Laser Fluence and Exposure Time Effects on Optoacoustic Signal from Gold Nanorods for Enhanced Medical Imaging

The aim of the present work is to assess the effects of different laser fluence and exposure time values on the signal enhancement obtained using of Gold Nanorods (AuNRs) as contrast agent for optoacoustic imaging. In fact, until now, extremely few relevant studies have assessed the optoacoustic behavior of AuNRs under repeatable experimental conditions. A dedicated experimental set-up was developed in order to quantify the contribution of independent parameters to the optoacoustic signal (OAS) produced by 100-mu l solution of AuNR at different concentrations (50, 100 and 200 pM), deposited in a custom-designed tissue-mimicking phantom and irradiated by an appropriate Near Infrared (NIR) light source at variable working conditions in order to generate plasmonic resonance in the AuNRs. Analysis of the OAS recorded by a single-channel ultrasound (US) probe allowed the identification of the optimal and maximum duration of laser exposure, determined through a quantitative analysis of the progressive degradation of the signal emitted by AuNRs under irradiation and at different laser fluence levels. Similarly the effect of rising AuNR concentrations on OAS characteristics was verified, finding a direct proportionality between signal amplitude and AuNR concentration. We found the optimal and maximum laser exposure duration, in order to preserve the AuNR optoacoustic efficiency, were respectively equal to 20 s and 60 s for laser fluence up to 50 mJ/cm(2). Furthermore, the OAS generated upon laser irradiation of AuNRs was found directly proportional to the concentration employed and the optimal concentration of 200 pM was identified.