Photoacoustic Imaging in Biomedicine and Life Sciences

Recent advances in the development of multiplexed nanophotonic biosensors

The field of nanophotonics has advanced and can be utilized as a method to detect different infectious diseases. The introduction of multiplex nanophotonic diagnostics has enabled the speedy and simultaneous detection of viral infections and specific biomarkers. The quick reaction times, high sensitivity, and specificity of multiplex nanophotonic diagnostics enable real-time identification of viruses without the need for nucleic acid amplification. This review presents an overview of nanophotonic tools used to identify diseases and particular biomarkers. The paper also examines possible research areas for the development of unique, cutting-edge multiplex nanophotonic diagnostics capable of concurrently detecting various diseases or biomarkers/biomolecules. Furthermore, it discusses barriers to further advancement and offers insight into anticipated trends.

Laser-induced ultrasound in multiple thin layers-An analytical solution

Laser-induced ultrasound is based on the thermo-elastic conversion of absorbed short light pulses to pressure pulses. In the work presented here, laser-induced ultrasound in a planar structure of interconnected layers with variations in optical, thermal, and mechanical properties is studied. Layered structures can be used for generating wideband ultrasonic pulses specific to a chosen application. An analytical time-domain solution is derived for the resulting pressure transmitted from the layered structure. The solution is derived for an arbitrary number of layers with an arbitrary optical absorption profile. Free space Green's functions with image sources are used to derive the solution. A solution employing the Beer-Lambert law is also proposed. The simplification with reflections only at the boundaries is in agreement with previous published results. The spectral properties of the generated pulse are derived, where the effects of optical absorption coefficients and layer thicknesses are shown. The analytical solution is compared to one-dimensional (1D) simulations and a three-dimensional (3D) simulation, realised as a two-dimensional (2D) axially symmetric case, using the matlab toolbox k-Wave. The 3D simulation on-axis pressure agrees well with the 1D analytical solution when the diameter of the laser beam is larger by approximately 1 order of magnitude than the thickness of the planar layered structure.

Use of Photoacoustic Imaging to Study the Effects of Anemia on Placental Oxygen Saturation in Normoxic and Hypoxic Conditions

We aimed to evaluate fetal and placental oxygen saturation (sO2) in anemic and non-anemic pregnant rats throughout gestation using photoacoustic imaging (PAI). Female Sprague-Dawley rats were fed an iron-restricted or iron-replete diet before and during pregnancy. On gestational days 13, 18, and 21, PAI was coupled with high resolution ultrasound to measure oxygenation of the fetus, whole placenta, mesometrial triangle, as well as the maternal and fetal faces of the placenta. PAI was performed in 3D, which allowed sO2 to be measured within an entire region, as well as in 2D, which enabled sO2 measurements in response to a hypoxic event in real time. Both 3D and 2D PAI were performed at varying levels of FiO2 (fraction of inspired oxygen). Iron restriction caused anemia in dams and fetuses, a reduction in fetal body weight, and an increase in placental weight, but overall had minimal effects on sO2. Reductions in FiO2 caused corresponding reductions in sO2 which correlated to the severity of the hypoxic challenge. Regional differences in sO2 were evident within the placenta and between the placenta and fetus. In conclusion, PAI enables non-invasive measurement of sO2 both rapidly and with a high degree of sensitivity. The lack of overt changes in sO2 levels between control and anemic fetuses may suggest reduced oxygen extraction and utilization in the latter group, which could be attributed to compensatory changes in growth and developmental trajectories.

Advances in photoacoustic imaging aided by nano contrast agents: special focus on role of lymphatic system imaging for cancer theranostics

Photoacoustic imaging (PAI) is a successful clinical imaging platform for management of cancer and other health conditions that has seen significant progress in the past decade. However, clinical translation of PAI based methods are still under scrutiny as the imaging quality and clinical information derived from PA images are not on par with other imaging methods. Hence, to improve PAI, exogenous contrast agents, in the form of nanomaterials, are being used to achieve better image with less side effects, lower accumulation, and improved target specificity. Nanomedicine has become inevitable in cancer management, as it contributes at every stage from diagnosis to therapy, surgery, and even in the postoperative care and surveillance for recurrence. Nanocontrast agents for PAI have been developed and are being explored for early and improved cancer diagnosis. The systemic stability and target specificity of the nanomaterials to render its theranostic property depends on various influencing factors such as the administration route and physico-chemical responsiveness. The recent focus in PAI is on targeting the lymphatic system and nodes for cancer diagnosis, as they play a vital role in cancer progression and metastasis. This review aims to discuss the clinical advancements of PAI using nanoparticles as exogenous contrast agents for cancer theranostics with emphasis on PAI of lymphatic system for diagnosis, cancer progression, metastasis, PAI guided tumor resection, and finally PAI guided drug delivery.

Photoacoustic Spectral Response using Ultrasound and Interferometric Sensors: A Correlation Study for a High Bandwidth Real-Time Blood Vasculature Monitoring Application in a Chick-Embryo Chorioallantoic Membrane (CAM) Model

Photoacoustic (PA) spectral response technique has shown good promise in efficient preclinical tissue diagnosis by depicting mechano-biological properties due to high spatial resolution and penetration depth. The conventional PA-based system is a pump-probe technique that utilizes neodymium-doped yttrium aluminum garnet pulsed laser as a pump and an ultrasound sensor as a probe. For biomedical studies, high-speed PA signals need to be acquired, requiring higher bandwidth ultrasound sensors. While the bandwidth increases, they exhibit a very low signal-to-noise ratio that inhibits acquiring PA signals of biomedical samples. An interferometer-based probe has recently been investigated as a potential ultrasound probe for obtaining PA signals as an alternative. This optical PA detection technique offers high sensitivity by combining low acoustic impedance with high electromechanical coupling. However, there is a lack of exploration of the same for real-time biomedical studies. This work shows the development of a homodyne Mach-Zehnder interferometer-based PA spectral response (PASR) followed by a correlation study between the conventional ultrasound sensor and the interferometer-based sensor. Further, this study demonstrates the capability of continuous monitoring of vascular growth and the effect of an antidrug (Cisplatin) on the vasculature tested on a chick-embryo chorioallantoic membrane model. PASR was able to monitor growth changes within one day, which was not possible with conventional methods. This opens up potential possibilities for using this technique in biomedical applications.

In vivo photothermal therapy monitored by multi-position calibrated photoacoustic thermometer

With the ability of monitoring both temperature and photothermal agents, the photoacoustic (PA) imaging is a promising guiding tool for the photothermal therapy (PTT). The calibration line which depicts the relative variation of PA amplitude with the temperature should be obtained before using PA thermometer. In existing study, a calibration line was generated based on the data from one spatial position, and used in the whole region of interesting (ROI). However, the generalization of this calibration line in ROI was not verified, especially for ROI with heterogeneous tissues. Moreover, the relationship between the distributions of photothermal agents and effective treatment area is not clear, hindering using photothermal agents' distribution to optimize the administration-therapy interval. In this study, the distribution of effective photothermal agents and temperature in subcutaneously transplanted tumor mouse models were continuously monitored by 3D photoacoustic/ ultrasonic dual-modality imaging in 8 h after administration. With multiple micro-temperature probes in tumor and surrounding normal tissue, the PA thermometer was calibrated and evaluated at multiple spatial positions for the first time. The generalization in homologous tissue and tissue specificity in heterogeneous tissues of the PA thermometer calibration line were verified. Our study not only validated the effectivity of PA thermometer by proving the generalization of calibration line, but also removes a major obstacle that prevents applying the PA thermometer to a heterogeneous tissues ROI. The positive correlation between the proportion of effective treatment area and the proportion of effective photothermal agent area in the tumor was observed. Since the latter can be monitored with fast PA imaging, PA imaging can be employed as a convenient tool for seeking optimal administration-treatment interval.

Photoacoustic imaging of hidden dental caries using visible-light diode laser

BACKGROUND

Hidden caries is a type of tooth decay that is difficult to identify through visual diagnosis because teeth with hidden caries appear normal on the tooth surface but are damaged underneath.

METHODS

A photoacoustic imaging system based on visible light using a diode laser with a wavelength of 532 nm was developed to detect hidden caries in teeth.

RESULTS

The results indicate that the average of acoustic intensity level for healthy teeth is -74.2 ± 0.1 dB, and the average of acoustic intensity range for teeth with hidden caries is -81.2 ± 0.5 dB. The intensity level for the caries area varies depending on the severity of caries.

CONCLUSION

Based on the acoustic intensity level measured by the interaction of teeth with laser light, the photoacoustic imaging system in the study can accurately detect the presence of hidden caries and recognize the difference between caries teeth and healthy teeth. This research can be developed into a prototype of a simple device that makes it easy to operate in dental practice.

Confocal Raman Micro-Spectroscopy for Discrimination of Glycerol Diffusivity in Ex Vivo Porcine Dura Mater

Dura mater (DM) is a connective tissue with dense collagen, which is a protective membrane surrounding the human brain. The optical clearing (OC) method was used to make DM more transparent, thereby allowing to increase in-depth investigation by confocal Raman micro-spectroscopy and estimate the diffusivity of 50% glycerol and water migration. Glycerol concentration was obtained, and the diffusion coefficient was calculated, which ranged from 9.6 × 10^-6 to 3.0 × 10^-5 cm²/s. Collagen-related Raman band intensities were significantly increased for all depths from 50 to 200 µm after treatment. In addition, the changes in water content during OC showed that 50% glycerol induces tissue dehydration. Weakly and strongly bound water types were found to be most concentrated, playing a major role in the glycerol-induced water flux and OC. Results show that OC is an efficient method for controlling the DM optical properties, thereby enhancing the in-depth probing for laser therapy and diagnostics of the brain. DM is a comparable to various collagen-containing tissues and organs, such as sclera of eyes and skin dermis.

Polyethylene glycol-modified nanoscale conjugated polymer for the photothermal therapy of lung cancer

Killing tumor cells efficiently with photothermal therapy remains a huge challenge. In this study, we successfully prepared a novel polymer with photothermal conversion capability via a condensation reaction, and then subjected it to Polyethylene glycol (PEG) modification and ultrasonic nanocrystalline treatment to make it suitable forin vivophotothermal therapy applications. The conjugated polymer demonstrated good biocompatibility and photothermal conversion ability and was shown in cell experiments to be effective in killing tumor cells after laser irradiation. In addition, the conjugated polymer-based photothermal therapy, guided by photoacoustic real-time imaging and mediated by laser irradiation, of a tumor-bearing mouse model could effectively inhibit the growth of tumor tissue and demonstrated goodin vivobiosafety. Thus, photothermal therapy based on the conjugated polymer synthesized in this study provides a new idea and strategy for the treatment of lung cancer.