Thermoacoustic Imaging System Research Articles

RPCA-based thermoacoustic imaging for microwave ablation monitoring

Microwave ablation (MWA) is a potent cancer treatment tool, but its effectiveness can be hindered by the lack of visual feedback. This paper validates the feasibility of using microwave-induced thermoacoustic imaging (TAI) technique to monitor the MWA process. A feasibility analysis was conducted at the principle level and a high-performance real-time TAI system was introduced. To address the interference caused by MWA, a robust principal component analysis (RPCA)-based method for TAI was proposed. This method leverages the correlation between multiple signal frames to eliminate interference. RPCA’s effectiveness in TAI was demonstrated through three sets of different experiments. Experiments demonstrated that TAI can effectively monitors the MWA process. This work represents the first application of RPCA-related matrix decomposition methods in TAI, paving the way for the application of TAI in more complex clinical scenarios. By providing rapid and accurate visual feedback, this research advances MWA 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.

The Relationship Between Dielectric Properties, Thermoacoustic Signals and Temperature of Liver Tissues at 3.0 GHz

Purpose: This study aimed to investigate the relationship between dielectric properties (permittivity and conductivity), thermoacoustic signals (TAS) and temperature of liver tissues at 3.0 GHz.Materials and Methods: An open-ended coaxial probe was used to measure the dielectric properties of fresh porcine liver tissues in vitro, and a thermoacoustic imaging (TAI) system was used to collect the TAS. Porcine liver tissues were placed on a heating platform, and the dielectric properties measuring probe and temperature sensor were inserted into the liver tissues separated by 1.5 cm. First, the liver tissues were gradually heated by a heating platform from room temperature (30°C) to 60°C, and the dielectric properties and TAS were measured as the temperature increased approximately every 1°C. Second, after the temperature of the porcine liver tissue reached 60°C, the heating platform was turned off to naturally cool the porcine liver tissue to room temperature. During the process, the dielectric properties and TAS were also measured as the temperature decreased every 1°C. Finally, the changes in the dielectric properties and TAS of the liver tissues with temperature at 3.0 GHz frequency were analyzed.Results: During the process of heating the tissues up to 60°C, the conductivity of the porcine liver tissues decreased while the permittivity and TAS of the porcine liver tissues increased, and the relationships were nonlinearly correlated. Meanwhile, during the cooling process, the conductivity of the porcine liver tissues increased, while the permittivity and TAS decreased, and the relationships were also nonlinearly correlated.Conclusion: The dielectric properties and TAS of porcine liver tissue changed significantly with temperature, which makes it possible to differentiate the safety margin during liver thermal ablation with thermoacoustic imaging (TAI). While the relationship between temperature, dielectric properties and TAS needs to be further investigated, TAI has the potential to be utilized for safety margin screening during thermal ablation.

An Excitation-Reception Collinear Probe for Ultrasonic, Photoacoustic, and Thermoacoustic Tri-Modal Volumetric Imaging.

Imaging systems that integrate multiple modalities can reveal complementary anatomic and functional information as they exploit different contrast mechanisms, which have shown great application potential and advantages in preclinical studies. A portable and easy-to-use imaging probe will be more conducive to transfer to clinical practice. Here, we present a tri-modal ultrasonic (US), photoacoustic (PA), and thermoacoustic (TA) imaging system with an excitation-reception collinear probe. The acoustic field, light field, and electric field of the probe were designed to be coaxial, realizing homogeneous illumination and high-sensitivity detection at the same detection position. US images can provide detailed information about structures, PA images can delineate the morphology of blood vessels in tissues, and TA images can reveal dielectric properties of the tissues. Moreover, phantoms and in vivo human finger experiments were performed by the tri-modal imaging system to demonstrate its performance. The results show that the tri-modal imaging system with the proposed probe has the ability to detect small breast tumors with a radius of only 2.5 mm and visualize the anatomical structure of the finger in three dimensions. Our work confirms that the tri-modal imaging system equipped with a collinear probe can be applied to a variety of different scenarios, which lays a solid foundation for the application of the tri-modality system in clinical trials.

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.

First assessment of thermoacoustic tomography for in vivo detection of rheumatoid arthritis in the finger joints detection of rheumatoid arthritis in the finger joints.

The diagnosis of rheumatoid arthritis (RA) is complicated because of the complexity ofsymptoms and joint structures. Current clinical imaging techniques for the diagnosis of RA have strengths and weaknesses. Emerging imaging techniques need to be developed for the diagnosis or auxiliary diagnosis of RA. This study aimed to demonstrate the potential of thermoacoustic tomography (TAT) for in vivo detection of RA in the finger joints. Finger joints were imaged by a TAT system using three different microwave illumination methods including pyramidal horn antenna, and parallel in-phase and anti-phase microwave illuminations. Both diseased and healthy joints were imaged and compared when the three microwave illumination methods were used. Magnetic resonance imaging (MRI) of all the joints was performed to validate the TAT findings. In addition, two diseased joints were imaged at two time points by the pyramidal horn antenna-based TAT to track/monitor the progression of RA during a time period of 16 months. Three-dimensional (3-D) TAT images of the joints were also obtained. The TAT images of the diseased joints displayed abnormalities in bone and soft tissues compared to the healthy ones. The TAT images by pyramidal horn antenna and in-phase microwave illumination showed high similarity in image appearance, while the anti-phase-based TAT images provided different information about the disease. We found that the TAT findings matched well with the MRI images. The 3-D TAT images effectively displayed the stereoscopic effect of joint lesions. Finally, it was evident that TAT could detect the development of the lesions in 16 months. TAT can noninvasively visualize bone lesions and soft tissue abnormalities in the joints with RA. This first in vivo assessment of TAT provides a foundation for its clinical application to the diagnosis and monitoring of RA in the finger joints.

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.

A Low-Cost Compressive Thermoacoustic Tomography System for Hot and Cold Foreign Bodies Detection

Microwave-induced thermoacoustic tomography (MITAT) is suitable for detecting foreign bodies (FBs) in human body due to the sufficient microwave contrast between the FBs and surrounding human tissues. But detection of both hot and cold FBs by the same modality has not been demonstrated. In addition, traditional widely used imaging algorithms for MITAT requires highly intensive measurements, which is not cost-effective or time-efficient. This work reports the application of a low-cost compressive thermoacoustic tomography (CTAT) system for the detection of both hot and cold FBs embedded in tissues, referred to as FBD. We give detailed description of the theory of MITAT and CTAT. We show experimental demonstrations using different shaped metallic samples as the hot FBs and glass and wood samples as the cold FBs. We achieve successful detection of all the hot and cold FBs embedded in different tissues. The CTAT-FBD method is a new paradigm promising for a bunch of diagnosis or sensing applications. It is also possible to build a portable system for this technique.

Fabry–Pérot interference principle-based non-contact thermoacoustic imaging system for breast tumor screening

Microwave-induced thermoacoustic imaging (MTAI) is a potential nonionizing candidate for breast tumor detection due to its high contrast of tissue dielectric absorption and high resolution of ultrasonography. However, conventional MTAI systems need to contact the imaging targets through the ultrasound-coupling medium for the efficient transmission of thermoacoustic signals. This is a major drawback for potential applications such as navigation of open surgeries, burns, and ulcer testing. In this paper, we report a Fabry–Pérot (FP) interference principle-based non-contact thermoacoustic imaging system (NCTAIS). The system can infer the ultrasonic intensity by detecting the changes of the acoustically induced refractive index in the FP cavity. The miniaturized FP interferometer with a diameter of 5 mm achieves outstanding imaging sensitivity with a frequency response up to 1 MHz. An adaptive Stolt migration-based imaging reconstruction algorithm is proposed to solve the MTAI image distortion caused by the significant difference in sound velocity between air and a biological tissue. The NCTAIS is capable of achieving an axial resolution of 0.95 mm and a lateral resolution of 1.91 mm. The technical feasibility for breast tumor screening is validated with an in vitro simulated breast tumor model. The results demonstrate that the proposed NCTAIS offers high imaging sensitivity, high contrast, and deep imaging depth and will have more application scenarios than the conventional MTAI system, requiring the ultrasonic coupling medium to contact ultrasonic transducers.

Technical Note: Compact thermoacoustic imaging system based on a low-cost and miniaturized microwave generator for invivo biomedical imaging.

Most of existing thermoacoustic imaging (TAI) studies generally utilized a linear modulator-based high peak power magnetron generator (MG) for efficient TA signal excitation. However, a linear modulator-based MG is bulky and expensive. Here we present a low-cost and compact thermoacoustic imaging (TAI) system based on a miniaturized MG. The MG is based on solid-state modulator and operates at 3.05±0.025GHz, with a peak power of up to 60kW and adjustable pulse duration from 70 to 600ns. The dimensions and weight of this MG are 350×210×70mm3 (Width×Length×Height) and 7.5kg, respectively. The spatial resolution of the miniaturized MG-based TAI system is determined to be from 0.3 to 1.4mm using controlled phantom experiments. The system is further evaluated using invivo experiments where the finger joints and vasculature in the forearm and opisthenar of human participants are successfully imaged. This study suggests that the miniaturized MG based TAI systems can be used for invivo joint and vascular imaging with multiscale resolutions (0.3-1.4mm).

Integrated thermoacoustic and ultrasound imaging based on the combination of a hollow concave transducer array and a linear transducer array

To integrate the high resolution of ultrasound imaging (UI) and the high tissue specificity of thermoacoustic imaging (TAI) and to achieve an easy and precise co-registration of the two different imaging modalities, we present and demonstrate a hybrid thermoacoustic and ultrasound (TA/US) imaging system based on the combination of a novel hollow concave array and a commercial linear array. This TA/US imaging system can provide enhanced imaging of both tissues’ mechanical and dielectric properties. We verified the effective imaging performance of the hybrid TA/US system using tissue phantom experiments. In vivo TA/US imaging of the wrist and foot in healthy volunteers was also demonstrated using the hybrid system. This hollow concave array provided enhanced imaging performance for TAI because of its wide angular coverage with an optimal center frequency, showing a large effective imaging field of view (FOV) and improved images with high contrast and superior quality. Compared with stand-alone UI or TAI, the hybrid TA/US imaging presented more complete tissue anatomical structures, like skin, muscles, tendons, blood vessels, and bones for possible human disease diagnosis, although the US image quality using the hybrid system was slightly lower because the distance between the tissue and commercial ultrasound array was not ideal. This study suggests that the hybrid TA/US imaging approach has the potential to become a clinical tool for diagnosis of diseases in the wrist and foot.

Thermoacoustic tomography of germinal matrix hemorrhage in neonatal mouse cerebrum.

Microwave-induced thermoacoustic tomography (TAT) has potential for detecting germinal matrix hemorrhage (GMH). However, it has not been demonstrated in vivo. To demonstrate the feasibility of TAT for in vivo detecting GMH by using neonatal mouse. A cylindrical-scanning TAT system was developed with optimized microwave irradiation and ultrasound detection for neonatal mouse imaging. Neonatal mice were used to develop GMH model by injection of autologous blood into the periventricular region. After TAT experiments, the animals were sacrificed, frozen and excised to validate the TAT findings. The detailed comparative analyses of the TAT images and corresponding photographs of the excised brain tissues were conducted. Satisfactory matches are identified between the TAT images and corresponding histological sections, in terms of the shape and size of the brain tissues. Some organs and tissues were also identified. Particularly, comparing to the corresponding histological sections, using TAT enables to more accurately detect the hematoma region at different depths in the neonatal mouse brain. This study demonstrates for the first time that TAT can detect GMH in neonatal mouse cerebrum in vivo. This represents the first important step towards the in vivo diagnosis and grading of hemorrhage in the infant human brain.

A Microwave-Induced Thermoacoustic Imaging System With Non-Contact Ultrasound Detection.

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 μ Pa 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× 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.

Technical Note: Anti-phase microwave illumination-based thermoacoustic tomography of invivo human finger joints.

Thermoacoustic tomography (TAT) has been studied to image joints. While several joint tissues could be thermoacoustically imaged, tendons and bone could not be recovered completely or clearly. The purpose of this study was to overcome this limitation. We developed a novel TAT system based on anti-phase microwave illumination method to image the proximal interphalangeal joint and middle phalanx of a right middle finger from a healthy volunteer. The performance of this new system for imaging joints and tendons was compared with that by in-phase microwave illumination and a conventional pyramidal horn antenna. Anti-phase microwave illumination can produce relatively homogeneous electric (E)-Field distributions inside the joint tissues. The homogeneous E-Field distributions can enhance the detectability of flexor tendon and extensor tendon. Anti-phase microwave illumination could image the flexor tendon, and extensor tendon and bone, which were not clearly imaged by the in-phase microwave illumination or by the horn antenna. The images generated by the in-phase microwave illumination and pyramidal horn antenna were almost identical in terms of the tissue types they imaged. Anti-phase illumination can overcome the limitation associated with the conventional TAT by adding the ability of completely delineating tendons and bone in the joints. This study paves the way for us to continue the study and to validate its utility in detection of joint diseases.

Beamforming Microwave-Induced Thermoacoustic Imaging for Screening Applications

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.

Technical Note: Design of a handheld dipole antenna for a compact thermoacoustic imaging system.

Conventional antenna used in thermoacoustic imaging (TAI) is often bulky, making it difficult to translate TAI from the bench to the clinic. In this paper, we proposed a handheld dipole antenna which will be a good candidate for the construction of a compact TAI system. Here, we propose a TAI system based on a handheld dipole antenna with an aperture of 6cm and a weight of 230g. Compensation of the electric field attenuation for such a system was also investigated. Compared to the conventional open-ended waveguide and pyramidal horn antenna used in TAI, our results reveal that the electric field attenuation induced distortion is effectively reduced by using the attenuation factor 1/r2 for compensation and that the handheld dipole antenna provides the highest efficiency of microwave delivery. This study suggests that the handheld dipole antenna has the potential to be utilized for the construction of a compact TAI system, making TAI more suitable for preclinical and clinical applications.

A thermoacoustic imaging system with variable curvature and multi-dimensional detection adapted to breast tumor screening

Microwave-induced thermoacoustic (TA) imaging is efficient in detecting anomalous absorption areas, especially breast tumors. The operation of previous imaging systems has generally suffered from complexity, and early-stage tumors could be missed because of signal loss. In this paper, we report a variable-curvature and multi-dimensional screening method realized by combining a redesigned adaptive algorithm and an integrated TA breast screening system (ITBCS) comprising a flexible detector and a traction device designed for multi-axis rotation. In contrast to conventional TA imaging systems, the proposed system improves the ability to identify early-stage breast tumors, with a simplified system and operational process. Variable curvature is achieved by the flexible characteristics of the detector, which achieves good shape adaptation between the tissue and the detector. Moreover, multi-dimensional scanning is achieved by the multi-axis traction device and meets the requirements for angle transformation and dynamic imaging. The experimental results indicate that ITBCS exhibits a good performance in detecting breast tumors. Faster screening makes inspection more efficient, thereby improving clinical applicability and providing a convenient facility for large-scale physical examination.

A Single Simulation Platform for Hybrid Photoacoustic and RF-Acoustic Computed Tomography

In recent years, multimodal thermoacoustic imaging has demonstrated superior imaging quality compared to other emerging modalities. It provides functional and molecular information, arising due to electromagnetic absorption contrast, at ultrasonic resolution using inexpensive and non-ionizing imaging methods. The development of optical- as well as radio frequency (RF)-induced thermoacoustic imaging systems would benefit from reliable numerical simulations. To date, most numerical models use a combination of different software in order to model the hybrid thermoacoustic phenomenon. Here, we demonstrate the use of a single open source finite element software platform (ONELAB) for photo- and RF-acoustic computed tomography. The solutions of the optical diffusion equation, frequency domain Maxwell’s equations, and time-domain wave equation are used to solve the optical, electromagnetic, and acoustic propagation problems, respectively, in ONELAB. The results on a test homogeneous phantom and an approximate breast phantom confirm that ONELAB is a very effective software for both photo- and RF-acoustic simulations, and invaluable for developing new reconstruction algorithms and hardware systems.

Nonlinear Thermoacoustic Imaging Based on Temperature-Dependent Thermoelastic Response.

In this paper, a novel nonlinear thermoacoustic imaging (NTAI) system is developed based on the temperature-dependent thermoelastic response under microwave irradiation. Specifically, we consider the high-pulse repetition frequency (HPRF) microwave regime, where the tissue temperature increases after microwave irradiation. In this circumstance, the temperature-dependent thermodynamic parameters of the tissue change; thus, the influence of absorbed microwave energy on the thermoacoustic (TA) amplitude is reflected in the change of thermodynamic parameters. Hence, this temperature dependence yields a nonlinear relationship between the TA amplitude and the absorbed microwave energy in the HPRF regime. In this paper, we obtain the nonlinear coefficient as a new parameter to distinguish biological tissues by performing differential measurements in the NTAI system. We further experimentally extract the nonlinear coefficient in phantom and ex vivo mouse, revealing the feasibility of applications in biological tissues identification with high contrast.

Homogenizing microwave illumination in thermoacoustic tomography by a linear-to-circular polarizer based on frequency selective surfaces

A circularly polarized antenna, providing more homogeneous illumination compared to a linearly polarized antenna, is more suitable for microwave induced thermoacoustic tomography (TAT). The conventional realization of a circular polarization is by using a helical antenna, but it suffers from low efficiency, low power capacity, and limited aperture in TAT systems. Here, we report an implementation of a circularly polarized illumination method in TAT by inserting a single-layer linear-to-circular polarizer based on frequency selective surfaces between a pyramidal horn antenna and an imaging object. The performance of the proposed method was validated by both simulations and experimental imaging of a breast tumor phantom. The results showed that a circular polarization was achieved, and the resultant thermoacoustic signal-to-noise was twice greater than that in the helical antenna case. The proposed method is more desirable in a waveguide-based TAT system than the conventional method.