LAUSR

Fluorescence Lifetime Imaging Ophthalmoscopy: A New Era of Autofluorescence Imaging of the Human Retina More so than almost any other field in medicine, noninvasive imaging has been critical for progress in the diagnosis and treatment of retinal disease. From the exquisite hand drawings of pioneers of ophthalmology after funduscopy was invented in 1851 by Helmholz to the first photographic records of macular structure to fluorescein angiography to optical coherence tomography, each imaging advance has led to new insights into the physiology and pathophysiology of macular and retinal disorders. Imaging of the intrinsic autofluorescence of the human macula was first systematically studied by Delori et al1 in the 1970s, but it remained largely a research technique until high-sensitivity detectors were incorporated into digital fundus cameras and scanning laser ophthalmoscopes in the 1990s and 2000s. It soon became clear that patterns of fluorescence originating largely from lipofuscin in the retinal pigment epithelium could have great utility in diagnosing and monitoring progression of a variety of macular diseases such as age-related macular degeneration, Stargardt disease, and pattern dystrophies. Topographic mapping of autofluorescence intensity (AFI) distributions is now routinely performed in medical retina evaluations, but with continued improvements in detector technology, it has become clear that a wealth of additional new information can be gleaned from the fluorophores of the retina. The most notable advance has been the ability to distinguish between fluorophores either spectrally or by monitoring differences in kinetics of photon emission, a technique known as fluorescence lifetime imaging ophthalmoscopy (FLIO). Fluorescence lifetime imaging ophthalmoscopy is still an uncommon technique, however, due to the current rarity of the prototype instruments that are in use in just a handful of sites in Europe and the United States. In this issue of Retina, the group led by Professors Wolf and Zinkernagel in Bern, Switzerland, nicely demonstrates the value of FLIO for monitoring the progression of Stargardt disease over a period ranging from 3 to 45 months.2 Fluorescence originates when a chemical compound absorbs a photon of light and promotes an electron to a higher energy state. The excited electron can then transition to a relatively stable slightly lower energy state where it eventually decays to its ground state through emission of a red-shifted photon, typically over a period of picoseconds or nanoseconds (fluorescence) or much longer times (phosphorescence). Fluorescent compounds have characteristic absorption and emission spectra that can be quantified by exciting the chemicals with various wavelengths of light and through the use of high-resolution spectrophotometers. Because incorporation of a high-resolution spectrometer is generally not feasible in a clinical imaging instrument, the emitted photons are usually filtered through spectral windows and detected on a charge-coupled device array or photon counters. This is the basis of the AFI imaging devices currently on the market based either on fundus cameras or scanning laser ophthalmoscopes. These devices provide high-resolution images of distributions and relative intensities of retinal fluorophores, but they allow for only minimal discrimination between various fluorophores. This is where FLIO manifests its utility. By substituting the conventional continuous-wave blue laser of a scanning laser ophthalmoscope with a pulsed picosecond 473-nm laser and by using 2 high-resolution time-gated photon counters tuned to a short spectral channel (498–560 nm) and a long spectral channel (560–720 nm), investigators can now begin to distinguish between various fluorophores depending on their chemical properties and metabolic environments. Typical FLIO images of a normal macula show shortest lifetimes (usually depicted in orange-red color) at the fovea originating from the weak intrinsic fluorescence of the macular carotenoid pigment, intermediate lifetimes (usually depicted in yellow-green) from lipofuscin fluorescence, and long lifetimes (usually depicted in blue) from collagen and elastin of the optic nerve and blood vessels (Figure 1). None of the authors has any financial/conflicting interests to disclose. Heidelberg Engineering has provided an investigational FLIO device to the Moran Eye Center at no cost.