A simplified block diagram of the TAOSLO is shown in . The instruments consists of three main sub-systems: a retinal tracker with wide-field line-scanning laser ophthalmoscope (T-LSLO, or TSLO) to stabilize to fixed and repeatable retinal coordinates and correct for the adverse effects of eye motion, a flying-spot scanning laser ophthalmoscope (SLO) that produces high-magnification confocal images, and an adaptive optics (AO) component that senses wave-front distortion and corrects ocular aberrations. Control of hardware and acquisition and processing of images and data from the sub-systems is accomplished from a single software platform. The system therefore presents to the operator and/or clinician a wide-field view of a large portion of the retina, a high magnification view of a particular region of interest, and a view of the ocular aberrations in both the raw form of wave-front slopes and the refined form of wave-front error map and Zernike coefficient plot.
TAOSLO block diagram. Tracker components are shown in red, SLO components are shown in light blue, AO components are shown in green, and the external therapeutic/stimulus port is shown in yellow.
The TSLO sub-system is similar to that reported previously [1
]. The SLO creates an image by detection of a flying-spot raster focused on the retina in a confocal arrangement. The adaptive optics component uses a Hartmann-Shack wave-front sensor and deformable mirror to detect and correct ocular aberrations in a closed loop. The system also includes a port that can be used for delivery of near-diffraction-limited stimulus or therapeutic beams to the retina [7
]. Those beams require an external independent focus and are collimated into the port behind the deformable mirror.
2.2 Retinal tracker
The retinal tracker stabilizes high magnification images by driving two galvanometers placed at appropriate conjugates within the path of the AOSLO in a “master-slave” configuration. The input to the master control loop is x-y error signals generated from the low power track beam (~100 μW measured at the cornea, 1060-nm laser diode, LD) dithered on a retinal feature and detected from a confocal reflectometer. The input to the slave control loop is the scaled position signals from the master galvanometers. The slave tracking mirrors are placed at conjugates to the center of rotation of the eye. This allows line-of-sight tracking (i.e., simultaneous tracking of the pupil and retina from rotational eye motion) because the mirrors pivot about the true axis of rotation of the eye.
Integration of retinal tracking into the AOSLO in a master-slave configuration requires several considerations. The setup requires a one-time calibration (gain and offset) between master and slave mirrors. To allow for tracking on a number of potential targets it is desirable to make the fields of view of the tracker and AO systems disparate. Thus we track on targets anywhere within the >40 deg LSLO field while the AOSLO is designed with optics to produce little distortion over a smaller <15 deg field. Also, the galvanometer control electronics are scaled to the appropriate angular range to maximize dynamic range. We generally track on the lamina cribrosa in the optic disc and image near the fovea – features separated by ~15 deg. This separation results in geometric and torsional eye motion errors because the eye rotates about equatorial axes and cyclo-rotates about an axis centered near the posterior pole. The one-time calibration between master and slave eliminates geometric but not cyclo-rotational errors. The TSLO produces a wider field of view at the expense of increased aberrations that are small relative to the pixel size. Aberrations must be carefully minimized in the AOSLO path due to the high resolution and small pixel size. Therefore, the AOSLO raster cannot pass through the wide-field LSLO imaging optics and must be combined at the pupil in a different manner.
Our integration scheme also allows for nested pupil tracking by adjusting the offsets of the slave and AOSLO raster scanners in a coordinated manner. Because these mirrors are at different conjugates, walking these mirrors within limits with commands derived from both retinal and pupil coordinates (the latter from the HS-WS) can correct for translational head motion. However, to date we have not yet closed the pupil tracking loop. Instead, we examine the combined residual error over AO image sequences with retinal tracking alone in order to estimate the magnitude of pupil translation effects. Advanced tracking schemes including adaptive filtering and tracking scaled to the AOSLO image feature size may be implemented for improved tracking precision. These will be discussed in more detail in Section 5.
2.3 Optical setup
The optical setup for the TAOSLO is shown in . We developed a simplified optical arrangement with elements placed on both sides of a vertical plate. This enables access to all components with a smaller system footprint. Spherical mirrors are primarily used to limit back-reflections, minimize chromatic distortion, and reduce dispersive pulse broadening (when using therapeutic ultrafast laser pulses, for example). In our configuration, astigmatism that results from use of spherical mirrors is diminished by operation at near-normal angles of incidence with longer focal lengths to accommodate the desired AOSLO field angles (±6 deg). Pair-wise orthogonal mirror relays automatically cancel most of the astigmatism.
Fig. 2 TAOSLO optical layout. (a) Ocular interface which includes TSLO and front lens relay, (b,c) View of front and rear of vertical plate on which optics are mounted. Entire plate is mounted on a stage to control focus between front lens relay. Positions of (more ...)
The front-end TSLO uses a 909-nm superluminescent diode (SLD, ~200 μW, 20-nm bandwidth) for wide-field imaging. A line fanned out by a cylindrical lens (CL) is scanned on the retina and de-scanned back to a linear array detector (LAD) with a single galvanometer in a confocal manner [8
]. Custom objectives (O3 and O4) transfer retinal image back to the detector and an ophthalmoscopic lens (OL) relays and focuses the image onto the retina. The tracker uses 8-kHz resonant scanners (RS) to dither on a circle and a confocal reflectometer and phase-sensitive detection scheme to driver the master tracker galvanometers (TG) according to the motion of the eye. An InGaAs avalanche photodiode (APD) is used for tracker beam detection. An 8×8 LED array is used for fixation.
Although it is possible to use the SLO imaging beam to also sense ocular aberrations with the wave-front sensor, higher throughput and off-axis operation (to minimize corneal reflections) are achieved with two separate beams. The imaging beam (800±15-nm SLD, ~300 μW) and the wave-front sensing beam (also called the AO beacon, 670-nm LD, ~35 μW) are thus at different wavelengths. LSLO imaging, tracking, SLO, AO beacon and fixation wavelengths are combined with custom dichroic beamsplitters (D1-5). The power levels for completely overlapping beams are greater than a factor of 2 below the ANSI laser safety limits.
The AOSLO beams are directed from the vertical plate through a front pair of objectives (O1 affixed to the plate and O2). Both off-the-shelf and custom lenses have been used in the relay successfully. The custom lenses were designed to minimize spherical and chromatic aberrations and control field flatness. Focus for the high-magnification image is independent of TSLO focus and is accomplished by adjustment of the DM stage (fine) and the stage on which the plate rests (coarse), the latter controlling separation between O1 and O2. Coarse focus adjustments of up to 10 diopters are achieved. DM stage (fine) focus controls the separation between beacon focal plane and the AO raster focal plane allowing systematic shifts of focus, such as those due to longitudinal chromatic aberration, to be compensated.
A standard approach is used for the SLO optical setup, whereby a flying spot is scanned on the retina and de-scanned through the same optics back to a confocal pinhole, often with unequally-sized entrance and exit pupils [9
]. Our system is designed for a 6-mm pupil (entrance and exit) and uses either a 100 or 200-μm pinhole (15 or 30-μm referenced to the retina). The SLO raster is created with a 12-kHz resonant scanner in the fast horizontal axis (RSh) and a galvanometer in the slower vertical axis (Gv), both placed at pupil conjugates to pivot the retinal raster from the pupil. Because voltage offsets cannot be applied to a resonant scanner, we have placed this scanner on an additional large, slow galvanometer (OG). This, together with a voltage offset applied to Gv, allow the raster to be rapidly positioned under computer control anywhere within the field of view. This can be used for the acquisition of automatic montages described in Section 4. The slave galvanometers (SGh and v) are placed at conjugates to the center of rotation for line-of-sight tracking. Spherical mirrors (SM5-8) relay the SLO imaging beam and the AO beacon through the scan engine to the eye and back again. A pellicle beamsplitter (92%T/8%R) is used to couple the beacon into the instrument.
On the rear side of the plate, SM1-4 direct the SLO source (and stimulus or therapeutic laser source, if used) off the deformable mirror to the eye and return backscattered light from the retina to the wave-front sensor and SLO detector (APD) via confocal pinhole (100 and 200-μm diameter). The Hartmann-Shack wave-front sensor (HS-WS) is comprised of a 65×65-element lenslet array (0.4-mm pitch and 25-mm focal length) and a 1024×1024-pixel, 12-mm CCD camera (Dalsa Inc.) with a maximum frame rate of 60 Hz. A 141-element, 4.8-mm, MEMS-based, continuous-surface, electrostatic-actuator-driven DM (Boston Micromachines Inc.), with maximum stroke of ~4 μm, is used for wave-front correction. The spherical mirrors are chosen to de-magnify a 6-mm pupil to 3.8-mm at the resonant scanner and deformable mirror and magnify it to 7.7-mm at the wave-front sensor.
2.4 Electronics and instrumentation
Tracking functions are performed by a set of stacked electronics boards. The control and processing electronics use a field programmable gated array (FPGA) chip to perform digital lock-in amplification and other pre-processing steps and a digital signal processor (DSP) to execute two real-time proportional-integral-derivative (PID) control loops (for master and slave systems). The DSP has a loop rate of 62.5 kHz for a closed loop bandwidth in excess of 1 kHz (up to the mechanical limit imposed by the scanner response). Analog-to-digital and digital-to-analog converters (ADC and DACs) receive reflectometer and galvanometer position signals and output galvanometers drive signals. Communication between the tracking boards and host computer is accomplished via a USB interface. The main system software sends control and calibration parameters to the tracking board, which in turn passes reflectance, position, and error signals back to host processor. The control loop includes an automated blink detection and track re-lock algorithm that has been described previously [2
Besides the tracking boards, system electronics include three framegrabbers (analog for SLO, digital for LSLO and WS), galvanometer driver boards, custom LSLO camera board, LED fixation driver board, and custom system timing board. Since the master and slave control loops are closed via software, the electronic control loop resident on the driver boards is by-passed for the tracking galvanometers. All other imaging and offset galvanometers use the electronic control loop. The small electronic board that drives the LED array for fixation is controlled by simple commands over the serial port. The custom timing board provides horizontal (line) and vertical (frame) sync signals and a non-linear pixel clock to the analog framegrabber to automatically linearize the sinusoidal scan produced by the resonant scanner Small phase errors in the generated non-linear pixel clock and the actual scanner output can cause distortion on one side of the frame where the sinusoidal scan is flat and the pixel clock sampling is sparse (i.e., approximately the first 50 pixels). The sync signals also drive the WS external synchronization signals and the SLO SLD source for modulation, which creates a blanking region for the analog framegrabber. Tracking in this configuration prohibits a fixed physical mask to be used for blanking. The LSLO and SLO systems are not synchronized since they use entirely independent optical paths. The LSLO camera acquires a 512×512-pixel frame at 15 frames/sec. The 12-kHz SLO resonant scanner frequency enables a 512×512-pixel frame rate up to ~25 frames/sec. In practice, we operate at half that speed since the images from all three components (LSLO, SLO, and WS) are acquired and displayed by a single software platform.
2.5 AO control software and user interface
The system software performs two primary tasks: adaptive optics control and acquisition, display, and processing of the LSLO, SLO, and WS images. All operations were performed on a single 3.1 GHz-processor computer. The retinal tracking control loop runs in real-time independently of the host, so processor resources are freed for essential AO and imaging operations. The adaptive optics operations include WS spot position and slope calculation, AO (WS/DM) calibration, AO closed-loop operation, and wave aberration and Zernike coefficient calculation. Secondary software tasks include communication with the tracking board, display and logging of tracking signals, streaming to disk of videos (from all three cameras), and single image acquisition in a variety of formats. When a live video is streamed to disk, two additional files are saved that contained track data (reflectance, x-y track mirror positions, and either the x-y error signals or the x-y slave mirror positions) in binary format and a text file that contained all system parameters and calibration factors.
For AO-correction using the continuous-surface DM, a one-time calibration is performed subsequent to system alignment to find the influence of each actuator on its neighbors and to establish a baseline for calculation of slopes. The software uses a standard algorithm for spot centroid determination that operates at the frame rate of the WS camera up to 30 Hz. During AO-correction, the local wave-front slopes are found and inverted with a pseudo-inverse function and fed to the DM driver. The wave aberration function and Zernike coefficients can be calculated in real-time, though this is not usually done during measurement to preserve processor resources for more critical functions. The closed-loop bandwidth of the AO system is ~2–3 Hz. Streaming videos requires processing and often slightly reduces the frame rate further (to the 8-Hz frame rate presented in this paper). The reduction in frame rate, however, did not lower the overall rate for the wave-front sensing and AO correction from 12 Hz.