Читать книгу Age-related macular degeneration - Christopher Schütze - Страница 5

Optical coherence tomography (OCT)³:

2.1 Imaging the human retina by OCT in ophthalmology:

OCT is a non-invasive and non-contact imaging modality capable of generating cross-sectional images of the retina, the optic nerve, and the vitreous. Further, anterior segment OCT is capable of imaging anatomical structures of the anterior section of the eye (i.e. the cornea, the anterior chamber, or the lens). OCT is analogous to ultrasonography, however the technology uses light waves for imaging instead of sound waves. During an examination the eye is scanned transversely using a light beam. Cross-sectional intensity-based images can be generated and analyzed by means of a false color or gray scale image. Using this technology, the echo time delay and the amount of reflected or backscattered light is measured using low coherence interferometry (Kanski et al., 2008).

Indications for retinal OCT imaging:

- Diagnostics of pathological alterations in the macular area such as macular holes, central serous retinopathy, epiretinal membranes, macular edema, or vitreomacular traction (Hee et al., 1995b, Hee et al., 1995c, Hwang et al., 2012, Hee et al., 1995a, Tsunoda et al., 2012, Kanski et al., 2008).

- Documentation of disease progression and treatment response, i.e. retinal thickness measurements in neovascular AMD and its development during treatment follow-up (Sulzbacher et al., 2013, Golbaz et al., 2011, Kanski et al., 2008).

- Differentiation of retinal detachment of longer duration and retinoschisis (Kanski et al., 2008).

- Retinal nerve analysis and evaluation of retinal nerve fiber layer (RNFL) thickness, i.e. in patients with glaucoma (Chauhan et al., 2012, Kanski et al., 2008).

There are several color and gray scales used for representing intensity-based retinal images in OCT imaging. To mention only one example, images acquired by i.e. Cirrus SD-OCT (Cirrus HD-OCT; Carl. Zeiss Meditec, Dublin, California, USA) are displayed in the following way: light colors usually represent highly reflective layers (red and white), whereas retinal structures with low reflectivity are visualized in dark colors (blue and black). Structures with intermediate reflectivity are illustrated in green color. The plexiform and nerve fiber layer are visualized in red, yellow, or light green color.

Using Cirrus SD-OCT imaging as a representative example, the inner (IPL) and outer plexiform layers (OPL) are represented in light green color. The inner (INL) and outer nuclear layers (ONL) usually appear in blue or black color. The junction between the outer and inner segments of photoreceptors is displayed as a thin red structure in the outer retina. High-resolution OCT (i.e. Cirrus OCT) is further capable of differentiating retinal structures like the external limiting membrane (ELM) and the ganglion cell layer (GCL) (Kanski et al., 2008) (Fig.1).

Fig.1 shows an SD-OCT image of a healthy retina, displaying a cross-sectional image of retinal layers.

In essence, the bandwidth of the light source used in OCT imaging determines the axial resolution (Drexler, 2008). Enhanced axial OCT resolution can be achieved when using broad bandwidth light or light sources with a low coherence length (Drexler, 2008a). Image resolution of modern OCT machines used in clinical practice ranges between 5-8μm (Drexler, 2008a).