Scale variation is an important property in retinal imaging, since during longitudinal epidemiological studies, fundus images can be taken with different equipment, at different times, and with different parameter settings such as resolution and magnification (field of view -- FOV). Thus, a measure that is scale-invariant and that finely detects changes in turns and twists, regardless of the equipment used to take it, is a challenge.
The study employed high quality non-mydriatic (i.e. pupil dilation is not required) fundus photography (FP) (Oculus Nidek AFC-230, 45 FOV) and a infra-red reflectance (IR) en face image obtained by scanning laser ophthalmoscope (SLO) (Heidelberg Spectralis, 30 FOV) to produce representative images of the retina and optic nerve. In order to prove the scale invariance property of tortuosity metrics, ophthalmological photographs from 18 subjects, both eyes, were randomly extracted from the LIFE-Adult-Study, 36 FP images and 36 IR (fovea centered) images, a total of 72 images of the ocular fundus.