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Ophthalmic Lasers Expand from Surgery to Detection and Diagnosis

BioPhotonics
Jul 2011
Marie Freebody, Contributing Editor, marie.freebody@photonics.com

Lasers are an established surgical tool in an ophthalmologist’s armory against glaucoma, cataracts and macular degeneration, but developments in OCT will have doctors turning to lasers before the patient ever reaches the operating room.

The development of lasik surgery has led to a rapid growth in the vision correction market over the past two decades. But lasers are now starting to find a place in the early detection and diagnosis of certain eye conditions, particularly when used in optical coherence tomography systems.

The first application of femtosecond lasers in ophthalmology was for lasik refractive surgery procedures, in which incredibly short pulses (around 700 fs) give surgeons the accuracy and predictability necessary for delicate tissue vaporization.

During a procedure, a femtosecond laser is used to create the corneal flap, approximately 150 µm thick, which is lifted prior to application of an excimer laser that remodels the shape of the cornea.

According to Graham Cox, a professional services manager at Ultralase, one of the leading refractive surgery providers in the UK and Ireland, the femtosecond laser creates corneal flaps, which are thinner and planar (of even thickness) compared with manual incisions. What’s more, the complication rate is greatly reduced, from around 1 in 300 using a traditional bladed microkeratome, to 1 in 2500 using a laser.

There are six companies that manufacture and sell femtosecond lasers for ophthalmology: Abbott Medical Optics of Santa Ana, Calif., which acquired Intra-Lase in 2008; Ziemer Ophthalmic of Port, Switzerland, which produces the FemtoLDV femtosecond laser; Carl Zeiss Meditec of Jena, Germany, which offers the VisuMax laser; Technolas Perfect Vision of Munich, Germany, which offers the Technolas Femtosecond Workstation; Alcon Laboratories of Fort Worth, Texas, which acquired LenSx Lasers in 2010; and LensAR of Winter Park, Fla., which sells the LensAR Laser System.

“Of these, the Intralase laser currently holds the overwhelming market share, although this may change in the future as the other lasers are developed for additional specialist applications,” Cox said. “Although there have been minor enhancements to this [Intralase-based lasik] application, such as increased frequency of the laser, giving a consequent reduction in the treatment time, it is likely that a plateau has been reached in this field, given the very high accuracy and predictability of modern lasik procedures.”

In 2010, Carl Zeiss Meditec introduced a new approach to refractive surgery using its VisuMax laser to correct myopia and myopic astigmatism. Rather than ablating corneal tissue with thousands of excimer laser pulses, the femtosecond lenticule extraction (ReLEx) procedure enables tissue to be removed in one piece through either a complete flap opening or a small 3- to 5-mm incision.


The first step in the ReLEx procedure is to anesthetize the eye using special drops. The VisuMax femtosecond laser then creates a thin lenticule in the intact cornea and makes an access cut for its removal. Once the surgeon manually removes the lenticule to change the refractive power of the cornea, the top corneal layer is flipped back into its original position. Images courtesy of Carl Zeiss Meditec.


“The disadvantage of lasik treatments is that the cornea has to be fully opened by the flap. This may potentially result in biomechanical instability. In addition, the opening leads to the dry eye syndrome in some individual cases,” said Dirk Mühlhoff, head of refractive lasers at Carl Zeiss Meditec. “For this reason, we at Carl Zeiss Meditec sought a minimally invasive approach using the femtosecond laser to cut the tissue to be removed from the cornea in a single piece called an intrastromal lenticule. There is no longer any need to create a flap, and the cornea is protected.”

The new approach is already marketed, and Mühlhoff reveals that extraction of the corneal lenticule through a tiny keyhole incision is scheduled for market launch in the second half of 2011.

Diagnosis and surgical guidance

The growth in OCT imaging procedures over the past 10 years has been astounding, said Eric Swanson, one of the co-inventors of OCT and co-founder of Advanced Ophthalmic Devices (an ophthalmic OCT imaging company that was acquired by Carl Zeiss Meditec and formed the basis of Zeiss’s ophthalmic OCT product line) as well as editor of www.octnews.org, an OCT news site.

Swanson and Dr. David Huang, OCT co-inventor and a leading researcher in the field, calculated that for US Medicare patients, there were fewer than half a million procedures related to OCT in 2000, but by 2010 this figure had reached nearly 9 million. Worldwide, they speculate that more than 20 million clinical OCT scans were carried out in 2010 and billed at a cost of more than $1 billion.

The meteoric rise can be attributed in part to the fact that OCT enables cross-sectional and volumetric imaging of the retina and cornea at resolutions that cannot be achieved by any other means except biopsy and histology. The images that OCT provides dramatically improve the ability to identify retinal diseases at an early, treatable stage.

In addition to being accepted as a standard diagnostic technique by ophthalmologists, it is also rapidly gaining acceptance by optometrists.

“Today there are over 10 companies supplying OCT systems for the ophthalmology and optometry markets. These companies are in fierce fights for market share and are investing lots of development dollars to achieve differentiation and drive sales,” Swanson said.

“As a result, there have been numerous advances in speed, accuracy, higher lateral resolution, higher longitudinal resolution, image processing, compactness, ease of use, functional imaging (e.g., Doppler detection) and lowering capital equipment costs. This has brought, and will continue to bring, benefits to both physicians and end patients,” he continued.

For OCT pioneer and professor James Fujimoto at MIT, one of the most exciting developments in the technology is the use of narrowband swept lasers. Swept lasers enable operation at long wavelengths such as 1000 and 1300 nm, where high-speed camera technology is not readily available.


Ultrahigh-speed OCT imaging enables 3-D volumetric data sets to be acquired. Images of the anterior eye (shown here) enable measurement of refractive parameters of the eye. Courtesy of MIT.


“These long wavelengths have improved image penetration in scattering tissues,” he said. “In ophthalmology, there is research interest in using swept-source OCT at 1000 nm because it is less sensitive to cataracts and enables imaging in scattering tissues, such as the choroid, the vascular network behind the retina.”

Swept-source OCT can image around 10 times faster than commercial spectral OCT instruments, or almost 1000 times faster than the first-generation OCT technology, which used superluminescent diode light sources.

Since swept-source OCT requires the use of a narrow-bandwidth light source that can be frequency swept at high repetition rates, there has been a renewed research interest in tunable light sources.

“There are new developments in swept-source OCT using MEMS [microelectromechanical systems] tunable vertical-cavity surface-emitting lasers (VCSELs),” Fujimoto said. “In many ways, the VCSEL is an ideal light source for OCT. It operates with a single longitudinal mode and can be rapidly tuned using a MEMS mirror. The linewidth is very narrow, so it is possible to perform OCT imaging over centimeter-depth ranges.”

What’s more, because tuning speed is limited in part by the cavity lengths, the short cavity of the VCSEL enables it to tune at record speeds.

Praevium Research in Santa Barbara, Calif., together with Thorlabs in New Jersey and Fujimoto’s group at MIT, recently demonstrated OCT imaging with 760-kHz axial scan speeds as well as record imaging depths. The group presented its work at CLEO 2011. According to Fujimoto, the VCSEL can be scaled to operate in different wavelength regions and is a promising light source for future-generation OCT.


GLOSSARY
cornea
The transparent front layer of the eye. Light entering the eye is refracted (converged) by the outer surface of the cornea.
excimer laser
A rare-gas halide or rare-gas metal vapor laser emitting in the ultraviolet (126 to 558 nm) that operates on electronic transitions of molecules, up to that point diatomic, whose ground state is essentially repulsive. Excitation may be by E-beam or electric discharge. Lasing gases include ArCl, ArF, KrCl, KrF, XeCl and XeF.
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