An increasingly important tool for medical diagnosis and biomedical research, Optical Coherence Tomography (OCT) enables two and three dimensional visualization of the internal structure and morphology of tissue [1
]. High sensitivity, large dynamic range, and micron level resolution imaging are achieved with OCT by interferometric detection of backscattered light from the sample. In ophthalmology, OCT can perform non-invasive structural and quantitative imaging of the retina and anterior segment, which enables the identification of pathologies for disease diagnosis or monitoring responses to therapy [2
The earliest implementations of OCT used low coherence interferometry with time domain detection in which the echo delay of backscattered light was measured by mechanically sweeping a mirror in a reference arm [3
]. Commercial ophthalmic OCT instruments using this time domain approach operated with imaging speeds of 400 axial scans per second and acquired individual cross-sectional OCT images of the retina. The development of Fourier domain detection allowed dramatic improvements in imaging speeds, which made acquisition of three dimensional data sets feasible.
Fourier domain OCT can be implemented either with a swept laser source [6
] or spectrometer based system [8
]. Both Fourier methods not only offer faster imaging speeds, but also a fundamental sensitivity advantage [10
] when compared to time domain methods. Swept source ophthalmic imaging has been demonstrated in research systems with axial scan rates typically in the 10 kHz - 50 kHz range. Examples of such demonstrations include systems with an axial scan rate of 18.8 kHz centered at 1050 nm [13
], 43.2 kHz at 855 nm [14
], 43 kHz at 840 nm [15
],16 kHz at 850 nm [16
], and 30 kHz at 1050 nm [17
]. Ultrahigh speed swept source retinal imaging was recently performed using a Fourier domain model locked (FDML) laser operating at 1050 nm with an axial scan rate of 236 kHz [18
] and at 1060 nm with an axial scan rate of 249 kHz [19
]. OCT imaging speeds of 5 MHz using a stretched pulse super-continuum source [20
] and 60 MHz using optical demultiplexers [21
] have been demonstrated in the laboratory, but do not have adequate sensitivity at the maximum allowed incident light levels for ophthalmic imaging applications. Swept source OCT systems have reduced sensitivity roll-off with imaging depth when compared to spectrometer based systems [16
] and can readily operate in the 1050 nm wavelength range, which may have advantages for deeper tissue penetration [22
]. Swept source systems require rapidly tunable, narrow linewidth lasers, but have the advantage that they use high speed A/D converters and single point detectors, rather than bulky spectrometers.
Most of the current generation ophthalmic OCT systems use spectral / Fourier domain detection based on a spectrometer design and there are currently no FDA approved swept source systems commercially available. Spectrometer based systems can make use of superluminescent diode (SLD) light sources with broad bandwidths for high axial resolutions, as well as leverage the existing infrastructure of economically priced high speed line scan cameras and frame grabbers used in machine vision. High-speed retinal imaging using spectral / Fourier domain detection was first demonstrated in 2002 at an acquisition rate of 1 axial scan every 19 ms [24
]. High speed acquisition of 29,000 axial scans per second with better than 6 μm
axial image resolution was demonstrated in 2003 [25
]. Ultrahigh image resolutions of 2.1 to 3.5 μm
in the retina were achieved at acquisition rates of 10,000 to 16,000 axial scans per second in 2004 [26
]. Manufacturers began introducing spectrometer based commercial instruments for ophthalmic imaging beginning in 2006. Specifications vary, but most commercial systems have axial resolutions of 5-7 μm
and imaging speeds of 25,000 axial scans per second.
The sensitivity and dynamic range requirements for ophthalmic OCT imaging are quite demanding because the incident power is limited by safety considerations. This governs the selection of suitable linear sensor arrays for spectral / Fourier domain OCT. The standard sensor for spectrometer based systems has been a low noise, high sensitivity, and high dynamic range CCD line scan camera. Examples include the e2v (previously known as Atmel) Aviiva SM2 CL2014 (2048 pixels with 28 kHz maximum line rate), SM2 CL4010 (4096 pixels at 14 kHz maximum line rate), M4 CL2014 (2048 pixels at 53 kHz line rate), Basler L104K (2048 pixels with a 29.2 kHz maximum line rate), and others. Line scan rates for this class of camera are generally in the 10 kHz to 55 kHz range. Using such cameras, ophthalmic spectral / Fourier domain OCT has been demonstrated in research systems at an axial scan rate of 29 kHz with 2048 camera pixels [28
], 75 kHz with 512 camera pixels [31
], and 12 kHz with 4096 camera pixels [32
]. It has been recognized for some time that CMOS technology has the potential to achieve faster sustained imaging speeds than CCD technology because it is possible to directly integrate digital communication circuitry, gain stages, A/D converters, and photosensitive pixels on the same chip. However, CMOS has traditionally suffered from lower sensitivity and higher noise than CCD [33
], reducing its suitability for OCT. Recent advances in CMOS imaging technology and sensor architecture are beginning to address these issues with a next generation of high speed line scan cameras.
This paper presents and compares several ultrahigh speed OCT system designs based on a recently developed CMOS line scan camera (Sprint spL4096-140k from Basler Vision Technologies) that exhibits exceptional sensitivity and noise characteristics while running at high speeds. The OCT system designs were chosen to investigate trade-offs in acquisition speed, sensitivity, sensitivity roll-off, resolution and imaging depth. The designs investigated differ in number of pixels used on the camera, light source/spectrum, spectrometer design, ophthalmic imaging module optics, and line rate/exposure time. Acquisition speeds for the four configurations tested range from 70,000 axial scans per second to 312,500 axial scans per second. A first configuration demonstrates improved sensitivity roll-off with imaging depth by using a high resolution spectrometer configuration while imaging at a speed of 70,000 axial scans per second. A second configuration demonstrates that it is possible to simultaneously achieve a higher axial resolution and larger spectrometer pixel count than any currently available commercial ophthalmic OCT system, while imaging at over 100,000 axial scans per second. A third configuration images at 250,000 axial scans per second, which is an order of magnitude faster than commercial systems, while maintaining good sensitivity performance. A fourth configuration images at 312,500 axial scans per second, which is to the best of our knowledge the fastest reported in vivo imaging of the human eye by any OCT method. Even higher speeds are possible by trading off resolution, sensitivity and imaging depth. Each configuration is characterized through optical testing and the trade-offs between acquisition speed, sensitivity, sensitivity roll-off, and resolution demonstrated with in vivo imaging of the fovea and optic disk in the human retina.
The high imaging speeds obtained with this new camera technology facilitate densely sampled, raster scanned, volumetric acquisition with minimal motion artifacts. Data sets of the fovea and optic disk show detailed fundus images with no visually detectable eye motion in the transverse plane. Registration scans that are oriented along the slow axis scan direction are used to correct for residual axial motion between B-scans, resulting in densely sampled three dimensional data sets which preserve the true global topography of the retina. Multiple volumetric data sets acquired over time can then be precisely registered and used to longitudinally track subtle changes in the retinal structure and promise to improve the monitoring of disease progression or response to therapy. The extremely fast imaging speeds also enable repeated sequential imaging of densely sampled volumes at a rapid refresh rate. This capability enables the visualization of small structures without the need for eye tracking, as well as the visualization of dynamic processes. As an example, we show that high speed image acquisition enables the visualization of individual photoreceptors (cones located at approximately 5 degrees peripheral to the fovea) in some areas of the retina without using a bite bar or adaptive optics. Registering the images of the cones between sequential volumes allows measurement of ocular motion in the transverse and axial directions. We then show how a similar repeated volumetric imaging technique can be used to image the capillary network surrounding the inner nuclear layer (INL) of the retina. Finally, we summarize and discuss the results of this study in the context of how increased acquisition speeds might improve clinical performance, functional imaging research, and investigation of disease pathogenesis. While this manuscript focuses on ophthalmic applications of high speed OCT imaging, this technology promises to have applications in many other clinical and biomedical research areas.