A novel laser more closely matches oxyhemoglobin absorption for photocoagulation treatment.
Andrew Masters, Coherent Inc.
Age-related macular degeneration (AMD) is a leading cause of blindness and diminished eyesight and is associated with bleeding in the retina. There are two recognized forms of AMD: wet and dry. The wet form is characterized by abnormal growth of blood vessels at the back of the eye, specifically beneath the macula. When blood and fluid leak from these vessels, photoreceptor cells are damaged, causing vision problems.
This simulation shows regular vision on the left and, on the right, how vision is affected in typical sufferers of wet AMD.
Early stages of the wet form can be treated by lasers in two ways: When the problem blood vessels are grouped behind the fovea, a type of photodynamic therapy is preferred; when the blood vessels are outside the fovea and are allowing blood to leak into the eye, photocoagulation can be employed. Photocoagulation uses the laser as a precise tool to produce controlled local cauterization that destroys the tiny culprit vessels and prevents further bleeding.
There are currently two types of laser-based photocoagulators available to ophthalmic surgeons: single- and three-color. Single-color systems have been available longer and originally incorporated a 514-nm argon-ion laser. More recently, they switched to 532-nm diode-pumped solid-state lasers. Three-color photocoagulators are based on three types of frequency-doubled Nd:YAG lasers, with outputs at 532 nm (green), 561 nm (green-yellow) and 659 nm (red).
These wavelengths are chosen because successful photocoagulation requires tissue selectivity and spatial confinement so that coagulation is maximized while overall thermal loading in the eye is minimized. This can be accomplished by matching the laser wavelength to the oxyhemoglobin absorption spectrum, which has a weak absorption peak at 532 nm (targeted by single-color instruments) and a strong peak at 577 nm (targeted by the 561-nm laser in three-color systems).
In the latter systems, the choice of 532 or 561 nm is made on a case-by-case basis, depending on the exact location of the problem and the relationship with surrounding material. The red laser is used when significant blood is already present and is located between the laser and the target coagulation site. This works because the red laser matches an absorption minimum in oxyhemoglobin and, thus, is not blocked by blood in the transmission path.
There are two problems associated with the use of 561-nm yellow lasers in this application, however. They poorly match the blood-absorption peak centered at 577 nm, and the fundamental 1122-nm Nd:YAG line is a very weak emission wavelength, so these lasers cannot be easily scaled in power. (Because of losses in the system, the laser source must be able to produce several watts of output power.)
In response to this situation, engineers developed a yellow laser based on optically pumped semiconductor technology. This compact technology is scalable in both power and wavelength. The power is increased by using either more pump diodes or more powerful ones, and the wavelength is arbitrarily set by the design of the quantum wells in the active chip. Because of their attributes, the lasers are already widely used at many wavelengths in various types of bioinstrumentation.
Using this technology, Coherent Inc. is delivering photocoagulation lasers with up to 3 W of output at the optimum 577-nm wavelength, a power that can be readily scaled up if required by the application. Photocoagulator manufacturers are set to adopt this new laser technology as a replacement for the existing 561-nm lasers in their three-color instruments.
The 577-nm optically pumped solid-state laser also may become the tool of choice in one-color systems. Traditionally, 532-nm lasers were used simply because they were available with much higher power than their 561-nm counterparts, even though the oxyhemoglobin absorption peak is relatively weak at the shorter wavelength. This situation changes with the advent of higher power 577-nm lasers, so it seems likely that manufacturers will take advantage of this new technology.
Meet the author
Andrew Masters is director of marketing at Coherent Inc. in Santa Clara, Calif.; e-mail: [email protected].