The axial misalignment between ocular lens and cornea observed by MRI (I)--at fixed accommodative state.

Lens tilt and decentration relative to the corneal axis are two of the factors which cause aberration in human eyes. Although many techniques have been developed to quantitatively measure these factors, however, they are mostly confined in 2D observations. To extend the view from 2D to 3D, MR imaging technique becomes a good candidate due to its capability of 3D-image reconstruction and with fairly good spatial resolution for that purpose. In this study a total of six eyes of six young students at this Institute were examined by using a 1.5T MRI machine incorporating with a commercial 3-in. surface coil at Taipei Veterans General Hospital. From a 45 degrees flipped reflective mirror, the subject could focus to a target at a distance of 60 cm from the eye for MR imaging in monocular vision. Quadric surface models were used to fit cornea and lens surfaces in the post image processes. Tilts of the two lens surfaces and the decentration of lens centre with respect to corneal axis were determined through coordinate transformations. One month later, retest was carried out on five of the six subjects. The results show that the average tilts of anterior and posterior lens surfaces, and lens decentration are 3.7+/-2.5 degrees, 3.3+/-1.4 degrees and 0.11+/-0.07 mm (H), -0.06+/-0.38 mm (V), respectively, for the test; 2.1+/-2.4 degrees, 1.9+/-1.8 degrees and -0.02+/-0.28 mm (H), -0.45+/-0.28 mm (V) for the retest. No statistically significant difference (by Wilcoxon Signed Ranks test) is revealed for the tilts of both lens surfaces (p(phia)=0.375, p(phip)=0.225) and for decentration (p(H)=0.343, p(V)=0.345) between test and retest in this longitudinal observation. The surface shapes of the ocular anterior components do not seem to be unified by only one type but ellipsoid, elliptic paraboloid, hyperboloid and paraboloid are all possible. Changes in shape type in the ocular anterior components were also found when measured in a period of one month.