Elimination of Nontargeted Photoacoustic Signals for Combined Photoacoustic and Ultrasound Imaging.

As a molecular imaging modality, photoacoustic (PA) imaging has been in the spotlight because it can provide an optical contrast image of physiological information and a relatively deep imaging depth. However, its sensitivity is limited despite the use of exogenous contrast agents due to the background PA signals generated from nontargeted absorbers, such as blood and boundaries between different biological tissues. In addition, clutter artifacts generated in both in-plane and out-of-plane imaging region degrade the sensitivity of PA imaging. We propose a method to eliminate the nontargeted PA signals. For this study, we used a dual-modal ultrasound (US)-PA contrast agent that is capable of generating both the backscattered US and PA signals in response to the transmitted US and irradiated light, respectively. The US images of the contrast agents are used to construct a masking image that contains the location information about the target site and is applied to the PA image acquired after contrast agent injection. In vitro and in vivo experimental results demonstrated that the masking image constructed using the US images makes it possible to completely remove nontargeted PA signals. The proposed method can be used to enhance the clear visualization of the target area in PA images.