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

BACKGROUND

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Thermoacoustic Imaging-Guided Thermo-Chemotherapy for Hepatocellular Carcinoma Sensitized by a Microwave-Responsive Nitric Oxide Nanogenerator

Imaging-guided percutaneous microwave thermotherapy has been regarded as an important alternative nonsurgical therapeutic strategy for hepatocellular carcinoma (HCC) that provides excellent local tumor control and favorable survival benefit. However, providing a high-resolution, real-time, and noninvasive imaging technique for intraoperative guidance and controlling postoperative residual tumor recurrence are urgent needs for the clinical setting. In this study, a cisplatin (CDDP)-loaded nanocapsule (NPs@CDDP) with microwave responsive property was prepared to simultaneously serve as a contrast agent of emerging thermoacoustic imaging and a sensitizing agent of microwave thermo-chemotherapy. Accompanying the enzymolysis in the tumor microenvironment, the NPs@CDDP responsively release l-arginine (l-Arg) and CDDP. l-Arg with excellent microwave-absorbing property allowed it to serve as a thermoacoustic imaging contrast agent for accurately delineating the tumor and remarkably increasing tumor temperature under ultralow power microwave irradiation. Apart from the chemotherapeutic effect, CDDP elevated the intracellular H₂O₂ level through cascade reactions and further accelerated the continuous transformation of l-Arg to nitric oxide (NO), which endowed the NPs@CDDP with NO-generation capability. Notably, the high concentration of intracellular NO was proved to aggravate lipid peroxidation and greatly improved the efficacy of microwave thermo-chemotherapy. Thereby, NPs@CDDP was expected to serve as a theranostic agent integrating the functions of tumor microenvironment-responsive drug delivery system, contrast agent of thermoacoustic imaging, thermal sensitizing agent, and NO nanogenerator, which was promising to provide a potential imaging-guided therapeutic strategy for HCC.

Technical Note: Compact thermoacoustic imaging system based on a low-cost and miniaturized microwave generator for in vivo biomedical imaging

PURPOSE

Most of existing thermoacoustic imaging (TAI) studies generally utilized a linear modulator-based high peak power magnetron generator (MG) for efficient TA signal excitation. However, a linear modulator-based MG is bulky and expensive. Here we present a low-cost and compact thermoacoustic imaging (TAI) system based on a miniaturized MG.

METHODS

The MG is based on solid-state modulator and operates at 3.05 ± 0.025 GHz, with a peak power of up to 60 kW and adjustable pulse duration from 70 to 600 ns. The dimensions and weight of this MG are 350 × 210 × 70 mm³ (Width × Length × Height) and 7.5 kg, respectively.

RESULTS

The spatial resolution of the miniaturized MG-based TAI system is determined to be from 0.3 to 1.4 mm using controlled phantom experiments. The system is further evaluated using in vivo experiments where the finger joints and vasculature in the forearm and opisthenar of human participants are successfully imaged.

CONCLUSIONS

This study suggests that the miniaturized MG based TAI systems can be used for in vivo joint and vascular imaging with multiscale resolutions (0.3-1.4 mm).

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.

Pancreatic Cancer detection via Galectin-1-targeted Thermoacoustic Imaging: validation in an in vivo heterozygosity model

Purpose: To investigate the feasibility of microwave-induced thermoacoustic imaging (MTAI) in detecting small pancreatic tumors (< 10 mm in diameter) and to complement the limitation of current clinical imaging methods.

Methods: A home-made MTAI system composed of a portable antenna and pulsed microwave generator was developed. The thermoacoustic nanoparticles were composed of the galectin-1 antibody for targeting pancreatic tumors and Fe₃O₄ nanoparticles as microwave absorbers (anti-Gal1-Fe₃O₄ nanoparticles). The microwave absorption properties of the nanoparticles were measured with a vector network analyzer and the resolving power of MTAI was investigated by imaging excised pancreatic tumors of different sizes (diameters of 1.0 mm, 3.1 mm, 5.0 mm, 7.2 mm). To simulate actual imaging scenarios, an in vivo heterozygosity model was constructed by covering the pancreatic tumors (~ 3 mm in diameter) in BALB/c nude mice with biologic tissue (~ 5 cm in depth). MTAI images of the heterozygosity model were acquired with/without the injection of the anti-Gal1-Fe₃O₄ nanoparticles and the thermoacoustic contrast from pancreatic tumors was evaluated with Student's paired t test. The data were analyzed with analysis of variance and nonparametric statistics.

Results: Following intravenous infusion, anti-Gal1-Fe₃O₄ nanoparticles efficiently accumulated in the tumor. The MTAI contrast enhancement in pancreatic tumors with anti-Gal1-Fe₃O₄ nanoparticles was verified in vitro and in vivo. The pancreatic tumors were visible in nude mice examined with MTAI with a mean contrast enhancement ratio of 2.3 ± 0.15 (standard error of the mean) (P =. 001) at 6 h post-injection of the nanoparticles. MTAI identified tiny pancreatic tumors in deep tissues with high fidelity.

Conclusion: MTAI offers deep imaging depth and high contrast when used with anti-Gal1-Fe₃O₄ nanoparticles. It can identify pancreatic tumors smaller than 5 mm, which is beyond the identification limit size (~10 mm) of other nondestructive clinical imaging methods. Thus, MTAI has great potential as an alternative imaging modality for early pancreatic cancer detection.

[Thermoacoustic imaging and its application in breast cancer detection and therapy]

Thermoacoustic imaging (TAI) is a new non-invasive, non-ionization and nondestructive modality capable of high microwave contrast and high ultrasound resolution, and it has attracted extensive attention in recent years. This review introduces the technical principle, imaging system and imaging characteristics of TAI, and then introduces the application of TAI for breast cancer detection as an example. This review introduces the advantages of TAI in solving corresponding clinical problems in view of its high resolution and high contrast. In addition, it also explains the roles of TAI in medical diagnosis and treatment. Finally, the potential applications of TAI in medical diagnosis is introduced from many aspects and multiple perspectives. The future development of TAI in the challenges of current medical diagnosis is also prospected.

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.

Pulsed Microwave-Pumped Drug-Free Thermoacoustic Therapy by Highly Biocompatible and Safe Metabolic Polyarginine Probes

Serious side effects are plaguing traditional chemotherapy, and the development of drug-free treatment is expected to ease the dilemma. Herein, drug-free polyarginine probes are fabricated from the co-polymerization of arginine monomer and slight amount of rhodamine B monomer, which are efficient for thermoacoustic imaging and therapy with high biocompatibility and safe metabolism. Polyarginine can be strongly pumped upon pulsed microwave irradiation, generating significant thermoacoustic shockwaves, namely thermocavitation, which can in situ destroy mitochondria to initiate programmed cancer cell apoptosis. In vivo explorations demonstrate the high theranostic efficiency for cancer thermoacoustic imaging and cancer inhibition, exhibiting low systemic cytotoxicity and good biocompatibility after systemic administration. Herein, pulsed microwave-pumped biocompatible polyarginine is promising for drug-free precision theranostics without any detectable side effects, and the deep penetration potency of microwave makes it potentially able to treat deep-seated diseases in future biomedicine.

Thermo-Acoustic Ultrasound for Detection of RF-Induced Device Lead Heating in MRI

Patients who have implanted medical devices with long conductive leads are often restricted from receiving MRI scans due to the danger of RF-induced heating near the lead tips. Phantom studies have shown that this heating varies significantly on a case-by-case basis, indicating that many patients with implanted devices can receive clinically useful MRI scans without harm. However, the difficulty of predicting RF-induced lead tip heating prior to scanning prevents numerous implant recipients from being scanned. Here, we demonstrate that thermo-acoustic ultrasound (TAUS) has the potential to be utilized for a pre-scan procedure assessing the risk of RF-induced lead tip heating in MRI. A system was developed to detect TAUS signals by four different TAUS acquisition methods. We then integrated this system with an MRI scanner and detected a peak in RF power absorption near the tip of a model lead when transmitting from the scanner's body coil. We also developed and experimentally validated simulations to characterize the thermo-acoustic signal generated near lead tips. These results indicate that TAUS is a promising method for assessing RF implant safety, and with further development, a TAUS pre-scan could allow many more patients to have access to MRI scans of significant clinical value.

Handheld Thermoacoustic Scanning System Based on a Linear-array Transducer

To receive the information necessary for imaging, traditional microwave-induced thermoacoustic imaging systems (MITISs) use a type of circular-scanning mode using single or arc detectors. However, the use of MITISs for body scanning is complicated by restrictions in space and imaging time. A linear-array detector, the most widely used transducer in medical ultrasound imaging systems for body scanning, is a possible alternative to MITISs for scanning biological tissues, such as from the breast or limbs. In this paper, a handheld MITIS, based on a linear-array detector and a multiple data acquisition system, is described, and the capacity of the system is explored experimentally. First, the vertical and lateral resolution of the system is discussed. Next, real-time imaging of a moving object, obtained with an image capture rate of 20 frame/s, is described. Finally, a phantom experiment is detailed, investigating the overall imaging capability. The results show that this system achieves rapid scanning with a large field of view. The system has the obvious advantages of being handheld, not using coupled fluids, and achieving real-time imaging with a large field of view, which make this MITIS more suitable for clinical applications.

Remote temperature estimation in intravascular photoacoustic imaging

Intravascular photoacoustic (IVPA) imaging is based on the detection of laser-induced acoustic waves generated within the arterial tissue under pulsed laser irradiation. In general, laser radiant energy levels are kept low (20 mJ/cm(2)) during photoacoustic imaging to conform to general standards for safe use of lasers on biological tissues. However, safety standards in intravascular photoacoustic imaging are not yet fully established. Consequently, monitoring spatio-temporal temperature changes associated with laser-tissue interaction is important to address thermal safety of IVPA imaging. In this study we utilize the IVUS-based strain measurements to estimate the laser-induced temperature increase. Temporal changes in temperature were estimated in a phantom modeling a vessel with an inclusion. A cross-correlation-based time delay estimator was used to assess temperature-induced strains produced by different laser radiant energies. The IVUS-based remote measurements revealed temperature increases of 0.7+/-0.3 degrees C, 2.9+/-0.2 degrees C and 5.0+/-0.2 degrees C, for the laser radiant energies of 30 mJ/cm(2), 60 mJ/cm(2) and 85 mJ/cm(2), respectively. The technique was then used in imaging of ex vivo samples of a normal rabbit aorta. For arterial tissues, a temperature elevation of 1.1 degrees C was observed for a laser fluence of 60 mJ/cm(2) and lesser than 1 degrees C for lower energy levels normally associated with IVPA imaging. Therefore, the developed ultrasound technique can be used to monitor temperature during IVPA imaging. Furthermore, the analysis based on the Arrhenius thermal damage model indicates no thermal injury in the arterial tissue, suggesting the safety of IVPA imaging.