Purpose:
The use of multiple wavelengths in a scanning laser ophthalmoscope (SLO) may provide more information than monochromatic imaging in macular disease. Additionally, a multiple wavelengths in a scanning laser ophthalmoscope can be used to assess optical density of various absorbers in the retina.
Methods:
A spectral SD-OCT (Spectralis™ HRA+OCT, Heidelberg Engineering, Heidelberg, Germany) has been fitted with four light sources of different wavelengths. The wavelengths used are 486 nm, 515 nm, 639 nm and 820 nm. The instrument allows simultaneous or separate imaging with one (487 nm, 515 nm, 820 nm), two (487 nm and 515 nm), and three (487nm, 515nm, 639 nm or 487nm, 515nm, 820 nm wavelength combinations. Using a band pass filter in the light detection pathway allows for autofluorescence imaging of multi-wavelength autofluorescence images. This feature was used to compare blue and green light autofluorescence images and to calculate macular pigment optical density (MPOD). Images using three wavelengths simultaneously allow for reconstruction RGB or IRGB color images. Different images fundus features like retinal hemorrhages, cotton wool spots, and drusen were analyzed and their appearance compared with color fundus photographs.
Results:
We have imaged a total of 78 subjects aged between 21 and 93 years. Diagnosis included healthy subjects, diabetic retinopathy, age related maculopathy, geographic atrophy, exsudative macular degeneration, and retinal vascular occlusion. In all subjects measuring of macular pigment using the simultaneous two wavelengths mode (486nm and 515nm) was possible. Measurement of MPOD showed a mean value of 0.68 ± 0.20 density units (DU) within a 1 degree circle around the fovea. In cases with geographic atrophy blue light autofluorescence images allowed to image small central pathologic changes without disturbances due to macular pigment absorption. The RGB and IRGB images showed differences to color fundus photographs. Due to the confocal mode of the scanning laser system the optic disc and drusenoid pigment epithelium detachments appeared relatively dark. Hemorrhages, microaneurysma, retinal vessels, and cotton wool spots appeared similar to color photographs. Especially, reticular drusen are better visible in IRGB images than in color fundus photographs.
Conclusions:
The SD-OCT equipped with four light sources of different wavelength expands the range of image modalities. The simultaneous imaging of autofluorescence with green and blue light enables measurement of macular pigment optical density. RGB and IRGB images may allow to detect special fundus features like reticular drusen.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)