Search

In this paper, we have developed a limited-view scanning microwave-induced thermoacoustic computed tomography (CT) system based on the multi-element phase-controlled focus technique. A 320-element linear transducer array was relatively rotated to capture thermoacoustic signals within limited detection views, which effectively eliminated the problem that the focused ultrasonic transducer can but receive finite signal from boundaries of tissues which are nearly perpendicular to the axis of the transducer. Using phase-controlled focus technique to collect thermoacoustic signals, the data need not be averaged because of a high signal-to-noise ratio, resulting in a data acquisition time of less than 5 s in each view. The collected limited-view data was utilized to reconstruct the geometrical details based on limited-field-filtered back-projection algorithm. The system may provide a reliable approach to thermoacoustic imaging, which can potentially be developed as a powerful diagnostic tool for early-stage breast caners.

Nanoparticles have drawn great attention as targeted imaging and therapeutic agents recently. However, so far, no research on radio-frequency absorption contrast agents in soft tissues has been reported. A microstructure agent, carbonyl iron, which has strong microwave absorption, is applied to microwave-induced thermoacoustic imaging. The size of this microsphere is approximately 2mum that is safe for biological and medical application. The carbonyl iron emulsion is prepared by the Sol-Gel process which makes carbonyl iron particles more hydrophilic and with good electromagnetic properties. A 320-element linear transducer array with a centre frequency of 3.5 MHz was adopted to capture thermoacoustic signals within limited field around the sample. Using phase-controlled focus technique to collect thermoacoustic signals, the data need not be averaged because of a high signal-to-noise ratio, resulting in a data acquisition time of less than 5 s at each step. The collected data was utilized to reconstruct the geometrical details based on limited-field-filtered back-projection algorithm. The effect of solid particles for enhancing thermoacoustic grey scale images was successfully investigated in tissue phantoms, the image brightness, i.e., mean grey scale level, increased with particle concentration. This particle may have potential to improve sensitivity and specificity for structural and functional thermoacoustic imaging and broadly expand the thermoacoustic imaging capability as a powerful medical diagnostic tool. This paper provides the first demonstration of a microwave contrast enhancing agent for thermoacoustic imaging.

We investigate the feasibility of using iron oxide nanoparticles as a contrast agent for radiofrequency (RF) induced thermoacoustic tomography. Aqueous colloids of iron oxide (Fe3O4) nanoparticles have been synthesized and characterized. The synthesis method yielded citrate-stabilized, spherical particles with a diameter of approximately 10 nm. The complex permittivity of the colloids was measured with a coaxial probe and vector network analyzer, and the microwave absorption properties were calculated by using a relationship between the complex permittivity and absorption coefficients. Using our pulsed thermoacoustic imaging system at 3 GHz, the time-resolved thermoacoustic responses of those colloids were measured and compared to that of deionized water. Finally, two-dimensional thermoacoustic images were acquired from iron oxide colloids in a tissue phantom. The iron oxide colloids produced an enhancement in RF absorption of up to three times that of deionized water at 3 GHz. The enhancement increased rapidly with decreasing frequency of the RF excitation source. A corresponding increase in time-resolved thermoacoustic signal of more than two times was demonstrated. Our results indicate that iron oxide nanoparticles have the potential to produce enhanced thermoacoustic signals and to provide molecular imaging with functionalized contrast agents for thermoacoustic tomography.

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.

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.

We developed a fast microwave-induced thermoacoustic tomography system based on a 320-element phase-controlled linear transducer array. A 1.2GHz microwave generator transmits microwave with pulse duration of 0.5 us and an incident energy density of 0.45 mJ/cm 2 . Compared to single transducer collection, the system with multi-element linear transducer array can eliminate the mechanical rotation of the transducer, so can effectively reduce the image blurring and improve the image resolution. Using phase-controlled technique to collect thermoacoustic signals, the data need not be averaged because of the high signal-to-noise ratio, resulting in the total data acquisition time of less than 5 s. The system thus provide a rapid and reliable approach to thermoacoustic imaging. which can potentially be developed as a powerful diagnostic tool for early-stage breast caners.

Atherosclerosis is a systemic disease characterized by the development of a plaque leading to several acute coronary syndromes. Imaging of plaque structure and composition is important in diagnosing the disease and further guiding coronary interventions. Currently, there is no clinical imaging technique capable of providing comprehensive morphological and functional information of the plaques. Several emerging techniques including palpography and thermography are under investigation for intravascular imaging of atherosclerosis. In this paper we present photoacoustic imaging as a means to assess the composition of plaques based on the optical properties of tissue. Specifically, we investigate the feasibility of obtaining intravascular photoacoustic (IVPA) images using a high frequency intravascular ultrasound (IVUS) imaging catheter. Indeed, the combination of intravascular photoacoustic (IVPA) imaging with clinically available intravascular ultrasound (IVUS) is desired for simultaneous functional and morphological imaging of the plaque. The imaging studies were performed using a tissue- mimicking phantom to model the arterial vessel with atherosclerotic plaque. The results of our study suggest that combined IVUS and IVPA imaging has the potential to detect and differentiate atherosclerosis based on both structure and composition of the plaque.

The effects of acoustic heterogeneities on thermoacoustic tomography (TAT) are examined and corrected. One assumption made in the existing reconstruction algorithms for thermoacoustic tomography is that biological tissue is acoustically homogeneous. In medical imaging applications, this assumption can cause blurring and distortion in the reconstructed images. This degradation of image quality can be compensated for by using an approximate distribution of the acoustic speed in the tissue. A method based on transmission ultrasound tomography, which is compatible with our thermoacoustic imaging setup, is developed to correct those effects. Experiments verify the validity of this method. The technique can be used to improve the image quality of thermoacoustic tomography.

We report a preliminary study of breast cancer imaging by microwave-induced thermoacoustic tomography. In this study, we built a prototype of breast cancer imager based on a circular scan mode. A 3-GHz 0.3~0.5-μs microwave is used as the excitation energy source. A 2.25-MHz ultrasound transducer scans the thermoacoustic signals. All the measured data is transferred to a personal computer for imaging based on our proposed back-projection reconstruction algorithms. We quantified the line spread function of the imaging system. It shows the spatial resolution of our experimental system reaches 0.5 mm. After phantom experiments demonstrated the principle of this technique, we moved the imaging system to the University of Texas MD Anderson Cancer Center to image the excised breast cancer specimens. After the surgery performed by the physicians at the Cancer Center, the excised breast specimen was placed in a plastic cylindrical container with a diameter of 10 cm; and it was then imaged by three imaging modalities: radiograph, ultrasound and thermoacoustic imaging. Four excised breast specimens have been tested. The tumor regions have been clearly located. This preliminary study demonstrated the potential of microwave-induced thermoacoustic tomography for applications in breast cancer imaging.

Since October 2001, we have published three peer-reviewed journal articles in IEEE Transactions on Medical imaging a top imaging journal, published one conference proceedings article, and delivered 9 invited talks. The combination of ultrasound and microwave has provided us a unique opportunity for early-cancer imaging with high resolution and high contrast. We have made significant technical progress in thermoacoustic imaging including data acquisition and imaging reconstruction. Specifically, our accomplishments include (1) an exact and an approximate time-domain reconstruction algorithm for thermoacoustic tomography in a spherical geometry was derived and published, (2) an exact frequency-domain reconstruction algorithm for thermoacoustic tomography in a planar geometry was derived and published, (3) an exact frequency-domain reconstruction algorithm for thermoacoustic tomography in a cylindrical geometry was derived and published, and (4) high- resolution and high-contrast images were obtained and published. The reconstruction is an inverse source problem similar to that in PET (positron emission tomography); however, the reconstruction in PET is based on geometric optics whereas the reconstruction in thermoacoustic imaging is based on diffractive/wave optics. We have successfully imaged biological tissue with high resolution and high contrast. We will advance this

We report results of a reconstruction algorithm for three-dimensional photoacoustic computed tomography. A universal back-projection formula is presented for three types of imaging geometries: planar, spherical, and cylindrical surfaces. A solid-angle weighting factor is introduced in the back-projection formula to compensate for the variations of detection views. A method for implementing this algorithm is described. Numerical simulation is used to demonstrate the performance of the algorithm.

We have utilized a prototype Thermoacoustic Computed Tomography Small Animal Imaging System to acquire images of athymic mice with bilateral tumors implanted in the cranial mammary fat pads. The breast tumor cell lines used in the study, which are MCF7, and MCF7 transfected with Vascular Endothelial Growth Factor (VEGF), exhibit distinctly contrasting levels of vascularization. Three dimensional images of the mice, acquired using pulses of NIR stimulating light, demonstrate the ability of the system to generate high resolution images of the vascular system up to one inch deep in tissue, and at the same time, differentiate tissue types based on the infrared absorption properties of the tissue; a property related in part to blood content and oxygenation levels. We have processed images acquired at different stimulating wavelengths to generate images representative of the distribution of oxygenated and deoxygenated hemoglobin throughout the tumors. The images demonstrate the in vivo capabilities of the imaging system and map system structure as well as the total, oxygenated and deoxygenated hemoglobin components of the blood.

In this paper, we present time-domain reconstruction algorithms for the thermoacoustic imaging of biological tissues. The algorithm for a spherical measurement configuration has recently been reported in another paper. Here, we extend the reconstruction algorithms to planar and cylindrical measurement configurations. First, we generalize the rigorous reconstruction formulas by employing Green's function technique. Then, in order to detect small (compared with the measurement geometry) but deeply buried objects, we can simplify the formulas when two practical conditions exist: 1) that the high-frequency components of the thermoacoustic signals contribute more to the spatial resolution than the low-frequency ones, and 2) that the detecting distances between the thermoacoustic sources and the detecting transducers are much greater than the wavelengths of the high-frequency thermoacoustic signals (i.e., those that are useful for imaging). The simplified formulas are computed with temporal back projections and coherent summations over spherical surfaces using certain spatial weighting factors. We refer to these reconstruction formulas as modified back projections. Numerical results are given to illustrate the validity of these algorithms.

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.

A study of pulsed-microwave-induced thermoacoustic tomography in biological tissues is presented. A backprojection algorithm based on rigorous theory is used to reconstruct the cross-sectional image from the thermoacoustic measurement in a circular configuration that encloses the sample under study. The results demonstrate the possibility of application in detecting small tumors buried in biological tissues using microwave absorption contrast and ultrasound spatial resolution. Finally, the method is compared with laser-induced thermoacoustic tomography.

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.

Purpose: Previous studies demonstrated that thermoacoustic imaging (TAI) has great potential for breast tumor detection. However, large field of view (FOV) imaging remains a long-standing challenge for three-dimensional (3D) breast tumor localization. Here, the authors propose a practical TAI system for noninvasive 3D localization of breast tumors with large FOV through the use of ultrashort microwave pulse (USMP). Methods: A USMP generator was employed for TAI. The energy density required for quality imaging and the corresponding microwave-to-acoustic conversion efficiency were compared with that of conventional TAI. The microwave energy distribution, imaging depth, resolution, and 3D imaging capabilities were then investigated. Finally, a breast phantom embedded with a laboratory-grown tumor was imaged to evaluate the FOV performance of the USMP TAI system, under a simulated clinical situation. Results: A radiation energy density equivalent to just 1.6%–2.2% of that for conventional submicrosecond microwave TAI was sufficient to obtain a thermoacoustic signal with the required signal-to-noise ratio. This result clearly demonstrated a significantly higher microwave-to-acoustic conversion efficiency of USMP TAI compared to that of conventional TAI. The USMP TAI system achieved 61 mm imaging depth and 12 × 12 cm2 microwave radiation area. The volumetric image of a copper target measured at depth of 4–6 cm matched well with the actual shape and the resolution reaches 230 μm. The TAI of the breast phantom was precisely localized to an accuracy of 0.1 cm over an 8 × 8 cm2 FOV. Conclusions: The experimental results demonstrated that the USMP TAI system offered significant potential for noninvasive clinical detection and 3D localization of deep breast tumors, with low microwave radiation dose and high spatial resolution over a sufficiently large FOV.

Microwave-induced thermoacoustic (TA) imaging combines the dielectric contrast of microwave imaging with the resolution of ultrasound imaging. Prior studies have only focused on time-domain techniques with short but powerful microwave pulses that require a peak output power in excess of several kilowatts to achieve sufficient signal-to-noise ratio (SNR). This poses safety concerns as well as to render the imager expensive and bulky with requiring a large vacuum radio frequency source. Here, we propose and demonstrate a coherent stepped-frequency continuous-wave (SFCW) technique for TA imaging which enables substantial improvements in SNR and consequently a reduction in peak power requirements for the imager. Constructive and destructive interferences between TA signals are observed and explained. Full coherency across microwave and acoustic domains, in the thermo-elastic response, is experimentally verified and this enables demonstration of coherent SFCW microwave-induced TA imaging. Compared to the pulsed technique, an improvement of 17 dB in SNR is demonstrated.

Purpose: Numerical study of microwave imaging and microwave-induced thermoacoustic imaging utilizes finite difference time domain (FDTD) analysis for simulation of microwave and acoustic interaction with biological tissues, which is time consuming due to complex grid-segmentation and numerous calculations, not straightforward due to no analytical solution and physical explanation, and incompatible with hardware development requiring circuit simulator such as SPICE. In this paper, instead of conventional FDTD numerical simulation, an equivalent electrical circuit model is proposed to model the microwave acoustic interaction with biological tissues for fast simulation and quantitative analysis in both one and two dimensions (2D). Methods: The equivalent circuit of ideal point-like tissue for microwave-acoustic interaction is proposed including transmission line, voltage-controlled current source, envelop detector, and resistor-inductor-capacitor (RLC) network, to model the microwave scattering, thermal expansion, and acoustic generation. Based on which, two-port network of the point-like tissue is built and characterized using pseudo S-parameters and transducer gain. Two dimensional circuit network including acoustic scatterer and acoustic channel is also constructed to model the 2D spatial information and acoustic scattering effect in heterogeneous medium. Results: Both FDTD simulation, circuit simulation, and experimental measurement are performed to compare the results in terms of time domain, frequency domain, and pseudo S-parameters characterization. 2D circuit network simulation is also performed under different scenarios including different sizes of tumors and the effect of acoustic scatterer. Conclusions: The proposed circuit model of microwave acoustic interaction with biological tissue could give good agreement with FDTD simulated and experimental measured results. The pseudo S-parameters and characteristic gain could globally evaluate the performance of tumor detection. The 2D circuit network enables the potential to combine the quasi-numerical simulation and circuit simulation in a uniform simulator for codesign and simulation of a microwave acoustic imaging system, bridging bioeffect study and hardware development seamlessly.

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.

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.

The authors evaluated images obtained with a prototypic thermoacoustic computed tomographic (CT) scanner constructed for use at 434 MHz, a promising radio frequency for detecting breast cancer. In one excised porcine kidney, acoustic energy emanating from the kidney was detected with transducers. The resultant electric signals were used to create a three-dimensional data set. Two-dimensional images reconstructed in multiple planes were compared with state-of-the-art T1- and T2-weighted magnetic resonance images. The renal outline, parenchyma, and collecting system were clearly delineated on the thermoacoustic CT images.

Magnetically mediated thermo-acoustic effect is predicted in theory and demonstrated in phantom studies in this letter. By applying transient current to a compact magnetically resonant coil at radio frequency below 20 MHz, large electric field is inducted by magnetic field inside conductive objects which then undergoes joule heating and emanates acoustic signal thermo-elastically. The magnetic mediation approach with low radio frequency can provide deeper penetration into conductive objects which may extend thermoacoustic imaging to deep laid human organs. Both incoherent time domain and coherent frequency domain approaches are discussed with the latter demonstrated potential for portable imaging system. Published version

Purpose: To propose an alternative method of thermoacoustic wave generation based on heating of magnetic nanoparticles (MNPs) using alternating magnetic field (AMF). Methods: The feasibility of thermoacoustic wave generation from MNPs by applying a short-burst of AMF or a frequency-modulated AMF is theoretically analyzed. As the relaxation of MNPs is strongly dependent upon the amplitude and frequency of AMF, either an amplitude modulated, fixed frequency AMF (termed time-domain AMF) or a frequency modulated, constant amplitude AMF (termed frequency-domain AMF) will result in time-varying heat dissipation from MNPs, which has the potential to generate thermoacoustic waves. Following Rosensweig's model of specific power loss of MNPs in a steady-state AMF, the time-resolved heat dissipations of MNPs of superparamagnetic size when exposed to a short bursting of AMF and/or to a linearly frequency chirped AMF are derived, and the resulted acoustic propagation is presented. Based on experimentally measured temperature-rise characteristics of a superparamagnetic iron-oxide nanoparticle (SPION) matrix in a steady-state AMF of various frequencies, the heat dissipations of the SPION under time-domain and frequency-domain AMF configurations that could have practical utility for thermoacoustic wave generation are estimated. Results: The initial rates of the temperature-rise of the SPION matrix were measured at an iron-weight concentration of 0.8 mg/ml and an AMF frequency of 88.8 kHz to 1.105 MHz. The measured initial rates of temperature-rise were modeled by Rosensweig's theory, and projected to 10 MHz AMF frequency, at which a 1μs bursting corresponding to a 1.55 mm axial resolution of acoustic detection could contain 10 complete cycles of AMF oscillation and the power dissipation is approximately 84 times of that at 1 MHz. Exposing the SPION matrix to a 1 μs bursting of AMF at 10 MHz frequency and 100 Oe field intensity would produce a volumetric heat dissipation of 7.7 μJ/cm3 over the microsecond duration of the AMF burst. If the SPION matrix is exposed to a 1 ms long AMF train at 100 Oe field intensity that chirps linearly from 1 to 10 MHz, the volumetric heat dissipation produced over each 2π phase change of the AMF oscillation is estimated to increase from 0.15 to 1.1 μJ/cm3 within the millisecond duration of the chirping of AMF. Conclusions: The heat dissipations upon SPION (∼1 mg/ml iron-weight concentration) by a 1μs bursting of 100 Oe AMF at 10 MHz and a 1 ms train of 100 Oe AMF that chirps linearly from 1 to 10 MHz were estimated to determine the potential of thermal-acoustic wave generation. Although thermoacoustic wave generation from MNPs by time- or frequency-domain AMF applications is predicted, the experimental generation of such a wave remains challenging.

In this letter, thermoacoustic resonance effect is predicted from theoretical analysis with series resistor-inductor-capacitor resonance circuit model and then observed experimentally using muscle tissue illuminated by multi-pulse microwave source. Through model fitting, the circuit parameters are extracted to characterize quantitatively the resonant response of the tissue. Coherent demodulation is applied to obtain the enhanced signal-to-noise ratio and spatial information by treating tissue as a communication channel. This physical phenomenon shows significantly higher sensitivity than conventional single microwave pulse induced thermoacoustic effect, enabling the potential design of low-power thermoacoustic imaging device for portable and on-site diagnosis. Published version

Microwave-induced thermoacoustic tomography (TAT) is a noninvasive, nonionizing modality based on the inherent differences in microwave absorption of malignant breast tissues and normal adipose-dominated breast tissues. In this paper, a TAT system based on multielement acquisition system was built to receive signals. Slices from different layers in the sample were composed into a three-dimensional (3D) volume. Based on the 3D volume, inherent differences in microwave absorption between different biological tissues can be converted into structure information. Our experimental results of some mimicked and human tumors indicate that TAT may potentially be used to detect early-stage breast cancers with high contrast.

We report on experimental demonstration of thermoacoustic tomography for quantitatively reconstructing the conductivity images of heterogeneous media. Thermoacoustic images are obtained from a series of tissue-like phantom experiments using a finite element-based reconstruction algorithm coupled with a scanning thermoacoustic imaging system. The experimental results show that conductivity images can be quantitatively reconstructed based on the finite-element (FE) solution to the Helmholtz electromagnetic wave equation coupled with the thermoacoustic wave equation.

Purpose: To demonstrate the feasibility of thermoacoustic (TA) imaging to detect microvasculature surrounding tumors embedded 6+ centimeters deep. Methods: Ex vivo imaging of porcine kidneys in a custom TA testbed demonstrates 6cm depth penetration. Chilled glycine solution serves as acoustic couplant. Submicrosecond radiofrequency pulses with carrier frequency 108 MHz and peak power of 40kW generate TA pressures. Step‐ and‐shoot volumetric scanning is performed by rotating and translating the kidney while the transducer remains stationary. Translation increment is 3.6mm between slices. Focused single‐element videoscan transducers with center frequencies of 1 MHz and 2.25 MHz collect signals, which are amplified by a 54dB preamp. Lightly filtered single‐slice sinograms are backprojected in‐plane to reconstructimages. TA signal generated by vials of blood product and pure anticoagulant are compared with TA signal from pure water. Vials are manually positioned, diminishing repeatability of these experiments, so multiple realizations of each are presented. Results: Reconstructions of the kidneys clearly track progression of calyces from one slice to the next. In vivo, calyces are filled with urine, which can have widely varying electrical conductivity. TA signal strength is directly proportional to electrical conductivity but is also a function of other tissue parameters: bulk modulus, thermal expansion coefficient, and specific heat capacity. In our testbed, the calyces fill with glycine, which has low conductivity, causing signal dropout in the calyces. TA pressures generated by both blood product and pure anticoagulant generate orders of magnitude stronger signal than pure water. Conclusions: We demonstrate whole organ TA imaging is possible with 108 MHz RF irradiation. Furthermore, at 108 MHz, blood is highly conductive and generates strong signal, supporting the hypothesis that TA may be able to detect microvasculature surrounding aggressive tumors. This work was supported by NIH grant 5 R21 CA137364‐02.

The main task of thermoacoustic imaging is the estimation of a function, denoted by , which depends on the electromagnetic absorption function and the optical scattering properties of the tissue. In the absence of acoustic dissipation, the parameter function can be estimated from one of the three types of projections (spherical, circular or planar). In the case of acoustic dissipative wave propagation in tissue, it is no longer possible to explicitly calculate the projection of from the respective pressure data (measured by point, planar or line detectors). The goal of this paper is to derive for each of the three types of pressure detectors, an integral equation that allows estimation of the respective projections of . The advantage of this approach is that known reconstruction formulas for using the respective projection data can then be exploited.

Thermal and elastic properties of materials are imaged by transmission or reflection thermoacoustic microscopes. These two operating modes are compared. Applications to non-destructive evaluation of simulated defects in metallic samples are presented.

Photoacoustic imaging (also called optoacoustic or thermoacoustic imaging) has the potential to image animal or human organs, such as the breast and the brain, with simultaneous high contrast and high spatial resolution. This article provides an overview of the rapidly expanding field of photoacoustic imaging for biomedical applications. Imaging techniques, including depth profiling in layered media, scanning tomography with focused ultrasonic transducers, image forming with an acoustic lens, and computed tomography with unfocused transducers, are introduced. Special emphasis is placed on computed tomography, including reconstruction algorithms, spatial resolution, and related recent experiments. Promising biomedical applications are discussed throughout the text, including (1) tomographic imaging of the skin and other superficial organs by laser-induced photoacoustic microscopy, which offers the critical advantages, over current high-resolution optical imaging modalities, of deeper imaging depth and higher absorptioncontrasts, (2) breast cancerdetection by near-infrared light or radio-frequency–wave-induced photoacoustic imaging, which has important potential for early detection, and (3) small animal imaging by laser-induced photoacoustic imaging, which measures unique optical absorptioncontrasts related to important biochemical information and provides better resolution in deep tissues than optical imaging.

A thermoacoustic imaging system including an electromagnetic radiation source for irradiating said tissue to stimulate a thermoacoustic response, a coupling media for acoustically coupling the response to an acoustic sensor array, and an acoustic sensor array. The array comprises sensors arranged on a surface, which is rotatable about an axis to position said sensors in a plurality of positions for detecting the thermoacoustic response. The angular extent of the surface about the axis, subtends an angle that is less than a full revolution, streamlining the device and permitting flexibility in positioning the radiation source and other elements of the device. The source of electromagnetic radiation for irradiating the tissue is a plurality of sources arranged about the tissue and producing synchronized electromagnetic radiation in varying polarizations or phases to irradiate said tissue with electromagnetic radiation of a desired polarization. A thermoacoustic imaging system sized to be held within the human hand is also disclosed.

Thermoacoustic imaging is a new technique that permits the detection and imaging of microscopic surface and subsurface fea- tures in a sample. We present a detailed theoretical treatment of the thermoacoustic process with particular emphasis on the roles of ther- mal and elastic parameters in both one and three dimensions. Exam- ples are presented of thermoacoustic images obtained with a scanning electron microscope. These include images of subsurface mechanical defects, metallic grains and grain boundaries, and of dislocation net- works and dopant regions in semiconductor crystals.

A special-purpose acoustic imaging technique is described in which heat pulses are generated within the region of interest by deeply penetrating radiation, and the resultant thermal expansions produce acoustic waves which are detected at the surface of any body sufficiently uniform for conventional ultrasonic imaging. By employing signal-averaging of successive pulses, only a fraction of a microdegree temperature rise is required in each pulse. If electromagnetic heating radiation is employed, images will sharply differentiate regions of differing electrical conductivity. Both electromagnetic and ionizing radiation show changes of thermal expansion coefficient and thermal heat capacity. In the soft-tissue regions of the body, the technique provides (a) a novel non-invasive identification of tissue characteristics which may provide a valuable supplement to the information derived from conventional imaging modalities and (b) a method to verify treatment plans in the positioning of therapeutic radiation dosage. In inanimate materials, the technique offers a new method of non-destructive evaluation (NDE) which can differentiate and locate regions of differing electrical conductivity, thermal expansion coefficient, and heat capacity not sensed by conventional NDE imaging.

Thermoacoustic emissions were induced in phantoms inside a microwave cavity by pulses from a 2450 MHz magnetron. These phantoms consisted o f graphite and lucite plates acting as interfaces in a small water tank and also of three components of tissue-simulatin g material. The acoustic emissions were received by an a irbacked 1-MHz resonant PZT5 transducer with two quarter-wave transformers for broadened bandwidth and increased e fficiency. The transducer's output was amplified by a very low noise amplifier and signal averaged by using a signal averager and microcomputer under CAMAC control. The results support the f easibility of thermoacoustic imaging of soft tissue by a transducer-ampli fieraveraging s ystem optimized for this application which might give information complementary to conventional imaging systems.

Thermoacoustic imaging permits the detection of surface and subsurface inhomogeneities in solid samples. In the scanning transmission microscope the periodic heating of the sample is realized by a power modulated laser. The thermally induced elastic waves are detected, on the opposite face of the sample, by an heterodyne laser probe. The noncontact testing system is described and compared to the optical microscope. The images of defects in solid samples are presented.

In transmission thermoacoustic scanning microscopy, the thermally induced bulk waves are detected on the opposite surface of the sample by a piezoelectric transducer or a laser probe. The use of a compact heterodyne interferometric laser probe enables high resolution and wide frequency range imaging. Such a microscope is described in this paper. Results obtained for inhomogeneous or layered samples are presented and discussed, showing potential applications.

Thermoacoustic waves were induced in vivo and in soft tissue phantoms consisting of layers of muscle tissue phantom gel and vegetable oil by 0.4 microsecond duration electric current pulses. The acoustic signals were detected by a 0.5 MHz transducer after digital signal averaging of typically 10 to 103 waveforms. The observed signals had the amplitude and time delay characteristics expected for thermoacoustic emission. The results support the feasibility of thermoacoustic imaging of soft-tissue by a transducer-amplifier-averaging system optimized for this application which might give information complementary to conventional ultrasonic and x-ray systems.

Thermoacoustic imaging techniques are considered as a means of detecting surface and subsurface thermal inhomogeneities in solids. Localized heating of the sample is usually produced by means of a modulated laser or an electron beam. Thermal waves1,8 or acoustic waves4,9 may be detected in several ways. Thermal waves essentially depend on optical and thermal properties of the material and elastic waves bring out extra information depending on its elastic constants. Scanning of the sample produces a thermal and/or acoustical image of inhomogeneities in the tested material.

The origins of the contrast observed in thermoacoustic imaging of material microstructure has received attention since the first such images were presented by CARGILL [1]. Two types of contrast are typically observed in images showing grain structure in metals — the contrast at the grain boundaries and the contrast between the grain interiors. Other possible sources of contrast in thermoacoustic images of microstructure include second phase structures such as martensite [2] and the presence of plastic deformation [3]. The contrast at the grain boundaries has been shown to be dependent on the thermal diffusion length in the material [4], but there has been much discussion as to whether the origin of the contrast between grain interiors has a thermal or an elastic origin. Observations by DAVIES and HOWIE [5] showed that bright grains (large thermoacoustic signal) in an electron-acoustic image of a Cu-Zn-Al specimen tended to have orientations close to [110] and [111] in the standard stereographic triangle while the dark grains (smaller signal) tended towards a [100] orientation. ROSENCWAIG and OPSAL [6] developed an expression for the ratio of the thermoelastic displacements in the [100] and [110] directions in a cubic single crystal and find that this ratio is dependent on the degree of elastic anisotropy and predict a greater displacement in the [100] direction.

Recent developments in thermal wave or more generally modulated source imaging using laser-, electron-, and ion-beam sources are discussed. Thermal wave imaging is further classified on the basis of the type of detection process used, and recent work relating to contrast mechanisms in the several classes of thermal imaging is discussed.

There has developed over the past few years some controversy [1] over the meaning and use of terms such as “thermal wave imaging” vs. “thermoacoustic imaging” or “Thermal Wave Microscopy” as opposed to “Scanning Electron Acostic Microscopy” (SEAM). The issue in question is the extent of the role played by acoustics directly on the imaging. In the present work, SEAM studies of a prepared test standard are presented which show that macroscopic elastic properties can play an important role in imaging. The SEAM technique uses Coordinate Modulation (CM) of the electron beam [2] rather than the more commonly used intensity modulation via beam blanking. Defects as deep as 26 thermal diffusion lengths in stainless steel have been clearly imaged with this technique. This work strongly supports the Jackson -Amer thermal bending model [3] for low frequency (up to approximately 100kHz in small specimens) acoustic detection in the solid as compared to the theory of Opsal and Rosencwaig [4] which does not incorporate total specimen response to the thermal source. This problem has more recently been theoretically investigated by Favro [5,6], who has developed a more general theory encompasing both the Opsal-Rosencwaig short wavelength limit and the Jackson-Amer long wavelength limit. Favro’s theory also accounts for features in the image shown.

Mammography and ultrasound are commonly used techniques for detection and follow-up of breast cancer. Microwave induced thermoacoustic imaging is a new non-ionizing imaging modality that can facilitate early detection and follow-up of cancer. Here we present development of an experimental microwave induced thermoacoustic imaging system along with preliminary test results.

We present tomographic imaging of biological tissues by use of microwave-induced acoustic signal. It was demonstrated that the acoustic signal was proportional to the intensity of the incident microwave and was related to the absorption property of microwave in the medium. Pulsed microwave radiation was used to illuminate the samples. Absorbed microwave energy caused thermoelastic expansion that radiated acoustic waves. A focused ultrasonic transducer detected the time-resolved acoustic signals. Each acoustic signal was converted into a one-dimensional image. A linear scanning of the ultrasonic transducer yielded multiple one-dimensional images, which formed a two-dimensional image. The imaging contrast is based on the difference in the dielectric constants among biological tissues. Because of the large contrast in microwave absorption among different tissue types, microwave-induced acoustic tomography could potentially provide a new modality for detecting early-stage cancers.

We study the effect of microwave pulse width on the spatial resolution of microwave-induced thermoacoustic imaging for early breast cancer detection. The microwave-induced thermoacoustic effect of the breast tumor under the irradiation of microwave pulse with different duration is simulated. The imaging results reconstructed by employing confocal imaging algorithm verify that the microwave pulse width indeed has an effect on the spatial resolution of microwave-induced thermoacoustics, and the shorter the microwave pulse width the better the spatial resolution. For a 14 mm times 14 mm detection aperture, when the duration of microwave pulse is 0.2 mus, the spatial resolution in 1 mm can be achieved.

Purpose: Thermoacoustic imaging at microwave excitation frequencies is limited by the low differential contrast exhibited by high water content tissues. To overcome this, exogenous thermoacoustic contrast agents based on gadolinium compounds, iron oxide, and single wall carbon nanotubes have previously been suggested and investigated. However, these previous studies did not fully characterize the electric, magnetic, and thermodynamic properties of these agents thus precluding identification of the underlying sources of contrast. To address this, measurements of the complex permittivity, complex permeability, DC conductivity, and Gruneisen parameter have been made. These measurements allowed the origins of the contrast provided by each substance to be identified. Methods: The electric and magnetic properties of the contrast agents were characterized at 3 GHz using two rectangular waveguide cavities. The DC conductivity was measured separately using a conductivity meter. Thermoacoustic signals were then acquired and compared to those generated in water. Finally, 3D electromagnetic simulations were used to decouple the different contributions to the absorbed power density. Results: It was found that the gadolinium compounds provided appreciable electric contrast but not originating from the gadolinium itself. The contrast was either due to dissociation of the gadolinium salt which increased ionic conductivity or its nondissociated polar fraction which increased dielectric polarization loss or a combination of both. In addition, very high concentrations were required to achieve appreciable contrast, to the extent that the Gruneisen parameter increased significantly and became a source of contrast. Iron oxide particles were found to produce low but measurable dielectric contrast due to dielectric polarization loss, but this is attributed to the coating of the particles not the iron oxide. Single wall carbon nanotubes did not provide measurable contrast of any type. Conclusions: It is concluded that gadolinium based contrast agents, iron oxide particles, and single walled carbon nanotubes have little intrinsic merit as thermoacoustic contrast agents. Simple electrolytes such as saline which yield high contrast based on ionic conductivity provide much higher dielectric contrast per unit solute concentration and are likely to be significantly more effective as contrast agents.

The acoustic wave generated by sudden thermal stress is used to obtain information non-invasively on the composition and structure of the stressed body. One or more acoustic transducers (1 and 1') are coupled with the surface of the body to intercept the acoustic wave and generate a corresponding electrical signal. The sudden thermal stress is induced by a pulse of radiation which deposits energy causing a rapid, but very small, rise of temperature. The radiation may be ionizing radiation, such as high energy electrons, photons (X-rays), neutrons, or other charged particles. The radiation may also be non-ionizing radiation, such as RF(2) and microwave electromagnetic radiation (3) and ultrasonic radiation (4). The electrical signal from the acoustic transducer (1) is amplified (5) and supplied to a digitizer (6), which provides a continuous stream of digital words corresponding to samples of the amplified signal. Because in most situations of practical interest the S/N ratio of a single pulse is much less than unity, it is necessary to signal-average the signals from many successive pulses. This is accomplished with a minicomputer or data processor ( 8) suitably interfaced (7) with the digitizer. The resulting data can then be suitably displayed as an image on a CRT display (9) or plotted or numerically printed out.