Microwave-induced Thermo-acoustic Tomography System Research Articles

Design and Implementation of an Ultra-Wideband Water Immersion Antenna for Underwater Ultrasonic Sensing in Microwave-Induced Thermoacoustic Tomography

Microwave-induced thermoacoustic tomography (MITAT) holds significant promise in biomedical applications. It creates images using ultrasonic sensors to detect thermoacoustic signals induced by microwaves. The key to generating thermoacoustic signals that accurately reflect the fact is to achieve sufficient and uniform microwave power absorption of the testing target, which is closely tied to the microwave illumination provided by the antenna. In this article, we introduce a novel design and implementation of an ultra-wideband water immersion antenna for an MITAT system. We analyze and compare the advantages of selecting water as the background medium. Simulations are conducted to analyze the ultra-wideband characteristics in impedance matching, axial ratio, and radiation pattern of the proposed antenna. The measured |S11| shows good agreement with the simulated results. We also simulate the microwave power absorption of tumor and brain tissue, and the uniform microwave power absorption and high contrast between the tumor and brain indicate the excellent performance of the proposed antenna in the MITAT system.

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.

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.

Iterative reconstruction method with optimal time-of-flights for imaging microwave absorption properties of tissues in acoustically heterogeneous environment in microwave-induced thermoacoustic tomography (MITAT)

In order to reduce the effects of acoustic heterogeneity and therefore accurately reconstruct the microwave absorption properties, a large number of measured data are required to estimate sound velocity distribution in microwave-induced thermoacoustic tomography (MITAT). However, the process of estimating the sound velocity distribution of heterogeneous tissue will consume lots of computing time and memory. This problem becomes serious especially for three-dimensional (3-D) imaging. To solve this problem, an improved iterative reconstruction method is proposed. Based on the variation trends of time-of-flights (TOFs) of ultrasonic signals, the spatial area where the acoustic heterogeneity of tissue may exist is estimated. Then the optimal TOFs are selected according to the estimated heterogeneous area. Using the selected optimal TOFs, the sound velocity distribution of tissues is iteratively estimated until convergence occurs. Finally, the microwave absorption distribution of tissues is reconstructed with the estimated sound velocity distribution. Contrary to the traditional iterative reconstruction method, which uses all the TOFs, the proposed method only uses the optimal TOFs to estimate the sound velocity distribution of tissues. It efficiently reduces the redundant information and therefore saves computing time and memory. Both numerical simulations and data experimentally measured from MITAT system are used to demonstrate the capability and effectiveness of the proposed method.

MICROWAVE-INDUCED THERMO-ACOUSTIC TOMOGRAPHY SYSTEM USING TRM-PSTD TECHNIQUE

Time reversal imaging method based on full wave numerical technique for likely breast tumors biological tissue in the Microwave-Induced Thermo-Acoustic Tomography (MITAT) system is discussed. In this paper, the mechanism of microwave-induced thermo-acoustic is strictly described based on thermodynamics and thermo-difiusion principles; the equivalent relationship between the absorbed microwave energy distribution of the biological tissue and the induced thermo-acoustic source distribution is used as the basis of the imaging algorithm. Due to its unique noise suppression feature and the stability of the algorithm, the Time Reversal Method (TRM) based on the Pseudospectral Time-Domain (PSTD) technique is applied to image heterogeneous phantom tissues from low Signal-to-Noise-Ratio (SNR) thermo-acoustic signals. Thereafter, an integrated MITAT prototype system is presented to obtain the thermo-acoustic signals from some biologic tissue with millimeter scale. The proposed TRM method is based on PSTD technique produced two-dimensional images, presented to study the performances of the MITAT in terms of contrast and resolution. These images prove predominant advantages in both contrast and resolution compared with conventional microwave and ultrasound imaging systems for malignant tumor detection. Based on the current results, our TRM-PSTD MITAT system provides evidence to predict breast tumor in an early stage and millimeter scale.

Development of microwave-induced thermo-acoustic tomography prototype system

Due to its advantages in both contrast and resolution compared with conventional microwave or ultrasound imaging system, microwave-induced thermo-acoustic tomography (MITAT) has received more and more concerns in biologic tissue image field. In this paper, an integrated prototype of MITAT system is presented. Using this system, some basic experiments for biologic tissue objects with high water-content imbedded in fatty material have been done. In the MITAT experiments, the signals generated by two porcine muscle tissue strips with millimeter order scale in cross-section were collected. Images with both good contrast and resolution were obtained. In order to demonstrate the advantages of MITAT, some ultrasonic experiments were studied using a commercial ultrasonic linear array system. The comparisons of the results of both systems show the good performance of the MITAT prototype system for the detection of high water content targets, which is generally the same as the tumor in biologic tissue.

Application of time reversal mirror technique in microwave-induced thermo-acoustic tomography system

Microwave-induced thermo-acoustic tomography (MITAT) is a promising technique with great potential in biomedical imaging. It has both the high contrast of the microwave imaging and the high resolution of the ultrasound imaging. In this paper, the proportional relationship between the absorbed microwave energy distribution and the induced ultrasound source distribution is derived. Further, the time reversal mirror (TRM) technique based on the pseudo-spectral time domain (PSTD) method is applied to MITAT system. The simulation results show that high contrast and resolution can be achieved by the TRM technique based on PSTD method even for the received signals with very low signal-to-noise ratio (SNR) and the model parameter with random fluctuation.