High frequency ultrasound and photoacoustic imaging for tissue characterization in vivo

The photoacoustic (PA) technology combined with high frequency ultrasound (HF -US) is a versatile tool to assess real-time and non-invasively the morpho-functional and molecular information of a target tissue, and to map the distribution of contrast agents. Recently a plethora of novel nanomaterials were developed as contrast agents and/or drug delivery systems. In this work, we report the use of micro-bubbles and gold nanorods (GNRs) as vascular contrast agents for the characterization of tumor micro-environment, combining HF-US and PA imaging. A bolus of 50μL of untargeted micro-bubbles and a bolus of 150μL of GNRs solution were injected intravenously in two male colon cancer xenograft mice and detected with a commercial PA-US imaging system. The subcutaneous tumor dimensions were assessed by means of B-mode images (146.48 mm²and 24.56 mm²); different perfusion parameters were estimated tracking the micro-bubbles (peak enhancement map; time to peak 51,3s, 58,7s and 39s and wash-in rate 35,5s, 5,4s and 109,4s for different tumor regions); the real time distribution of GNRs in the tumor was assessed by means of the amplitude variation of the PA signal during time (30 % increase of the PA signal 28 min after injection). The combination of HF-US and PA imaging could become an efficient and non invasive tool for the in vivo and real-time evaluation of the tissue viability, to visualize the map of perfusion and monitoring the efficacy of drug/nano-particle delivery. The use of targeted microbubbles and nanoparticles could open new avenues for the real-time mapping of specific cell epitopes and phenotypes.