Microwave-induced Thermoacoustic Imaging Research Articles

Portable Microwave-Acoustic Coaxial Thermoacoustic Probe With Miniaturized Vivaldi Antennas for Breast Tumor Screening.

Microwave-induced thermoacoustic (TA) imaging (MTAI), which exploits dielectric contrasts to provide images with high contrast and spatial resolution, holds the potential to serve as an additional means of clinical diagnosis and treatment. However, conventional MTAI usually uses large and heavy metal antennas to radiate pulsed microwaves, making it challenging to image different target areas flexibly. In this work, we presented the design and evaluation of a portable microwave-acoustic coaxial TA probe (51 mm × 63 mm × 138 mm) that can flexibly image the region of interest. The TA probe contains two miniaturized symmetrically distributed Vivaldi antennas (7.5 g) and a 128-element linear ultrasonic transducer. By adjusting the geometry of the antennas and the ultrasonic transducer, the TA probe's acoustic field and microwave field can be designed to be coaxial, which helps achieve homogeneous microwave illumination and high-sensitivity ultrasonic detection. The practical feasibility of the proposed probe was tested on an in vitro ewe breast and a healthy volunteer. The results demonstrate that the MTAI system with the proposed TA probe can visualize the anatomical structure of the breast tumor in ewe breast and a healthy volunteer breast with resolutions in hundreds of microns (transverse: 910 μm, axial: 780 μm) and an excellent signal-to-noise ratio can be obtained in deep adipose tissue (10 dB in 6 cm fat). The miniaturized portable TA probe takes a solid step forward in translating MTAI technology to clinical breast tumor diagnosis.

Biomedical microwave-induced thermoacoustic imaging

Microwave thermoacoustic imaging (MTAI) is an exciting imaging technique rooted from the underlying principle of exploiting the distinct electrical properties of biological tissues. By using short-pulsed microwaves as a stimulation source and their interaction with the human body, MTAI has paved the way for revolutionary advancements in medical imaging. When microwaves are absorbed by polar molecules and ions within the tissues, an ingenious thermoelastic effect gives rise to ultrasound waves. These ultrasound waves, brimming with invaluable pathological and physiological insights, propagate outward, carrying the essence of the composition and functionality of biological tissue. Through a meticulous collection of ultrasound signals from all directions surrounding the tissue, it becomes possible to reconstruct intricate internal structures and visualize the tissue's functional dynamics. The MTAI excels in non-invasiveness, capable of delving several centimeters beneath the surface with a microscopic resolution on the order of micrometers. The magic lies in converting microwave energy into ultrasound waves, entering into the hidden depths of tissues without causing harm. This groundbreaking imaging modality unlocks a realm of possibilities for acquiring profound insights into the intricate structures and functionality of deep-seated tissues. Furthermore, the inherent polarization characteristics of microwaves empower MTAI to capture additional dimensions of information, unraveling the intricate polarization properties and illuminating a richer understanding of the tissue's complexity. The great potential of MTAI extends far and wide within the medicine field. It has made remarkable achievements in non-invasive imaging of brain structures, screening breast tumors, visualizing human arthritis, and detecting liver fat content. These accomplishments have laid a solid foundation, firmly establishing MTAI as a trailblazing medical imaging technique. The present study offers a comprehensive and in-depth exploration of the physical principles underpinning MTAI, the sophisticated system devices involved, and the recent groundbreaking research breakthroughs. Moreover, it delves into the exciting prospects and challenges that lie ahead in the future development of MTAI. As the technology continues to progress by leaps and bounds, MTAI is ready to break down barriers, and usher in a new era of unmatched imaging quality and performance. This, in turn, will open the floodgates for transformative innovations and applications in medical diagnosis and treatment. The anticipation is palpable as MTAI strives to make substantial contributions to the ever-developing medical imaging field, bestowing upon humanity more accurate, reliable, and life-enhancing diagnostic capabilities.

A high-efficient excitation-detection thermoacoustic imaging probe for breast tumor detection.

Microwave-induced thermoacoustic (TA) imaging (MTAI) is a promising alternative to biomedical imaging due to its high resolution, deep imaging depth, and minimal biohazard. To provide images of different anatomical regions and apply them to different clinical scenarios, the development of miniaturized portable TA probes is imperative. This study is aimed to propose a highly efficient handheld non-reflective microwave-acoustic coaxial TA probe to advance the translation of MTAI for the clinical use. The TA probe integrates a hollowed microwave antenna with a forward radiating uniform microwave field and a linear ultrasonic transducer array which is placed in the hole in the middle of the antenna that has almost no effect on the microwave distribution. The integrated probe was evaluated for properties, including the excitation efficiency of the microwave and the reception efficiency of the acoustic signal. Finally, an isolated EMT6 cell tumor was embedded in a sheep mammary gland to simulate the natural breast tumor environment, and the tumor was detected with the proposed MTAI probe to evaluate its practical feasibility. Compared with the previous TA imaging probe, it has improved the detection efficiency of TA signals by up to 41%, contributing to an improved signal-to-noise ratio (SNR) of the image. The proposed TA probe successfully detected tumors embedded in the breast with a contrast ratio 3.27 times higher than the surrounding tissue in phantom experiments. The proposed TA probe with the features of microwave illumination and ultrasonic detection of coaxial, avoiding TA signal attenuation due to reflection, enables high-efficient TA signal excitation and detection. The proposed TA probe is essential for improving the excitation and detection efficiency of TA signals, and increasing the flexibility of the probe, providing a bright future for the clinical application of MTAI technology.

Microwave-induced thermoacoustic imaging for the early detection of canine intracerebral hemorrhage.

Purpose: This study aimed to investigate the feasibility and validation of microwave-induced thermoacoustic imaging (TAI) for the early detection of canine intracerebral hemorrhage. Methods: A TAI system was used to record the thermoacoustic signal (TAS) of canine intracerebral hemorrhage in the study. First, the difference in TAS between deionized water, fresh ex vivo porcine blood and brain tissue was explored. Second, the canine hemorrhagic stroke model was established, and canine brain ultrasound examination and TAI examination were performed before modeling and at 0.5h, 1h, 2h, 3h, 4h, 4.5h, 5h and 6h after modeling. Finally, pathology and ultrasound were used as the reference diagnoses to verify the accuracy of the thermoacoustic imaging data. Results: The results showed that significant differences were observed in TASs among deionized water, fresh ex vivo porcine blood and brain tissue. The intensity of the thermoacoustic signal of blood was significantly higher than that of ex vivo porcine brain tissue and deionized water. The intracerebral hemorrhage model of five beagles was successfully established. Hematomas presented hyperintensity in TAI. Considering ultrasound and pathology as reference diagnoses, TAI can be used to visualize canine intracerebral hemorrhage at 0.5h, 1h, 2h, 3h, 4h, 4.5h, 5h and 6h after modeling. Conclusion: This is the first experimental study to explore the use of TAI in the detection of intracerebral hemorrhage in large live animals (canine). The results indicated that TAI could detect canine intracerebral hemorrhage in the early stage and has the potential to be a rapid and noninvasive method for the detection of intracerebral hemorrhage in humans.

Microwave-induced thermoacoustic imaging with functional nanoparticles

As an emerging hybrid imaging modality, microwave-induced thermoacoustic imaging (MTAI), using microwaves as the excitation source and ultrasonic signals as the information carrier for combining the characteristics of high contrast of electromagnetic imaging and high resolution of ultrasound imaging, has shown broad prospects in biomedical and clinical applications. The imaging contrast depends on the microwave-absorption coefficient of the endogenous imaged tissue and the injected MTAI contrast agents. With systemically introduced functional nanoparticles, MTAI contrast and sensitivity can be further improved, and enables visualization of biological processes in vivo. In recent years, functional nanoparticles for MTAI have been developed to improve the performance and application range of MTAI in biomedical applications. This paper reviews the recent progress of functional nanoparticles for MTAI and their biomedical applications. The challenges and future directions of microwave thermoacoustic imaging with functional nanoparticles in the field of translational medicine are discussed.

Deep-Learning-Enabled Microwave-Induced Thermoacoustic Tomography Based on Sparse Data for Breast Cancer Detection

As a rapidly developing novel electromagnetic imaging technique, microwave-induced thermoacoustic tomography (MITAT) has found many applications and attracted tremendous research interest. Using sparse data to reconstruct images is very challenging for MITAT. This work proposes a novel deep-learning-enabled MITAT (DL-MITAT) modality to address the sparse data reconstruction problem and applies it in breast cancer detection. The applied network is a domain transform network called feature projection network (FPNet) + ResU-Net. Detailed structure and implementation method of the network is described. We conduct both simulation and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex vivo</i> experiments with breast phantoms to test the validity of the DL-MITAT approach. The obtained images given by the trained network exhibit much better quality and have much less artifacts than those obtained by a traditional imaging algorithm. We show that only 15 measurements can still reliably recover an image of the breast tumor for both full-view and limited-view configurations in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex vivo</i> experiments. We also provide detailed discussions on the capability and limitations of the proposed scheme. This work presents a new paradigm for MITAT based on sparse data and can be applied in all related applications of MITAT, including biomedical imaging, nondestructive testing, and therapy guidance.

Biomedical microwave-induced thermoacoustic imaging

Microwave-induced thermoacoustic imaging (MTAI) has emerged as a potential biomedical imaging modality with over 20-year growth. MTAI typically employs pulsed microwave as the pumping source, and detects the microwave-induced ultrasound wave via acoustic transducers. Therefore, it features high acoustic resolution, rich electromagnetic contrast, and large imaging depth. Benefiting from these unique advantages, MTAI has been extensively applied to various fields including pathology, biology, material and medicine. Till now, MTAI has been deployed for a wide range of biomedical applications, including cancer diagnosis, joint evaluation, brain investigation and endoscopy. This paper provides a comprehensive review on (1) essential physics (endogenous/exogenous contrast mechanisms, penetration depth and resolution), (2) hardware configurations and software implementations (excitation source, antenna, ultrasound detector and image recovery algorithm), (3) animal studies and clinical applications, and (4) future directions.

Multiple Back Projection With Impact Factor Algorithm Based on Circular Scanning for Microwave-Induced Thermoacoustic Tomography

Back-projection (BP) algorithm is a common image reconstruction method for microwave-induced thermoacoustic tomography (MITAT). According to the thermoacoustic (TA) signals obtained, which are superimposed with the corresponding time inversion of each pixel in the region of interest (ROI), the BP algorithm can function in a quick response and is suitable for fast real-time MITAT applications. However, high artifacts and background noises caused by incomplete sampling of TA signals seriously affect the image quality. In this paper, a multiple back-projection with impact factor (MBP-IF) algorithm based on circular scanning geometry is proposed to address this problem. Compared with the BP algorithm, the MBP-IF approach calculates not only the initial acoustic pressure of the TA signal of each pixel (i.e., the image obtained by BP), but also the complete acoustic pressure distributions at different time points during the thermoacoustic wave propagation. Then impact factors can be obtained and used to effectively suppress artifacts and eliminate noises. Numerical simulation and phantom experiments prove that the MBP-IF algorithm is advantageous in suppressing image artifacts and noises. Some figures of merit are also obtained to quantify the results. In addition, due to the independence of acoustic pressure distributions at different time points, the MBP-IF can be computed in parallel by GPU, which can be widely used in real-time clinical MITAT system construction.

Wideband frequency-dependent dielectric properties of rat tissues exposed to low-intensity focused ultrasound in the microwave frequency range

Tissue dielectric properties can vary upon the incident of an acoustic wave. The goal of this study is to quantify this change due to the acoustoelectric effect (AE), and to obtain the frequency-dependent dielectric properties of tissues exposed to low-intensity focused ultrasound (LIFU). The dielectric properties of the blood, brain, chest muscle, heart, kidney, leg muscle, liver, lung, pancreas, and spleen of rats were measured by an open-ended coaxial probe method. The acoustic intensity of LIFU focus was 2.97 MPa (67.6 W/cm2), 3.95 MPa (120 W/cm2), and 5.17 MPa (204 W/cm2), respectively, and the measurement frequency band was 0.1–7.08 GHz. The measurement results show that with the LIFU modulation, the conductivity and dielectric constant decreased in the high-frequency band, and on the contrary, they increased in the low-frequency band, and the larger the acoustic intensity was, the more obvious the phenomenon was. This work contributes to a better understanding of the mechanisms by which ultrasound acts on the dielectric properties of biological tissues. It is expected that the findings from this study will provide a basis that the response of tissue to LIFU modulation can be monitored by noninvasive techniques such as microwave-induced thermoacoustic imaging (MTI) and microwave imaging, present a new idea for improving the endogenous contrast between different biological tissues in MTI and acoustoelectric imaging, and possibly lead to the development of a new imaging method based on the relaxation time of tissue after LIFU modulation.

A Handheld Microwave Thermoacoustic Imaging System With an Impedance Matching Microwave-Sono Probe for Breast Tumor Screening.

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.

Multi-Watt-Level 4.9-GHz Silicon Power Amplifier for Portable Thermoacoustic Imaging

Microwave-induced thermoacoustic (TA) imaging, combining high microwave contrast with high ultrasonic resolution has the potential to revolutionize applications such as continuous healthcare monitoring, point-of-care imaging, and biometric authentication. However, the size, cost, and integration of a high-power microwave transmitter is a key bottleneck in making TA imaging truly portable, affordable, and ubiquitous. Toward that end, this work presents a compact 4.9-GHz pulsed power amplifier (PA) with a 4.87-mm² active area implemented in a 55-nm BiCMOS technology, operating in a duty-cycled mode and achieving 37.3-dBm peak output power&#x2014;the highest demonstrated peak power in PAs fabricated on a silicon substrate with deep submicron CMOS integration. We also reconstruct the first known high-fidelity TA images of tissue phantoms using an integrated silicon PA.

Polarization microwave-induced thermoacoustic imaging for quantitative characterization of deep biological tissue microstructures

Polarization optical imaging can be used to characterize anisotropy in biological tissue microstructures and has been demonstrated to be a powerful tool for clinical diagnosis. However, the approach is limited by an inability to image targets deeper than ∼ 1 mm due to strong optical scattering in biological tissues. As such, we propose a novel polarization microwave-induced thermoacoustic imaging (P-MTAI) technique to noninvasively detect variations in deep tissue by exploiting the thermoacoustic signals induced by four pulsed microwaves of varying polarization orientations. The proposed P-MTAI method overcomes the penetration limits of conventional polarization optical imaging and provides submillimeter resolution over depths of several centimeters. As part of the paper, the structural characteristics of tissues were quantified using a new parameter, the degree of microwave absorption anisotropy. P-MTAI was also applied to the noninvasive detection of morphological changes in cardiomyocytes as they transitioned from ordered to disordered states, providing a potential indication of myocardial infarction.

Microwave-induced thermoacoustic signal characteristics in a dynamic temperature environment

We conduct a simulation-based study to investigate the impact of a dynamic temperature environment on the characteristics of microwave-induced thermoacoustic signals. We investigate thermoacoustic signals that are generated using an interstitial microwave ablation antenna powered by a microsecond pulsed microwave source. Two temperature regimes are examined: first, a spatially uniform temperature throughout the medium to experimentally validate the simulation model, and second, the realistic, spatially nonuniform temperature profiles that arise during microwave ablation. We employ a multi-physics model that considers electromagnetics, heat transfer, and acoustic physics to simulate the coupled processes of microwave absorption and heating of the medium and thermoacoustic signal generation and propagation. An interstitial coaxial antenna is used to generate microsecond microwave pulses that simultaneously induce microwave heating and excite thermoacoustic signals via microwave pulse absorption. We find that thermoacoustic signal characteristics are highly temperature-dependent and thus change significantly within an environment where temperature varies through space and time. Furthermore, the temperature-dependent properties within the active region of the antenna drive the evolution of thermoacoustic signal characteristics. Temperature-dependent thermoacoustic signal characteristics can be exploited to track the progress of microwave ablation. Consequently, microwave-induced thermoacoustic imaging is a promising method for monitoring microwave ablation in real-time.

Manganous-manganic oxide nanoparticle as an activatable microwave-induced thermoacoustic probe for deep-located tumor specific imaging in vivo

Deep-located tumor specific imaging has broad clinical applications in improving the accuracy of tumor diagnosis. Microwave-induced thermoacoustic imaging (MTAI), combining the high-contrast of microwave imaging with the high-resolution of ultrasound imaging, is a potential candidate for noninvasive tumor detection. Herein, a deep-located tumor specific MTAI method by tumor microenvironment (TME) activated nanoprobe is reported. In principle, manganous-manganic oxide-based nanoprobe can be triggered by TME with overexpressed glutathione and weak acidity, causing to release manganese ions and increase conductivity. With pulsed microwaves, manganese ions move repeatedly in gigahertz alternating electric field, resulting in a transient heating and thermoelastic expansion through the Joule effect, which yields a strong thermoacoustic (TA) wave in tumor site. In vitro and in vivo experiments demonstrate that manganous-manganic oxide-based nanoprobe could high-selectively amplify the TA signal in deep-located tumor. Our proposed tumor-specific MTAI method based on TME activation provides a potential approach for deep-located tumor detection.

Microwave-Induced Thermoacoustic Imaging of Small Animals Applying Scanning Orthogonal Polarization Excitation

The traditional microwave-induced thermoacoustic imaging (MITAI) technique usually suffers from the problem of inhomogeneous microwave power distribution in the sample under test, which in turn renders the loss of some features of the sample in the image. To address this problem, circularly polarized antennas (CPA) have been used in MITAI to improve the image quality. However, the advantage of CPA in homogenizing the field distribution has been only validated using small samples with very simple structures. For complicated biological samples like small animals or human breasts featured with highly inhomogeneous tissue distributions, the performance of CPA-based MITAI is significantly worse than expected. Moreover, the applied CPAs generally have low efficiency, limited power capacity and narrow bandwidth. Thus, a more feasible and efficient method for homogenizing the power distribution is demanded. This work proposes 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, which uses a linearly polarized antenna. Two small animal samples, a small mouse and a frog, are experimentally investigated by the MITAI-SOP method. The results indicate that the SOP excitation mechanism can completely reveal the structure of the small animals due to improved power homogeneity and the proposed 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.

Ultrashort-Pulse-Microwave Excited Whole-Breast Thermoacoustic Imaging With Uniform Field of Large Size Aperture Antenna for Tumor Screening.

Microwave-induced thermoacoustic imaging (MTAI) has been widely used in biomedical science, and has the potential as an auxiliary measure for clinical diagnosis and treatment. Recently, there are increasing interests in using ultrashort microwave-pumped thermoacoustic imaging techniques to obtain high-efficiency, high-resolution images. However, the traditional imaging system can only provide uniform radiation in a relatively small area, which limits their large field of view in clinical applications (such as whole-breast imaging, brain imaging). To address this problem, we propose an ultrashort pulse microwave thermoacoustic imaging device with a large size aperture antenna. The system can provide a microwave radiation area of 40 cm × 27 cm and a uniform imaging view of 14 cm × 14 cm. With 7 cm imaging depth and a 290 μm resolution. The practical feasibility of the system for breast tumor screening is tested in phantoms with different shapes and in an ex vivo human breast tumor which is embedded in the excised breast of an ewe (π × 5 cm × 5 cm). The tumor can be identified with a contrast of about 1:2. The results demonstrate that the dedicated MTAI system with the uniform large field of view, high imaging resolution, and large imaging depth have the potential for clinical routine breast screening.

Research Progress and Prospect of Microwave-Induced Thermoacoustic Imaging Technology

Research Progress and Prospect of Microwave-Induced Thermoacoustic Imaging Technology

2-D Noninvasive Temperature Measurement of Biological Samples Based on Compressive Thermoacoustic Tomography

There is a great need in temperature monitoring in biomedical applications. The current available temperature measurement techniques have some deficiencies in noninvasiveness, applicability, sensitivity and resolution. Microwave-induced thermoacoustic tomography (MITAT) has found diversified applications in biomedical related disciplines. It has been shown that MITAT can be used to estimate the temperature in a noninvasive manner. However, the applicability of the technique is still limited. This work presents experimental demonstration of two-dimensional (2-D) temperature detection of biological samples based on compressive TAT, referred to as MICTAT. Detailed principle of the method is established. A phantom sample and a biological sample are tested by experiments and 2-D temperature maps are obtained. This work provides a new route of getting 2-D temperature maps of biological samples and may find many applications in biomedical related areas.

Developing a near-infrared spectroscopy and microwave-induced thermoacoustic tomography-based dual-modality imaging system.

Near-infrared spectroscopy (NIRS) techniques can provide noninvasive in vivo hemoglobin oxygenation information but suffer from relatively low resolution in biological tissue imaging. Microwave-induced thermoacoustic tomography (TAT) can produce high-resolution images of the biological tissue anatomy but offer limited physiological information of samples because of the single species of the chromophore it maps. To overcome these drawbacks and take advantage of the merits of the two independent techniques, we built a dual-modality system by combining a NIRS system and a TAT system to image biological tissues. A series of phantom trials were carried out to demonstrate the performance of the new system. The spatial resolution is about 1mm, with a penetration depth of at least 17.5mm in the human subject. A cohort of five healthy subjects was recruited to conduct real-time forearm venous and arterial cuff occlusion experiments. Numerous results showed that this dual-modality system could measure oxygen metabolism and simultaneously provide anatomical structure changes of biological tissues. We also found that although the hemoglobin concentration varied consistently with many other published papers, the TAT signal intensity of veins showed an opposite variation tendency in the venous occlusion stage compared with other existing work. A detailed explanation is given to account for the discrepancy, thus, providing another possibility for the forearm experiments using TAT. Furthermore, based on the multiple types of information afforded by this dual-modality system, a pilot clinical application for the diagnosis of anemia is discussed.

Enhanced pulsed thermoacoustic imaging by noncoherent pulse compression

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.