Microwave-induced Thermoacoustic Imaging Research Articles

Quantitative microwave‐induced thermoacoustic tomography

Microwave-induced thermoacoustic tomography (MI-TAT) is an imaging modality that exploits dielectric contrast while producing images with high ultrasound resolution. Existing reconstruction algorithms for MI-TAT are qualitative and can image only the distribution of the absorbed microwave energy or power loss density. Here the authors describe a method for quantitative MI-TAT and obtain the distribution of dielectric property which directly correlates with tissue structural and functional information. The authors implement the quantitative MI-TAT method based on the finite-element (FE) solution to the Helmholtz equation for electromagnetic field coupled with the thermoacoustic wave equation. Regularization techniques are also used in the FE-based reconstruction algorithm. Simulation results are obtained under various practical scenarios including different noise levels, different contrast levels between the heterogeneity and background region, and multiple targets with various sizes and shapes. The quantitative MI-TAT method described can provide accurate recovery of conductivity distribution in heterogeneous media and is insensitive to noise effect.

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.

In vivo detection and imaging of low-density foreign body with microwave-induced thermoacoustic tomography

Radiography or computed tomography is the most widely available imaging tool for foreign body detection. However, the detectability of low-density substances by x ray is very poor when located in soft tissues. Various dielectric loss factors of foreign bodies contribute great microwave absorption heterogeneity compared with the surrounding tissue. A fast thermoacoustic tomography system at 1.2 and 6 GHz was developed to detect foreign targets in small animals. The 6 GHz system had a much higher signal-to-noise ratio in near-surface imaging but smaller imaging depth than the 1.2 GHz system. The effects of microwave distribution inhomogeneity on nonuniform excitation of acoustic pressure were studied and a corresponding calibration algorithm for image distortion was provided and experimentally examined. Thermoacoustic images of radiolucent objects including glass fiber, wood, and bamboo hidden in phantom and residual in living mice were compared with radiography and ultrasonography. Good contrast was obtained between the foreign bodies and the tissue surrounding it, and the location and size of the lesion targets in thermoacoustic images were in good agreement with the actual sample. The experimental results demonstrate that thermoacoustic tomography may become the ideal modality for radiolucent foreign body detection and imaging in animals and human.

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.

Toward contrast-enhanced microwave-induced thermoacoustic imaging of breast cancer: an experimental study of the effects of microbubbles on simple thermoacoustic targets

Microwave-induced thermoacoustic tomography (MI-TAT) is an imaging technique that exploits dielectric contrast at microwave frequencies while creating images with ultrasound resolution. We propose the use of microbubbles as a dielectric contrast agent for enhancing the sensitivity of MI-TAT for breast cancer detection. As an initial investigation of this concept, we experimentally studied the extent to which the microwave-induced thermoacoustic response of a dielectric target is modified by the presence of air-filled glass microbubbles. We created mixtures of ethylene glycol with varying weight percentages of microbubbles and characterized both their microwave properties (0.5–6 GHz) and thermoacoustic response when irradiated with microwave energy at 3 GHz. Our data show that the microbubbles considerably lowered the relative permittivity, electrical conductivity and thermoacoustic response of the ethylene glycol mixtures. We hypothesize that the interstitial infusion of microbubbles to a tumor site will similarly create a smaller thermoacoustic response compared to the pre-contrast-agent response, thereby enhancing sensitivity through the use of differential imaging techniques.

Spectral Difference Between Microwave Radar and Microwave-Induced Thermoacoustic Signals

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This letter presents a spectral content comparison of the signals generated in the microwave radar (MR) and microwave-induced thermoacoustic (MIT) imaging systems. Two physical processes occur when microwave interacts with a lossy dielectric object. First, due to the contrast in the complex permittivity between the embedded object and background medium, reflection of microwave energy occurs at the interface. Second, due to the contrast in the absorption coefficient between the object and background medium and the consequent thermal expansion, microwave induces an acoustic wave. As a method of comparison between the inputs and outputs of the MR and MIT processes, we define the MR and MIT channels. We use a two-dimensional (2-D) example to demonstrate that the MR channel over the ultrawideband (UWB) spectrum from 3.1 to 10.6 GHz manifests different fading from the MIT channel over the ultrasonic spectrum from 0.32 to 1.10 MHz. The two output signals are distinctive, but they are co-registered to the same object. Hence, using the information provided by both could enhance the imaging modality. We apply this dual-physics scheme in the context of breast tumor detection. </para>

Prospects of photoacoustic tomography

Commercially available high-resolution three-dimensional optical imaging modalities-including confocal microscopy, two-photon microscopy, and optical coherence tomography-have fundamentally impacted biomedicine. Unfortunately, such tools cannot penetrate biological tissue deeper than the optical transport mean free path (approximately 1 mm in the skin). Photoacoustic tomography, which combines strong optical contrast and high ultrasonic resolution in a single modality, has broken through this fundamental depth limitation and achieved superdepth high-resolution optical imaging. In parallel, radio frequency-or microwave-induced thermoacoustic tomography is being actively developed to combine radio frequency or microwave contrast with ultrasonic resolution. In this Vision 20/20 article, the prospects of photoacoustic tomography are envisaged in the following aspects: (1) photoacoustic microscopy of optical absorption emerging as a mainstream technology, (2) melanoma detection using photoacoustic microscopy, (3) photoacoustic endoscopy, (4) simultaneous functional and molecular photoacoustic tomography, (5) photoacoustic tomography of gene expression, (6) Doppler photoacoustic tomography for flow measurement, (7) photoacoustic tomography of metabolic rate of oxygen, (8) photoacoustic mapping of sentinel lymph nodes, (9) multiscale photoacoustic imaging in vivo with common signal origins, (10) simultaneous photoacoustic and thermoacoustic tomography of the breast, (11) photoacoustic and thermoacoustic tomography of the brain, and (12) low-background thermoacoustic molecular imaging.

Effects of acoustic heterogeneities on transcranial brain imaging with microwave-induced thermoacoustic tomography

The effects of acoustic heterogeneities on transcranial brain imaging with microwave-induced thermoacoustic tomography were studied. A numerical model for calculating the propagation of thermoacoustic waves through the skull was developed and experimentally examined. The model takes into account wave reflection and refraction at the skull surfaces and therefore provides improved accuracy for the reconstruction. To evaluate when the skull-induced effects could be ignored in reconstruction, the reconstructed images obtained by the proposed method were further compared with those obtained with the method based on homogeneous acoustic properties. From simulation and experimental results, it was found that when the target region is close to the center of the brain, the effects caused by the skull layer are minimal and both reconstruction methods work well. As the target region becomes closer to the interface between the skull and brain tissue, however, the skull-induced distortion becomes increasingly severe, and the reconstructed image would be strongly distorted without correcting those effects. In this case, the proposed numerical method can improve image quality by taking into consideration the wave refraction and mode conversion at the skull surfaces. This work is important for obtaining good brain images when the thickness of the skull cannot be ignored.

Image distortion in thermoacoustic tomography caused by microwave diffraction

We report an intrinsic image distortion in microwave-induced thermoacoustic tomography. The distortion, due to microwave diffraction in the object to be imaged, leads to nonuniform excitation of acoustic pressure during microwave illumination. Both numerical simulations and phantom experiments demonstrate this phenomenon. A method of partial correction is also provided.

The Prototype of Microwave-Induced Thermo-Acoustic Tomography Imaging by Time Reversal Mirror

Microwave-Induced Thermo-Acoustic Tomography (MITAT) is a promising technique in biologic tissue imaging. It has predominant advantages in both contrast and resolution compared with conventional microwave or ultrasound imaging system for malignant tumors. In this paper, an integrated prototype MITAT system is briefly introduced. And Time Reversal Mirror (TRM) imaging technique based on Pseudo-Spectrum Time Domain (PSTD) is also first time applied to MITAT system. Two tissue strips of porcine muscle are used as targets in the MITAT experiment to demonstrate the imaging potential of the system. TRM and Filtered Back-Projection (FBP) imaging results are both presented to compare the performance of the TRM and other imaging techniques in contrast and resolution for MITAT.

Detection of foreign body using fast thermoacoustic tomography with a multielement linear transducer array

Current imaging modalities face challenges in clinical applications due to limitations in resolution or contrast. Microwave-induced thermoacoustic imaging may provide a complementary modality for medical imaging, particularly for detecting foreign objects due to their different absorption of electromagnetic radiation at specific frequencies. A thermoacoustic tomography system with a multielement linear transducer array was developed and used to detect foreign objects in tissue. Radiography and thermoacoustic images of objects with different electromagnetic properties, including glass, sand, and iron, were compared. The authors' results demonstrate that thermoacoustic imaging has the potential to become a fast method for surgical localization of occult foreign objects.

Fast Microwave-Induced Thermoacoustic Tomography Based onMulti-Element Phase-Controlled Focus Technique

We develop a fast microwave-induced thermoacoustic tomography systembased on a 320-element phase-controlled focus linear transducer array.A 1.2-GHz microwave generator transmits microwave with a pulse width of0.5 μs and an incident energy density of 0.45 mJ/cm2, and themicrowave energy is delivered by a rectangular waveguide with a crosssection of (80.01±0.02)×10−4 m2. Compared to singletransducer collection, the system with the multi-element lineartransducer array can eliminate the mechanical rotation of thetransducer, hence can effectively reduce the image blurring and improvethe image resolution. Using a phase-controlled focus technique to collectthermoacoustic signals, the data need not be averaged because of a highsignal-to-noise ratio, resulting in a total data acquisition time ofless than 5 s. The system thus provides a rapid and reliable approachto thermoacoustic imaging, which can potentially be developed as apowerful diagnostic tool for early-stage breast caners.

Rhesus monkey brain imaging through intact skull with thermoacoustic tomography

Two-dimensional microwave-induced thermoacoustic tomography (TAT) is applied to imaging the Rhesus monkey brain through the intact skull. To reduce the wavefront distortion caused by the skull, only the low-frequency components of the thermoacoustic signals (< 1 MHz) are used to reconstruct the TAT images. The methods of signal processing and image reconstruction are validated by imaging a lamb kidney. The resolution of the system is found to be 4 mm when we image a 1-month-old monkey head containing inserted needles. We also image the coronal and axial sections of a 7-month-old monkey head. Brain features that are 3 cm deep in the head are imaged clearly. Our results demonstrate that TAT has potential for use in portable, cost-effective imagers for pediatric brains.

Time-domain reconstruction for thermoacoustic tomography in a spherical geometry.

Reconstruction-based microwave-induced thermoacoustic tomography in a spherical configuration is presented. Thermoacoustic waves from biological tissue samples excited by microwave pulses are measured by a wide-band unfocused ultrasonic transducer, which is set on a spherical surface enclosing the sample. Sufficient data are acquired from different directions to reconstruct the microwave absorption distribution. An exact reconstruction solution is derived and approximated to a modified backprojection algorithm. Experiments demonstrate that the reconstructed images agree well with the original samples. The spatial resolution of the system reaches 0.5 mm.

Microwave-induced thermoacoustic tomography: reconstruction by synthetic aperture.

We have applied the synthetic-aperture method to linear-scanning microwave-induced thermoacoustic tomography in biological tissues. A nonfocused ultrasonic transducer was used to receive thermoacoustic signals, to which the delay-and-sum algorithm was applied for image reconstruction. We greatly improved the lateral resolution of images and acquired a clear view of the circular boundaries of buried cylindrical objects, which could not be obtained in conventional linear-scanning microwave-induced thermoacoustic tomography based on focused transducers. Two microwave sources, which had frequencies of 9 and 3 GHz, respectively, were used in the experiments for comparison. The 3 GHz system had a much larger imaging depth but a lower signal-noise ratio than the 9 GHz system in near-surface imaging.

Microwave-induced thermoacoustic tomography using multi-sector scanning.

A study of microwave-induced thermoacoustic tomography of inhomogeneous tissues using multi-sector scanning is presented. A short-pulsed microwave beam is used to irradiate the tissue samples. The microwave absorption excites time-resolved acoustic waves by thermoelastic expansion. The amplitudes of the acoustic waves are strongly related to locally absorbed microwave-energy density. The acoustic waves may propagate in all spatial directions. A focused ultrasonic transducer is employed to acquire temporal acoustic signals from multiple directions. Each detected signal is converted into a one-dimensional (1D) image along the acoustic axis of the transducer. The cross-sectional images of the tissue samples are calculated by combining all of the 1D images acquired in the same planes.

Signal processing in scanning thermoacoustic tomography in biological tissues.

Microwave-induced thermoacoustic tomography was explored to image biological tissues. Short microwave pulses irradiated tissues to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves were detected with a focused ultrasonic transducer to obtain two-dimensional tomographic images of biological tissues. The dependence of the axial and the lateral resolutions on the spectra of the signals was studied. A reshaping filter was applied to the temporal piezoelectric signals from the transducer to increase the weight of the high-frequency components, which improved the lateral resolution, and to broaden the spectrum of the signal, which enhanced the axial resolution. A numerical simulation validated our signal-processing approach.

Scanning microwave-induced thermoacoustic tomography: signal, resolution, and contrast.

Scanning thermoacoustic tomography was explored in the microwave region of the electromagnetic spectrum. Short microwave pulses were used to induce acoustic waves by thermoelastic expansion in biological tissues. Cross sections of tissue samples were imaged by a linear scan of the samples while a focused ultrasonic transducer detected the time-resolved thermoacoustic signals. Based on the microwave-absorption properties of normal and cancerous breast tissues, the piezoelectric signals in response to the thermoacoustic contrast were investigated over a wide range of electromagnetic frequencies and depths of tumor locations. The axial resolution is related to the temporal profile of the microwave pulses and to the impulse response of the ultrasonic transducer. The lateral resolution is related to the numerical aperture of the ultrasonic transducer as well as to the frequency spectra of the piezoelectric signals in the time window corresponding to the axial resolution. Gain compensation, counteracting the microwave attenuation, was applied to enhance the image contrast.

Scanning thermoacoustic tomography in biological tissue.

Microwave-induced thermoacoustic tomography was explored to image biological tissue. Short microwave pulses irradiated tissue to generate acoustic waves by thermoelastic expansion. The microwave-induced thermoacoustic waves were detected with a focused ultrasonic transducer. Each time-domain signal from the ultrasonic transducer represented a one-dimensional image along the acoustic axis of the ultrasonic transducer similar to an ultrasonic A-scan. Scanning the system perpendicularly to the acoustic axis of the ultrasonic transducer would generate multi-dimensional images. Two-dimensional tomographic images of biological tissue were obtained with 3-GHz microwaves. The axial and lateral resolutions were characterized. The time-domain piezo-electric signal from the ultrasonic transducer in response to the thermoacoustic signal was simulated theoretically, and the theoretical result agreed with the experimental result very well.