Magnification changes in scanning laser tomography.

PURPOSE

It is important when evaluating glaucomatous optic disc progression in longitudinal images that image magnification remains unchanged. We studied the effect of changed lens power on magnification in scanning laser tomography. The relative contribution to magnification of axial length, eye-scanner distance, and image-to-image scaling was also assessed.

METHODS

A simulated optic disc in a model eye was imaged using the Heidelberg Retina Tomograph. Lens power was alterable by exchanging intraocular lenses (IOL) mounted at the lens plane of the eye to mimic changes in the crystalline lens. IOL power of +20.0D and axial length of 21.5 mm was compatible with emmetropia. The optic disc was imaged through IOLs differing in power (+16.0D to +25.0D) but with axial length kept constant. IOL power was then held constant and imaging was repeated for various axial lengths (17.5-23.5 mm). Model eye-scanner distance was varied with each test sequence. The distances between landmarks on the disc was measured before and after contour lines were exported.

RESULTS

Image size varied with IOL power and axial length (r > 0.98; P < 0.0001), with the magnification effect of a +1D increase in lens power equivalent to a third the magnification effect of a 1-mm increase in axial length. Magnification tended to increase with myopia (IOL power > +20.0D) and was accentuated by longer eye-scanner distances. Image-to-image scaling corrected some magnification though this varied with ametropia.

CONCLUSIONS

Changed lens power, axial length, and eye-scanner distance can affect the size of the optic disc in scanning laser tomography images. The exported contour line partly compensates for changed magnification.