Calibrated photoacoustic spectrometry with an imaging system

Photoacoustic (PA) contrast agents are usually characterized with spectrophotometry or uncalibrated PA imaging systems, leading to partial assessment of their PA efficacy. To perform calibrated PA spectroscopy with a PA imaging system, we developed a method that both corrects for the spectral energy distribution of excitation light and performs a conversion from arbitrary to spectroscopic units, using a reference solution of cupric sulfate. The method was implemented on an imaging setup based on a tunable laser and a 5MHz ultrasound array. We demonstrated robust calibrated spectroscopy on 15$\mu$L sample volumes of known chromophores and commonly used PA contrast agents, and for multiple samples simultaneously. The detection was linear with the absorption and the sensitivity below 0.08cm-1

Photoacoustic and Optical spectral transformations in Solid Lipid Nanoparticles labelled with increasing concentrations of Photoacoustic NIR BODIPY

Solid lipid nanoparticles labelled with a photoacoustic BODIPY dye exhibit complex but continuous transformation for both optical and photoacoustic spectra as the BODIPY content increases. Gaussian decomposition of the spectra reveals the interplay between three optical absorption bands and unveils a band-dependent and concentration-dependant photoacoustic generation efficiency above 1

Calibrated photoacoustic spectrometer based on a conventional imaging system for in vitro characterization of contrast agents

International audiencePhotoacoustic (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 uncali-brated PA imaging systems, leading to partial assessment of their PA efficiency. To enable quantitative PA spectroscopy of con-trast 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 acces-sible tool for the development of new and efficient PA contrast agents by improving their quantitative characterization

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

Volumetric dual photacoustic (PA) / ultrasonic (US) imaging with precise spatial and temporal coregistration can provide valuable and complementary information for diagnosis and monitoring. Considerable research has sought to combine 3D PA/US imaging in configurations that can be transferred to clinical application but technical compromises currently result in poor image quality either for photoacoustic or ultrasonic modes. Simultaneous 3D PA/US tomography was implemented here by interlacing PA and US acquisitions during the rotate-translate scan of a 5-MHz linear array (12 angles and 30 mm translational range to image a cylindrical volume of 21 mm diameter and 19 mm length within 21 seconds). Volumetric image reconstruction was performed with synthetic aperture approaches. An original calibration method was developed to estimate 6 geometrical parameters and 1 temporal off-set providing sharpest and best superimposed reconstructions. Calibration thread phantom design and choice of metrics to build the cost function were based on analysis of a numerical phantom and the final selection demonstrates a high estimation accuracy of the 7 parameters. Experimental estimations validated the calibration repeatability. Experiments in an additional phantom showed a superposition distance between thread centers identified in the PA and US images to be smaller than 10% of the acoustic wavelength, and a spatial resolution on the order of the wavelength. Dual mode 3D imaging with high-quality co-registration and excellent, uniform spatial resolution was further demonstrated on phantoms with complementary contrasts, and should contribute to more sensitive and robust imaging to detect and follow biological changes or the accumulation of nanoagents in living systems

Fine Tuning of the Photoacoustic Generation Efficiency by Aggregation-Caused Quenching and Excitation Energy Transfer in Bodipy-Labeled Polylactide Nanoparticles

The relationship between the fluorescence decrease by Aggregation-Caused Quenching (ACQ) and the Photoacoustic Generation Efficiency in polylactide-Bodipy nanoparticles (PLA-Bodipy NPs) is demonstrated. PLA NPs with different PLA-Bodipy loading (from 2.5% to 50% by weight) were studied using a calibrated photoacoustic spectrophotometer. We demonstrate the presence of two photoacoustic emission regimes thanks to the determination of the Global PhotoAcoustic Efficiency (GPAE) which represents the averaged Photoacoustic Generation Efficiency (PGE) over the whole studied band (680-870 nm). In the monomer regime, below 10% by weight of PLA-Bodipy, the fluorescence emission from the Bodipy monomer limits the GPAE. Above 10% of PLA-Bodipy, in the ACQ regime, non-radiative deactivations from the aggregates are predominant and GPAE reaches a high value of 93%. We also introduce the photoacoustic brightness BPA, as the produce of the GPAE by the molar extinction coefficient of NPs. When Bodipy is aggregated, high nanoparticular extinction coefficients (2.4 x 108 L.mol-1.cm-1 for NP-50%) and high GPAE values are reached leading to ultrabright NPs (22 x 107 L.mol-1.cm-1). Finally, we show that high laser fluences (1-3.5 mJ.cm-2) can significantly reduce the photoacoustic signal by ground state depopulation at the band maximum excitation (-20% for NP-2.5%). This non-linear effect can be highly reduced with NPs in the ACQ regime. Above 10% PLA-Bodipy, NPs exhibit intense photoacoustic brightness and low signal loss by nonlinear effects. A mathematical fit of the absorption and photoacoustic excitation spectra allowed to introduce the Band PhotoAcoustic Efficiency (BPAE) to spectrally differentiate the averaged photoacoustic efficiency at the band maximum BPAEred and in the vibrational shoulder BPAEblue. The BPAEred highly depends on the laser fluence due to high ground state depopulation, whereas the BPAEblue does not change with the fluence thanks to a lower laser fluence saturation

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 metric 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 colloïdal 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 nanoagent

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

International audienceBiomedical 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