The feasibility of using microwave-induced thermoacoustic tomography for detection and evaluation of renal calculi

Thermoacoustic Imaging Using Single-Channel Data Acquisition System for Non-Invasive Assessment of Liver Microwave Ablation: A Feasibility Study

Microwave ablation (MWA) plays a crucial role in non-surgical liver cancer treatment, but the existing efficacy evaluation tools lack the characteristics of being real-time, non-invasive, and efficient. As an emerging imaging technology, thermoacoustic imaging (TAI) has attracted extensive clinical attention for its excellent merits, which combine the advantages of high contrast in microwave imaging and high resolution in ultrasound imaging. Particularly, the application of a circular scanned single-channel data acquisition system maximizes the capture of thermoacoustic signals, thereby providing more comprehensive image information and rendering reconstructed images closer to reality. This study aimed to verify the feasibility of TAI in non-invasive evaluation of the efficacy of MWA on ex vivo porcine liver and in vivo rabbit liver. During the experiments, ultrasound is used to cross-verify the results of TAI to ensure the accuracy and reliability of the method. Additionally, by altering the thickness of porcine liver tissue to increase the distance (from 0 mm to 80 mm) between the horn antenna and the target (soy sauce tube), TAI is used to observe the change of the image signal-to-noise ratio to preliminarily explore the imaging depth of TAI. The results of ex and in vivo experiments can not only promote the clinical application of TAI, but also be expected to provide a more accurate and reliable efficacy assessment method for MWA in liver cancer treatment.

Microwave-induced thermoacoustic microscopy based on short-pulse microwave and high-frequency point-focused ultrasonic transducer

BACKGROUND

As an emerging hybrid imaging modality, microwave-induced thermoacoustic imaging (MITAI) provides high contrast and deep tissue penetration, and has been extensively applied in cancer diagnosis, arthritis detection, and brain research. However, the previous studies had a limited spatial resolution of about 0.45-1.5 mm.

PURPOSE

Here, we describe a microwave-induced thermoacoustic microscopy (MITAM) system to help overcome the resolution limitation of current MITAI to image more subtle tissue features. On this basis, this paper applies MITAM to the thin skin and to demonstrate the potential of MITAM in detecting scleroderma.

METHODS

To achieve high resolution, short pulse width microwave (pulse width: 70 ns) and high-frequency ultrasonic point-focused transducer (center frequency: 25 MHz) were used to build the MITAM system. Two parallel copper wires with a diameter of 90 μm in the X/Y plane and Y/Z plane were imaged to estimate X/Y/Z resolution. Nine Balb/c mice were randomly divided into three groups and injected with different concentrations of bleomycin to induce scleroderma models. Their ex vivo skins were then imaged by our MITAM system. Visual observations were performed on the 3-dimensional skins MITAM images. And the mean value, Standard deviation, quartile distance, and signal-to-noise ratio were calculated to verify the results of the qualitative observations. Hematoxylin-Eosin (HE) and Masson staining were used to validate the findings of the MITAM.

RESULTS

The thickness of each imaged skin was measured to be about 450 μm. As an organ composed of multiple layers of tissues, the skin needs to be imaged at high resolution for the detection of related diseases. The results obtained showed that the improved resolution (68 μm in the Z-axis and 135 μm in the X-axis/Y-axis) of MITAM over conventional MITAI allowed us to differentiate scleroderma skins from normal skins and to identify the severity of scleroderma skins, consistent with the pathological findings of these skins.

CONCLUSIONS

The preliminary results obtained indicate that the MITAM can relieve the resolution limitation of traditional MITAI and has the potential to detection scleroderma. However, the transmission-type MITAM mentioned in this paper is difficult to image in vivo due to the narrow area between the antenna and the transducer. In the future, a reflective scanning MITAM will be constructed to detect scleroderma in vivo.

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

BACKGROUND

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.

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

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.

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 m 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 (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.

Microwave thermoacoustic tomographic (MTT) imaging

Microwave thermoacoustic tomography (MTT) uses microwave pulse-induced thermoelastic pressure waves to form planar or tomographic images. Since the generation and detection of thermoelastic pressure waves depends on dielectric permittivity, specific heat, thermal expansion, and acoustic properties of tissue, microwave thermoacoustic imaging possesses the characteristic features of a dual-modality imaging system. The unique attributes of the high contrast offered by microwave absorption and the fine spatial resolution furnished by ultrasound are being explored to provide a nonionizing and noninvasive imaging modality for characterization of tissues, especially for early detection of breast cancer. This paper reviews the research being conducted in developing MTT imaging for medical diagnosis. It discusses the science of thermoelastic wave generation and propagation in biological tissues, the design of prototype MTT systems, the reconstruction of tomographic images, and the application and accomplishment of prototype MTT systems in phantom models and experimental subjects.

Sound Out the Deep Colors: Photoacoustic Molecular Imaging at New Depths

Photoacoustic tomography (PAT) has become increasingly popular for molecular imaging due to its unique optical absorption contrast, high spatial resolution, deep imaging depth, and high imaging speed. Yet, the strong optical attenuation of biological tissues has traditionally prevented PAT from penetrating more than a few centimeters and limited its application for studying deeply seated targets. A variety of PAT technologies have been developed to extend the imaging depth, including employing deep-penetrating microwaves and X-ray photons as excitation sources, delivering the light to the inside of the organ, reshaping the light wavefront to better focus into scattering medium, as well as improving the sensitivity of ultrasonic transducers. At the same time, novel optical fluence mapping algorithms and image reconstruction methods have been developed to improve the quantitative accuracy of PAT, which is crucial to recover weak molecular signals at larger depths. The development of highly-absorbing near-infrared PA molecular probes has also flourished to provide high sensitivity and specificity in studying cellular processes. This review aims to introduce the recent developments in deep PA molecular imaging, including novel imaging systems, image processing methods and molecular probes, as well as their representative biomedical applications. Existing challenges and future directions are also discussed.

Reducing the effects of acoustic heterogeneity with an iterative reconstruction method from experimental data in microwave induced thermoacoustic tomography

PURPOSE

An iterative reconstruction method has been previously reported by the authors of this paper. However, the iterative reconstruction method was demonstrated by solely using the numerical simulations. It is essential to apply the iterative reconstruction method to practice conditions. The objective of this work is to validate the capability of the iterative reconstruction method for reducing the effects of acoustic heterogeneity with the experimental data in microwave induced thermoacoustic tomography.

METHODS

Most existing reconstruction methods need to combine the ultrasonic measurement technology to quantitatively measure the velocity distribution of heterogeneity, which increases the system complexity. Different to existing reconstruction methods, the iterative reconstruction method combines time reversal mirror technique, fast marching method, and simultaneous algebraic reconstruction technique to iteratively estimate the velocity distribution of heterogeneous tissue by solely using the measured data. Then, the estimated velocity distribution is used subsequently to reconstruct the highly accurate image of microwave absorption distribution. Experiments that a target placed in an acoustic heterogeneous environment are performed to validate the iterative reconstruction method.

RESULTS

By using the estimated velocity distribution, the target in an acoustic heterogeneous environment can be reconstructed with better shape and higher image contrast than targets that are reconstructed with a homogeneous velocity distribution.

CONCLUSIONS

The distortions caused by the acoustic heterogeneity can be efficiently corrected by utilizing the velocity distribution estimated by the iterative reconstruction method. The advantage of the iterative reconstruction method over the existing correction methods is that it is successful in improving the quality of the image of microwave absorption distribution without increasing the system complexity.

Thermoacoustic contrast of prostate cancer due to heating by very high frequency irradiation

Applying the thermoacoustic (TA) effect to diagnostic imaging was first proposed in the 1980s. The object under test is irradiated by high-power pulses of electromagnetic energy, which heat tissue and cause thermal expansion. Outgoing TA pressure pulses are detected by ultrasound transducers and reconstructed to provide images of the object. The TA contrast mechanism is strongly dependent upon the frequency of the irradiating electromagnetic pulse. When very high frequency (VHF) electromagnetic irradiation is utilized, TA signal production is driven by ionic content. Prostatic fluids contain high levels of ionic metabolites, including citrate, zinc, calcium, and magnesium. Healthy prostate glands produce more ionic metabolites than diseased glands. VHF pulses are therefore expected to generate stronger TA signal in healthy prostate glands than in diseased glands. A benchtop system for performing ex vivo TA computed tomography with VHF energy is described and images are presented. The system utilizes irradiation pulses of 700 ns duration exceeding 20 kW power. Reconstructions frequently visualize anatomic landmarks such as the urethra and verumontanum. TA reconstructions from three freshly excised human prostate glands with little, moderate, and severe cancerous involvement are compared with histology. TA signal strength is negatively correlated with percent cancerous involvement in this small sample size. For the 45 regions of interest analyzed, a reconstruction value of 0.4 mV provides 100% sensitivity but only 29% specificity. This sample size is far too small to draw sweeping conclusions, but the results warrant a larger volume study including comparison of TA images to the gold standard, histology.

Transcranial thermoacoustic tomography: a comparison of two imaging algorithms

Thermoacoustic tomography (TAT) is a novel, non-invasive medical imaging technique but has encountered obstacles in imaging through the cranium. In this paper we present two methods for transcranial TAT: Kirchhoff migration (KM) and reverse-time migration (RTM). The two methods' imaging qualities are verified and compared based on both synthetic and experimental data. RTM proves to have better velocity variance and imaging quality, and little noise with spatial aliasing. RTM is a promising approach for achieving transcranial TAT in further studies.

Three-dimensional thermoacoustic imaging for early breast cancer detection

PURPOSE

Microwave-induced thermoacoustic tomography (TAT) is a noninvasive modality based on the differences in microwave absorption of various biological tissues. In this paper, the feasibility of the early breast tumor detection by TAT system has been discussed and validated experimentally.

METHODS

A fast TAT system, which based on three 128-elements transducers, a 384-64ch switch and a parallel data acquisition system (DAS), was developed to reconstruct the three-dimensional (3D) image of a breast model with similar microwave absorption coefficient to breast tissue. A novel method to explore the ability of TAT system to distinguish absorption coefficient was introduced and the minimum absorption coefficient difference that can be distinguished clearly by our TAT system is 12 m(-1).

RESULTS

The potential applications of the TAT system were clearly demonstrated by successfully mapping breast model with mimicked tumors and microcalcification. An imaging experiment of human breast tumor embedding in the breast model was performed and the tumor was visualized by the 3D thermoacoustic volume.

CONCLUSIONS

The thermoacoustic images match well with the samples and achieve penetration depth of 6 cm. The experimental results indicate that TAT has a great potential to be used for detecting early-stage breast cancers with high contrast and high resolution.