Frequency Analysis of the Photoacoustic Signal Generated by Coronary Atherosclerotic Plaque

The identification of unstable atherosclerotic plaques in the coronary arteries is emerging as an important tool for guiding percutaneous coronary interventions and may enable preventive treatment of such plaques in the future. Assessment of plaque stability requires imaging of both structure and composition. Spectroscopic photoacoustic (sPA) imaging can visualize atherosclerotic plaque composition on the basis of the optical absorption contrast. It is an established fact that the frequency content of the photoacoustic (PA) signal is correlated with structural tissue properties. As PA signals can be weak, it is important to match the transducer bandwidth to the signal frequency content for in vivo imaging. In this ex vivo study on human coronary arteries, we combined sPA imaging and analysis of frequency content of the PA signals. Using a broadband transducer (-3-dB one-way bandwidth of 10-35 MHz) and a 1-mm needle hydrophone (calibrated for 1-20 MHz), we covered a large frequency range of 1-35 MHz for receiving the PA signals. Spectroscopic PA imaging was performed at wavelengths ranging from 1125 to 1275 nm with a step of 2 nm, allowing discrimination between plaque lipids and adventitial tissue. Under sPA imaging guidance, the frequency content of the PA signals from the plaque lipids was quantified. Our data indicate that more than 80% of the PA energy of the coronary plaque lipids lies in the frequency band below 8 MHz. This frequency information can guide the choice of the transducer element used for PA catheter fabrication.