Chicago—The Heidelberg Retina Tomograph II (HRT II, Heidelberg Engineering GmbH), the most widely available generation of this technology, creates a two-dimensional picture of a three-dimensional structure, such as the optic nerve, by acquiring images in a manner similar to that of computed tomography. Jeffrey D. Henderer, MD, explained the basics of this technology and its relevance to glaucoma at the American Academy of Ophthalmology annual meeting.
The scans that the HRT acquires are reconstructed by the computer into a three-dimensional map of the optic nerve head.
"The advantage of this type of analysis is that the height of various regions of the optic nerve and the surrounding tissue in the retina can be quantified. Statistics can then be applied against a normative database to determine the areas where the optic nerve is normal or abnormal. The instrument does this by acquiring three topographic images that are then merged to create a mean image and a standard deviation among the three images," Dr. Henderer explained. He is assistant professor of ophthalmology, Wills Eye Hospital, Glaucoma Service, Philadelphia.
"It does this by using a reference plane set 50 µm below the temporal retinal surface," Dr. Henderer said. "The nice thing about having this kind of information is that the interpreter can compare the information with a normative database. The machine uses an algorithm that was developed at Moorfields Hospital, London, that looks at the neuroretinal rim area for each sector of the optic nerve and compares it with the expected rim area controlling for the overall disc area."
The instrument then highlights an abnormality using either a yellow exclamation point, which indicates borderline, or an X, indicating abnormal.
Dr. Henderer explained that when he looks at an HRT II scan, he first wants to determine that what he is viewing is reliable. There are two issues with reliability-is the scan image quality satisfactory, and is the optic nerve correctly outlined?
With regard to image quality, he demonstrated an example of an unreliable HRT II scan in which the standard deviation was 70 µm. The cross-section through the optic nerve cup revealed a highly jagged and irregular cup floor, resulting in a splotchy color pattern of the cup. He explained that he normally looks for a standard deviation of less than 40 µm and prefers the standard deviation to be less than 20 μm.
To be sure that the nerve is correctly outlined, Dr. Henderer likes to have the HRT image available when examining the patient to be sure the outline is drawn correctly. In a second case, when he examined the patient, his impression was that the cup was fairly large; however, the instrument did not show this.
"In this case, there was a subtle shift in the location of the contour line, which resulted in a huge shift in the appearance of the optic nerve," Dr. Henderer explained.
When following patients to determine progression, the HRT II compares previously acquired images with those currently acquired to create a change probability map. He showed a patient with images from 1994 and 1998 that were compared by the instrument.
"The machine then takes the change probability map and assigns differences to areas that are either sinking away from the camera (coded red) or those that are rising toward the camera (coded green). This information appears on the printout and helps highlight areas of altered topography. The HRT II offers topographic measurements of the optic nerve head and has a normative database to analyze the rim area. Software is provided to determine change," he concluded.