Microwave-induced Thermoacoustic Imaging Research Articles

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.

Large field homogeneous illumination in microwave-induced thermoacoustic tomography based on a quasi-conical spiral antenna

Conventional helical and horn antennas based on frequency selective surfaces have been used to provide microwave illumination in microwave-induced thermoacoustic tomography (TAT). However, the electromagnetic waves radiated from the conventional antennas are not circularly polarized and thus impair image quality. In addition, conventional antennas can provide uniform radiations only within a relatively small area and thus limit their clinical applications (e.g., breast imaging). To address these problems, we propose a quasi-conical log-spiral antenna for homogenous illumination over a large field. We theoretically and experimentally validated this approach. Tissue-mimicking phantoms were imaged. The antenna produced not only an electric field with a circular polarization but also a homogeneous illumination area with a 10 cm diameter. Accordingly, our method has advanced TAT by improving microwave illumination.

Microwave induced thermoacoustic tomography based on probabilistic reconstruction

The performance of the existing reconstruction algorithms based on compressive sensing (CS) in microwave induced thermoacoustic tomography (MITAT) is influenced by the positions of detectors. Besides, some a priori information, such as target distribution or the correlation among thermoacoustic signals, has not been taken into account. In this letter, a probabilistic reconstruction algorithm in MITAT based on sparse Bayesian learning is proposed. Different from norm-based point estimation algorithms in CS, the sound pressure distribution which needs to be estimated is provided by probability distributions in the probabilistic reconstruction algorithm and an image is reconstructed based on the posterior density. Compared with the widely used norm-based point estimation algorithms (GPSR, Lasso) whose solution is not always the sparsest, the sparse Bayesian learning framework is globally convergent which can produce the sparsest solution at the posterior mean. Therefore, the robustness of the probabilistic reconstruction is better than that of norm-based point estimation algorithms. In addition, the estimations of the initial pressure distributions can be more accurately provided if the correlation of thermoacoustic signals can be considered, especially under the condition of low signal to noise ratio (SNR). Simulations and experiments on real breast tumors demonstrate that the proposed algorithm improves the robustness of reconstruction and show better performance at low SNRs.

Microwave-Induced Thermoacoustic Imaging of Subcutaneous Vasculature With Near-Field RF Excitation

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.

Mitigating acoustic heterogeneous effects in microwave-induced breast thermoacoustic tomography using multi-physical K-means clustering

Microwave-induced thermoacoustic tomography shows great potential for early-stage breast tumor detection, but imaging quality usually suffers due to acoustic heterogeneity of breast tissue. To mitigate this problem, conventional methods estimate the distribution of speeds of sound but at a heavy cost of system complexity or computation burden. We propose an imaging reconstruction method that incorporates dielectric and acoustic properties of tissues as a-priori information and reformulates the velocity estimation problem as a data clustering problem. The proposed method is validated by imaging anatomically realistic numerical breast phantoms and real biological tissues. Both simulation and experimental results demonstrate that the proposed method is robust and significantly improves image fidelity with less computational burden than conventional methods. These results make our approach a promising candidate for clinical breast cancer detection.

Block based compressive sensing method of microwave induced thermoacoustic tomography for breast tumor detection

Microwave induced thermoacoustic tomography (MITAT) is a developing non-ionized technique which has great potential in early breast tumor detection. In our previous work, an imaging method, CS-MITAT, was proposed, which applied the compressive sensing theory in MITAT and achieved a good image. The method converts a signal model into an unconstrained optimization problem with ℓ1 norm regularization, which only exploits the spatial sparsity of targets. In this paper, based on the block sparsity of thermoacoustic signals and target distribution in MITAT, the signals to be detected can be grouped into several blocks and the summation of ℓ2 norm regularization is used to replace the ℓ1 norm regularization of the CS-MITAT method. The combination of ℓ2 and ℓ1 norm regularizations helps the aggregation of nonzero elements which are accumulated in blocks. A priori structural constraint is added to form a more realistic signal model which can improve the image quality. Compared with the conventional approach of time reversal mirror and the method of gradient projection for sparse reconstruction, the alternating direction method of multipliers is applied to solve the convex optimization problem. Simulations and experiments on a real breast tumor demonstrate the effectiveness of the proposed method.

Peak-Power-Limited Frequency-Domain Microwave-Induced Thermoacoustic Imaging for Handheld Diagnostic and Screening Tools

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.

A review of microwave-induced thermoacoustic imaging: Excitation source, data acquisition system and biomedical applications

Microwave-induced thermoacoustic imaging (TAI) is a noninvasive modality based on the differences in microwave absorption of various biological tissues. TAI has been extensively researched in recent years, and several studies have revealed that TAI possesses advantages such as high resolution, high contrast, high imaging depth and fast imaging speed. In this paper, we reviewed the development of the TAI technique, its excitation source, data acquisition system and biomedical applications. It is believed that TAI has great potential applications in biomedical research and clinical study.

Thermoacoustic tomography of in vivo rat brain

We present for the first time in vivo imaging of rat brain using microwave-induced thermoacoustic tomography (TAT). The in vivo imaging of rat brain was realized through an unconventional delivery of microwave energy from the front of rat brain (while the transducer was scanned along coronal plane of the animal brain), which maximized the microwave penetration into the brain. In addition, we found that the imaging contrast was highly dependent on the direction of the electric field polarization (EFP) and that more tissue structures/compositions could be revealed when both [Formula: see text]- and [Formula: see text]-EFPs were used for TAT. The in vivo TAT images of rat brain obtained were compared with the 3.0 T MRI images and histological photographs, and numerous important brain anatomical structures were identified. An example of our TAT approach for imaging a foreign object embedded in a rat brain was also demonstrated. This study suggests that TAT has a great potential to be used in neuroscience studies and in noninvasive imaging of brain disorders.

Microwave-excited ultrasound and thermoacoustic dual imaging

We designed a microwave-excited ultrasound (MUI) and thermoacoustic dual imaging system. Under the pulsed microwave excitation, the piezoelectric transducer used for thermoacoustic signal detection will also emit a highly directional ultrasonic beam based on the inverse piezoelectric effect. With this beam, the ultrasonic transmitter circuitry of the traditional ultrasound imaging (TUI) system can be replaced by a microwave source. In other words, TUI can be fully integrated into the thermoacoustic imaging system by sharing the microwave excitation source and the transducer. Moreover, the signals of the two imaging modalities do not interfere with each other due to the existence of the sound path difference, so that MUI can be performed simultaneously with microwave-induced thermoacoustic imaging. In the study, the performance characteristics and imaging capabilities of this hybrid system are demonstrated. The results indicate that our design provides one easy method for low-cost platform integration and has the potential to offer a clinically useful dual-modality tool for the detection of accurate diseases.

Comparative Effects of Linearly and Circularly Polarized Illumination on Microwave-Induced Thermoacoustic Tomography

Homogeneous illumination is important in microwave-induced thermoacoustic tomography (TAT) because it directly determines the quantification of the reconstructed images of objects. In this work, we studied the effects of both linearly and circularly polarized microwave illuminations on TAT images. A customized horn antenna and a helical antenna working at 3 GHz were used to provide linearly polarized and circularly polarized illuminations, respectively. Based on our simulated and experimental results from tumor phantoms, the imaged patterns of the targets showed dependence on their permittivity. The target size and microwave wavelength within the target affected the microwave power distribution, which resulted in different reconstructed images. We analyzed the effect of the microwave penetration depth on TAT images as well. The images under circularly polarized illumination presented better quality than those under linearly polarized illumination. Hence, circularly polarized illumination can potentially better detect breast tumors in clinical applications.

ANALYSIS OF SHORT PULSE IMPACTING ON MICROWAVE INDUCED THERMO-ACOUSTIC TOMOGRAPHY

Microwave induced thermo-acoustic tomography (MITAT) is a developing technique for biomedical applications, especially for early breast cancer detection. In this paper, impacts of short microwave pulse on thermo-acoustic (TA) signals are analyzed and verified through some experimental comparisons. In these experiments, short microwave pulses with widths of 10 ns and 500 ns are employed as radiation resources. TA signals generated from a cubic sample are analyzed in both time- and frequency-domain. A trapezoid sample is also performed for experimental comparing. Different from previous literature, the effects of rising edge of radiation microwave pulse have been intensively studied. Experimental results demonstrate that shorter rising edge duration conducts broader bandwidth of TA signal, which give rise to better spatial resolution for tomography imaging.

Targeted Fe-filled carbon nanotube as a multifunctional contrast agent for thermoacoustic and magnetic resonance imaging of tumor in living mice

Microwave-induced thermoacoustic imaging (TAI) can map the microwave absorption distribution of targets, which depends on the electrical and magnetic properties. Although carbon nanotubes (CNTs) with good electrical properties have been used as TAI contrast agents, the negligible magnetic absorption hinders its application for sensitive detection. In order to exploit CNTs with electrical and magnetic absorption properties as agent of TAI, the ferromagnetic material-filled multi-walled CNTs (MMWCNTs) are investigated. In this study, the folic acid conjugated plain multiwalled CNTs (MWCNTs) and MMWCNTs were injected through the tail-vein of mice separately, and TAI and magnetic resonance imaging (MRI) were performed. The results show the MMWCNTs can clearly image the size and edge of the tumor with the TAI contrast enhancement of 67% and T2 signal intensity decrease of four fifths compared to MWCNTs. This study demonstrated the hybrid particles have the potential to be a high-sensitive contrast agent for accurate tumor detection. From the Clinical EditorNovel imaging modalities are emerging. Microwave-induced thermoacoustic imaging (TAI) relies on the absorption distribution of microwave of targets. In this article the authors investigate the use of ferromagnetic material-filled multi-walled CNTs as contrast agents for both TAI and MRI in an in-vivo model for tumors. The positive findings would imply that the application of dual-modality probe could provide more accurate imaging for the clinical setting.

Shape-adapting thermoacoustic imaging system based on flexible multi-element transducer

Microwave-induced thermoacoustic (TA) imaging is efficient in detecting the anomaly that has difference in microwave absorption with normal tissue, particularly for breast tumor and foreign objects. However, the traditional circular-scanning TA system needs a large operating space for rotation of the transducers and bulk of coupling medium. The linear-scanning system can overcome these problems partially but usually lose some information and cause image distortion. In order to overcome these limitations, a shape-adapting TA imaging system based on flexible multi-element transducer is proposed in this letter. The experimental results show that this system provides obvious advantages, including shape adaptation, information integrity, and efficient transmission, which make it a preferred choice for biomedical applications, especially for breast tumor detection.

Handheld Thermoacoustic Scanning System Based on a Linear-array Transducer.

To receive the information necessary for imaging, traditional microwave-induced thermoacoustic imaging systems (MITISs) use a type of circular-scanning mode using single or arc detectors. However, the use of MITISs for body scanning is complicated by restrictions in space and imaging time. A linear-array detector, the most widely used transducer in medical ultrasound imaging systems for body scanning, is a possible alternative to MITISs for scanning biological tissues, such as from the breast or limbs. In this paper, a handheld MITIS, based on a linear-array detector and a multiple data acquisition system, is described, and the capacity of the system is explored experimentally. First, the vertical and lateral resolution of the system is discussed. Next, real-time imaging of a moving object, obtained with an image capture rate of 20 frame/s, is described. Finally, a phantom experiment is detailed, investigating the overall imaging capability. The results show that this system achieves rapid scanning with a large field of view. The system has the obvious advantages of being handheld, not using coupled fluids, and achieving real-time imaging with a large field of view, which make this MITIS more suitable for clinical applications.

Interferogram-based breast tumor classification using microwave-induced thermoacoustic imaging.

Microwave-induced thermoacoustic (TA) imaging combines the dielectric/conductivity contrast in the microwave range with the high resolution of ultrasound imaging. Lack of ionizing radiation exposure in TA imaging makes this technique suitable for frequent screening applications, as with breast cancer screening. In this paper we demonstrate breast tumor classification based on TA imaging. The sensitivity of the signal-based classification algorithm to errors in the estimation of tumor locations is investigated. To reduce this sensitivity, we propose to use the interferogram of received pressure waves as the feature basis used for classification, and demonstrate the robustness based on a finite-difference time-domain (FDTD) simulation framework.

S-band microwave-Induced thermo-acoustic tomography system

The technology of microwave-induced thermo-acoustic tomography that transmits electromagnetic wave pulses to the object and makes it absorb energy, can cause a rapid temperature rise in it. At the same time, a pressure wave will be generated instantaneouly, corresponding to generating an ultrasonic signal which can be detected by an ultrasonic sensor. After the ultrasonic signal is sampled and an image reconstructed, the image can reflect the characteristics of the electromagnetic energy absorbed by the object. The method combines a microwave imaging of high contrast and high resolution ultrasound imaging characteristics, hence verifies theoretically the feasibility of the thermo-acoustic imaging techniques for early breast cancer detection. In this study, we use S-band microwave pulse radiation source to radiate the biological tissue, and also make use of the circling mechanical motion systems to scan the tissue. In order to verify the imaging performance of the simulation experiments, we use both tumors, body and actual biological tissue as the samples of the experiments. The imaging reconstruction and comparative analysis can verify that the experimental system detects and distinguishes the tumor phantoms and the real biological tissue effectively. Results of the performance of high-resolution images and high contrast by the methods can provide further theoretical support for early detecting of breast cancer.

Experimental Validation of a Numerical Model for Thermoacoustic Imaging Applications

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.

Pulsed microwave energy spatial distribution imaging by means of thermoacoustic tomography

Microwave-induced thermoacoustic imaging is a noninvasive, high contrast, high resolution, and cost effective method for cancer detection. It has the potential to serve as a routine breast tumor screening. In the present study, simulation and experiment have been used for pulsed microwave energy spatial distribution investigation. The target to be imaged is a 99 square array composed of tubes 3 mm in diameter and 8 mm in separation. The simulation and experimental results both indicat that far away from the antenna, more tubes could be thermoacoustically recovered, which means a larger radiation area obtained. The thermoacooustically recovered tubes are 3.1 mm in diameter and 7.7 mm in separation. Obtained results suggest that it is feasible to detect microwave energy spatial distribution with the thermoacoustic imaging, which has paved the way to solve the inhomogeneous microwave energy problem in traditional quantitative thermoacoustic tomography.

Evaluation of contrast enhancement by carbon nanotubes for microwave-induced thermoacoustic tomography.

Microwave-induced thermoacoustic tomography (MITAT) is a hybrid method which constructs images with ultrasound spatial resolution while exploiting dielectric contrast at microwave frequency. It has great potential in biomedical imaging especially in early breast cancer detection. The detection of early stage breast tumor in MITAT is challenged by the moderate endogenous dielectric contrast between malignant and healthy glandular tissues. In order to overcome this limitation, the performance of using carbon nanotubes (CNTs) as an imaging contrast enhancement agent is evaluated. First, the influences in dielectric and acoustic properties caused by CNTs are measured. Second, based on the measurements and the published data, numerical breast phantom is created and then used to explore the contrast enhancing effect of CNTs for MITAT, by an integrated simulation approach in both electromagnetic and acoustic field. With an experimental MITAT system, the thermoacoustic responses of tissue mimicking materials with different CNTs concentrations are also quantitatively investigated. Finally, the effectiveness of the contrast agent is also validated experimentally by using a MITAT system. The results show that the using of the dielectric contrast agent can effectively enhance the contrast of the MITAT image.