Fluorescence Imaging In Vivo: Raster Scanned Point-Source Imaging Provides More Accurate Quantification than Broad Beam Geometries

Two fluorescence imaging systems were compared for their ability to quantify mean fluorescence intensity from surface-weighted imaging of tissue. A broad beam CCD camera system was compared to a point sampling system that raster scans to create the image. The effects of absorption and scattering in the background tissue volume were shown to be similar in their effect upon the signal, but the effect of the three-dimensional shape of the tissue was shown to be a significant distortion upon the signal. Spherical phantoms with Intralipid and blood for absorber and scatterer were used with a fixed concentration of aluminum phthalocyanine fluorophore to illustrate that the mean intensity observed with the broad beam system increased with size, while the mean intensity observed with the raster scanned system was not as significantly affected. Similar results were observed in vivo with mice injected with the fluorophore and imaged multiple times to observe the pharmacokinetics of the drug. The fluorescence in the tumor observed with the broad beam system was higher than that observed with the raster scanned system. Based upon the phantom and animal observations in this study, it should be concluded that using broad beam fluorescence imaging systems to quantify fluorescence in vivo may be problematic when comparing tissues with different three dimensional characteristics. In particular, the ratio of fluorescence from tumor to normal tissue can yield inaccurate results when the tumor is large. However, similar measurements with a narrow beam system that is raster scanned to create the images are not as significantly affected by the three dimensional shape of the tissue. Raster scanned imaging appears to provide a more uniform and accurate way to quantify fluorescence signals from distributed tissues in vivo.