Purpose: Intraocular scattering reduces the contrast of fundus images, especially in flood-illumination imaging systems. Since macular pigment density (MPD) is often estimated from the analysis of multispectral retinal images, this contrast reduction may affect the results. We performed experiments to better understand the severity of this problem and to overcome it by compensating scatter, thus obtaining a more accurate estimate of the MPD.
Methods: A new multispectral fundus imaging system based on the double pass principle, also capable of measuring intraocular scattering at various wavelengths, was built. In addition, a computational method to estimate MPD from retinal images after compensation of light scattering was developed. The experimental setup includes a xenon lamp filtered by band-pass filters and homogenized by diffusers,a motorized iris conjugated to the subject’s retina and an electron multiplying CCD camera. To avoid reflections and backscattered light from the lens and the cornea, the imaging and illumination arms are spatially separated by means of rectangular apertures conjugated to the pupil plane. Intraocular scattering was estimated using an optical integrator method by projection of uniform disks ranging from 0.4 to 6 degrees radius (Ginis, et al (2012), Journal of Vision, 12(3), 1-10). For the MPD calculation, a pair of foveal images was recorded at two wavelengths, 460 and 550 nm. These images were then corrected for the effect of scatter by using a standard deconvolution technique and the scatter free value of MPD was calculated according to a formula (Delori et al (2001), J. Opt. Soc. Am. A, 18, 1212-1230).
Results: The complete procedure was successfully applied to 2 young subjects, LH and DC, with no known eye pathology. The observed maximum value of the MPD was 0.34 and 0.28 D.U. for DC and LH respectively, whereas without any scatter compensation the respected vales were 0.18 and 0.16 D.U., exhibiting an underestimation at the order of 45% for the maximum value and 35% for the mean value of the total MPD. The effect depends highly on the overall scattering but also on the difference in scattering for blue and for green.
Conclusions: Initial results from two young, healthy subjects have shown an important underestimation of the MPD when the effect of scattering is not taken into account. This effect is expected to be more important in elderly subjects with elevated intraocular straylight.