Imaging
Retinal Nerve Fiber Layer Imaging with Spectral-domain Optical Coherence Tomography (OCT)—A Review of the Cirrus HD-OCT
Elaine To, MSc, Dennis Lam, MD, FRCOphth and Christopher Kai-shun Leung, MD, MBChB Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong
Abstract
High-speed, high-resolution imaging of the retinal nerve fiber layer (RNFL) with spectral-domain optical coherence tomography (OCT) has become an essential tool for evaluation of glaucoma. The Cirrus HD-OCT (Carl Zeiss Meditec, Inc. Dublin, CA) is a spectral-domain OCT that provides visualization of the distribution pattern and measurement of RNFL abnormalities in a 6x6mm2 optic disc region. Analysis of the RNFL thickness deviation map—a color-coded map displaying areas of RNFL abnormalities—detects glaucoma with high sensitivity and specificity. Trend analysis of average and sectorial RNFL thicknesses, and event analysis of the RNFL thickness maps and the RNFL thickness profiles can be used to detect and follow diffuse and focal RNFL progression. RNFL measurement with spectral-domain OCT could provide important information for use in formulating treatment plans and evaluating disease prognosis in the management of glaucoma.
Keywords
Spectral-domain optical coherence tomography (OCT), retinal nerve fiber layer (RNFL), Cirrus HD-OCT, RNFL thickness map, RNFL thickness deviation map, glaucoma progression, RNFL progression
Disclosure: Christopher Kai-shun Leung, MD, MBChB, has received honoraria for conference presentations and research support from Carl Zeiss Meditec, Inc. The other authors have no conflicts of interest to declare. Received: November 2, 2010 Accepted: January 29, 2011 Citation: US Ophthalmic Review, 2011;4(1):12–6 Correspondence: Christopher Kai-shun Leung, MD, MBChB, Department of Ophthalmology & Visual Sciences, The Chinese University of Hong Kong, Hong Kong Eye Hospital, 147K Argyle Street, Kowloon, Hong Kong. E:
tlims00@hotmail.com
Support: The publication of this article was funded by Carl Zeiss Meditec, Inc. The views and opinions expressed are those of the authors and not necessarily those of Carl Zeiss Meditec, Inc.
Optical coherence tomography (OCT) is a non-contact optical technique that allows imaging and measurement of the retinal nerve fiber layer (RNFL). Since 1991,1
Meditec, Inc.)—a spectral-domain OCT—as an example to illustrate the advances in OCT technology to detect and follow glaucoma.
OCT has undergone rapid evolution for use in
detection and monitoring of glaucoma and macular diseases. Based on the principle of low-coherence interferometry, OCT measures the intensity of backscattered infrared light. Time-domain OCT operates by using a moving reference mirror to measure the consecutive sequence of echo time delays. By contrast, the reference mirror in spectral-domain OCT is stationary. The interference spectrum data is detected by a spectrometer and Fourier transformed to generate axial measurements.2
This means that spectral-domain OCT has a
much faster scan speed (at least 20,000 A-scans per second) than time-domain OCT (up to 400 A-scans per second for the Stratus OCT, Carl Zeiss Meditec, Inc.).
High-speed imaging allows volumetric datasets covering a region of the optic disc to be captured in a few seconds. The axial resolution of approximately 5μm means that the RNFL can be measured with low variability. These features greatly facilitate the imaging of RNFL for glaucoma evaluation. This paper uses the Cirrus HD-OCT (Carl Zeiss
12 The Optic Disc Cube Scan
The Cirrus HD-OCT (Carl Zeiss Meditec, Inc.) uses a super luminescent diode with a center wavelength of 840nm and has an acquisition rate of 27,000 A-scans per second. The transverse and axial resolutions are 15μm and 5μm, respectively. A line scanning ophthalmoscope with a frame rate of 20Hz provides a ‘fundus viewpoint’ to indicate the retinal area addressed by the OCT scan. The ‘optic disc cube’ scan protocol is used to image the optic disc and the RNFL over the 6x6mm2 parapapillary region using 200x200 axial measurements. The RNFL at each scan point is then segmented and measured, and an RNFL thickness map is generated in the analysis printout (see Figure 1).
The RNFL thickness map provides an overview on the distribution profile of the RNFL over the optic disc. The RNFL thickness map and the RNFL thickness deviation map facilitate visualization of RNFL defects (see Figure 2). The RNFL thickness map is composed of 200x200 pixels and the RNFL thickness deviation map is composed of 50x50 super-pixels
© TOUCH BRIEFINGS 2011
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