines and holds the potential to serve as an auxiliary measure for diagnosis and treatment in clinics. Conventional imaging algorithms for MITAI, for example, back-projection (BP), require exceedingly intensive sampling of thermoacoustic signals stemming from a sample under test, and thus, suffer from deficiencies such as low time efficiency, high system cost, and more microwave radiation received by patients. Combining compressive sensing (CS) and MITAI, referred to as CS-MITAI, is promising to address the problem. Reported experiments in two dimensions and simulations in three dimensions (3D) have proved that the CS-MITAI approach can reliably reconstruct images with much fewer measurements than the BP algorithm. However, there is a pressing need for the experimental validation of the CS-MITAI technique in a real three-dimensional (3-D) environment, which has profound implications for its practical applications and further improvement. This article experimentally investigates the 3-D CS-MITAI technique by imaging a sample with 3-D features. Establishment of a dictionary is crucial for the successful implementation of the 3-D CS-MITAI approach. An analytical method is proposed to build the dictionary with much higher efficiency than other reported measures, and its detailed derivation is provided. In addition, modeling work and parametric studies are performed to explore the robustness of the 3-D CS-MITAI mechanism in dealing with different cases. Both simulation and experimental results show that the 3-D CS-MITAI modality can offer comparable imaging quality as the BP method but requires 200 and 68 times fewer measurements, respectively.

Portable and easy-to-use imaging systems are in high demand for medical, security screening, nondestructive testing, and sensing applications. We present a new microwave-induced thermoacoustic imaging system with non-contact, airborne ultrasound (US) detection. In this system, a 2.7 GHz microwave excitation causes differential heating at interfaces with dielectric contrast, and the resulting US signal via the thermoacoustic effect travels out of the sample to the detector in air at a standoff. The 65 dB interface loss due to the impedance mismatch at the air-sample boundary is overcome with high-sensitivity capacitive micromachined ultrasonic transducers with minimum detectable pressures (MDPs) as low as ${278~\mu {\mathrm{ Pa}}_{\mathrm{ rms}}}$ and we explore two different designs—one operating at a center frequency of 71 kHz and another at a center frequency of 910 kHz. We further demonstrate that the air–sample interface presents a tradeoff 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 $10\times $ smaller than that predicted by the traditional speed/bandwidth limit. A piecewise synthetic aperture radar (SAR) algorithm modified for US imaging and enhanced with signal processing techniques is used for image reconstruction, resulting in mm-scale lateral and axial image resolution. Finally, measurements are conducted to verify simulations and demonstrate successful system performance.

Noncontact thermoacoustic imaging (TAI) has several desirable characteristics for applications such as explosive detection in high-water-content media. In this letter, we report a detection technique using millimeter-wave interferometry based on sensitive phase detection at ${W}$ -band. The displacement sensitivity of the proposed ${W}$ -band vibrometer at 95 GHz is of the order of 1 nm. We also analyze the effect of phase noise on the sensitivity of the vibrometer. Unlike laser-based sensors, a ${W}$ -band sensor has several advantages; it can easily penetrate surface obscurants such as fur or cloth and it does not require a highly reflective surface of the target to detect the thermoacoustic vibrations.

Improvised explosive devices (IEDs) have always been a major threat to personnel safety with continuous evolutions and new variants. Traditional methods for detecting IEDs, such as X-ray, have some drawbacks, so safe and efficient new detection techniques are highly desired. Microwave-induced thermoacoustic imaging (MITAI), being widely investigated for biomedical applications, is a promising candidate for detecting explosives embedded in high-water content materials due to its intrinsic advantages of nonionizing radiation, high contrast, and easy imaging procedure. Proof-of-concept demonstration is theoretically and experimentally carried out by successful detection of an explosive surrogate sample immersed in water. Good accordance between the experimental and simulation results validates the effectiveness of this approach. A parametric study of some key parameters is then performed to investigate their impact on detection efficiency and to evaluate the performance of a practical MITAI system for explosive detection.

This paper presents a beamforming microwave-induced thermoacoustic (TA) imaging system for screening applications. Near-field/quasi-near-field beamforming with multiple applicators focuses the radio-frequency (RF) energy to the target location in depth and enhances the TA signal generation, thereby lowering the peak-power requirements of the solid-state RF power amplifier (PA). More importantly, the proposed beamforming method reduces surface heating at the skin interface by distributing the excitation power over multiple sites and, hence, reducing the per-element peak at a similar acoustic signal-to-noise ratio (SNR). A 3-D printed setup in a conformal configuration is designed and used to demonstrate signal enhancement with RF focusing. By tuning the phase of each channel, the system can focus RF energy to specific locations and steer the focal point location across the region of interest. With a four-element array, measurements confirm a 15.8-dB acoustic intensity SNR improvement compared with a single power-limited excitation and 3.8-dB improvement compared to a PA with the same total peak power (i.e., $4\times P_{\text {out}}$ ). The results herein suggest the potential benefits of the proposed beamforming system and imply further enhancements once scaled to a larger array.

Magnetically mediated thermoacoustic imaging (MM-TAI) is an emerging medical electrical parameter imaging technology; it has the advantages of high contrast of electrical impedance imaging and high resolution of ultrasound imaging; it can realize the early diagnosis of disease in the tissues comparing with structure imaging technology. The magnetically mediated thermoacoustic (TA) effect is analyzed in this paper through the theoretical inference, numerical calculation, and experiment. Pulsed excitation is considered in the analysis, which is different from previously reported research. The pulsed magnetic field is applied to the low conductivity sample such as gel phantoms (0.2, 4, and 2.5 S/m); then the Joule heat is generated inside the objects and an acoustic signal is excited due to heat expansion. The acoustic signal is detected and the image is reconstructed from TA source to conductivity. This paper has laid the foundation for the MM-TAI in biological tissues.

Imaging of subcutaneous vasculature is of great interest for biometric security and point-of-care medicine. We investigate the feasibility of microwave-induced thermoacoustic (TA) tomography as a safe, compact, low-power, and cost-effective imaging technique for subcutaneous vasculature by means of application-specific design of near-field, radio frequency (RF) applicators. Using commercial transducers, we demonstrate proof-of-concept TA imaging of synthetic phantoms, plant vasculature, and earthworm blood vessels with only 50 W of peak power, or 42 mW average power, at 300 $\mu \text{m}$ resolution. The proposed RF applicator design enabled uniform, orientation-independent illumination of vasculature phantoms with only 10% variation. Finally, we show that the benefits of microwave contrast make possible the distinction of actual blood vessels, in an earthworm, from surrounding tissue within a modest receiver dynamic range of 40 dB.

Coherent frequency-domain microwave-induced thermoacoustic (TA) imaging is investigated as an alternative to the traditional pulsed-based TA imaging approaches that use extremely high-energy short excitation pulses, often produced by vacuum RF sources or spark-gap generators. It is shown that under peak-power limitation of the RF source, dictated by the goals of form factor and portability of this paper, the frequency-domain approach can achieve the required signal-to-noise ratio (SNR) without sacrificing resolution or other performance metrics. Theoretical and experimental comparison of the time-domain and frequency-domain TA methods is provided with the ultimate goal of implementing the imager with solid-state hardware. Using a microwave source with 120 W of peak power, frequency-domain TA demonstrates 27-dB SNR improvement over the pulse method while the average specific absorption ratio remains below 10 W/kg. Finally, we demonstrate TA images of multilayer relatively complex muscle-fat structures. To the best of our knowledge, these are the first TA images obtained with solid-state electronic sources.

We report the design of a real-time thermoacoustic (TA) scanner dedicated to imaging deep breast tumors and investigate its imaging performance. The TA imaging system is composed of an ultrashort microwave pulse generator and a ring transducer array with 384 elements. By vertically scanning the transducer array that encircles the breast phantom, we achieve real-time, 3D thermoacoustic imaging (TAI) with an imaging speed of 16.7 frames per second. The stability of the microwave energy and its distribution in the cling-skin acoustic coupling cup are measured. The results indicate that there is a nearly uniform electromagnetic field in each XY-imaging plane. Three plastic tubes filled with salt water are imaged dynamically to evaluate the real-time performance of our system, followed by 3D imaging of an excised breast tumor embedded in a breast phantom. Finally, to demonstrate the potential for clinical applications, the excised breast of a ewe embedded with an ex vivo human breast tumor is imaged clearly with a contrast of about 1:2.8. The high imaging speed, large field of view, and 3D imaging performance of our dedicated TAI system provide the potential for clinical routine breast screening.

Thermoacoustic imaging (TAI) is a non-ionizing, high-contrast and high-resolution modality for biomedical applications. The resolution of the reconstructed image by traditional imaging algorithm, such as back-projection (BP), is dependent on the input microwave pulse width. In this work, it is experimentally demonstrated that the quality of images reconstructed based on compressive sensing (CS), a novel imaging algorithm that can reconstruct images using fewer measurements, is largely independent on the microwave pulse width which is taken into account when building the dictionary. Quantitative analysis reveals that the peak signal to noise ratio of the reconstructed images based on CS is at least 6 dB higher than those via BP, and the correlation coefficients of the images based on CS show higher correlation than those via BP.

Owing to its intrinsic advantages of favorable contrast and spatial resolution, microwave-induced thermoacoustic imaging (TAI) has drawn great attention in biomedical imaging applications, such as breast cancer detection. Many experimental studies have demonstrated the promising potential of TAI. Several TAI modeling studies have also been published that facilitate the design and optimization of TAI systems. However, experimental validation of the modeling results is rarely seen; thus it is highly desirable to prove the effectiveness of the modeling approach. In this letter, the TAI modeling approach previously described by our group is validated by experiments. A three-dimensional printed polymer slab with featured structures is used as the sample to be investigated by both the model and the experiment. Images are obtained to reveal the featured structures in the slab from both the modeling and experimental results. Rigorous comparisons between the modeling and experimental imaging results are carried out. The achieved good agreement between the images corroborates the validity of the TAI modeling approach and thereby encourages more applications of it.

Thermoacoustic imaging is a promising modality for breast cancer detection. Acoustic signals generated by tumor targets with different sizes subject to microwave pulses with various widths and waveforms are analytically studied in this letter. Time- and frequency-domain profiles of the acoustic signals are achieved. Some key parameters of the acoustic signals, including peak value and peak-to-peak time interval of the time-domain acoustic pressure and frequency of the global peak and magnitude ratio of the second highest peak to the global peak of the frequency-domain pressure, are discussed. Impact of pulse width on acoustic signal peak-to-peak interval and image spatial resolution is developed. Square pulse is demonstrated to be superior in acquiring higher image resolution and signal-to-noise ratio. This study is helpful to optimizing the microwave pulse, selecting acoustic transducer, and evaluating image resolution.

Thermoacoustic imaging is a new technique that permits the detection and imaging of microscopic surface and subsurface fea- tures in a sample. We present a detailed theoretical treatment of the thermoacoustic process with particular emphasis on the roles of ther- mal and elastic parameters in both one and three dimensions. Exam- ples are presented of thermoacoustic images obtained with a scanning electron microscope. These include images of subsurface mechanical defects, metallic grains and grain boundaries, and of dislocation net- works and dopant regions in semiconductor crystals.