Principles of Optical Coherence Tomography Angiography for the Front of the Eye
We used the commercially available spectral-domain OCT RT XR Avanti with the AngioVue software (Optovue, Inc., Fremont, CA). The instrument used for OCTA images was based on the AngioVue Imaging System to obtain amplitude-decorrelation angiography images. The Avanti OCT operates at an 840-nm wavelength range and generates 70,000 axial-scans per second. Each OCTA volume contains 304 × 304 A-scans with two consecutive B-scans captured at each fixed position before proceeding to the next sampling location. Split-spectrum amplitude-decorrelation angiography (SSADA) was used to extract the OCTA information.7 To obtain a scan of the anterior segment, we used the AngioVue OCTA system using the anterior segment optical adaptor lens (L-CAM). A specific anterior module (angiocornea) was used to perform anterior segment scans. The best scans were processed automatically to reduce motion artifacts such as transversal saccadic and residual axial motion in the internal software. Using the CAM lens, the scan size was 6 × 6 mm and was obtained with rapid acquisition (4–5 seconds per scan).
17.3 Technical Issues
Despite continuous improvements, the quality of the anterior segment scans is still not as good as the retinal images even after reducing motion artifacts. Anterior segment OCTA does not tolerate any eye movement of the patient because even micromovements create transverse artifacts on the final images. Therefore, scans cannot be performed when patients are unable to fixate, or have continuous eye or eyelid movements such as nystagmus or symptoms causing abnormal blinking rate or blepharospasm. Another issue of the anterior segment module is that it still does not allow quantitative analyses. In particular, it does not provide numerical data about corneal or conjunctival flow areas as it may allow at the level of the posterior segment.
17.4 Optical Coherence Tomography Angiography in Corneal Diseases
Scans were performed for various corneal diseases, such as corneal neovascularization in corneal graft rejection, pterygium, postherpetic, fungal keratitis, or limbal stem cell deficiency. Evaluation of abnormal corneal and limbal vessels is important considering the visual impairment it may cause. One major interest is to use the anterior segment module of the OCTA to evaluate corneal neovascularization, as new blood vessels may accompany or even precede graft rejection, and are strongly associated with immune and inflammatory reactions. Subtle changes may be missed or underestimated by slit-lamp examination, which makes OCTA a major tool for early evaluation of such threatening complication. As seen in ▶ Fig. 17.1 and ▶ Fig. 17.2, OCTA is able to clearly define the corneal vessels invading the corneal graft, in a much more visible way than on the slit-lamp photographs. It also shows clearly the abnormal vascular loops and the demarcation between normal and abnormal vessels. OCTA shows the vessel organization at the graft interface; in particular, it precisely shows the vessels passing the graft–host junction ( ▶ Fig. 17.1d).
Fig. 17.1 (a) Slit-lamp image of corneal graft rejection. (b) Optical coherence tomography angiography (OCTA) of corneal graft rejection in the nasal quadrant, (c) superior quadrant, (d) temporal quadrant.
Fig. 17.2 (a) Slit-lamp image of corneal graft rejection with abnormal corneal vessels on the superior quadrant. (b) Optical coherence tomography angiography clearly showing superior conjunctival vessels invading the corneal graft: abnormal vascular loops typical of active neovascularization.
OCTA can also be extremely useful to describe abnormal neovascularization in stromal keratitis, especially when stromal scarring causes loss of transparency and masks new blood vessels. ▶ Fig. 17.3 shows a central corneal lesion secondary to herpes simplex virus in a 45-year-old man with a long history of recurrent stromal keratitis in his right eye. The slit-lamp photography shows epithelial and stromal edema, whereas OCTA images show more precisely the abnormal vessels. ▶ Fig. 17.4 shows clearly on the OCTA scan a penetrating vessel in the stroma secondary to a fungal ulcer, 1 month after treatment. These findings are of particular importance when persistence of infection is suspected in inflammatory eyes in which keratoplasty is envisaged.
Fig. 17.3 (a) Slit-lamp image of postherpetic stromal keratitis. Note the low density of visible vessels. (b) Optical coherence tomography angiography (OCTA) showing the abnormal vessels invading the corneal stroma. Note the discrepancy between slit-lamp or infrared images and vessel density in OCTA.
Fig. 17.4 (a) Slit-lamp image of infectious keratitis with intense neovascularization. (b) Optical coherence tomography angiography showing blood vessels invading the cornea. Note the extremely high density of tightly and homogeneously arranged vessels.