Ophthalmic & Medical OCT
EXALOS is the leading supplier of wide-bandwidth and high-power SLEDs (EXS) for ophthalmic and medical OCT applications.
EXALOS is also offering high-performance driver boards for SLEDs (EBD) featuring ultra-low-noise and ultra-high current stability with zero long-term drift.
EXALOS is also a key supplier of high-speed swept sources (ESS), for wavelength ranges from 800 to 1600 nm, and of ultra-low-noise balanced receivers (EBR).
O Optical coherence tomography (OCT) is a powerful enabling technology for producing real-time, high-resolution cross-sectional images of tissue with a resolution of a few microns .
Other medical applications of OCT such as gastrointestinal, pulmonary, dental or skin tissue examinations are in early-stage clinical use which show the potential of OCT being deployed in multiple areas of medicine.
EXALOS´ industry-leading, innovative products for OCT
COMBINED BROADBAND SLED MODULES
BROADBAND LIGHT SOURCES
Today’s ophthalmic OCT systems use SLEDs in the 800-to-900-nm wavelength range for retinal applications and SLEDs in the 1300-nm regime for cornea examinations. The 1060-nm spectrum is also gaining traction in recent years for whole-eye imaging with emphasis on retinal and choroidal layers due to a favorable combination of relatively low water absorption, low dispersion, lower scattering and higher laser safety power limits. Non-ophthalmic medical OCT typically operates in the 1300-nm regime.
Time-domain (TD-) OCT systems, see Fig. 1, require SLEDs with good spectral shape, for example first-order Gaussian, such that the coherence function features good sidelobe suppression. Axial or depth scanning is achieved by moving the reference mirror. The optical detector is a simple photodiode (PD) or photoreceiver that is connected to a data acquisition (DAQ) card, which is sampling the OCT signal and forwarding the data to a host PC. TD-OCT is typically limited to a few kHz, which is why time-domain have been replaced in many medical applications by faster and more sensitive Fourier-domain systems.
Fig. 1 Generic setup of a time-domain (TD) OCT system with a reflective Michelson interferometer.
Spectral-domain (SD-) OCT systems, see Fig. 2, use short-wavelength SLEDs having a spectrum with a flat-top shape and a bandwidth of 50 to 100 nm. The optical detection of the OCT signal is accomplished using a spectrometer featuring a silicon detector array or a so-called line-camera that is connected to a frame grabber card, which is recording the received optical spectrum and forwarding the data to a host PC. The reference mirror is not moving but fixed and axial or depth information is achieved through Fourier transform of the spectral data into the spatial domain.
Fig. 2 Generic setup of a spectral-domain (SD) OCT system with a reflective Michelson interferometer.
Typical ophthalmic SD-OCT systems employ an 840-nm SLED with a 3-dB bandwidth of 50 nm and a 10-dB bandwidth of 75 nm. The spectrometer typically spans 80 nm and comprises a 2000- pixel camera with a refresh rate of 25 to >100 kHz.
Not considering any spectral windowing, this spectral bandwidth corresponds to an axial resolution of 8-9 microns in air or 5-7 microns in tissue. Using 2048 sampling points for the Fourier transform, an imaging depth of 4.5 mm in air or 3.3 mm in tissue is achieved. In many cases advanced signal processing, e.g., spectral windowing, is performed such that sidelobes are less critical. Still, spectral shape matters in order to have a rather constant power spectral density over a wide wavelength range.
Fig. 3 Generic setup of a swept-source (SS) OCT system with a non-reflective reference arm.
Swept-source (SS-) OCT systems, see Fig. 3, employ, instead of a broadband SLED, a swept source as an optical source. The swept source is typically a laser featuring a narrow linewidth that is capable of performing fast wavelength sweeps across 80 to 160 nm at repetition rates of 1 to 200 kHz. Here, the linewidth of the laser defines the coherence length and hence the imaging depth of the SS-OCT system. The optical sweep range defines the axial resolution. The optical detection of the OCT signal is accomplished using a balanced receiver that is connected to a high-speed DAQ card, which is sampling and forwarding the data to the host PC.
EXALOS is the leading supplier of high-bandwidth and high-power SLEDs (EXS, ) for ophthalmic and medical OCT applications. EXALOS is also offering high-performance driver boards for SLEDs (EBD, ) featuring ultra-low-noise and ultra-high current stability with zero long-term drift. EXALOS is also a key supplier of high-speed swept sources (ESS, ), for wavelength ranges from 800 to 1600 nm, and of ultra-low-noise balanced receivers (EBR, ).
The following OCT images were taken with EXALOS SLEDs, its combined broadband light sources (EBS, ), allowing resolutions down to two microns in tissue, or with EXALOS’ high-speed and broadband swept sources.
Fig. 4 3D OCT cross-section of the macular hole of a human retina, recorded with an 840-nm broadband SLED EXS210022 and a high-resolution spectrometer (courtesy of Prof. Rainer A. Leitgeb, Center of Biomedical Engineering and Physics, Medical University of Vienna).
Fig. 5 3D volume tomogram of Fig. 4.
Fig. 6 2D OCT cross-section (B-scan) of a human retina, recorded with a 1070-nm broadband SLED source EBS4C34 and a high-resolution spectrometer with an InGaAs camera (courtesy of Prof. Wolfgang Drexler, Center of Biomedical Engineering and Physics, Medical University of Vienna).
Fig. 7 Detailed excerpt of Fig. 6, demonstrating the high resolution of the individual retina layers and the good penetration into the choroid.
Fig. 10 2D OCT cross-section (B-scan) of a human retina, recorded with a 1060-nm high-speed 100-kHz Swept Source ESS320026 (courtesy of Prof. Rainer A. Leitgeb, Center of Biomedical Engineering and Physics, Medical University of Vienna).