Ophthalmic & Medical OCT

Optical coherence tomography (OCT) is an emerging technology for producing real- time, high-resolution cross-sectional medical images of tissue with a resolution of a few microns [1]. The analogy of OCT is traditional ultrasound with the difference being that the OCT has a resolution that is typically at least a factor of one hundred better. On the other hand, ultrasound has a penetration depth of more than ten centimeters while OCT has an imaging depth of only a few millimeters – the near- infrared light used in OCT penetrates human tissue typically only millimeters before absorption and scattering become too high. Most OCT systems used today are in the field of ophthalmology (e.g., retina or cornea examinations) or in combination with endoscopy, for example for cardiovascular medicine. Other medical applications of OCT like dental or skin tissue examinations are still in development but show the potential of OCT being deployed in other areas of medicine. (SLED, ESS)

01

Fig. 1    Generic setup of a time-domain (TD) OCT system with a reflective Michelson interferometer.

 

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. New wavelengths like the 1050-nm low-absorption window of water have been recently proposed for retinal OCT as well. Non-ophthalmic medical OCT is typically operating 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. This is typically limited to a few kHz, which is why time-domain have been replaced in many medical applications by faster Fourier-domain systems. 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.

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. 

02

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 70 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. 
 
03

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, [2]) for ophthalmic and medical OCT applications. EXALOS is also offering high-performance driver boards for SLEDs (EBD, [3]) 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, [4]), for wavelength ranges from 800 to 1600 nm, and of ultra-low-noise balanced receivers (EBR, [5]). 

 The following OCT images were taken with EXALOS SLEDs, its combined broadband light sources (EBS, [6]), allowing resolutions down to two microns in tissue, or with EXALOS’ high-speed and broadband swept sources. 
 

 Fig4 3D OCT

  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).

 

 

Fig5 3D volume

 Fig. 5   3D volume tomogram of Fig. 4.

 

 

 Fig6 2D OCT cross-section

  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).

 

 Fig7 Detailed excerpt of Fig. 6

Fig. 7   Detailed excerpt of Fig. 6, demonstrating the high resolution of the individual retina layers and the good penetration into the choroid.

 

 

 

Fig 8 2D OCT cross-section (B-scan) of a finger nail

 Fig. 8   2D OCT cross-section (B-scan) of a finger nail, recorded with a 1310-nm ultra-broadband 150-nm swept source ESS320027  (courtesy of Michelson Diagnostics Ltd).

 

 

 

Fig 9 2D OCT cross-section (B-scan) of a finger tip

  Fig. 9   2D OCT cross-section (B-scan) of a finger tip, recorded with a 1310-nm ultra-broadband 150-nm swept source ESS320027  (courtesy of Michelson Diagnostics Ltd).

 

 

 Fig6 2D OCT cross-section (B-scan)

Fig 10 2D OCT cross-section (B-scan) of a human retina

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).