Xiaofeng Qi

In vivo measurement of erythrocyte velocity and retinal blood flow using adaptive optics scanning laser ophthalmoscopy

In vivo measurement of retinal blood flow is obtained by measuring the blood velocity of erythrocytes and lumen diameters of the blood vessels using an adaptive optics scanning laser ophthalmoscope. Erythrocyte velocity is measured by tracking erythrocytes moving across a horizontal scanning line. This approach provides high temporal bandwidth measurements, allowing the fluctuation of blood flow during cardiac cycles to be measured. The technique is most applicable to medium-sized blood vessels.

Wavefront-aberration sorting and correction for a dual-deformable-mirror adaptive-optics system

Many next-generation adaptive optics (AO) systems for vision will have two deformable mirrors (DMs) instead of one: a high-stroke, low-resolution mirror (the woofer) and a low-stroke, high-resolution mirror (the tweeter). We developed a zonal wavefront-control algorithm and validated it using simulations. Rather than separating the woofer and tweeter corrections into two independent control processes or using a modal decomposition, the algorithm we proposed uses wavefront slope measurements from a single Shack-Hartmann wavefront sensor to generate control signals for both deformable mirrors within a single zonal control. A Lagrange multiplier is chosen to integrate two DMs into a single-DM wavefront control, and a damped least-squares control is employed to suppress the correlation between the two DMs.

Woofer-tweeter adaptive optics scanning laser ophthalmoscopic imaging based on Lagrange-multiplier damped least-squares algorithm

We implemented a Lagrange-multiplier (LM)-based damped least-squares (DLS) control algorithm in a woofer-tweeter dual deformable-mirror (DM) adaptive optics scanning laser ophthalmoscope (AOSLO). The algorithm uses data from a single Shack-Hartmann wavefront sensor to simultaneously correct large-amplitude low-order aberrations by a woofer DM and small-amplitude higher-order aberrations by a tweeter DM. We measured the in vivo performance of high resolution retinal imaging with the dual DM AOSLO. We compared the simultaneous LM-based DLS dual DM controller with both single DM controller, and a successive dual DM controller. We evaluated performance using both wavefront (RMS) and image quality metrics including brightness and power spectrum. The simultaneous LM-based dual DM AO can consistently provide near diffraction-limited in vivo routine imaging of human retina.

Stokes vector analysis of adaptive optics images of the retina.

A high-resolution Stokes vector imaging polarimeter was developed to measure the polarization properties at the cellular level in living human eyes. The application of this cellular level polarimetric technique to in vivo retinal imaging has allowed us to measure depolarization in the retina and to improve the retinal image contrast of retinal structures based on their polarization properties.

A Clinical Planning Module for Adaptive Optics Slo Imaging

Purpose. To develop a clinical planning module (CPM) to improve the efficiency of imaging subjects with a steerable wide-field adaptive optics scanning laser ophthalmoscope (AOSLO) and to evaluate the performance of this module by imaging the retina in healthy and diseased eyes. Methods. We developed a software-based CPM with two submodules: a navigation module and a montage acquisition module. The navigation module guides the AOSLO to image identified retinal regions from a clinical imaging platform using a matrix-based mapping between the two. The montage acquisition module systematically moves the AOSLO steering mirrors across the retina in predefined patterns. The CPM was calibrated using a model eye and tested on five normal subjects and one patient with a retinal nerve fiber layer defect. Results. Within the central ±7° from the fixation target, the CPM can direct the AOSLO beam to the desired regions with localization errors of <0.3°. The navigation error increases with eccentricity, and larger errors (up to 0.8°) were evident for regions beyond 7°. The repeatability of CPM navigation was tested on the same locations from two subjects. The localization errors between trials on different days did not differ significantly (p > 0.05). The region with a size of approximately 13° × 10° can be imaged in about 30 min. An approximately 12° × 4.5° montage of the diseased region from a patient was imaged in 18 min. Conclusions. We have implemented a clinical planning module to accurately guide the AOSLO imaging beam to desired locations and to quickly acquire high-resolution AOSLO montages. The approach is not only friendly for patients and clinicians but also convenient to relate the imaging data between different imaging platforms.

Dual electro-optical modulator polarimeter based on adaptive optics scanning laser ophthalmoscope

We constructed a high speed and high-resolution Stokes vector retinal imaging polarimeter with dual electro-optical modulators based on adaptive optics scanning laser ophthalmoscope. By varying the voltages on the EO crystals line by line, we were able to measure over 500,000 Stokes vectors per second. We used this system in three human subjects demonstrating the capability of the system to be employed in vivo. The high speed effectively decreases the adverse impact of eye motion induced errors in polarization calculations, improving the contrast of retinal structures based on their polarization properties.

Improving wavefront boundary condition for in vivo high resolution adaptive optics ophthalmic imaging

An ophthalmic adaptive optics (AO) imaging system is especially affected by pupil edge effects due to the higher noise and aberration level at the edge of the human pupil as well as the impact of head and eye motions on the pupil. In this paper, a two-step approach was proposed and implemented for reducing the edge effects and improving wavefront slope boundary condition. First, given an imaging pupil, a smaller size of sampling aperture can be adopted to avoid the noisy boundary slope data. To do this, we calibrated a set of influence matrices for different aperture sizes to accommodate pupil variations within the population. In step two, the slope data was extrapolated from the less noisy slope data inside the pupil towards the outside such that we had reasonable slope data over a larger aperture to stabilize the impact of eye pupil dynamics. This technique is applicable to any Neumann boundary-based active /adaptive modality but it is especially useful in the eye for improving AO retinal image quality where the boundary positions fluctuate.

Adapting AO systems for clinical Imaging

Modern adaptive optics systems for retinal imaging represent a blend between optical design and software control. The problems faced for real-world clinical imaging include the need to obtain high quality data rapidly in less than ideal conditions, including variable size and shape moving pupils, the need to control low order aberrations of 10’s of microns while also correcting high order aberrations to RMS values on the order of 20-40 ums.

Adaptive Optics Instrumentation

Adaptive optics imaging of the retina presents unusual design challenges. AO instruments allowing steering of the beam across the retina, large amounts of defocus, and variable pupil sizes will be discussed.

Wavefront Aberration Correction for Dual-Deformable-Mirror Adaptive Optics Systems

A control algorithm for a Woofer-Tweeter dual deformable-mirror (DM) adaptive optics system is developed for correcting the low-order aberration with the Woofer DM and high-order aberration with the Tweeter DM.