Faculty Bibliography

Iris retraction syndrome (IRS), a potential sign of retinal detachment, is a clinical phenomenon noted in both non-surgical and post-surgical settings. The mechanism of IRS remains unclear and has been attributed to decreased aqueous humor formation and increased fluid clearance through retinal pigment epithelium (RPE) pump action. If IRS is present, prompt pupillary dilation and a thorough exam of the peripheral retina is indicated

In this manuscript we describe the design and optimization of ultrahigh resolution spectral/Fourier domain OCT systems for three applications in retinal imaging: imaging of the normal retina, three-dimensional (3D) imaging of retinal pathologies, and 3D imaging of the rodent retina. Seven spectrometer configurations were tested for resolution and sensitivity drop with depth, and CCD pixel crosstalk was characterized. The human retina was imaged in vivo with five different axial resolutions between 2 and 10 microns, and with three different transverse resolutions. Information from these experiments enabled the optimization of OCT systems for the above applications. Results include clinical 3D data of retinal pathologies, high quality cross-sectional images of the normal retina with different axial and transverse resolutions and 3D data from the rat and mouse retinas. Factors affecting the sensitivity fall-off are discussed and theoretical predictions are compared with experimental measurements. Different retinal imaging applications necessitate different system designs, depending on the requirements of speed, axial resolution, axial measurement range, transverse resolution, and field of view. While axial resolution is the dominant factor in image quality, a smaller transverse spot size can reduce speckle size and improve contrast at boundaries such as the boundary between the ganglion cell layer and the inner plexiform layer. The effect of reducing the transverse spot size is most pronounced in images with 5-10 um axial resolution. In addition, we characterize all factors responsible for the sensitivity drop with depth in spectral/Fourier domain OCT.

AIM: To create a new, automated method of evaluating the quality of optical coherence tomography (OCT) images and to compare its image quality discriminating ability with the quality assessment parameters signal to noise ratio (SNR) and signal strength (SS). METHODS: A new OCT image quality assessment parameter, quality index (QI), was created. OCT images (linear macular scan, peripapillary circular scan, and optic nerve head scan) were analysed using the latest StratusOCT system. SNR and SS were collected for each image. QI was calculated based on image histogram information using a software program of our own design. To evaluate the performance of these parameters, the results were compared with subjective three level grading (excellent, acceptable, and poor) performed by three OCT experts. RESULTS: 63 images of 21 subjects (seven each for normal, early/moderate, and advanced glaucoma) were enrolled in this study. Subjects were selected in a consecutive and retrospective fashion from our OCT imaging database. There were significant differences in SNR, SS, and QI between excellent and poor images (p = 0.04, p = 0.002, and p<0.001, respectively, Wilcoxon test) and between acceptable and poor images (p = 0.02, p<0.001, and p<0.001, respectively). Only QI showed significant difference between excellent and acceptable images (p = 0.001). Areas under the receiver operating characteristics (ROC) curve for discrimination of poor from excellent/acceptable images were 0.68 (SNR), 0.89 (IQP), and 0.99 (QI). CONCLUSION: A quality index such as QI may permit automated objective and quantitative assessment of OCT image quality that performs similarly to an expert human observer.

AIM: To describe the appearance of the non-exudative forms of age related macular degeneration (AMD) as imaged by ultrahigh resolution optical coherence tomography (UHR-OCT). METHODS: A UHR-OCT ophthalmic imaging system, which utilises a femtosecond laser light source capable of approximately 3 mum axial resolution, was employed to obtain retinal cross sectional images of patients with non-exudative AMD. Observational studies of the resulting retinal images were performed. RESULTS: 52 eyes of 42 patients with the clinical diagnosis of non-exudative AMD were imaged using the UHR-OCT system. 47 of the 52 (90%) eyes had the clinical diagnosis of drusen and/or retinal pigment epithelial (RPE) changes. In these patients, three patterns of drusen were apparent on UHR-OCT: (1) distinct RPE excrescences, (2) a saw toothed pattern of the RPE, and (3) nodular drusen. On UHR-OCT, three eyes (6%) with a clinical diagnosis of non-exudative AMD had evidence of fluid under the retina or RPE. Two of these three patients had findings suspicious for subclinical choroidal neovascularisation on UHR-OCT. CONCLUSION: With the increased resolution of UHR-OCT compared to standard OCT, the involvement of the outer retinal layers are more clearly defined. UHR-OCT may allow for the detection of early exudative changes not visible clinically or by angiography.

BACKGROUND: Limited data are available to guide optimal positioning of glaucoma drainage devices (GDD) in relation to the limbus and optic nerve. The authors aim to provide guidelines for appropriate and safe GDD implantation. METHOD: The optimal positioning of five different GDD were evaluated using necropsy eyes of varying axial lengths. The dependent variable that was measured was the maximum distance that a GDD could be placed posterior to the limbus while remaining 2 mm away from the optic nerve. RESULTS: The average maximum distance posterior to the limbus of the anterior plate edge ranged between 9.0-15.0 mm in the superotemporal quadrant for the GDD tested. The distances for superonasal, inferonasal, and inferotemporal quadrants ranged between 8.0-14.0 mm, 9.0-14.0 mm, and 11.0-17.0 mm, respectively. The Molteno device could be placed most posteriorly while remaining 2 mm away from the nerve. The Ahmed FP7 and S2 were the least amenable to posterior placement before encroaching on the 2 mm limit. CONCLUSION: The maximum distance that a GDD can be placed posterior to the limbus, before encroachment around the optic nerve, varies between different devices and quadrants of placement. Taking a measurement of the exact distance of the plate from the limbus during GDD surgery is recommended.