The Development of Adaptive Optics and Its Application in Ophthalmology
More than 25 years ago, the first in vivo measurement of the eye’s wave aberration was demonstrated at the University of Heidelberg, using the Shack-Hartmann method. The continuous measurement of ocular aberrations paved the way to employ adaptive optics for the correction of ocular aberrations in ophthalmoscopy. Various new diagnostic and treatment modalities have since been successfully introduced, e.g. adaptive optics scanning laser ophthalmoscopy (AO-SLO), adaptive optics optical coherence tomography (AO-OCT) and adaptive optics two-photon ophthalmoscopy (AO-TPO). Employing adaptive optics techniques, the limits of human vision can be assessed. With the development of wavefront-guided laser refractive surgery, the quest for optically perfect vision is being pursued over the last 20 years.
KeywordsAdaptive optics Optical aberrations Shack-Hartmann sensor Wavefront Optical image quality Visual acuity Wavefront-guided laser refractive surgery
Wavefront technology was originally developed nearly 50 years ago for astronomical applications. It was used to measure wavefront distortions that occurred when light traveling through the atmosphere entered an optical telescope. By applying adaptive optical closed-loop controls the speckle patterns of the star images could be improved towards diffraction-limited performance. Most of the technology was developed in association with research towards anti-missile defense systems in the late nineteen-hundred seventies. The same technology can be used to correct the ocular imperfections.
These complex distortions can be assessed with the wavefront technology. In recent years different wavefront sensors based on several principles have been developed, and the most important ones being Tscherning ray tracing and Shack-Hartmann sensors. More recently, the application of wavefront sensing for pre-operative evaluation of refractive surgical procedures has been proposed. Adaptive optical closed loop systems can be used to subjectively measure and compensate the higher-order optical aberrations of the human eye to guide the surgeon in the selection of parameters of the procedure.
16.2 Brief History of Adaptive Optics
Starting in 1978, the principle of wavefront measurement and compensation of wavefront aberrations was adopted at the University of Heidelberg for ophthalmic applications. The technique is based on Shack-Hartmann sensing, measuring the optical path of light rays through the eye to detect all aberrations at all points in the optical system of the human eye. Adaptive optical systems were developed to measure and compensate wave aberrations of the human eye with closed-loop control [1, 2].
History of adaptive optics closed-loop control
Adaptive optics in astronomy
Bille et al.: Image restoration by adaptive optical phase compensation. sixth ICPR
Bille: U.S Patent, 4,579,430—Method and apparatus for forming an image of the ocular fundus
Clafin et al.: Configuring an electrostatic membrane mirror by least squares fitting with analytically derived influence functions 
In 1989, Dreher along with Bille and Weinreb attempted to measure and correct monochromatic aberrations using an active mirror to correct ocular aberrations and provided improved depth resolution retinal images using scanning laser ophthalmoscope (SLO) . The clinical adoption of Shack-Hartmann wavefront sensor to measure eyes wave aberration was demonstrated in early 90s at the University of Heidelberg, in Bille’s laboratory with Liang working as a graduate student . This led to the key development of closed-loop adaptive optics systems for ophthalmology. Later, Liang with Williams at the University of Rochester built the first closed-loop adaptive optics system that could correct higher-order aberrations of the eye and achieve a supernormal vision and visualization of single cells in the human retina . Thereafter, wavefront-guided laser refractive surgery was introduced as a clinical treatment for refractive correction . Although there are many methods to measure the ocular aberrations, Shack-Hartmann is considered the finest method to precisely measure the aberrations in the human eye and is generally employed in clinical aberrometers. The wavefront technology advancement in recent years has advanced to produce accurate measurements and diagnoses of higher-order aberrations. And this leads to wavefront designed glasses, contact lenses, intraocular implants, and wavefront-guided laser vision correction. Also, fundus cameras, SLO, Optical coherence tomography (OCT) and two-photon ophthalmoscopy (TPO) have incorporated adaptive optics to achieve diffraction-limited imaging system [9, 10, 11].
16.3 Higher-Order Aberrations
Any imperfection in the eye will lead to distorted images and decrease in visual performance. This is commonly referred to as optical or wavefront aberrations. The lower order aberrations and higher-order aberrations are the two types of wavefront aberrations of the eye. The lower order aberrations such as myopia, hyperopia, and astigmatism can be corrected with glasses, contact lenses or refractive surgery. Higher-order aberrations are the imperfections that cannot be corrected by these technologies. There are some degrees of higher-order aberrations present in each eye and this can be measured using the wavefront technology. The wavefront aberrations of the human eye are generally defined mathematically by a series of Zernike polynomials (for a detail description of Zernike polynomials, see Chap. 18).
16.4 Principle of Aberration Measurement
16.5 Definitions of Optical Imaging Quality
For the description of the performance of an optical system, there are several parameters in use. Some of them are applied to the human eye as well. A short overview of some scales used in ophthalmology will be given in this section.
16.5.1 Root Mean Square (RMS)
The RMS of the wavefront is a very simple criterion. It is nothing but the integrated root mean square of the differences between the wavefront surface and the mean value of the surface. The complex phenomenon of aberration is packed into a single number. This makes it so convenient for ophthalmology. The RMS can be calculated directly from the Zernike polynomials.
The RMS can be calculated simply as a root of the sum of coefficients. The peak to valley (PTV) is closely connected to the RMS. While the PTV depends heavily on just two extreme values, the RMS is a kind of mean value received from the complete set of data points. This makes the RMS much more stable against deviations.
16.5.2 Optical Aberration Index (OAI) .
The OAI has values between zero and one. Zero stands for an optical system that is perfect and 1 for infinite aberrations. The RMS-value is given as a fraction of the wavelength of light. The OAI is very sensitive in the typical range for higher order aberrations. It was introduced as an even simpler scale for the optical quality of an eye.
16.5.3 Modulation Transfer Function (MTF) .
16.5.4 Point Spread Function (PSF)
The point response of an optic should still be a point. Even if the optic is perfect the response is a pattern due to diffraction. In a real system, the aberrations widen the image up to a spot. The spot is represented by a two-dimensional distribution. This is described by the PSF.
If the aberrations are smaller than 0.25λ (Rayleigh criterion) the diffraction pattern provides a good description of the PSF . Up to about 2λ, it is appropriate to consider the manner in which the aberration affects the diffraction pattern. For larger wavefront aberrations illumination described by ray tracing is sufficient for description.
16.5.5 Application of the Performance Indices in a Normal Human Eye
16.6 Principle of Closed-Loop Adaptive Optical Control
16.6.1 Adaptive Optics in Astronomy
16.6.2 History of Adaptive Optics at the University of Heidelberg
16.7 Demonstration of Adaptive Optics Aberrometer
16.7.1 Clinical Prototype Adaptive Optics Aberrometer
16.7.2 A Case Study on a Refractive-Surgical Patient with Clinical Prototype
16.8 The Limits of Human Vision
Campbell and Green published a seminal paper on the neuronal limit of human vision in 1965 . The neuronal limit of human vision is related to the size of the retinal photoreceptors. In the fovea, the retina exhibits cones with smallest dimensions of 2.2 μm, corresponding to a visual angle of 0.5 arcminutes, i.e. visual acuity of 20/10. At the limit of resolution, the lateral inhibition is disabled, i.e. the initial neuronal processing of the retina is rendered ineffective. An individual neuronal sensitivity curve is sketched in Fig. 16.21 in yellow color. Related to this individual characteristic, a contrast of 0.8 would be necessary to achieve a 20/10 vision. A contrast of 0.8 at 60 cycles/degree is above the diffraction-limited characteristic. Thus, the achievable improvement of the visual acuity through adaptive optics compensation of the optical aberrations of the human eye is limited by the individual neuronal threshold. Our clinical studies have shown that the neuronal threshold characteristics can be enhanced by adaption, based on a lengthy learning process.
However, as demonstrated above (see Fig. 16.21, MTF characteristic of patient WE(OD)), the optical quality of human eyes can be optimized to a smallest PSF of approximately 1.25 μm spot-size (‘airy-disc’), which is essential for high-resolution imaging of the retina of the human eye (see Chap. 17).
16.9 Aberration-Free Retinal Imaging
Retinal imaging has been an integral part of every ophthalmic examination for early diagnosis and follow-up the retinal diseases. Fundus cameras, SLOs, and OCT systems provide a macroscopic view of the retina , however, these instruments lack in transverse resolution to reveal the microscopic structures of the retina. Improving the resolution of the retinal images has always been the concern for the researchers . The main limitation is the pupil size, where the diffraction dominates the PSF in small pupil size and the aberrations in large pupil size . Adaptive optics facilitated the retinal imaging techniques by improving the image quality. Adaptive optics was first applied in a fundus camera by Liang et al. in 1997 to resolve the cone photoreceptors . Since then adaptive optics has been implemented by many researchers for aberration-free retinal imaging. In 2002, Burns et al. used phase plates with confocal SLO for higher-order aberrations and achieved a 26% increase in contrast of the retinal blood vessels . Meantime, Roorda et al. presented the first adaptive optics confocal scanning laser ophthalmoscope (cSLO) using a Shack-Hartmann sensor providing a real-time microscopic view of the human retina. The resolution achieved was 2.5 μm lateral and 100 μm axial compared to 5 μm lateral and 300 μm axial in conventional SLO . Further, the OCT has also been integrated with adaptive optics for aberration-free retinal imaging to improve the axial and lateral resolution. Adaptive optics was integrated with ultrahigh resolution OCT and spectral domain OCT [10, 23].Likewise, the adaptive optics has been incorporated with TPO aimed at detecting the early onset of retinal diseases .
16.10 Wavefront-Guided Laser Refractive Surgery (CustomVue)
Wavefront-guided laser refractive surgery (CustomVue) was founded with a pilot study at the Augenpraxisklinik (Eye clinics) Heidelberg in the year 2000, performing the first fifty wavefront-guided eye corrections, world-wide. Subsequently, five hundred patients were treated in a multi-center FDA study in the United States.
The introduction of wavefront technology in ophthalmology allows determining the optical aberrations of the human eye, far beyond the sphero-cylindrical refractive error. Based on WaveScan technology the reproducibility and accuracy of the new technique were established in worldwide multicenter clinical studies, of which, one of the most powerful clinical applications is the wavefront-guided refractive surgery. In this chapter, it was demonstrated that closed-loop adaptive optical control allows for improved spatial resolution of aberration measurements, increasing the resolution limit by two orders of magnitude over, e.g. Shack-Hartmann technology. Also, adaptive optics has proven its ability to resolve the microstructures of the retina by correcting the optical aberrations down to diffraction-limit. The aberration-free retinal imaging with adaptive optics will improve our understanding of the visual system in the normal and diseased eye.
The historical background and the basics of adaptive optics aberration measurements were published previously in reference .
- 1.Bille JF, Freischlad K, Jahn G, Merkle F. Image restauration by adaptive optical phase compensation. In Proceedings 6th international conference on pattern recognition, Munich, Germany. 1982.Google Scholar
- 2.Bille JF. US4579430A – Method and apparatus for forming an image of the ocular fundus. Google Patents. 1986.Google Scholar
- 12.Resan M, Vukosavljevi M, Milivojevi M. Wavefront aberrations. In: Advances in ophthalmology. Croatia: InTech; 2012.Google Scholar
- 13.Bille JF, Grimm B, Liang J, Mueller K. Imaging of the retina by scanning laser tomography. New methods in microscopy and low light imaging. SPIE. 1989;1161:417.Google Scholar
- 14.Liang J. A new method to precisely measure the Wave Aberrations of the Human Eye with a Hartmann-Shack Sensor. PhD thesis, University of Heidelberg, Germany. 1991.Google Scholar
- 17.Bille JF. Method for programming an active mirror to mimic a wavefront. US Patent 6,220,707 B1. 2001.Google Scholar
- 18.von Pape U. Wavefront sensing in the human eye. PhD thesis, University of Heidelberg, Germany. 2002.Google Scholar
- 20.Parthasarathy MK, Lakshminarayanan V. A brief history of aberrometry applications in ophthalmology and vision science. Singapore: Springer; 2017. p. 31–9.Google Scholar
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