Clinical Applications and Advancements in the Understanding of Visual Image Quality Metrics



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PURPOSE To use the visual image quality metric VSX (the visual Strehl ratio) to optimize objective refraction and to evaluate ophthalmic corrections across modalities (spectacles, conventional and wavefront-guided scleral lenses). In doing so, to identify aspects of VSX that could be evolved towards improving its application to real world conditions as well as to individual eyes. METHODS Five experiments are described. (1) The sphere, cylinder, and axis spectacle corrections that objectively optimized VSX for 40 eyes (20 subjects) were compared with subjective refraction. Visual acuity and short-term preference with the two corrections were also compared. (2) Sphere, cylinder, and axis best-corrected normative values of VSX were determined using the same objective optimization method for 146 eyes (146 subjects) ranging from 20 to 80 years at fixed 7, 6, 5, 4, and 3 mm pupil sizes. (3) VSX as well as higher order root mean square (HORMS) wavefront error, visual acuity, and letter contrast sensitivity, were used to evaluate eyes with corneal ectasia corrected by conventional and wavefront-guided scleral lenses. In the final two experiments, the neural weighting component of VSX was (4) evolved to a model that respects the effects of retinal illuminance and age and (5) personalized using measured neural contrast sensitivity functions (measured using a custom orientation-specific Maxwellian-view laser interferometer) of individual eyes (both typical and those with keratoconus). RESULTS (1) For 36 myopic eyes, the VSX correction provided (statistically and clinically) equivalent visual acuity to subjective refraction and was preferred by 72% of eyes over subjective refraction. In four habitually undercorrected hyperopic eyes, the VSX correction was substantially more positive in power than subjective refraction and was not preferred. (2) Normative best-corrected values of VSX as a function of age and fixed pupil size were published. (3) While aberrations (HORMS), visual acuity, and letter contrast sensitivity reached typical levels for most eyes with ectasia when wearing wavefront-guided scleral lenses, these eyes did not reach the normative values of VSX. While the magnitude of higher-order aberrations was within typical levels, the distribution of residual aberration terms was markedly different. Secondary astigmatism was frequently elevated in these corrected ectatic eyes and is particularly devastating to visual image quality. (4) Best-corrected VSX calculated using the model of neural contrast sensitivity that accounted for both retinal illuminance and age, tracked changes in best-corrected visual acuity as a function of age (from literature) better than using a model that only accounted for retinal illuminance and better than using no neural weighting factor. (5) Measured neural contrast sensitivity functions of typical eyes did not substantially differ from that defined by the model. Eyes with keratoconus showed rotationally-asymmetric sensitivities that were comparable at some orientations to typical eyes in milder stages of disease but decreased (especially at higher spatial frequencies and at particular orientations) as disease severity worsened. Personalizing the neural weighting component of VSX did not make a clinical difference to the optimal sphere, cylinder, and axis correction. Measured neural contrast sensitivity functions of eyes with keratoconus apodize (remove the tails / feet of) the PSF more substantially than the photopic function of typical eyes, which over-estimates the detrimental effects of highly-aberrated optics. CONCLUSIONS The visual image quality metric VSX objectively identified spectacle corrections that were clinically equivalent to subjective refraction for myopic eyes and served as a method of evaluating corrections across modalities that is more robust than residual diopters (over-refraction) and RMS wavefront error. Because measurement of orientation-specific neural contrast sensitivity is time-consuming and the gains over the model (that defines neural contrast sensitivity as a function of retinal illuminance and age) were not clinically significant, it is recommended that the model be incorporated into calculation of the metric.



Wavefront error, Aberrations, Visual image quality, Keratoconus, Neural contrast sensitivity


Portions of this document appear in: Hastings, Gareth D., Jason D. Marsack, Lan Chi Nguyen, Han Cheng, and Raymond A. Applegate. "Is an objective refraction optimised using the visual Strehl ratio better than a subjective refraction?." Ophthalmic and Physiological Optics 37, no. 3 (2017): 317-325. This paper is included in this dissertation with permission from John Wiley and Sons via RightsLink Copyright Clearance Center under license number: 4647850600380. And in: Hastings, Gareth D., Jason D. Marsack, Larry N. Thibos, and Raymond A. Applegate. "Normative best-corrected values of the visual image quality metric VSX as a function of age and pupil size." JOSA A 35, no. 5 (2018): 732-739. And in: Hastings, Gareth D., Raymond A. Applegate, Lan Chi Nguyen, Matthew J. Kauffman, Roxana T. Hemmati, and Jason D. Marsack. "Comparison of wavefront-guided and best conventional scleral lenses after habituation in eyes with corneal ectasia." Optometry and Vision Science 96, no. 4 (2019): 238-247.