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

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.

Real-Time Volumetric Thermoacoustic Imaging and Thermometry Using a 1.5-D Ultrasound Array

Noninvasive thermal therapies for the treatment of breast cancer depend on accurate monitoring of tissue temperature to optimize treatment and ensure safety. This work describes a real-time system for 3-D thermoacoustic imaging and thermometry (TAI-TAT) for tracking temperature in tissue samples during heating. The study combines a 2.7-GHz microwave pulse generator with a custom 1.5-D 0.6 MHz ultrasound array for generating and detecting TA signals. The system is tested and validated on slabs of biological tissue and saline gel during heating. Calibration curves for relating the TA signal to temperature were calculated in saline gel (3.40%/°C), muscle (1.73%/°C), and fat (1.15%/°C), respectively. The calibrations were used to produce real-time, volumetric temperature maps at ~3-s intervals with a spatial resolution of approximately 3 mm. TAT temperature changes within a region of interest were compared to adjacent thermocouples with a mean error of 17.3%, 13.2%, and 20.4% for muscle, gel, and fat, respectively. The TAT algorithm was also able to simultaneously track temperatures in different tissues. With further development, noninvasive TAI-TAT may prove to be a valuable method for accurate and real-time feedback during breast cancer ablation therapy.

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.

Increasing dielectric loss of a graphene oxide nanoparticle to enhance the microwave thermoacoustic imaging contrast of breast tumor

Microwave-induced thermoacoustic imaging (MTAI) utilizes the high dielectric contrast of tissues in the microwave range, and, combined with the high resolution of ultrasound imaging, can noninvasively obtain anatomical and functional structure information. However, adequate contrast in certain indications (e.g., breast tumor in their early stages) can be difficult to achieve due to the minimal dielectric difference in the target tissues. A tumor specific targeting nanoparticle with high dielectric loss would enhance MTAI contrast in such applications. In this work, we propose a physics-principle-based technique that increases the dielectric loss of a nanoparticle by increasing its atomic defect to generate an amplified thermoacoustic signal. In principle, bovine serum albumin loaded on graphene oxide (GO) serves as a reductant to create additional vacancies in GO to produce more electric dipoles. Upon pulsed microwave irradiation, the defect dipoles are polarized repeatedly, causing transient heating and thermoelastic expansion, and ultimately generating an amplified acoustic wave. This hypothesis was tested in vitro and in vivo with a breast tumor animal model. The results demonstrate that the nanoparticle can effectively enhance the MTAI contrast of breast tumors. The physics-principle-based technique is likely to contribute to early breast tumor imaging.

Nonlinear Thermoacoustic Imaging Based on Temperature-Dependent Thermoelastic Response

In this paper, a novel nonlinear thermoacoustic imaging (NTAI) system is developed based on the temperature-dependent thermoelastic response under microwave irradiation. Specifically, we consider the high-pulse repetition frequency (HPRF) microwave regime, where the tissue temperature increases after microwave irradiation. In this circumstance, the temperature-dependent thermodynamic parameters of the tissue change; thus, the influence of absorbed microwave energy on the thermoacoustic (TA) amplitude is reflected in the change of thermodynamic parameters. Hence, this temperature dependence yields a nonlinear relationship between the TA amplitude and the absorbed microwave energy in the HPRF regime. In this paper, we obtain the nonlinear coefficient as a new parameter to distinguish biological tissues by performing differential measurements in the NTAI system. We further experimentally extract the nonlinear coefficient in phantom and ex vivo mouse, revealing the feasibility of applications in biological tissues identification with high contrast.

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.