Adaptive complementary neighboring sub-aperture beamforming for thermoacoustic imaging

BACKGROUND

When applied to thermoacoustic imaging (TAI), the delay-and-sum (DAS) algorithm produces strong sidelobes due to its disadvantages of uniform aperture weighting. As a result, the quality of TAI images recovered by DAS is often severely degraded by strong non-coherent clutter, which restricts the development and application of TAI.

PURPOSE

To address this issue, we propose an adaptive complementary neighboring sub-aperture (NSA) beamforming algorithm for TAI.

METHODS

In NSA, we introduce a coordinate system transformation when calculating the normalized cross-correlation (NCC) matrix. This approach enables the computation of the NCC coefficient within the specified kernel without complex coordinate calculations. We first conducted the numerical simulation experiment to validate NSA using a tree branch phantom. In addition, we also conducted phantom (five sauce tubes), ex vivo (ablation needle in ex vivo porcine liver), and in vivo (human arm) TAI experiments using our TAI system with a center frequency of 3 GHz.

RESULTS

In the numerical simulation experiment, the structural similarity index (SSIM) value for NSA is increased from 0.37828 for DAS to 0.75492. In the point target phantom TAI experiment, the generalized contrast-to-noise ratio (gCNR) value for NSA is increased from 0.936 for DAS to 0.962. The experimental results show that NSA can recover clearer thermoacoustic images compared to DAS. In the ex vivo TAI experiment, the full width at half maxima (FWHM) of an ablation needle (diameter = 1.5 mm) for coherence factor (CF) weighted DAS and NSA are 0.9 and 1.3 mm, respectively. Furthermore, in the in vivo TAI experiment, CF reduces the signals within the arm compared to NSA. Therefore, compared with CF, NSA can maintain the integrity of target information in TAI while effectively suppressing non-coherent background clutter.

CONCLUSIONS

NSA can effectively reduce non-coherent background noise while ensuring the completeness of the target information. So, NSA offers the potential to provide high-quality thermoacoustic images and further advance their clinical application.

A one-step method for quantitative microwave-induced thermoacoustic tomography

BACKGROUND

Electrical conductivity directly correlates with tissue functional information such as blood and water contents, and quantitative extraction of tissue conductivity is of significant importance for disease detection and diagnosis using microwave-induced thermoacoustic tomography (TAT).

OBJECTIVE

The existing quantitative TAT (qTAT) approaches capable of extracting tissue conductivity require two steps for the recovery of conductivity. Such two steps approaches depend on an accurate knowledge of the microwave energy loss distribution in tissue and offer a slow computational convergence rate. The purpose of this study is to develop a new algorithm to reconstruct tissue conductivity with higher reconstruction accuracy and greater computational efficiency.

METHODS

We propose an improved qTAT method for direct recovery of tissue conductivity from thermoacoustic data measured along the boundary with only one step without the dependence of microwave energy loss information. The feasibility of our one-step qTAT method is validated in both simulated and tissue-mimicking phantom experiments with single-target and multi-target configurations with different contrast levels.

RESULTS

Compared with the previous two-step methods, our one-step qTAT method improves the accuracy of conductivity recovery with approximately one-fold reduction in the mean absolute error (MAE) and root mean square error (RMSE) with p-values greater than 0.05. In addition, the convergence rate is improved by more than two folds for the one-step method.

CONCLUSIONS

The study demonstrates that new method can quantitatively reconstruct conductivity of tissue more accurately and efficiently over the existing qTAT methods, leading to potentially enhanced accuracy for disease detection and diagnosis.

Developing a near-infrared spectroscopy and microwave-induced thermoacoustic tomography-based dual-modality imaging system

Near-infrared spectroscopy (NIRS) techniques can provide noninvasive in vivo hemoglobin oxygenation information but suffer from relatively low resolution in biological tissue imaging. Microwave-induced thermoacoustic tomography (TAT) can produce high-resolution images of the biological tissue anatomy but offer limited physiological information of samples because of the single species of the chromophore it maps. To overcome these drawbacks and take advantage of the merits of the two independent techniques, we built a dual-modality system by combining a NIRS system and a TAT system to image biological tissues. A series of phantom trials were carried out to demonstrate the performance of the new system. The spatial resolution is about 1 mm, with a penetration depth of at least 17.5 mm in the human subject. A cohort of five healthy subjects was recruited to conduct real-time forearm venous and arterial cuff occlusion experiments. Numerous results showed that this dual-modality system could measure oxygen metabolism and simultaneously provide anatomical structure changes of biological tissues. We also found that although the hemoglobin concentration varied consistently with many other published papers, the TAT signal intensity of veins showed an opposite variation tendency in the venous occlusion stage compared with other existing work. A detailed explanation is given to account for the discrepancy, thus, providing another possibility for the forearm experiments using TAT. Furthermore, based on the multiple types of information afforded by this dual-modality system, a pilot clinical application for the diagnosis of anemia is discussed.

Integrated thermoacoustic and ultrasound imaging based on the combination of a hollow concave transducer array and a linear transducer array

To integrate the high resolution of ultrasound imaging (UI) and the high tissue specificity of thermoacoustic imaging (TAI) and to achieve an easy and precise co-registration of the two different imaging modalities, we present and demonstrate a hybrid thermoacoustic and ultrasound (TA/US) imaging system based on the combination of a novel hollow concave array and a commercial linear array. This TA/US imaging system can provide enhanced imaging of both tissues' mechanical and dielectric properties. We verified the effective imaging performance of the hybrid TA/US system using tissue phantom experiments.In vivoTA/US imaging of the wrist and foot in healthy volunteers was also demonstrated using the hybrid system. This hollow concave array provided enhanced imaging performance for TAI because of its wide angular coverage with an optimal center frequency, showing a large effective imaging field of view (FOV) and improved images with high contrast and superior quality. Compared with stand-alone UI or TAI, the hybrid TA/US imaging presented more complete tissue anatomical structures, like skin, muscles, tendons, blood vessels, and bones for possible human disease diagnosis, although the US image quality using the hybrid system was slightly lower because the distance between the tissue and commercial ultrasound array was not ideal. This study suggests that the hybrid TA/US imaging approach has the potential to become a clinical tool for diagnosis of diseases in the wrist and foot.

A stimulated liquid-gas phase transition nanoprobe dedicated to enhance the microwave thermoacoustic imaging contrast of breast tumors

Microwave-induced thermoacoustic imaging (MTAI), combining the advantages of the high contrast of microwave imaging and the high resolution of ultrasonic imaging, is a potential candidate for breast tumor detection. MTAI probes have been used to extend thermoacoustic imaging to molecular imaging. However, due to the high content of water molecules in tissues, the thermoelastic expansion-based probes used in conventional MTAI are not capable of adequate enhancement. Herein, an MTAI nanoprobe for amplification of thermoacoustic (TA) signals by the stimulated liquid-gas phase transition mechanism has been developed, providing significantly higher signal amplitude than that from the conventional mechanism of thermoelastic expansion. The nanoprobe consists of liquid perfluorohexane (PFH) and tungsten disulfide (WS2) nanoparticles rich in defect electric dipoles. When irradiated with pulsed microwaves, the defect electric dipoles in WS2 were repeatedly polarized by gigahertz. This results in localized transient heating and an acoustic shockwave, which destroys the van der Waals forces between PFH molecules. Ultimately, liquid PFH droplets undergo a liquid-gas phase transition, generating dramatically enhanced TA signals. The practical feasibility was tested in vitro and in a breast tumor animal model. The results show that the proposed nanoprobe can greatly improve the contrast of tumor imaging. It will be a new generation probe for MTAI.

An improved method for quantitative recovery of conductivity using tomographically measured thermoacoustic data

Noninvasive extraction of tissue conductivity distribution is important in brain imaging and cancer detection. Here we present an improved method that can accurately image tissue conductivity using tomographically measured microwave-induced thermoacoustic data. Our reconstruction algorithm is first tested using simulations, and then validated using tissue phantom experiments where saline-containing tubes are used as target(s) with various target sizes, positions and conductivities. The average error of reconstruction for the simulations is reduced from 4.87% to 1.38% compared with the previous algorithm. The experimental results obtained suggest that accurate quantitative thermoacoustic imaging would provide a potential tool for precision medicine.

Thermoacoustic tomography of germinal matrix hemorrhage in neonatal mouse cerebrum

BACKGROUND

Microwave-induced thermoacoustic tomography (TAT) has potential for detecting germinal matrix hemorrhage (GMH). However, it has not been demonstrated in vivo.

OBJECTIVE

To demonstrate the feasibility of TAT for in vivo detecting GMH by using neonatal mouse.

METHODS

A cylindrical-scanning TAT system was developed with optimized microwave irradiation and ultrasound detection for neonatal mouse imaging. Neonatal mice were used to develop GMH model by injection of autologous blood into the periventricular region. After TAT experiments, the animals were sacrificed, frozen and excised to validate the TAT findings. The detailed comparative analyses of the TAT images and corresponding photographs of the excised brain tissues were conducted.

RESULTS

Satisfactory matches are identified between the TAT images and corresponding histological sections, in terms of the shape and size of the brain tissues. Some organs and tissues were also identified. Particularly, comparing to the corresponding histological sections, using TAT enables to more accurately detect the hematoma region at different depths in the neonatal mouse brain.

CONCLUSIONS

This study demonstrates for the first time that TAT can detect GMH in neonatal mouse cerebrum in vivo. This represents the first important step towards the in vivo diagnosis and grading of hemorrhage in the infant human brain.

Thermoacoustic elastography: recovery of bulk elastic modulus of heterogeneous media using tomographically measured thermoacoustic measurements

Background

Tissue mechanical parameters such as elasticity are of great significance for the assessment of biological histopathological and physiological characteristics. Here, we propose a new approach called thermoacoustic elastography (TAE) for imaging tissue elastic modulus.

Methods

Central to TAE is an image reconstruction algorithm that allows the recovery of both microwave energy loss and elastic modulus distributions. The algorithm is first evaluated using simulated data under various practical scenarios, including a varied range of microwave energy loss and elastic modulus between the heterogeneity and background region, different noise levels, and multiple targets. The feasibility of the proposed TAE was then validated by imaging the elastic modulus distribution of agar phantoms with various elastic modulus and microwave energy loss.

Results

The results from both the simulated and phantom experiments show that the recovered elastic modulus by TAE agree well with the exact values, having an average error of less than 12.74%.

Conclusions

The findings from this study suggest that TAE provides a new addition to the family of elasticity imaging and may have broad application prospects, such as cirrhosis and atherosclerosis detection.

Thermoacoustic Tomography of In Vivo Human Finger Joints

OBJECTIVE

the purpose of this study was to demonstrate the potential of thermoacoustic tomography (TAT) to reveal anatomic structures of in vivo human finger joints.

METHODS

all the participating volunteers provided written informed consent. Eight healthy middle and index fingers from five volunteers were imaged in vivo by our TAT imaging system. Axial T1-weighted MR imaging (3.0 T) was used to validate the TAT findings. Comparative analyses between TAT and MRI images were performed in two dimension for all the fingers imaged. Three-dimensional (3-D) images and animations were also obtained for some of the fingers thermoacoustically scanned.

RESULTS

various intra- and extra-articular tissues were identified in TAT images in high fidelity. These TAT images matched well with the MRI images. Both the 3-D images and animations effectively displayed the extension and transformation of the entire finger along the axis.

CONCLUSION

TAT can noninvasively visualize anatomic structures of the finger joints based on the electrical properties of the joint tissues. The results obtained indicate that TAT may have the potential to contribute to the detection of joint and bone diseases.

SIGNIFICANCE

this study represents the first for TAT of in vivo human joints and fingers. This study reveals that TAT can effectively recover both soft and hard tissues of the healthy interphalangeal joints, which provides a foundation for its clinical application to detection and diagnosis of joint and bone diseases.

In vivo tumor detection with combined MR–Photoacoustic-Thermoacoustic imaging

Here, we report a new method using combined magnetic resonance (MR)–Photoacoustic (PA)–Thermoacoustic (TA) imaging techniques, and demonstrate its unique ability for in vivo cancer detection using tumor-bearing mice. Circular scanning TA and PA imaging systems were used to recover the dielectric and optical property distributions of three colon carcinoma bearing mice While a 7.0-T magnetic resonance imaging (MRI) unit with a mouse body volume coil was utilized for high resolution structural imaging of the same mice. Three plastic tubes filled with soybean sauce were used as fiducial markers for the co-registration of MR, PA and TA images. The resulting fused images provided both enhanced tumor margin and contrast relative to the surrounding normal tissues. In particular, some finger-like protrusions extending into the surrounding tissues were revealed in the MR/TA infused images. These results show that the tissue functional optical and dielectric properties provided by PA and TA images along with the anatomical structure by MRI in one picture make accurate tumor identification easier. This combined MR–PA–TA-imaging strategy has the potential to offer a clinically useful triple-modality tool for accurate cancer detection and for intraoperative surgical navigation.

Toward Quantitative Whole Organ Thermoacoustics With a Clinical Array Plus One Very Low-Frequency Channel Applied to Prostate Cancer Imaging

Thermoacoustics has the potential to provide quantitative images of intrinsic tissue properties, most notably electrical conductivity in Siemens/meter, much as shear wave elastography provides tissue stiffness in kilopascal. Although thermoacoustic imaging with optical excitation has been commercialized for small animals, it has not yet made the transition to clinic for whole organ imaging in humans. The purpose of this work was to develop and validate specifications for a clinical ultrasound array for quantitative whole organ thermoacoustic imaging. Imaging a large organ requires exciting thermoacoustic pulses throughout the volume and broadband detection of those pulses because tomographic image reconstruction preserves frequency content. Applying the half-wavelength limit to a [Formula: see text] inclusion inside a 7.5-cm diameter organ requires measurement sensitivity to frequencies ranging from 4 MHz to 10 kHz, respectively. A dual-transducer system utilizing a P4-1 array connected to a Verasonics V1 system as well as a focused single-element transducer sensitive to lower frequencies was developed. Very high-frequency (VHF) irradiation generated thermoacoustic pulses throughout a [Formula: see text] volume. In the VHF regime, electrical conductivity drives thermoacoustic signal production. Simultaneous acquisition of thermoacoustic pulses by both transducers enabled comparison of transducer performance. Data from the clinical array generated a stack of 96 images with a separation of 0.3 mm, whereas the single-element transducer imaged only in a single plane. In-plane resolution and quantitative accuracy were quantified at isocenter. The array provided volumetric imaging capability with superior resolution whereas the single-element transducer provided superior quantitative accuracy in axial images. Combining axial images from both transducers preserved resolution of the P4-1 array and improved image contrast. Neither transducer was sensitive to frequencies below 50 kHz, resulting in a dc offset and low-frequency shading over fields of view exceeding 15 mm. Fresh human prostates were imaged ex vivo and volumetric reconstructions reveal structures rarely seen in diagnostic images. In conclusion, quantitative whole-organ thermoacoustic tomography will be feasible by sparsely interspersing transducer elements sensitive to the low end of the ultrasonic range.

Magnetothermoacoustics from magnetic nanoparticles by short bursting or frequency chirped alternating magnetic field: a theoretical feasibility analysis

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 1 μs 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∕cm(3) 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∕cm(3) within the millisecond duration of the chirping of AMF.

CONCLUSIONS

The heat dissipations upon SPION (∼1 mg∕ml iron-weight concentration) by a 1 μs 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.

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.