Fourier-domain OCT of the macula has been shown to provide greater detail (1125 A-scans vs. 512 A-scans) than commercially available time-domain OCT systems image in a shorter period of time (0.072 vs. 1.23 s) (43). This dramatically decreases motion artifact, which appears as undulating of the retina in the slower time-domain OCT image. Because motion-correcting algorithms are not required, the images better represent the true topography of the retina. Moreover, the faster scanning time allow a larger area to be scanned and offers more precise registration. It is also possible to acquire three-dimentional OCT data that achieve comprehensive retinal coverage and allow correlation between OCT images and clinical fundus features (44). The significant advantages of the Fourier-domain OCT will likely be the basis of the next generation of retinal OCT systems.

Doppler OCT has been used for measurement of blood flow using both time-domain (45, 46). and Fourier-domain OCTs (47, 48). The Doppler shift is localized in depth by use of joint time–frequency analysis algorithms, which generate depth-resolved Doppler frequency spectra of the reflected light. Rapid acquisition of retinal flow data in a few milliseconds allows the extraction of dynamic flow properties, such as the retinal vascular response to changes in perfusion pressure or oxygen content. Doppler OCT offers superior spatial resolution compared with Doppler ultrasound and Doppler scanning laser ophthalmoscopy. Unlike angiography, Doppler OCT is more quantitative and does not require injection of a contrast agent.

The Role of OCT in Diabetic Macular Edema

OCT retinal thickness measurements in diabetic patients have been shown in a number of studies to be highly reproducible. Retinal thickness measurements reproducible to within ±5% and ±6% were found for normal and diabetes subjects with diabetic macular edema, respectively (49)−24 Thus, changes in central retinal thickness greater than 6% for healthy patients and greater than 10% for diabetic patients are likely to be due to true changes in retinal thickness rather than inconsistencies in the OCT measurements.

Changes in macular thickness may be reported in absolute values before and after treatment or in percentage change. However, no uniform method currently exists for reporting changes in macular thickness. Chan and Duker suggested a standardized method for reporting changes in macular thickness as a percentage of total possible change based on normative OCT data (50). Standardizing reported changes in macular thickening may be required to better evaluate the efficacy of therapeutic intervention, as well as to compare various treatment strategies.

Some variability in retinal thickness measurements may be observed due to artifacts that impair the correct detection of retinal boundaries by the OCT analysis software (49, 51). For example, intraretinal exudates appear as spots of high reflectivity with areas of lowreflective shadowing behind them, and are found primarily in the outer retinal layers (Fig. 12). Large collections of hard exudates can confuse the analysis algorithm. Epiretinal membrane appears as highly reflective horizontal signal at the anterior surface of the retina (Fig. 13). Thus, epiretinal membranes or a low lying posterior hyaloidal face can confuse the analysis algorithm and incorrectly increase reported retinal thickness (52).

Fig. 12. OCT scan of intraretinal exudates appearing as spots of high reflectivity with areas of low-reflective shadowing behind them.

Fig. 13. OCT scan of epiretinal membrane appearing as highly reflective horizontal signal on the inner surface of the retina, with irregularities of the retinal surface beneath.

Morphologic Patterns of Diabetic Macular Edema

Studies have described the presence of at least five different morphologic patterns of diabetic macular edema seen on OCT (7, 29, 53–55). Diffuse retinal thickening appears as increased sponge-like retinal thickness greater than 200 m with reduced intraretinal reflectivity, particularly in the outer retinal layers (Fig. 14). Cystoid macular edema appears as small, round or oval, hyporeflective lacunae with highly reflective septae bridging the retinal layers and separating the cystoid-like cavities (Fig. 15). The cystoid spaces are located primarily in the outer retinal layers, leaving a thin outer layer in the fovea. Some morphologic differences exist between newly developed and long-standing cystoid macular edema (53). In early cystoid macular edema, cystoid spaces primarily are located in the outer retinal layers, and the inner retinal layers are relatively preserved. In chronic cystoid macular edema, the septa of each cystoid space disappear, forming confluent large cystoid cavities (Fig. 16). Large cystoid spaces may involve the entire retinal layer, appearing as retinoschisis.

Posterior hyaloidal traction is defined as a highly reflective signal arising from the inner retinal surface and extending toward the optic nerve or peripherally (Fig. 17) (54). Subretinal fluid or serous retinal detachment appears as a shallow elevation of the retina resembling a dome, with an optically clear space between the retina and the RPE, and

Fig. 14. OCT scan of diffuse macular edema appearing as increased sponge-like retinal thickening with reduced intraretinal reflectivity, particularly in the outer retinal layers. Note the intraretinal exudates.

Fig. 15. OCT scan of cystoid macular edema appearing as round or oval, hyporeflective cystoid-like intraretinal spaces with septae bridging the retinal layers and separating the cavities. The cystoid spaces are located primarily in the outer retinal layers.

Fig. 16. OCT scan of chronic cystoid macular edema showing a large intraretinal cystoid cavity with loss of septae.

a distinct outer border of the detached retina (Fig. 18). The identification of the highly reflective posterior border of detached retina distinguishes subretinal from intraretinal fluid. Finally, tractional retinal detachment is defined as a peak-shaped detachment of the retina with an area of low signal underlying the highly reflective border of the neurosensory retina, and is accompanied by posterior hyaloidal traction (Fig. 19).

Fig. 17. OCT scan of posterior hyaloidal traction showing a highly reflective signal arising from the inner retinal surface and extending in an anterior–posterior direction. Note the diffuse macular edema.

Fig. 18. OCT scan of subretinal fluid appearing as a shallow elevation of the retina with an optically clear space between the retina and the RPE. Cystoid macular edema is seen as well.

Fig. 19. OCT scan of a shallow tractional retinal detachment with posterior hyaloidal traction. A highly reflective signal corresponding to posterior hyaloidal traction arises from the inner retinal surface. A peak-shaped detachment of the retina can be seen with a hyporeflective area underneath the highly reflective border of the neurosensory retina.

While diffuse retinal thickening and cystoid macular edema may be detected by biomicroscopy, OCT seems particularly helpful in the analysis of the vitreomacular relationship. OCT is much more accurate than biomicroscopy in determining the status of the posterior hyaloid when it is only slightly detached from the macular surface (35, 56–58). Evidence of posterior hyaloidal traction on OCT indicates vitreomacular traction