Development of multiphoton photoacoustic spectroscopy for noninvasive tissue diagnostics

Abstract

In this paper we describe the development of a novel non-invasive multispectral imaging technique, multiphoton photoacoustic spectroscopy (MPPAS). In this technique, a tunable high-power laser is used to perform multiphoton excitation events, which are then detected as an acoustic (i.e. ultrasonic) signal, using a commercial ultrasonic piezoelectric (PZT) transducer. The signal from the PZT is then corrected for laser power fluctuations, resulting in normalized MPPAS signal intensity. Because MPPAS relies on non-radiative relaxation of the absorbing species, unlike single or multiphoton fluorescence spectroscopies, it is capable of monitoring non-fluorescent species. In addition, since the majority of the energy imparted to most molecules upon the absorption of light is released through non-radiative pathways, sensitive measurements of even fluorescent molecules, can be performed. A test solution of aqueous rhodamine 6G was used to obtain the MPPAS spectrum. It matches well with that of the steady state absorbance of the same solution. In addition, concentration dependent studies of rhodamine 6G have shown that the technique is even sensitive to nanomolar concentrations and below.