Quantitative, precise and multi-wavelength evaluation of the light-to-heat conversion efficiency for nanoparticular photothermal agents with calibrated photoacoustic spectroscopy.

Biomedical photothermal therapy with optical nanoparticles is based on the conversion of optical energy into heat through three steps: optical absorption, thermal conversion of the absorbed energy and heat transfer to the surrounding medium. The light-to-heat conversion efficiency (LHCE) has become one of the main metrics to quantitatively characterize the last two steps and evaluate the merit of nanoparticules for photothermal therapy. The estimation of the LHCE is mostly performed by monitoring the temperature evolution of a solution under laser irradiation. However, this estimation strongly depends on the experimental set-up and the heat balance model used. We demonstrate here, theoretically and experimentally, that the LHCE at multiple wavelengths can be efficiently and directly determined, without the use of models, by calibrated photoacoustic spectroscopy. The method was validated using already characterized colloidal suspensions of silver sulfide nanoparticles and maghemite nanoflowers and an uncertainty of 3 to 7% was estimated for the LHCE determination. Photoacoustic spectroscopy provides a new, precise and robust method of analysis of the photothermal capabilities of aqueous solutions of nanoagents.

Volumetric and Simultaneous Photoacoustic and Ultrasound Imaging With a Conventional Linear Array in a Multiview Scanning Scheme.

Volumetric, multimodal imaging with precise spatial and temporal coregistration can provide valuable and complementary information for diagnosis and monitoring. Considerable research has sought to combine 3-D photoacoustic (PA) and ultrasound (US) imaging in clinically translatable configurations; however, technical compromises currently result in poor image quality either for PA or ultrasonic modes. This work aims to provide translatable, high-quality, simultaneously coregistered dual-mode PA/US 3-D tomography. Volumetric imaging based on a synthetic aperture approach was implemented by interlacing PA and US acquisitions during a rotate-translate scan with a 5-MHz linear array (12 angles and 30-mm translation to image a 21-mm diameter, 19 mm long cylindrical volume within 21 s). For coregistration, an original calibration method using a specifically designed thread phantom was developed to estimate six geometrical parameters and one temporal offset through global optimization of the reconstructed sharpness and superposition of calibration phantom structures. Phantom design and cost function metrics were selected based on analysis of a numerical phantom and resulted in a high estimation accuracy for the seven parameters. Experimental estimations validated the calibration repeatability. The estimated parameters were used for the bimodal reconstruction of additional phantoms with either identical or distinct spatial distributions of US and PA contrasts. The superposition distance of the two modes was within < 10% of the acoustic wavelength, and a wavelength-order uniform spatial resolution was obtained. This dual-mode PA/US tomography should contribute to more sensitive and robust detection and follow-up of biological changes or the monitoring of slower-kinetic phenomena in living systems such as the accumulation of nanoagents.

Evaluation of relative efficiency of PDT photosensitizers in producing hydroxyl radicals and singlet oxygen in aqueous media using a UV-visible spectroscopy pNDA dosage.

In order to improve the performance of PDT, it is important to develop new photosensitizers that induce the formation of both hydroxyl radicals and singlet oxygen. In this work, we developed and validated the experimental conditions and reproducibility for the evaluation of relative efficiency of hydroxyl radicals and singlet oxygen production by studying the bleaching of p-nitrosoaniline (pNDA) using a continuous flow UV-visible spectroscopy method in presence of photosensitizers in PBS media. Rapid data sampling made possible to analyze the kinetics of the bleaching by using a mathematical modeling. The pNDA dosage is specific of hydroxyl radicals' production without l-histidine and of singlet oxygen production in presence of l-histidine. A statistical approach is used to precisely evaluate the reliability of the results and to be able to compare different photosensitizers between them such as Methylene Blue and Brillant Blue G.

Calibrated Photoacoustic Spectrometer Based on a Conventional Imaging System for In Vitro Characterization of Contrast Agents.

Photoacoustic (PA) imaging systems are spreading in the biomedical community, and the development of new PA contrast agents is an active area of research. However, PA contrast agents are usually characterized with spectrophotometry or uncalibrated PA imaging systems, leading to partial assessment of their PA efficiency. To enable quantitative PA spectroscopy of contrast agents in vitro with conventional PA imaging systems, we have developed an adapted calibration method. Contrast agents in solution are injected in a dedicated non-scattering tube phantom imaged at different optical wavelengths. The calibration method uses a reference solution of cupric sulfate to simultaneously correct for the spectral energy distribution of excitation light at the tube location and perform a conversion of the tube amplitude in the image from arbitrary to spectroscopic units. The method does not require any precise alignment and provides quantitative PA spectra, even with non-uniform illumination and ultrasound sensitivity. It was implemented on a conventional imaging setup based on a tunable laser operating between 680 nm and 980 nm and a 5 MHz clinical ultrasound array. We demonstrated robust calibrated PA spectroscopy with sample volumes as low as 15 µL of known chromophores and commonly used contrast agents. The validated method will be an essential and accessible tool for the development of new and efficient PA contrast agents by improving their quantitative characterization.