**2. Ultra-high resolution OCT**

#### **2.1 Technical aspects**

Optical coherence tomography (OCT) is a non-contact image acquisition technology first developed by Huang et al. [14]. It produces an 'optical biopsy': detailed cross-sectional images of biological tissue [14]. Signal acquisition and processing methods determine image speed and resolution [15]. As resolution and tissue penetrance have improved, clinical applications continue to evolve [16].

The time-domain OCT (TD-OCT) was the first available anterior segment OCT, with the prototypes Visante and SL-OCT (Heidelberg Engineering, Heidelberg, Germany). Both acquired 2000 A-scans per second and had a resolution of 10–18 μm. These earlier models could scan the entire anterior segment, but were slow with poor resolution [1, 2]. Resolution improved to 5 μm with the SD-OCT, but scan width was limited at 3–6 mm. Available SD-OCTs included the Cirrus (Carl Zeiss Meditec, Inc.), Spectralis (Heidelberg Engineering, Dossenheim, Germany), RTVue (Optovue, Meridianville, AL), 3D OCT (Topcon Medical Systems, Oakland, NJ), and Bioptigen SD-OCT (Bioptigen Inc., Research Triangle Park, NC) [1, 2]. Swept-source (SS) Fourier-domain (FD) OCT was available in 2008 and the prototype was Casia SS OCT (Tomey, Nagoya, Japan). This SS-OCT had a scan width of 16 mm, depth of 6 mm and A-scan rate of 30,000 per second which then allowed for three-dimensional scanning of the entire anterior segment [1].

The first use of UHR-OCT was reported by Drexler et al. in 2001 with 2–3 resolution imaging of Bowman's membrane [17]. Current axial resolution of the UHR-OCT is 1–4 μm with a scan width of 5–12 mm [9, 10, 18]. OCT resolution was improved with a light source with a broad bandwidth greater than 100 nm and a specifically designed spectrometer which detected fringes collected from both reference and sample arms [2]. These changes allowed for *in vivo* imaging of individual corneal layers (microlayer), the tear film, tear meniscus and contact lens interfaces [9, 18, 19].

Most published data using UHR-OCT was acquired from custom-built machines, and availability. The Bioptigen Envisu (Bioptigen Inc., Research Triangle Park, NC, USA) and the SOCT Copernicus HR (Optopol Technologies SA, Zawiercie, Poland) are current commercially available models of UHR-OCT.

#### **2.2 Corneal anatomy**

The cornea is the transparent structure which, along with the tear film, provides about two-thirds of the refractive power of the eye [20]. The central cornea on average is 551–565 μm thick and the peripheral cornea ranges from 612 to 640 μm thick [21]. The cornea receives its nutrients mainly from the aqueous humor, as it is an avascular structure. Cellular components of the cornea include epithelial cells, keratocytes, and endothelial cells. Acellular components of the cornea form a matrix of collagen and glycosaminoglycans. Corneal transparency is based on uniformity of collagen fibril diameter and packing [22].

#### **2.3 Healthy cornea parameters**

UHR-OCT of the healthy cornea is important in order to recognize changes in disease states (**Figure 1**). UHR-OCT parameters of the normal tear film were first reported by Werkmeister et al. [10]. The group reported an average central tear

**5**

cornea.

**Figure 1.**

*Corneal Microlayer Optical Tomography Review DOI: http://dx.doi.org/10.5772/intechopen.84750*

film thickness of 4.79 μm [23]. Other reported central tear film thicknesses include 3.4 μm [24], 5.1 μm [25] and a range of 3–8 μm [26]. The central cornea measured

*A prototypical cross-sectional UHR-OCT image of a healthy human cornea. Epithelium (1b), Bowman's layer (1c), stroma (1d), and endothelium/ Descemet membrane complex (1e) can be distinguished. The topmost* 

Data from a SD-OCT with 3.9 μm axial resolution showed that central corneal epithelium thickness is not statistically significant between subjects less than or greater than 40 years old (48.3 and 48.8 μm, respectively) [27]. This conclusion was supported by previous studies which showed no alteration in epithelial cell density with age [28]. Epithelial thickness varied over the vertical and horizontal meridians

Bowman's layer has been reported as 18 [10] and 18.7 μm [25] thick with an uneven thickness distribution over the horizontal meridian of the cornea [25]. The central and midperiphery, nasal, temporal Bowman's layer thickness was 17.7, 20.0 and 19.8 μm, respectively [25]. Thickness gradually increases from temporal to nasal

The Endothelium/Descemet membrane complex (En/DM) is made of Descemet membrane (DM), endothelium, and retro-corneal membranes. These layers are typically indistinguishable so the thickness measured from UHR-OCT is different from that from pathology [30]. DM in healthy young subjects is seen with UHR-OCT as a single, opaque, smooth line. The same structure is a band of two smooth opaque lines surrounding a translucent space in normal elderly subjects [9]. Bizheva et al. reported UHR-OCT data from healthy subjects, and revealed the average thickness was 6.6 for pre-Descemet's layer, 10.4 μm for Descemet's membrane and 4.8 μm for endothelium [31]. Another group published an average central Descemet's membrane thicknesses of 10 and 16 μm in the young and elderly healthy

Corneal imaging has been significantly improved by the improvements in speed and resolution in the current UHR-OCT. The high axial resolution of UHR-OCT systems allows precise delineation of the corneal microlayers. There are a wide variety of clinical applications of UHR-OCT for the diagnosis and management of corneal disease [12, 13, 16, 17]. In this review, we summarize the clinical applications of imaging the ocular surface and cornea based on microlayers of the

585 μm, with epithelial thicknesses of 55 [10] and 55.8 μm [25].

*highly reflective layer in the tomogram represents the pre-corneal tear film (1-a).*

from 42.9 to 55.2 μm and 58.6 to 59.3 μm, respectively [29].

**2.4 Applications of UHR-OCT for corneal microlayers**

and from inferior to superior [25].

groups, respectively [9].

*Corneal Microlayer Optical Tomography Review DOI: http://dx.doi.org/10.5772/intechopen.84750*

#### **Figure 1.**

*A Practical Guide to Clinical Application of OCT in Ophthalmology*

Optical coherence tomography (OCT) is a non-contact image acquisition technology first developed by Huang et al. [14]. It produces an 'optical biopsy': detailed cross-sectional images of biological tissue [14]. Signal acquisition and processing methods determine image speed and resolution [15]. As resolution and tissue penetrance have improved, clinical applications continue to evolve [16].

The time-domain OCT (TD-OCT) was the first available anterior segment

Heidelberg, Germany). Both acquired 2000 A-scans per second and had a resolution of 10–18 μm. These earlier models could scan the entire anterior segment, but were slow with poor resolution [1, 2]. Resolution improved to 5 μm with the SD-OCT, but scan width was limited at 3–6 mm. Available SD-OCTs included the Cirrus (Carl Zeiss Meditec, Inc.), Spectralis (Heidelberg Engineering,

Dossenheim, Germany), RTVue (Optovue, Meridianville, AL), 3D OCT (Topcon Medical Systems, Oakland, NJ), and Bioptigen SD-OCT (Bioptigen Inc., Research Triangle Park, NC) [1, 2]. Swept-source (SS) Fourier-domain (FD) OCT was available in 2008 and the prototype was Casia SS OCT (Tomey, Nagoya, Japan). This SS-OCT had a scan width of 16 mm, depth of 6 mm and A-scan rate of 30,000 per second which then allowed for three-dimensional scanning of the

The first use of UHR-OCT was reported by Drexler et al. in 2001 with 2–3 resolution imaging of Bowman's membrane [17]. Current axial resolution of the UHR-OCT is 1–4 μm with a scan width of 5–12 mm [9, 10, 18]. OCT resolution was improved with a light source with a broad bandwidth greater than 100 nm and a specifically designed spectrometer which detected fringes collected from both reference and sample arms [2]. These changes allowed for *in vivo* imaging of individual corneal layers (microlayer), the tear film, tear meniscus and contact

Most published data using UHR-OCT was acquired from custom-built machines, and availability. The Bioptigen Envisu (Bioptigen Inc., Research

Triangle Park, NC, USA) and the SOCT Copernicus HR (Optopol Technologies SA, Zawiercie, Poland) are current commercially available models of UHR-OCT.

The cornea is the transparent structure which, along with the tear film, provides about two-thirds of the refractive power of the eye [20]. The central cornea on average is 551–565 μm thick and the peripheral cornea ranges from 612 to 640 μm thick [21]. The cornea receives its nutrients mainly from the aqueous humor, as it is an avascular structure. Cellular components of the cornea include epithelial cells, keratocytes, and endothelial cells. Acellular components of the cornea form a matrix of collagen and glycosaminoglycans. Corneal transparency is based on uniformity of

UHR-OCT of the healthy cornea is important in order to recognize changes in disease states (**Figure 1**). UHR-OCT parameters of the normal tear film were first reported by Werkmeister et al. [10]. The group reported an average central tear

OCT, with the prototypes Visante and SL-OCT (Heidelberg Engineering,

**2. Ultra-high resolution OCT**

**2.1 Technical aspects**

entire anterior segment [1].

lens interfaces [9, 18, 19].

**2.2 Corneal anatomy**

collagen fibril diameter and packing [22].

**2.3 Healthy cornea parameters**

**4**

*A prototypical cross-sectional UHR-OCT image of a healthy human cornea. Epithelium (1b), Bowman's layer (1c), stroma (1d), and endothelium/ Descemet membrane complex (1e) can be distinguished. The topmost highly reflective layer in the tomogram represents the pre-corneal tear film (1-a).*

film thickness of 4.79 μm [23]. Other reported central tear film thicknesses include 3.4 μm [24], 5.1 μm [25] and a range of 3–8 μm [26]. The central cornea measured 585 μm, with epithelial thicknesses of 55 [10] and 55.8 μm [25].

Data from a SD-OCT with 3.9 μm axial resolution showed that central corneal epithelium thickness is not statistically significant between subjects less than or greater than 40 years old (48.3 and 48.8 μm, respectively) [27]. This conclusion was supported by previous studies which showed no alteration in epithelial cell density with age [28]. Epithelial thickness varied over the vertical and horizontal meridians from 42.9 to 55.2 μm and 58.6 to 59.3 μm, respectively [29].

Bowman's layer has been reported as 18 [10] and 18.7 μm [25] thick with an uneven thickness distribution over the horizontal meridian of the cornea [25]. The central and midperiphery, nasal, temporal Bowman's layer thickness was 17.7, 20.0 and 19.8 μm, respectively [25]. Thickness gradually increases from temporal to nasal and from inferior to superior [25].

The Endothelium/Descemet membrane complex (En/DM) is made of Descemet membrane (DM), endothelium, and retro-corneal membranes. These layers are typically indistinguishable so the thickness measured from UHR-OCT is different from that from pathology [30]. DM in healthy young subjects is seen with UHR-OCT as a single, opaque, smooth line. The same structure is a band of two smooth opaque lines surrounding a translucent space in normal elderly subjects [9]. Bizheva et al. reported UHR-OCT data from healthy subjects, and revealed the average thickness was 6.6 for pre-Descemet's layer, 10.4 μm for Descemet's membrane and 4.8 μm for endothelium [31]. Another group published an average central Descemet's membrane thicknesses of 10 and 16 μm in the young and elderly healthy groups, respectively [9].

#### **2.4 Applications of UHR-OCT for corneal microlayers**

Corneal imaging has been significantly improved by the improvements in speed and resolution in the current UHR-OCT. The high axial resolution of UHR-OCT systems allows precise delineation of the corneal microlayers. There are a wide variety of clinical applications of UHR-OCT for the diagnosis and management of corneal disease [12, 13, 16, 17]. In this review, we summarize the clinical applications of imaging the ocular surface and cornea based on microlayers of the cornea.
