Pedro M. Prieto

Adaptive-optics ultrahigh-resolution optical coherence tomography.

Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 microm) and improved transverse resolution (5-10 microm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm x 300 mm) closed-loop AO system, based on a real-time Hartmann-Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 microm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.

Analysis of the performance of the Hartmann–Shack sensor in the human eye

A description of a Hartmann-Shack sensor to measure the aberrations of the human eye is presented. We performed an analysis of the accuracy and limitations of the sensor using experimental results and computer simulations. We compared the ocular modulation transfer function obtained from simultaneously recorded double-pass and Hartmann-Shack images. The following factors affecting the sensor performance were evaluated: the statistical accuracy, the number of modes used to reconstruct the wave front, the size of the microlenses, and the exposure time.

Correction of the aberrations in the human eye with a liquid-crystal spatial light modulator: limits to performance.

We evaluated the performance of a liquid-crystal spatial light modulator for static correction of the aberrations in the human eye. By applying phase-retrieval techniques to pairs of double-pass images we first estimated the wave aberration of the eye to be corrected. Then we introduced the opposite phase map in the modulator, which was placed in a plane conjugated with the eyes pupil, and we recorded double-pass images of a point source before and after correction of the aberrations. In a slightly aberrated artificial eye a clear improvement was obtained after correction, and, although diffraction-limited performance was not achieved, the results were close to the theoretical predictions. In the two living eyes that we studied some benefit also appeared in the correction, but the performance was worse than that expected. We evaluated possible explanations for the relatively poor performance that was obtained in the human eye: an incorrect estimate of the ocular aberration, the limited spatial resolution of the modulator, and the dynamic changes in the ocular aberrations. Based on the results in the artificial eye, the first problem was not considered to be a major source of error. However, we showed that the spatial resolution of the liquid-crystal spatial light modulator limits the maximum correction to be attained. In addition, the changes in the ocular optics over time also impose a limit in the performance of static corrections.

Imperfect optics may be the eye's defence against chromatic blur

The optics of the eye cause different wavelengths of light to be differentially focused at the retina. This phenomenon is due to longitudinal chromatic aberration, a wavelength-dependent change in refractive power. Retinal image quality may consequently vary for the different classes of cone photoreceptors, cells tuned to absorb bands of different wavelengths. For instance, it has been assumed that when the eye is focused for mid-spectral wavelengths near the peak sensitivities of long- (L) and middle- (M) wavelength-sensitive cones, short-wavelength (bluish) light is so blurred that it cannot contribute to and may even impair spatial vision. These optical effects have been proposed to explain the function of the macular pigment, which selectively absorbs short-wavelength light, and the sparsity of short-wavelength-sensitive (S) cones. However, such explanations have ignored the effect of monochromatic wave aberrations present in real eyes. Here we show that, when these effects are taken into account, short wavelengths are not as blurred as previously thought, that the potential image quality for S cones is comparable to that for L and M cones, and that macular pigment has no significant function in improving the retinal image.

Three-dimensional adaptive optics ultrahigh-resolution optical coherence tomography using a liquid crystal spatial light modulator

A liquid crystal programmable phase modulator (PPM) is used as correcting device in an adaptive optics system for three-dimensional ultrahigh-resolution optical coherence tomography (UHR OCT). The feasibility of the PPM to correct high order aberrations even when using polychromatic light is studied, showing potential for future clinical use. Volumetric UHR OCT of the living retina, obtained with up 25,000A-scans/s and high resolution enables visualization of retinal features that might correspond to groups of terminal bars of photoreceptors at the external limiting membrane.

Adaptive optics with a programmable phase modulator: applications in the human eye.

Adaptive optics for the human eye has two main applications: to obtain high-resolution images of the retina and to produce aberration-free retinal images to improve vision. Additionally, it can be used to modify the aberrations of the eye to perform experiments to study the visual function. We have developed an adaptive optics prototype by using a liquid crystal spatial light modulator (Hamamatsu Programmable Phase Modulator X8267). The performance of this device both as aberration generator and corrector has been evaluated. The system operated either with red (633nm) or infrared (780nm) illumination and used a real-time Hartmann-Shack wave-front sensor (25 Hz). The aberration generation capabilities of the modulator were checked by inducing different amounts of single Zernike terms. For a wide range of values, the aberration production process was found to be linear, with negligible cross-coupling between Zernike terms. Subsequently, the modulator was demonstrated to be able to correct the aberrations of an artificial eye in a single step. And finally, it was successfully operated in close-loop mode for aberration correction in living human eyes. Despite its slow temporal response, when compared to currently available deformable mirrors, this device presents advantages in terms of effective stroke and mode independence. Accordingly, the programmable phase modulator allows production and compensation of a wide range of aberrations, surpassing in this respect the performance of low-cost mirrors and standing comparison against more expensive devices.

Ocular aberrations as a function of wavelength in the near infrared measured with a femtosecond laser

A compact mode-locked Ti:sapphire laser, emitting a broad spectrum of 277 nm bandwidth, centered at 790 nm, was used to measure the dependence of the aberrations of the human eye with wavelength in the near infrared region. The aberrations were systematically measured with a Hartmann-Shack wave-front sensor at the following wavelengths: 700, 730, 750, 780, 800, 850, 870 and 900 nm, in four normal subjects. During the measurements, the wavelengths were selected by using 10 nm band-pass filters. We found that monochromatic high order aberrations, beyond defocus, were nearly constant across 700 to 900 nm wavelength in the four subjects. The average chromatic difference in defocus was 0.4 diopters in the considered wavelength band. The predictions of a simple water-eye model were compared with the experimental results in the near infrared. These results have potential applications in those situations where defocus or higher order aberration correction in the near infrared is required. This is the case of many imaging techniques: scanning laser ophthalmoscope, flood illumination fundus camera, or optical coherence tomography.

Effect of optical correction and remaining aberrations on peripheral resolution acuity in the human eye.

Retinal sampling poses a fundamental limit to resolution acuity in the periphery. However, reduced image quality from optical aberrations may also influence peripheral resolution. In this study, we investigate the impact of different degrees of optical correction on acuity in the periphery. We used an adaptive optics system to measure and modify the off-axis aberrations of the right eye of six normal subjects at 20 degrees eccentricity. The system consists of a Hartmann-Shack sensor, a deformable mirror, and a channel for visual testing. Four different optical corrections were tested, ranging from foveal sphero-cylindrical correction to full correction of eccentric low- and high-order monochromatic aberrations. High-contrast visual acuity was measured in green light using a forced choice procedure with Landolt Cs, viewed via the deformable mirror through a 4.8-mm artificial pupil. The Zernike terms mainly induced by eccentricity were defocus and with- and against-the-rule astigmatism and each correction condition was successfully implemented. On average, resolution decimal visual acuity improved from 0.057 to 0.061 as the total root-mean-square wavefront error changed from 1.01 mum to 0.05 mum. However, this small tendency of improvement in visual acuity with correction was not significant. The results suggest that for our experimental conditions and subjects, the resolution acuity in the periphery cannot be improved with optical correction.

Binocular adaptive optics visual simulator

A binocular adaptive optics visual simulator is presented. The instrument allows for measuring and manipulating ocular aberrations of the two eyes simultaneously, while the subject performs visual testing under binocular vision. An important feature of the apparatus consists on the use of a single correcting device and wavefront sensor. Aberrations are controlled by means of a liquid-crystal-on-silicon spatial light modulator, where the two pupils of the subject are projected. Aberrations from the two eyes are measured with a single Hartmann-Shack sensor. As an example of the potential of the apparatus for the study of the impact of the eyes aberrations on binocular vision, results of contrast sensitivity after addition of spherical aberration are presented for one subject. Different binocular combinations of spherical aberration were explored. Results suggest complex binocular interactions in the presence of monochromatic aberrations. The technique and the instrument might contribute to the better understanding of binocular vision and to the search for optimized ophthalmic corrections.

Wave-aberration control with a liquid crystal on silicon (LCOS) spatial phase modulator

Liquid crystal on Silicon (LCOS) spatial phase modulators offer enhanced possibilities for adaptive optics applications in terms of response velocity and fidelity. Unlike deformable mirrors, they present a capability for reproducing discontinuous phase profiles. This ability also allows an increase in the effective stroke of the device by means of phase wrapping. The latter is only limited by the diffraction related effects that become noticeable as the number of phase cycles increase. In this work we estimated the ranges of generation of the Zernike polynomials as a means for characterizing the performance of the device. Sets of images systematically degraded with the different Zernike polynomials generated using a LCOS phase modulator have been recorded and compared with their theoretical digital counterparts. For each Zernike mode, we have found that image degradation reaches a limit for a certain coefficient value; further increase in the aberration amount has no additional effect in image quality. This behavior is attributed to the intensification of the 0-order diffraction. These results have allowed determining the usable limits of the phase modulator virtually free from diffraction artifacts. The results are particularly important for visual simulation and ophthalmic testing applications, although they are equally interesting for any adaptive optics application with liquid crystal based devices.