Ultrasound-Switchable Fluorescence Imaging via an EMCCD Camera and a Z-Scan Method

The spatial resolution of the conventional fluorescence imaging in centimeter-deep tissue is limited due to high light scattering. Ultrasound-switchable fluorescence (USF) imaging was recently developed to achieve high resolution in deep tissue. However, previous frequency-domain USF systems adopted a fiber bundle to collect the emitted fluorescence photons, which led to a few limitations, such as, first, low photon-collection efficiency; second, low efficiency for scanning a sample with an uneven surface; third, low imaging speed, because of the adoption of a raster scan and the long wait time for the tissue to cool down before starting the data acquisition at the next location. In this study, we proposed a camera-based USF imaging system to overcome these limitations. Thanks to the spatial information provided by the camera, a new scan method, Z-scan, was developed, and the imaging speed was improved four times over the raster scan. The USF imaging using the Z-scan for tissue samples was realized. The results were validated by a commercial micro-CT system.