Comparison between a fundus camera and scanning laser ophthalmoscope in acquiring fluorescence emission in vitro

To compare the fluorescence measurements acquired from a fundus camera with those from a scanning laser ophthalmoscope (SLO) camera.

The fundus camera and the SLO camera were used to capture images of 29 cuvettes each containing serially diluted sodium fluorescein dye in normal saline. The intensity levels of the resulting images were plotted as a function of concentration to compare the two cameras. Ten samples of serially diluted indocyanine green (ICG) dye in bovine serum were also measured.

Both cameras revealed that fluorescence intensity varied as a function of the logarithmic concentration of the dye, independent of the actual dye used, with expected decrease in fluorescence at very high concentrations of dye due to quenching of fluorescence. There were very small variations on repeated trials with the fundus camera, whereas the SLO camera exhibited marked variability, particularly at higher concentrations of dye. Measurements acquired with the SLO camera varied as a function of time, which did not occur with the fundus camera. The image averaging software on the SLO camera caused shifts in the grayscale values measured that depended on the initial amount of fluorescence measured in the raw samples.

Although these differences may cause modest qualitative differences in imaging the ocular fundus, the variation in data obtained from the SLO camera would seem problematic if quantification of the amounts of fluorescence is required.