Today, a wide array of laser technologies support an amazingly diverse range of medical and biomedical applications. In fact, it would take a large volume to discuss all the current uses of lasers in medicine. This article selects just four representative examples, and shows how key developments in disparate laser technologies have enabled or enhanced each particular procedure.
LASIK – The Advent of All-Laser LASIK
The very first medical laser application, developed nearly 50 years ago, was a treatment for glaucoma. Today, lasers are still employed extensively in ophthalmology. One of the bestknown applications is laser-assisted in Situ keratomileusis (LASIK), used to correct myopia and sometimes accompanying astigmatism. Here, a thin, hinged flap is lifted from the outer surface of the cornea, exposing the core material. An excimer laser is then used to remove material and hence reshape the cornea. The laser beam is passed through a variable aperture and projected on to the eye using fast scanning mirrors. The flap is then replaced, sealing and protecting the front of the eye. (See Figures 1a and b)
The excimer laser has proved ideal for this application because its short pulsewidth (a few nanoseconds) and short wavelength (193 nm) remove the corneal material in a relatively cold process called photoablation. The laser photons directly break the molecular bonds in the cornea atomizing the material without any thermal effects that could impact the cornea’s transparency. The excimer works by firing a high voltage discharge in a tube containing a mixture of halogen and inert gases. A key step in enabling widespread safe use of LASIK was to optimize this discharge so as to produce a beam with superior uniformity and extremely predictable pulse energy. Erratic pulse energy or hot spots in the beam profile could translate into errors in correcting the cornea.
A recent advance in LASIK is so called all-laser LASIK, or femtosecond LASIK. In this technique, a laser with femtosecond pulse duration is focused to a small spot just under the surface of the cornea. At the tightly focused beam waist, the instantaneous power is high enough to cause highly localized photoablation, even though the cornea material is nominally transparent at the 1053 nm laser wavelength. The focused spot is scanned in a pattern to create a flap replacing the microkeratome blade formerly used. There are several advantages to this approach. When first introduced, the all laser method proved capable of creating a much thinner flap, leaving more of the cornea’s original thickness for reshaping. This now allowed patients with thinner than average corneas to benefit from LASIK. But today, both blade and laser can create flaps as thin as 100 μm. The all laser method can also be more automated — the scanning of both lasers is controlled by software — reducing operator subjectivity in the way the procedure is performed, arguably leading to improved and more consistent outcomes.
A key enabler here was the development of simple, turnkey, ultrafast lasers, often called one-box lasers. These have allowed the development of robust, reliable instruments, which can be operated by those unfamiliar with laser technology.
Improved Photocoagulation for AMD
Photocoagulation is often used to treat the wet form of Age- Related Macular Degeneration (AMD), characterized by periodic ruptures of small blood vessels in the retina. The goal of laser photocoagulation is to seal the leaking vessel without damaging peripheral tissue. Since leaking blood is the main differentiating material between the target area and surrounding tissue, this goal can best be achieved by using a laser wavelength that is selectively and effectively absorbed by blood. Moreover, this should be a visible laser wavelength, allowing it to be delivered through the front of the eye without any damage to the lens or aqueous humor.
This has been commonly accomplished using the 532 nm output of diode-pumped, solid-state (DPSS) lasers, since this wavelength is near a weaker absorption of oxy-hemoglobin. However, the visible absorption spectrum of oxy-hemoglobin actually peaks at 577 nm. (See Figure 2) Fortunately, a relatively new type of visible laser—the optically pumped semiconductor laser (OPSL)—can be designed for any arbitrary wavelength over most of the visible spectrum.
Working with photocoagulation equipment companies, Coherent designed a laser to deliver three watts of output at this specific wavelength. Based on OPSL technology, this laser was also designed to deliver a circular beam with a Gaussian profile, called TEM00, which allows the beam to be focused to a small, well-behaved circular spot on the retina.
This new laser has now enabled improved vessel closure with reduced thermal loading on the eye. But beyond this important benefit, one of the inherent properties of OPSLs is leading researchers to investigate a further improvement in photocoagulation treatment. Specifically, OPSLs can be directly pulsed at very high rates (up to 100 kHz). The use of fast micropulsing can provide extreme dosing control that may enable laser treatment to initiate a wound-healing response without actually causing any trauma.
Skin Resurfacing and Retightening
Lasers have also have a long and varied history in aesthetic procedures. These include treatments to eliminate natural (e.g., age spots) and manmade (tattoos) pigmentations, and procedures to tighten and/or smooth facial skin, i.e., minimize wrinkles.
The mid-infrared output of the carbon dioxide laser is strongly absorbed by human skin and by any water in the skin. In theory, it could, therefore, be used as an alternative to mechanical and chemical peel methods for dermabrasion, i.e., to remove the top layer of scarred, sun/age-damaged skin, and a variety of lesions. But, early carbon dioxide lasers did not offer the required combination of output and cost characteristics necessary to make them practical for medical applications. For example, they did not enable fine control over dosing, which is critical because excessive cumulative heating can cause damage and even scarring rather than a positive outcome.
In response, Coherent developed the UltraPulse laser. Unlike earlier carbon dioxide lasers, this was based on a slab discharge, to provide a simple, rugged compact product. But most importantly, it offered pulsed output supported by a novel RF power supply with fast rise/fall times. This enabled short pulsewidth (as short as 600 microseconds), variable pulsewidths and power on demand pulsing – user control down to the single pulse level. Together with the laser’s TEM00 output beam, this combination provided the precise and subtle control needed to safely and effectively treat facial skin using a method called fractional skin resurfacing, which is less traumatic than earlier procedures and also is characterized by less patient discomfort and faster healing. In a fractional procedure the laser pulses are scanned over the skin creating small holes with a typical depth of a millimeter. The self-healing mechanism of these holes and the related collagen stimulation results in the desired skin resurfacing. Since then, several other pulsed lasers (e.g. the erbium laser) have been developed for these types of application. However, a slab discharge carbon dioxide laser is the only laser proven to tighten skin by tightening the underlying collagen.