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Recent advances in anterior segment imaging enable ophthalmic surgeons to evaluate, treat, and follow their cataract, refractive, and glaucoma patients better. Three different technologies-optical coherence tomography, a Scheimpflug camera, and a wide-field contact digital fundus camera-are now being used to assess those patients more precisely for diagnosis, possible treatment, and long-term follow-up.
One of the earliest anterior segment devices to be used and evaluated was ultrasound biomicroscopy (UBM). It was developed at the Princess Margaret Hospital, Toronto, in 1989, for imaging the cornea, anterior chamber, lens, and ciliary body.
To undergo UBM imaging, a patient is required to be in the supine position. A methylcellulose gel is then applied to the patient's eye. A special eyecup is used to ensure that the piezoelectric crystal of the UBM transducer does not touch the cornea.
"It has become a very useful research device," Dr. Ahmed pointed out.
"It is one of the first devices to be used and evaluated [for anterior segment imaging]," he said.
New technology similar to UBM (iUltrasound, iScience Interventional) is a higher-frequency device that allows for more detailed evaluation of the angle, including Schlemm's canal, he said.
Also, wide-field UBM devices (Artemis, Ultralink LLC) provide high-frequency digital anterior segment evaluation, Dr. Ahmed added.
More recent technologies such as optical coherence tomography (OCT) and the Scheimpflug camera are able to produce high-resolution images of the anterior segment in a patient-friendly manner.
The use of a wide-field contact digital fundus camera also may benefit surgeons by providing photodocumentation with an actual visual of the iridocorneal angle.
Dr. Ahmed discussed the applications, advantages, and limitations of those three instruments with Ophthalmology Times.
Versatility of OCT
Anterior segment OCT (Visante OCT, Carl Zeiss Meditec) provides excellent cross-sectional images of the cornea, which is particularly helpful for planning refractive and corneal surgeries, explained Dr. Ahmed.
"This new noncontact device has the ability to obtain cross-sectional images of the cornea quickly without distorting the anatomy," he said.
The OCT device uses near-infrared light with a wavelength of 1,310 nm and has a scan rate of about 4,000 axial scans per second. The patient's anterior chamber can be imaged in about 1/8 of a second, Dr. Ahmed said.
OCT is able to provide high-resolution images of the cornea, and identify and measure LASIK flap thickness and lamellar keratoplasties. Besides the cornea, the OCT can provide clear images of all the structures above the pigmented epithelium of the iris, including the angle, the sclera, and the lens. Anterior chamber structures can be measured with the device, providing the surgeon with an accurate assessment of the depth, angles, and diameter of the anterior chamber, he said.
Glaucoma surgeons can use the OCT for angle assessment, complementing traditional gonioscopy that can be subjective, prone to artifact, and dependent on the clinician's skill, Dr. Ahmed said. OCT allows a technician to visualize the angle anatomy easily, providing objective quantifiable measurements such as the angle opening distance from the scleral spur, the angle recess area, and the trabecular iris space area. Specialists then can determine whether the angle is open, narrow, or closed.
This ability also may be useful in the evaluation and planning of new glaucoma drainage devices.