Involved researchers: S. Lori BRIDAL and Jérôme GATEAU

Photoacoustic imaging provides molecular contrast based on the detection of ultrasonic waves generated through optical absorption. This biomedical imaging modality has been rapidly developing since the 2000s. It provides complementary sensitivity to ultrasound imaging through its sensitivity to colored molecules or nanoparticles, and benefits from the sub-millimeter spatial resolution of ultrasound detection.

We are particularly interested in two types of colored agents. First, hemoglobin, which gives blood its red color and whose color varies as a function of the blood oxygenation level. Local oxygenation mapping can provide information about tissue oxygen consumption. This information, which was previously difficult to obtain, can provide important data for understanding tumor metabolism and guiding treatment. We are also interested in therapeutic nanoparticles, which can be naturally pigmented or colored with a dye. Treatments using nanoagents are under development for the treatment of tumors and arthritis, two major public health issues. A major barrier to their application lies in the spatial and interpatient variability of their distribution which can evolve significantly over the course disease progression. Photoacoustic imaging could provide a solution to quantify this accumulation and thus help assess the potential efficacy of this form of treatment for each individual within an approach for personalized medicine.

Our work focuses on two complementary axes: (1) the development of volumetric and multispectral photoacoustic imaging to map optical absorbers in biological tissue in three dimensions while separating the different types of absorbers according to their colors (absorption spectrum); and (2) quantitative characterization of solutions of novel colored nanoparticles by the measurement of their ultrasonic emission and photoacoustic response.

For the first axis, we develop different methods to record and reconstruct, using the same ultrasonic sensor, volumetric molecular information provided by photoacoustic imaging and anatomical (ultrasound imaging) or functional (ultrasound localization microscopy, ULM, of microcirculation) information. For the second axis, we have developed a quantitative photoacoustic spectrometer that allows us to characterize aqueous solutions of absorbing agents to determine their ability to convert light into heat and ultrasound. This second research axis provides a more detailed understanding of the photoacoustic effect at the nanometric scale.

We collaborate with several research teams in the field of chemistry for particle synthesis, labeling, and characterization; in the field of mathematics for image reconstruction; and in the field of biology for biomedical applications.

Ongoing, key projects:

• Development of volumetric (3D) imaging methods combining photoacoustic imaging and ultrasound imaging (B-mode or ULM).
• Photoacoustic and quantitative characterization of photoacoustic agents in the near infrared.
• Spectral evaluation of photoacoustic parameters (light-to-heat conversion, heat-to-pressure conversion) of nanometric agents.