The Development and Applications of Optical Coherence Tomography

Rimayanti U¹*, Kiuchi Y² and Maulidia R³

¹Department of Nursing, UIN Alauddin Makassar, Indonesia

²Department of Ophthalmology and Visual Science, Hiroshima University, Japan

³Department of Ophthalmology, Awal Bros Hospital, Indonesia

*Corresponding author: Rimayanti U, Department of Nursing, Faculty of Health Sciences, UIN Alauddin Makassar, Jl. Sultan Alauddin 36, Samata, Gowa, 90222, Indonesia

Received: August 20, 2014; Accepted: September 15, 2014; Published: September 16, 2014

Abstract

Optical Coherence Tomography (OCT) was first applied for imaging of the eye, and until now, it has showed the largest benefit for ophthalmological examinations. The dynamic development of OCT has resulted in higher resolution, sensitivity, accuracy, and reproducibility of image of the detailed eye structures. This paper reviews the development of OCT and its applications to diagnose, monitor, and evaluate the treatment of eye diseases.

Keywords: Optical coherence tomography; Retina

Optical Coherence Tomography Development

Optical coherence tomography (OCT) is an emerging optical imaging technology that uses low-coherence light to perform high-resolution, cross-sectional imaging of the internal structure [1,2]. The imaging can be performed in-situ and in real time [3]. OCT was first applied for imaging of the eye, and recently, OCT has had the largest influence for clinical use in ophthalmology [4,5]. The reasons for that include the high transmittance of ocular media, the interferometric sensitivity and precision of OCT, which fits well the near-optical quality of many ophthalmological structures [6]. Thus, OCT has already become a routine tool for the investigation, mainly in the posterior segment of the eye. OCT can also be helpful to make the image of anterior part of eye, including details of corneal pathologies and structural changes of the chamber angle and iris [7,8].

OCT was first demonstrated by Huang D, et al. [1]. In 1993, the first in vivo tomograms of the human retina were published by Fercher et al. [9] and Swanson et al [10]. Initially, OCT techniques were based on low time-coherence interferometry (LCI) scans performed in the time domain[61]. Time-domain OCT (TD-OCT) generates cross-sectional images by measuring the echo time delay and intensity of light that is reflected or backscattered from intraocular structures [1]. To obtain the image of retinal layers, TD-OCT uses near-infrared light directed at the retina and also reflected from a reference mirror that is positioned at a known distance from each retinal layer. When light reflected from the retina combines with light reflected from the reference mirror, a pattern is formed. The reference mirror is then moved to different distances from the retina, thus producing different signals for the light reflected back from each respective retinal layer. A time delay is used to form a different signal (axial scan [A-scan]) for each retinal layer. Multiple A-scans are then combined to construct a two-dimensional image (B-scan) displaying a different signal for each retinal layer [11].

Two scans have to be performed in standard TD-OCT: the lateral OCT scan addresses laterally adjacent sample positions, and the OCT depth-scan to detect depth positions of light re-emitting sites in the sample [5]. The first commercially available OCT machine was the OCT 1000 marketed in 1996 by Carl Zeiss Meditec (Dublin, CA). The conventional TD-OCT system used moving reference mirror, which causes a physical constraint, provides imaging rate limited to approximately 400 A-scans per second, with an axial resolution of 10 μm [12]. Several improvements in OCT hardware have been introduced since the first commercial TD-OCT system became available. Better axial resolution [13-15] and increased scanning speed [16-22] are the two main advancements.

The advent of Spectral Domain OCT (SD-OCT) eliminated the need for the axial movement of the scanning mirror required in TD-OCT, yielding improved resolution and speed. The spectrometer-based SD-OCT, also known as Fourier domain OCT [23,24], uses a broadband light source and a low-loss spectrometer to measure the spectral oscillations from the different layers. SD-OCT obtains images by keeping the reference mirror in one position and collecting all of the backscattered light from the retina at a single point in time [25]. This technique relies on differences in the frequency spectrum of light reflected from the different layers [26].

Using the spectral technique, no OCT depth-scan is needed and, therefore, data acquisition can be very fast [6]. In a TDOCT system, increasing source bandwidth decreases the signal-to-noise ratio (SNR) as it requires increased electronic detection bandwidth, while the SNR of spectrally discriminated techniques in Fourier domain and swept source SD-OCT are independent of the source coherence length [25,27]. Therefore, both spectrometer- and swept source-based SDOCT systems show superior sensitivity advantage over the TD-OCT, leading to higher speed and scan. Current SD-OCT machines scan at up to 55,000 A-scans per second and provide an axial resolution lower than 5 μm [12,28].

Swept source-OCT (SS-OCT) obtains time-encoded spectral information by sweeping a narrow-bandwidth laser through a broad optical spectrum. Backscattered intensity is detected with a photodetector. This process is in contrast to SD-OCT, which uses a broad bandwidth light source and detects the interference spectra with a charge-coupled device (CCD) camera and spectrometer [29]. SD-OCT has been commonly used, but there are some benefits of using SS-OCT compared to SD-OCT systems. One of them is that SS-OCT has greater sensitivity and lower signal-to-noise ratios (SNR) at greater scanning depths. Another advantage of SS-OCT is that it does not require a CCD camera and spectrometer photodetector. Therefore, the complexity is decreased and the CCD array cost is abolished. To date, speeds of up to 400,000 A-scans/s have been attained in the eye using the SS-OCT [30]. Thus, reducing sampling errors, potential motion artifact, and the possibility of missing focal pathologies. Most of the SS-OCT systems are now operating at 1–1.3 μm wavelengths, only few studies demonstrating SS-OCT in the 800 nm range [31,32]. The use of 1–1.3 μm wavelengths resulted in lower axial resolution but deeper penetration into the posterior segment of the eye [33,34]. The water absorption window at 1.3 μm offers even deeper penetration of light and may be useful for cornea and anterior segment imaging [35,36]. The illustration about the differences between TD-OCT, SD-OCT, and SS-OCT is shown in Table (1).

Table 1: Comparison of Time-Domain Optical Coherence Tomography (TD-OCT), Spectral-Domain OCT, and Swept-Source OCT.



  
    Parameter
    TD-OCT
    SD-OCT
    SS-OCT
  
  
    Working    Principle
    Measure the    echo time delay and intensity of reflected light
    Measure the    wavelengths of reflected light
    Measure the    wavelengths of reflected light
  
  
    Light Source
    Broadband    and continuous wave
    Broadband    and continuous wave
    Narrow    instantaneous line width, rapidly swept wave
  
  
    Central    Wavelength
    830 nm
    830 nm
    1300 nm
  
  
    Axial    resolution/transverse resolution
    ~10 μm/20-25    μm
    ~5 to 7    μm/10-25 μm
    ~5 to 7 μm/~12.5 μm
  
  
    Scan Speed
    400 A-scans/second
    ~55,000 A-scans/second
    ~400,000 A-scans/second



Table 1:  Comparison of Time-Domain Optical Coherence Tomography (TD-OCT), Spectral-Domain OCT, and Swept-Source OCT.

Current applications of optical coherence tomography in ophthalmology

Glaucoma: The most common scan patterns in TD-OCT glaucoma imaging were a 3.4 mm scan around the optic nerve head (ONH) and six equally spaced radial scans through the macula (6 mm) and optic nerve (4 mm) [29]. Retinal nerve fiber layer (RNFL) thickness is obtained via automated RNFL segmentation in the circumpapillary scan protocol. The segmentation of the RNFL is performed automatically by detecting the internal limiting membrane and outer RNFL border in each A-scan of OCT frame and calculated as the distance between these two boundaries [37]. SD-OCT produces an RNFL thickness map by acquiring a 6 × 6 mm cube of signal data in the peripapillary region in less than 1.5 seconds [38]. Each cube of data consists of data points, which will be compared with the signal from an age-matched normative database in a deviation map. A 3.46-mm peripapillary circle of data points centered on the optic disc is then extracted to construct a peripapillary RNFL map. This map displays mean absolute thickness measurements of the different peripapillary regions according to the corresponding quadrant or segment. Each segment is also color-coded according to the relative thickness in that area compared with the normative age-matched database. Red, yellow, and green colors represent a less than 1%, 1% to 5%, 5% to 95% amount that the measured RNFL thickness is within normal range for an age-matched population, respectively. A white color means the RNFL thickness is thicker than 95% of age-matched database [11].

Macular thickness (Internal Limiting Membrane [ILM] to the photoreceptor inner segment-outer [IS–OS] segment junction) is automatically segmented in the macular scan pattern. The optic nerve scan is used to obtain cup area, disc area, rim area, cup diameter, and disc diameter [29]. In subjects with ocular hypertension, retinal nerve fiber layer defects and normal visual fields, the vertical cup-to-disc diameter ratio, total neuroretinal rim area, rim-to-disc area ratio, and cup-to-disc area ratio are the most valuable optic disc variables for early detection of glaucomatous optic nerve damage [39]. Another study noted that global parameters, namely cup and rim area and horizontal cup-disc ratio, were less likely to detect early focal glaucomatous optic nerve head changes, than parameters representing localised changes, such as minimum rim width within the 60 degrees and 90 degrees sectors across the vertical meridian [40]. Neuroretinal rim area declined at the rate of between 0.28% and 0.39% per year, while vertical optic cup diameter, optic cup area, and mean cup/disc diameter ratio increased with age [41]. Thus, age-related changes should be taken into account when assessing glaucoma diagnosis and progression.

The ONH and RNFL scan protocols have been used since TD-OCT became commercially available, and RNFL and ONH parameters have been shown to be significantly different between glaucomatous and healthy eyes [42,43]. Measurement using TD-OCT showed that the glaucoma-discriminating ability of circumpapillary RNFL and optic nerve head parameters were higher than macular volume and thickness [44-46]. The higher speed and resolution of SD-OCT improved the repeatability of macular imaging compared to standard TD-OCT. The addition of macular ganglion cell complex (GCC) parameters to SD-OCT gives benefit for glaucoma diagnosis. One commercially available system (RTVue, Optovue, Inc) has developed an approach for summarizing GCC, which consists of the retinal ganglion layer (retinal ganglion cell bodies), retinal nerve fiber (RGC axons), and inner plexiform layers (RGC dendrites). The GCC measurements are then directly compared to a normative database. The SD-OCT GCC average had significantly higher diagnostic power than macular retinal thickness measured by SD-OCT and TD-OCT in differentiating between perimetric glaucoma and normal groups [47]. Two of the GCC pattern parameters, GLV and FLV, sum up the volume of GCC loss in the macula with differing levels of locality. The Global Loss Volume (GLV) measures the average amount of GCC volume loss over the entire GCC field, while the Focal Loss Volume (FLV) is used to detect focal losses to correct for overall absolute changes [48]. FLV and GLV were found to have higher diagnostic accuracy than the simple average for the diagnosis of perimetric glaucoma [47-50]. The GCC, FLV, and GLV measured using RTVue is presented in Figure 1.

The Development and Applications of Optical Coherence Tomography

Abstract

Optical Coherence Tomography Development

Current applications of optical coherence tomography in ophthalmology

Conclusion

References