PhotoAcoustic Imaging (PAI) *

PhotoAcoustic (or optoacoustic) Imaging (PAI) is a novel but well-established technology that combines optical and ultrasound imaging. Briefly, the technique detects endogenous or exogenous chromophores that are excited through an illumination carried out with pulsed laser light, typically in the Red-NIR range. The non-radiative release of the absorbed energy produces a microheating and a thermoelastic expansion in the chromophore close surrounding, which can generate ultrasound. The acoustic waves are detected by a transducer and transformed into an image.

PAI combines the advantages of optical and US imaging technologies. The detection of the US signal makes it possible to achieve an increase of both spatial resolution (micrometric) and penetration depth with respect to optical imaging and reduces scattering phenomena. Other advantages include good sensitivity (comparable to that of optical imaging), safety, cheapness and ease of use.

A very large number of chromophores can be used to generate PAI contrast: endogenous molecules (e.g. oxy- and deoxy-hemoglobin, myoglobin, melanin), exogenous probes (e.g. organic dyes like fluorescein and ICG), nanosystems (e.g. noble metals-containing nanoparticles, quantum dots, carbon nanotubes, etc.) and also fluorescent proteins and gene reporters. Detection of different types of chromophores in the same anatomical region is also feasible.

As any tomographic technique, PAI allows to get 3D-high resolution images of soft tissues which provide anatomical, functional, and molecular information. Due to the not optimal penetration depth, PAI is particularly useful for preclinical imaging, whereas in humans it’s use is limited to superficial organs analysis (e.g. palmar vessels detection, breast analysis). The most important in vivo application of PAI is the detection of blood hemoglobin, which provides a different PA signal depending on its deoxy- or oxy-state, allowing it to evaluate the vascular volume and pO2 in normal and hypoxic conditions.

Other applications in the preclinical biomedical field include, but are not limited to:

• oncology (cancer detection, assessment of vascular volume and hypoxia, targeting experiments, assessment of drug distribution and therapy outcome)
• neurobiology (stroke, brain cancer, intracranial injection of drugs, functional imaging)
• cardiovascular biology (heart attack, detection of atherosclerotic plaques, hemodynamic and O2 perfusion)
• in developmental biology (vascular volume and oxygenation of placenta, image-guided embryo injection, fetal analysis)
• abdomen analysis (kidney diseases as renal microcirculation flow, renal obstruction analysis, gastrointestinal motility, organs’ perfusion)
• skin analysis (mainly detection of melanin in melanoma cells)
• image-guided injection of drug