Photoacoustic imaging: Development of imaging systems and molecular agents

Abstract

Photoacoustic imaging is a relatively new imaging modality with great promise to overcome most of the limitation of conventional optical imaging. By leveraging the conversion of short light pulses into ultrasound waves, it is possible to generate three-dimensional maps of a tissue with high spatial resolution and at a high tissue depth of penetration. Since the basic mechanism that gives rise to a photoacoustic signal is light absorption, several endogenous contrasts can be used for photoacoustic imaging of tissues, including hemoglobin and melanin. To allow photoacoustic imaging to reach its full potential, exogenous contrast agents that can target biomolecules in living tissues were developed, enabling molecular imaging studies. This chapter will review the physical basis of photoacoustic imaging, starting with the photoacoustic effect and the conditions needed to generate detectable ultrasonic waves from light excitation of an absorber. The different photoacoustic scanner implementations will then be discussed, including photoacoustic tomography (PAT) and microscopy systems and the biomedical applications to which they are best suited. Finally, the various exogenous contrast agents for photoacoustic imaging will be discussed and a general approach for contrast agent validation will be described.