New developments in OCT offer high-resolution images

"It has often been said that OCT is similar to ultrasound, except that OCT uses light rather than sound. However, unlike ultrasound, light waves travel so fast that a fundamentally different way is needed to process the acquired information," Dr. Chen said.

She demonstrated that the Stratus OCT (Carl Zeiss Meditec) has a super-luminescent diode light source that emits light through a beam splitter to the eye and to a reference mirror. The light then returns from the eye and the reference mirror to create an interference pattern, which is processed by a photodetector and produces the OCT image. Dr. Chen is assistant professor of ophthalmology, Harvard Medical School, Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Glaucoma Service, Boston.

"The macular area is where the retinal ganglion cell layer is more than one cell thick," she said.

Since the ganglion cells and the nerve fiber layer may contribute a third of the retina thickness in the macula, it is logical to expect that macular thinning is associated with glaucoma.

Nerve fiber layer thinning, however, is a better indicator of glaucoma, she pointed out. At least three concentric circular scans around the optic nerve create a map of the nerve fiber layer, showing the degree of nerve fiber layer thinning compared with age-matched patients with normal vision.

The third and most often used scan in glaucoma is a scan of the optic nerve head. In the Stratus OCT printout, the borders of the optic nerve head or disc are determined by where the retinal pigment epithelium ends, and the superior border of the cup is 150μm above the retinal pigment epithelium. "Using this method, the cup is shown in green and the disc in red," Dr. Chen said.

She explained that all of the currently commercially available OCT machines are based on time-domain OCT and are limited by a low resolution of 10μm and slow acquisition speed. "A new technology, that is, spectral-domain OCT, is a fundamentally more efficient way to acquire the data. The basic set-up is the same; however, the light returns from the eye and the reference mirror, and that information is processed by a spectrometer. This is similar to having 2,000 photodetectors in parallel, which is obviously more efficient," Dr. Chen explained. The information then undergoes Fourier transformation to produce the image. Therefore, this technology is also referred to as Fourier-domain OCT.

The advantages of spectral-domain OCT compared with conventional OCT technology are that the former has a more than 150-fold better light sensitivity, allowing higher resolution up to 2 to 3μm, and ultra-high acquisition speeds, such that the images can be obtained at video rate or faster without compromising image quality, according to Dr. Chen.

"For the first time, we are able to have simultaneous ultra-high-resolution and ultra-high-speed ophthalmic imaging without a significant increase in cost. We can now image larger areas of the eye and create better tomographic images of the foveal region and the optic nerve. Three-dimensional videos of the optic nerve and the foveal region can also be created. Spectral-domain OCT can also create nerve fiber layer maps with more information than those based on three concentric circular scans. Also, the flow of blood in the retina and optic nerve can be visualized," she said. This spectral-domain OCT technology is being developed by research collaborator Johannes F. de Boer, PhD, associate professor, Harvard Medical School, Massachusetts General Hospital, Boston.