**7. Types of OCT**

### **7.1 Stratus or time domain OCT**

From its inception, OCT images were acquired in a time domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30° apart. Time-domain OCT (TD-OCT) systems featured scan rates of 400 A-scans per second with an axial resolution of 8–10 μm in tissue [7]. Since the slices are 30° apart, there always is a possibility to miss pathology between the slices.

### **7.2 Fourier domain OCT (FD-OCT) or spectral domain OCT (SD-OCT)**

SD-OCT, on the other hand, achieves scan rates of 20,000–52,000 A-scans per second and a resolution of 5–7 μm in tissue [8, 9]. This increased scan rate and number diminishes the likelihood of motion artifact, enhances the resolution, and decreases the chance of missing lesions.

#### **7.3 Anterior segment OCT (AS-OCT)**

Anterior segment OCT utilizes higher wavelength light (1310 nm) as compared to 830 nm of traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration allowing clear images of the parts of the anterior segment (cornea, anterior chamber, iris, and angle). Currently, there are two commercially produced dedicated anterior segment devices, the slit lamp-OCT (SL-OCT: Heidelberg Engineering) and the Visante (Carl Zeiss Meditec, Inc.).
