Anterior-segment imaging for assessment of glaucoma

Abstract

This article summarizes the physics, technology and clinical application of ultrasound biomicroscopy (UBM) and optical coherence tomography (OCT) for assessment of the anterior segment in glaucoma. UBM systems use frequencies ranging from approximately 35 to 80 MHz, as compared with typical 10-MHz systems used for general-purpose ophthalmic imaging. OCT systems use lowcoherence, near-infrared light to provide detailed images of anterior segment structures at resolutions exceeding that of UBM. Both technologies allow visualization of the iridocorneal angle and, thus, can contribute to the diagnosis and management of glaucoma. OCT systems are advantageous, being noncontact proceedures and providing finer resolution than UBM, but UBM systems are superior for the visualization of retroiridal structures, including the ciliary body, posterior chamber and zonules, which can provide crucial diagnostic information for the assessment of glaucoma. †Author Glaucoma is a disease that is essentially an optic neuropathy. Great strides have been made in the last decade in imaging the retinal nerve fiber layer and the optic nerve using optical coherence tomography (OCT). OCT is a noninvasive technique allowing the visualization and measurement of these structures for the assessment of glaucoma progression [1][2][3][4][5]. While damage to the optic and retinal nerves is responsible for the degradation of vision, in most instances this damage is associated with elevated intraocular pressure (IOP). Elevation of IOP results from an imbalance between the production and drainage of aqueous fluid, specifically, increased outflow resistance. Thus, assessment of the anterior segment plays a major role in the diagnosis and treatment of glaucoma.The traditional tools for the assessment of the angle and anterior segment are the slit-lamp and gonioscopy, in combination with a Goldmann contact lens and classification using the system introduced by Shaffer [6]. Several newer technologies now exist for imaging of the anterior segment, including scanning Scheimpflug (e.g., Pentacam, Oculus, Inc., Lynnwood, WA, USA), and scanning slit-lamp systems (e.g., Orbscan, Orbtek Inc., Salt Lake City, UT, USA). The Pentacam can provide rapid and reproducible measurements of anterior chamber depth and angle width [7], but is limited to visualization of only the angle approach. Scheimpflug photography also does not display the retroiridal structures or the ciliary body, which are of great interest in glaucoma diagnosis [8]. The scanning peripheral anterior chamber depth analyzer is another optically based system. It has high reproducibility in screening for narrow angles, as with other systems using visible light, but may have difficulty in assessing the bottom of the angle, as can be accomplished by OCT and ultrasound biomicroscopy (UBM) [9]. While these visible light systems are undoubtedly useful, especially for screening for narrow angles, OCT (based on near-infrared light) and UBM systems allow imaging of the full-angle anatomy and, in the cas...

Pentacam anterior chamber parameters in young and middle‐aged Chinese

“…Since Chinese are more prone to have narrow angles, some screening tools would be beneficial to identify those patients at high risk for angle closure. Apart from gonioscopy, ultrasonic biomicroscopy and AS‐OCT are commonly used to evaluate the anterior chamber angle …”

Iridocorneal angle assessment of companion rabbits using gonioscopy, spectral‐domain optical coherence tomography (Optovue iVue®), high‐resolution ultrasound, and Pentacam®HR imaging

“…Recent advancement in ocular imaging technology allows for more objective analysis and measurement of the ICA . Although gonioscopy is considered a reliable, semi‐quantitative method for ICA analysis, it provides very limited objective information on ICA parameters.…”

Anterior Segment Imaging in Glaucoma

“…Peripheral laser iridoplasty will open up the angles in plateau iris syndrome patients (Figure 8). 5,58 UBM and AS-OCT demonstrate little iris bowing but rather a steep rise in the iris near its point of insertion. Anterior positioning of the ciliary processes and absence of the sulcus are best visualized with UBM ( Figure 8).…”

“…Therefore, rather than a pulse-echo technique, OCT uses a low-coherence light beam (typically a superluminescent diode emitting in the near-infrared region) to compare the delay of tissue reflections against a reference reflection. 5 The final image is produced by scanning a light beam laterally, creating a series of axial scans, and then combining these scans into a composite image.…”