Technicians & Nurses Program

Transcription

1 ASCRS ASOA Symposium & Congress Technicians & Nurses Program April 17-21, 2015 San Diego, California

2 What Do All These Colors Mean And who s paying for all the ink? BLUF: Red is bad. Frank W. Scribbick, MD University of Texas Health Science Center San Antonio, Texas Topography: The word topography is derived from the Greek words topos, meaning a place, and graphien, meaning to write. Thus, topography is the written, or drawn, description of a place. Some of the earliest known maps were made in Mesopotamia, in the area now known as Iraq, where a series of maps showing property boundaries were drawn in about 2400 B.C. for the purpose of land taxation. A Roman map dating from about A.D. showed such topographical features as roads, cities, rivers, and mountains. Physical maps commonly use color most dramatically to show changes in elevation. A palette of greens is often used to display common elevations. Dark green usually represents low-lying land with lighter shades of green used for higher elevations. In the higher elevations, physical maps will often use a palette of light brown to dark brown to show higher elevations. Such maps will commonly use reds or white or purples to represent the highest elevations on the map. In a similar fashion, corneal topography uses green to annotate normal corneal elevation, usually a keratometry measurement of 43.5, with red being steeper and blue flatter. Tomography: From the ancient Greek, tomos-slice, graphien meaning to write. Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. Since 1995 the devices commonly used to evaluate the anterior segment (Orbscan II and Pentacam) are more accurately described as tomographers rather than topographers. What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 1

3 Historical development of corneal topography: The modern day machines that we use in refractive surgery practices are the result of 400 years of scientific development and innovation. Below is a timeline of the significant events that led to our modern devices: 1619 Scheiner, convex mirrors, reflection of marbles from cornea 1854 Von Helmoltz, spheres 1882 Placido disc 1889 Javal placido disc & keratometer 1892 Theodor Scheimpflug, scheimpflug photography 1896 Gullstrand, photography & keratometer-lead to videokeratoscopy 1911 Gullstrand slit lamp biomicroscopy 1920 Vogt specular microscopy, not useful until the 1960s 1950 ultrasound pachymetry st Apple computer st placido based keratoscope 1990 confocal biomicroscopy 1991 OCT posterior segment IR 800nm 1993 VHF digital ultrasound biomicroscopy-artemis 1994 OCT anterior segment IR 1310nm 1995 slit scanning elevation topography-orbscan II 1995 scheimpflug imaging Pentacam Christoph Scheiner, an Austrian Jesuit priest, was a prolific scientist; among his many interests, he researched the eye, optics and telescopes. In 1619 he observed and calculated the radius of the cornea from the reflection of standard sized marbles. This property of the cornea acting as a convex mirror and the use of optical formulas was a pivotal discovery, and was the basis of corneal topography measurements until late 1980s. What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 2

4 Von Helmholtz, 1n 1851 revolutionized ophthalmology with the invention of the direct ophthalmoscope. He also developed spheres of known size and radius for use in determining the corneal radius and power. The development of the manual keratometer is credited to Von Helmhotz. The keratometer uses only 4 data points, 2 on the steep axis, 2 90 degrees away. The reflection of the 4 points is over a central zone of 2.88 to 4mm. This device is accurate for regular normal corneas, but not for irregular corneas. Also there is no information inside, outside, or between the 4 reference points. The manual keratometer is also less accurate for powers less than 36D and above 50D, although the theoretical range is 30-60D. Antonio Placido, a Portuguese ophthalmologist is considered the father of corneal topography, with his invention of the Placido disc in He created a disc of alternating white and black rings, which produced contour lines when reflected off the corneal tear film. It was accurate in the central and midperipheral areas, but not in the far periphery. It was not until the 1950s that a convex, or bowl shaped disc was produced to diminish the distortion produced by a flat disc and curved cornea. What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 3

5 Theodor Scheimpflug, an Austrian army Captain, in the early 1890s, developed a photographic method/technique to correct perspective distortion in aerial photography. This was born out of a wartime need for photographic maps that accurately displayed the places to be bombed, and the scheimplug principal is used in all modern corneal topographers. Allvar Gullstrand, a Swedish ophthalmologist, invented the first slit lamp biomicroscope in 1911, for this he won the Nobel Prize in medicine. He also advanced the use of keratometry and is credited with the schematic eye which we also still use today. What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 4

6 Ultrasound pachymetry, used to measure corneal thickness, began in the 1950s; ultrasound for medical purposes only began 10 years earlier, with no one person credited for the invention. The research into ultrasound technology took off in 1914 after the Titanic disaster, as a way to detect icebergs. Again, the multiple military applications led to advances in subsequent wars. Personal computer, late 1970s, revolutionized every aspect of human life, to include medicine. With the power of the computer, the images acquired from a placido-based topographer could be analyzed and interpreted in ways never before possible, and the era of modern corneal topography was born. The first placido-based keratoscope was commercially available in What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 5

7 Confocal biomicroscopy, introduced in 1990, was another unique way to examine the cornea, here not by topography which only looks at the anterior surface, but more accurately called corneal tomography, where the internal structures of the corneal tissue could be visualized. This led to OCT, which provides 3-D images of the tissue examined, and with the computer software, a mind-numbing amount of data points. OCT posterior segment, Ocular coherence tomography is based on low coherence interferometry, compares the time-delay of infrared light (1,310 nm in anterior segment models or 800 nm in posterior segment models) reflected from the anterior segment structures against a reference reflection. This interference pattern leads to a cross-sectional image of the eye with a high resolution in the range of microns (1/1000 of a mm). What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 6

8 OCT anterior segment, The time domain is capable of seeing all anterior segment structures in single image. Fourier domain OCT is faster and has better resolution, but can t see everything. Orbscan II, 1995 slit scanning elevation topography. It is one of the elevationbased methods for assessment of topography. Multiple complimentary slits are used to perform an assessment of the corneal surface. In the Orbscan, 40 slits (20 each from nasal and temporal side) are projected on the cornea to assess 240 points on each slit. The triangulation between the reference slit beam surface and the reflected beam captured by the camera can be used to analyze the anterior and posterior corneal curvature and the pachymetry. What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 7

9 Pentacam, 1995, scheimpflug imaging. The Pentacam can measure the anterior and also the posterior surface of the cornea. The Pentacam is a noncontact optical device with a rotating scheimpflug camera that takes up to 50 slitimages of the anterior segment of the eye in less than 2 seconds. These images are then reconstructed in a three-dimensional image, and the software generates different parameters. This technology was the second to assess the posterior corneal surface, after the Orbscan II (Bausch and Lomb Inc.). During the scan, the placido disk topography and scheimpflug images are acquired simultaneously, obtaining anterior and posterior corneal topography data, the complete corneal pachymetry, and the lens density. What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 8

10 Wavefront/Aberrations: A complex way to describe the total refractive error of the eye. Prior to this technology, which was developed for use in astronomy (Hubble telescope) we were limited to describing refractive error as sphere and cylinder, and could only correct these lower order aberrations. Wavefront analysis is the clinical application of the wave theory of light (light being described both as a ray and as a wave). Wavefront analysis is now done by one of four methods: Hartmann-Shack; Tscherning; thin-beam single ray tracing; and optical path difference. Each of these will provide a detailed report of lower-order aberrations (sphere and cylinder) as well as higher-order aberrations (spherical aberration, coma, and trefoil, among others). A low power laser beam is focused on the retina, this light is reflected out of the eye as waves, in a perfect eye, all the rays would emerge in parallel, and the wavefront would be a flat plane. In reality, that never happens, no eye is perfect. An array of lenses samples parts of the wavefront and focuses light on a detector and the optical aberrations can be resolved into a variety of shapes. Zernike polynomials are mathematical formulas used to describe surfaces, and through these we can generate 3-D shapes for the lower and higher order aberrations. It is important to remember that the wavefront aberrations are incorporating the entire eye, which includes the lens (which can change overtime) and is affected by pupil size. What Do All These Colors Mean/Scribbick/JCAHPO/ASCRS-San Diego 4/2015 9

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