Optical coherence tomography (OCT) interferometrically detects backscattered light to perform micron level two and three dimensional imaging of tissue with high sensitivity and dynamic range [1
]. In ophthalmology, OCT is widely used for non-invasive structural and quantitative imaging of the retina and anterior segment. OCT enables identification of pathologies for disease diagnosis and monitoring response to therapy [2
Early OCT systems used low coherence interferometry with a mechanically swept reference mirror to measure the echo delay of backscattered light (time domain OCT). Commercial ophthalmic OCT instruments operated at speeds of 100–400 axial scans per second. A limited number of cross sectional images could be acquired because blinking and eye motion limited data acquisition times. A different detection scheme, Fourier domain OCT, can be implemented using a broadband light source, spectrometer and line scan camera (spectral domain OCT) or using a wavelength swept laser, single or dual balanced detector and high speed A/D (swept source OCT) [3
]. Fourier domain detection has a fundamental sensitivity advantage which enables orders of magnitude faster imaging speeds compared to time domain OCT [4
The first demonstration of spectral/Fourier domain OCT for in vivo
retinal imaging was performed in 2002 [7
]. Ultrahigh axial resolutions of 2.1 to 3.5μm
in the retina at ~10kHz to ~29kHz axial scan rates were demonstrated in 2004 [8
]. In 2006, the first commercial ophthalmic OCT instruments using spectral/Fourier domain OCT were introduced. Spectral/Fourier domain OCT instruments measure interference spectra using a spectrometer and line scan camera. The limited number of camera pixels results in a design trade-off between OCT axial resolution and imaging range. Current commercial ophthalmic OCT instruments typically achieve ~5μm
axial resolution with ~25–27kHz axial scan rates over an imaging range of ~2.0–2.6mm. Instruments such as the Optopol Copernicus HR operate with 3μm
axial resolution at 52kHz. Other instruments, such as the Heidelberg Spectralis, have axial resolution of 7μm
at 40kHz. In these systems, performance tradeoffs in imaging speed, imaging range, and axial resolution have been governed by commercially available CCD linescan camera technology. Recently, research OCT instruments using high speed CMOS cameras have been demonstrated for retinal imaging at record 312kHz axial scan rates [11
]. However, operating at high imaging speeds requires short camera exposure times, resulting in reduced sensitivity. Although good quality images can be obtained in young subjects and normal eyes at imaging speeds from 70 to 312kHz axial scan rates with 850nm wavelengths [11
], the practicality of imaging at speeds much greater than ~100kHz with such technology in the clinic is not clear because many patients have cataracts and ocular opacities, which reduce signal strength and image quality.
Swept source/Fourier domain OCT [3
] offers several advantages over spectral/Fourier domain OCT, including reduced fringe washout, better sensitivity with imaging depth (lower sensitivity roll-off), longer imaging range, higher detection efficiencies (because there are no spectrometer grating losses), and ability to perform dual balanced detection. Early wavelength swept laser sources using a variety of laser tuning mechanisms, including rotating polygon mirrors [12
] and galvo driven grating filters [13
], achieved axial scan speeds in the ~10–50kHz range at wavelengths of ~1050nm [12
] and ~850nm [13
] for retinal imaging. The long length of these laser cavities limited the maximum laser sweep rates because of the time required for a sufficient number of laser round trips through the gain medium and filter to achieve saturation and suppression of ASE noise (amplified spontaneous emission) from the laser gain element [17
]. The introduction of Fourier domain mode locking (FDML) enabled dramatic increases in sweep speeds by using a long fiber optic delay line in the laser cavity to store the entire frequency sweep while synchronously tuning the intracavity filter with the sweep [18
]. Recently, by combining a fast laser sweep repetition rate with multiple spot imaging, an FDML laser operating at 1310nm was used to generate 3D-OCT volumes at record imaging rates of 20.8 Million axial scans per second with 13μm
axial resolution and record data acquisition rates of 4.5 GVoxel per second with 11μm
axial resolution in tissue [19
]. Retinal imaging cannot be performed at 1310nm because of water absorption. Ultrahigh speed swept source OCT retinal imaging using an FDML laser operating at 1060nm with an axial scan rate of 249kHz and an 8μm
axial resolution in tissue [20
] has been demonstrated. Anterior segment imaging at 200kHz with 9μm
axial resolution over a 2mm imaging range and 25μm
axial resolution over an 8mm imaging range using an FDML laser at 1310nm has also recently been performed [21
Increased penetration into the choroid and optic nerve head with reduced sensitivity to ocular opacities has been demonstrated for OCT imaging in the water absorption window at ~1050nm [14
] compared to 850nm wavelengths. Imaging at 1050nm can be performed with spectral/Fourier domain OCT using InGaAs linescan cameras or with swept source/Fourier domain OCT using swept lasers. The width of the 1050nm water absorption window limits the achievable axial resolution [24
]. Imaging at 850nm permits higher axial resolution and appears to provide higher inner retinal layer contrast than 1050nm [25
]. Although the clinical utility of 1050nm OCT imaging is yet to be determined, recent research results from prototype systems suggest 1050nm imaging may become an important clinical technology for ophthalmology due to enhanced imaging range and penetration through tissue and ocular opacities [26
In this paper, we demonstrate ultrahigh speed swept source/Fourier domain ophthalmic OCT imaging at speeds of 100,000–400,000 axial scan rates using a new commercially available swept laser centered at ~1050nm. Four imaging configurations are characterized and demonstrated for retinal and anterior segment imaging. A first configuration images at 100kHz axial scan rate with 5.3μm
axial resolution in tissue and uses an optically derived, variable frequency clock for the high speed A/D acquisition. This optical clocking method samples the nonlinearly swept waveform at equal wavenumber (k) intervals, thereby eliminating the need to perform numerical k calibration and sweep linearization. Sweep-to-sweep variation is accommodated in real time with this method. Two dimensional cross sectional OCT images and dense three dimensional raster scanned volumes of the macula, disc, and anterior segment are demonstrated. A second configuration images at 100kHz axial scan rate with a 1 GSPS A/D sample rate and uses numerical k calibration to linearize the fringe signal to achieve axial resolution of 6.0μm
over a long 7.5mm in tissue (10mm in air) imaging range. This configuration is used to image the anterior segment, enabling visualization of the cornea, iris, and anterior lens in a single image. A third configuration images at 200kHz axial scan rate by buffering and multiplexing the laser sweep using a long optical fiber. A fixed A/D sampling rate of 400 MSPS with numerical k calibration enables an extended imaging range over the optically clocked configuration. The high imaging speeds of this configuration are used to demonstrate wide field 12mmx12mm (1100×1100 axial scan) OCT volumetric raster scans that include the optic nerve head and macula in a single acquisition. Individual cone photoreceptors can be visualized in certain regions of the retina when zooming in to image small, high sampling density 700μm
volumes at 200kHz axial scan rate. Multiple small volumes were acquired and show increasing photoreceptor spacing from the fovea to optic nerve head. A fourth configuration achieves 400kHz axial scan rates by projecting two spots on the retina and detecting with two parallel interferometers. To the best of our knowledge, 400kHz axial scan rate is the fastest speed demonstrated for imaging the retina. The 400kHz axial scan rate is almost twice as fast as previously reported for 1050nm retinal imaging [20
] and twenty times faster than most current commercial OCT instruments. As demonstrated in this paper, multi-spot retinal imaging may be an enabling approach for obtaining wide field, large volume acquisitions within the short 2–3 second imaging times required for clinical ophthalmic imaging.
In all configurations, the long coherence length of the swept laser shows significantly improved imaging depth range and less sensitivity roll-off with depth when compared to spectral/Fourier domain OCT. It is common when imaging patients in a clinical setting that the eye move in the axial direction relative to the instrument due to heartbeat and breathing, as well as gross head movement within the instrument chin rest. The improved sensitivity roll-off performance enables high signal strength images to be obtained over an extended imaging depth range, which promises to improve performance in the clinic where patient head and eye movements in the axial direction can compromise data quality. The improved performance of 1050nm wavelength imaging through cataracts and ocular opacities, improved image penetration into the optic nerve head and choroid, high instrument sensitivity at 100kHz to 400kHz axial scan rates, and long imaging range promise to enable practical ultrahigh speed OCT imaging in the ophthalmology clinic. Furthermore, these results show that a single ultrahigh speed ophthalmic system can image both the retina and anterior eye at 1050nm to achieve a good balance between axial resolution and penetration/imaging range. Results suggest that ophthalmic OCT imaging systems using swept source/Fourier domain OCT detection, new swept laser technology, sweep buffering, and multi-spot imaging may have a powerful impact on clinical ophthalmic imaging, as well as in other applications.
All of the prototype ophthalmic OCT imaging configurations use swept source/Fourier domain detection. The systems are summarized in and the layouts of the optical systems are shown in . A commercially available swept laser at ~1050nm (Axsun Technologies, Inc.) with ~100nm full width sweep bandwidth operates at a 100kHz repetition rate, as shown in . The laser consists of a semiconductor gain element and high finesse tunable filter with a short cavity length to enable rapid tuning. As shown in , the laser is connected to a 50:50 fiber coupler (for output power attenuation) to generate a 100kHz swept laser source. As shown in , during the backward sweep, the laser is turned off. The duty cycle of the sweep is only slightly larger than 50 percent, so the effective repetition rate of the laser can be doubled by buffering and multiplexing the sweep, as shown in . The laser sweep is split by a 60:40 fiber coupler and the original sweep from the 40 percent output is directed to the 50:50 fiber coupler for multiplexing. A ~1km length of fiber is used to delay the sweep from the 60 percent output by one half of the sweep period such that a “copy” of the sweep can be combined with the original sweep during the time period when the laser is off, as shown in . Polarization controls are used to match the polarization states of the two sweeps. This generates a 200kHz repetition rate sweep at the outputs labeled 2 and 3 in . shows a schematic of the OCT interferometer and patient interface for retinal imaging. Light from output 1, 2, or 3 in is coupled into a 50:50 (Configuration C) or 70:30 (Configurations A, B, and D) fiber coupler in the interferometer(s). A portion of the light proceeds to the patient interface and the other portion to the reference arm. The average output power of the laser is ~18mW. Retinal exposures were 1.9mW or less, consistent with safe ocular exposure limits set by the American National Standards Institute (ANSI). When imaging at 100kHz and 200kHz, the exposure was 1.8mW. At 400kHz, the exposure in each of the two spots was 950μ W each, totaling 1.9mW. A 70:30 fiber coupler is used for the 100kHz and 400kHz configurations to attenuate the power before reaching the eye and also has the advantage of increasing the amount of light to the interferometer that is collected by the patient interface. The sweep buffering stage introduces considerable power attenuation. When imaging at 200kHz, a 50:50 fiber coupler is used to obtain 1.8mW on the eye. For anterior segment imaging, the patient interface module is modified to telecentrically scan a focused beam, as shown in . Light backscattered from the eye is collected by the patient interface module and interferes with the reference arm light at a 50:50 fiber coupler. A balanced detector with InGaAs photodiodes is sampled with a high speed 14 bit 400MHz A/D card (Innovative Integration X5-400M) or 8 bit digital storage scope (Tektronix DPO 7104) set to sample at 1 GSPS to record the interferometric signal. To achieve 400kHz axial scan rates, two separate imaging spots, each with its own interferometer, were projected onto the eye. Output number 2 in supplies light to the first interferometer and output number 3 in supplies light to the second interferometer. Two separate balanced detectors were used to measure the signals from the two interferometers. Because the laser sweep is nonlinear in time (red lines in show the sweep phase vs. sample number), the sweep must be either optically clocked or calibrated to be linear in k (or frequency). The sweep calibration can be determined experimentally by directing the reference arm light to a Michelson interferometer and detecting the output interference fringe signal with a single (non-balanced) channel of the detector, as shown in . The Michelson calibration technique requires manual fiber connections, but has the advantage of being dispersion balanced and time delay matched because the same detector and electrical path are used for calibration and imaging.
System Design Configurations and Performance Measures
Fig. 1 System layout. (A) Swept laser source for 100kHz OCT imaging. (B) Swept laser source for 200kHz and 400kHz OCT imaging. (C) System configuration for retinal imaging. (D) Patient interface for anterior segment imaging. (E) System configuration for sweep (more ...)
Fig. 2 Short cavity light source. (A) Spectrum measured with OSA. (B) Sweep trigger signal, laser output, and A/D clock signal from the swept laser source. (C) OCT fringe data acquired at fixed 400 MSPS clock rate (top) and optically derived, variable frequency (more ...)
A beam diameter of 1.4mm (1/e2 diameter) on the cornea enables a long depth of field (Raleigh range) for long depth range imaging. Changing the eye lens  to achieve 3.3mm beam diameter improves transverse resolution for imaging small features, however depth of field is reduced. For anterior imaging, the 0.035NA configuration achieves high lateral resolution, while the 0.024NA configuration achieves a long focal length for extended depth imaging. Beam diameters were measured with a beam profiler (DataRay WinCamD).
2.2. Sample clocking and wavelength calibration
Swept source OCT imaging systems typically have a nonlinear frequency sweep in time and require a numerical calibration of the raw fringe data to achieve equal sample spacing in k (or frequency) to linearize the fringe. Mach-Zehnder [18
] or Michelson [21
] interferometers can be used to generate a calibration trace at a fixed interferometer delay. The calibration can be applied to every laser sweep individually to account for sweep-to-sweep variation [21
] caused by dynamic instabilities in the high speed tunable optical filter, however this requires two simultaneous channels of high speed acquisition as well as substantial post processing. One representative calibration can be applied to multiple lines of acquisition [28
], however imaging performance can be degraded if the high speed tunable optical filter has mechanical resonances causing sweep-to-sweep variation. Attempts to linearize the sweep profile by optimizing the filter driving waveform trajectory can address sweep nonlinearity, but do not explicitly address sweep-to-sweep variation [29
By clocking the high speed A/D with a signal derived from an interferometer and conditioned with electronics both integrated in the commercial light source, the fringe can be automatically sampled at intervals with equal k (or frequency) spacing, to produce a linearized fringe signal. This allows a direct Fourier transform of the fringe data to generate OCT intensity images and eliminates the need for the Michelson or Mach-Zehnder calibration and fringe linearization procedure. Furthermore, provided that the optical clock is stable, the data is properly sampled, even with sweep-to-sweep variation. Similar approaches for A/D clocking to achieve uniform k-space sampling have been previously described [30
]. These approaches also reduce the data transfer and storage requirements for the raw fringe data because the slower frequency sweep portions of the fringe are not oversampled.
The commercially available short cavity swept laser used in this study provides an A/D clock signal output, which was used to clock the high speed A/D data acquisition board. A start-of-sweep trigger from the laser source was used to trigger the A/D data acquisition through a pulse generator/delay. shows the start-of-sweep trigger, laser output, and optically derived clock signal. The clock is synthesized from the laser output, so is only valid during the time when the laser is on. When the laser is off, the swept source generates an asynchronous clock signal, as shown in . The optical clock works for the specific A/D used in these studies, however some A/Ds require continuous clock signals with precise duty cycles, without dropouts or phase discontinuities. For the system used in this study, occasional clock instabilities occurred, resulting in distorted axial scans with loss of resolution and increased background levels. Example fringes acquired using a fixed frequency 400 MSPS sampling rate and the optical clocking scheme for a fringe generated from reflections at two depth positions are shown in . The red line indicates fringe phase vs. sample point. Note the nonlinearity in the phase curve for the fixed 400 MSPS data compared to the linearized fringe from the optically clocked data set. Results from a direct Fourier transform of the clock signal indicate a maximum clock frequency of only ~310MHz, which implies that the 400MHz sampling configuration should have a longer imaging range than the optically clocked configuration.
shows that the optically derived clock is only valid during the laser sweep. When buffering the laser to achieve a 200kHz repetition rate, the A/D is operated at a constant 400 MSPS sampling rate using an internal clock and software fringe linearization is performed. The reference trace for the software calibration is obtained from a Michelson interferometer, as shown in , which has the advantage of being dispersion balanced to separate the effects of sweep nonlinearity from dispersion mismatch. A similar software fringe linearization approach was used for the 100kHz axial scan rate configuration with digital storage scope acquisition set to sample at 1 GSPS and the 400kHz axial scan rate dual beam configuration.
2.3. Computer, data acquisition, electronics and control
Ultrahigh speed OCT can acquire large data sets within only a few seconds acquisition time. To allow data sets greater than the 4 Gigabytes supported by 32 bit operating systems, the instrument control computer used a 64 bit operating system (Windows Vista 64). The high speed A/D card can sample up to 400 MSPS at 14 bit resolution. Data acquisition at 100kHz to 200kHz is performed using one A/D card in the computer. However, although the A/D card has two analog input channels, the data throughput requirements of operating both channels at 100 percent duty cycle in parallel exceed the sustained data transfer rate of the PCIe data connection (8 lanes) and board drivers. To enable data capture on two channels at full acquisition rates, two identical A/D cards (X5-400M, Innovative Integration) were used simultaneously, as supported by the independence of PCIe architecture. Digital galvanometers (Cambridge Technology) were driven by a 16 bit D/A board (National Instruments). A custom user interface and data acquisition software was developed in C++ to coordinate instrument control and enable user interaction.
Improved performance for the retina and anterior angle imaging configurations was obtained by modifying a commercially available 350MHz InGaAs (Thorlabs) dual balanced detector to increase the transimpedance gain by 2X and reduce the bandwidth to ~200MHz. A stock 350MHz dual balanced detector (Thorlabs) was used for the long imaging range anterior segment imaging with the digital scope set to sample at 1 GSPS for data acquisition. Wavelength dependence in the 50:50 fiber coupler before the detectors causes an imbalance in the reference arm signal as the laser is swept, resulting in a significant background in the dual balanced detection. The low frequency background occupied 2–4 bits of the A/D range, leaving only 10–12 of the 14 bits to digitize the fringe. When the digital storage scope was used to record the signal at 1 GSPS, only 8 bit digitization was possible and a high pass filter (Mini-circuits 50MHz high pass filter) was used to remove the background. This filter also blocked interferometric signals near the zero delay, over the first 1mm imaging range.