Magnetic Resonance Imaging (MRI) Models of the Posterior Segment

Abstract

Purpose: To evaluate high-resolution MRI sequences for utility in defining the 3D shape of eyes by comparing emmetropic and myopic eyes of subjects born full-term to those with a history of retinopathy of prematurity (ROP), a disease associated with both short axial length and myopia.

Methods: We use T₁- and T₂-weighted spin echo sequences to image both eyes of all subjects at 0.5×0.5×0.5 mm isotropic resolution. To date, we have imaged 17 subjects: eight emmetropes born full-term (ET), six myopes born full-term (MT), one emmetrope with a history of ROP (EROP), and two myopes with a history of ROP (MROP). We measured axial length (AxL, mm) with an IOLMaster (Carl Zeiss AG) and derived spherical equivalent refraction (SE, D) from autorefractions (Grand Seiko). In Voxar3D (Toshiba Medical Visualization Systems), by tilt, rotation and panning, we exported two 2D cross-sections from each eye: the transverse plane that included both the pupil and optic nerve, and the largest orthogonal (coronal) cross-section. We segmented the vitreo-retinal boundary in each image and fit it with both circular (diameter) and elliptical (long and short axes) shape descriptors. Finally, we used Mimics Innovation Suite (Materialise) to segment and register T₁ and T₂ images.

Results: We found that a T₂ ‘Sampling Perfection with Application-optimized Contrasts using different flip angle Evolution’ (SPACE) sequence was excellent for vitreo-retinal boundary detection. A T₁ ‘Multi-Echo Magnetization Prepared Rapid Acquisition Gradient Echo’ (MEMPRAGE) sequence clearly showed the insertion points of the extraocular muscles (which were invisible in the SPACE images) and sometimes offered superior lens segmentation in Mimics. None of the eyes had staphylomata. Consistent with the peculiar features of ROP myopia, mean AxL in our subjects, from highest, was: MT 25.6 (SE -5.41), MROP 24.9 (SE -5.66), ET 23.5 (SE -0.24), and EROP 22.3 (SE +0.19). Coronal diameters followed the same rank order; the most perfectly circular eye at the equator was the EROP eye. In the transverse sections, the rank order (from the highest) of the ratio of the long to short elliptical axes was ET, EROP, MT, MROP.

Conclusions: T₁ and T₂ sequences provide complementary information and, in combination, are suitable for modeling the shape of the eye. Myopic eyes are enlarged at the equator, not just axially enlarged. In this small sample, ROP eyes are shorter than is typical for their SE.