Figure 2.1

Basic principles of time-domain OCT. Interference of reference and reflected light beams from different layers within the tissue occurs as a function of axial transition of the reference mirror.

As of this date, two different OCT technologies are still available, the older time-domain (TD) and the more recent spectral-domain (SD) OCT, also called Fourier-domain (FD) OCT [10]. The latter name refers to Jean-Baptiste Joseph Fourier (1768–1830), a French Professor at the École Polytechnique, who is known for his work on the Fourier Transform [11].

Time-domain (TD) optical coherence tomography

The basic components and setup of a TD-OCT system are detailed in Figure 2.1. As the name of this technology suggests, TD-OCT is based on the difference in the time delay of the sample light echoes reflected from the different retinal layers as a function of their depth within the tissue (here, the retina) and the reference beam echo, a single echo that is varied by changing the position of the mobile reference mirror (i.e., the reference arm-length).

Interference of light beams with low coherence only occurs in cases where the distance traveled by the light in both arms of the interferometer is equivalent to within the coherence length in order to allow interference by co-occurrence on the level of the photodetector. This phenomenon allows accurate measurement of the echo time delay. Light reflected from superficial structures of the retina or uppermost retinal layers has a shorter echo time delay than light reflected from deeper retinal structures (innermost layers) [2, 5]. The reference mirror is axially translocated in order to match echo time delays from various tissue layers. As the path length of the moving reference mirror is known, it is possible to calculate the depth of the tissue from which the fraction of reflected light arises based on the specific time delay. Based on the amplitude of the interference signals that arise from interference of the reflected light from retinal layers of different depths and the reference light from various path lengths, a single axial scan, the so-called A-scan, is deduced. Key to longitudinal scanning is the fact that the reference mirror can be mechanically mobilized, resulting in a shift of the reference beam, extending the reference path. The reference mirror moves with a specific distance- and time-interval constant, leading to multiple adjacent A-scans, which, in sum, generate the cross-sectional 2-D or longitudinal B-scan of the retina (Figure 2.1). In TD-OCT the cross-sectional image representing the different retinal layers is generated as a function of time delay of the reflected sample light beams. In turn, the time delay depends on the composition and depth of the different layers and the position or axial translation of the reference mirror. These basic principles have led to the name of this technology: time-domain OCT.

TD-OCT has an axial resolution of approximately 10 µm or less (Table 2.1). Image acquisition speed is limited in TD-OCT, however, because the reference mirror needs to be moved.

Table 2.1 Technical characteristics of commercially available OCT devices

Device	Fundus image			OCT image								Manufacturer	Web address
Device	Live image	Size	Optional modes	Technology	Light source	Scan Speed A-scans/sec	Transverse resolution**	Axial resolution**	Scan depth	Min. pupil diameter	Max. no. of A-scans/B-scan	Manufacturer	Web address
SPECTRALIS	cSLO	30°, 55°, 165°	AF, ICGA, FA, MC	SD	870 nm	40000	14 µm	7 µm	1.9 mm	2 mm	1536	Heidelberg Engineering	www.heidelbergengineering.com
Stratus	IR Cam	26° × 20.5°		TD	820 nm	400	20 µm	10 µm	2 mm	3.2 mm	768	Zeiss Meditec AG	www.meditec.zeiss.com
CIRRUS 4000	SLO	36° × 30°		SD	840 nm	27000	15 µm	5 µm	2 mm	2 mm	4096	Zeiss Meditec AG	www.meditec.zeiss.com
3D OCT 2000	IR Cam	45°	CF Camera,AF, FA	SD	840 nm	50000	20 µm	6 µm	2.3 mm	2.5 mm	1024	Topcon	www.topcon-medical.eu
DR 1	SLO	43°		SwS	1050 nm	100000	20 µm	8 µm		2.5 mm	1024	Topcon	www.topcon-medical.eu
iVue	IR Cam	32° × 23°		SD	840 nm	25000	15 µm	5 µm	2.3 mm	3 mm	1024	Optovue	www.optovue.com
RS-3000	SLO	40°x 30°		SD	880 nm	53000	20 µm	7 µm	2.1 mm	2.5 mm		Nidek	www.nidek-intl.com
OCT SLO	cSLO	29°		SD	830 nm	25000	20 µm	10 µm	2 mm	3 mm		Optos	www.optos.com
Copernicus HR	IR Cam	30°		SD	850 nm	52000	12 µm	3 µm	2 mm	3 mm	20000	Optopol	www.optopol.com
Canon OCT HS-100	SLO	44° × 33°		SD	855 nm	70000	20 µm	3 µm	2 mm	3 mm		Canon	www.canon-europe.com

cSLO confocal Scanning Laser Ophthalmoscope

SLO Scanning Laser Ophthalmoscope

AF Auto Fluorescence

ICGA Indocyanin Green Angiographay

FA Fluorescence Angiography

MC Multicolor

* only 3-D OCT FA Plus

** optical resolution

Spectral-domain (SD) optical coherence tomography

The most recent, so-called fourth-generation or spectral-domain (SD) OCT technology, is based on the mathematical Fourier transform equation. This methodology is, therefore, also known as Fourier-domain technology. The Fourier transform eliminates the need for a movement of the reference beam mirror. SD-OCT replaces the photodetector from TD-OCT with a spectrometer capable of analyzing the full spectrum of interference signals at one time point generated when the sample and the reference beam meet along the same path. It allows the analysis of all frequencies simultaneously. As opposed to TD-OCT, in SD-OCT the interference signal is a function of the different wavelengths and not of the different echo time delays. The whole wavelength spectrum is converted into time delay signals by the Fourier transform. In retinal OCT imaging, this allows the analysis of all echoes from the different retinal layers simultaneously. As a consequence, SD-OCT is much more rapid than its counterpart, while at the same time providing excellent resolution. In TD-OCT, high-resolution imaging can only be achieved at the expense of an increase in acquisition times. Scanning speed is 50–100 times faster with SD-OCT than with TD-OCT [10,11].

Rapid scanning allows larger numbers of B-scans per time interval, and high-speed macular scans with an increasing B-scan density have become possible with the latest devices (see Table 2.1). SD-OCT achieves an axial resolution in the range of 5–6 µm, while digital resolution can be even higher (Table 2.1). High-resolution scans together with real-time averaging significantly increase the signal-to-noise ratio (SNR), ensuring superior image resolution and quality in SD-OCT.

Currently, a number of TD- and SD-OCT devices are commercially available. Technical characteristics of different OCT devices are detailed in Table 2.1.

Only gold members can continue reading. Log In or Register to continue