High resolution MRI study of circumlental space in the aging eye.

Abstract

he controversy and confusion regarding the mechanism of presbyopia results in large part from an inability to directly measure the anatomical structures in question. In this study, magnetic resonance imaging (MRI) has been used to directly detect age related changes in circumlental space of 47 emmetropes and mild myopes. MRI offers unparalleled soft tissue contrast, providing differentiation between muscle and other soft tissue, thus allowing the ciliary muscle to be visualized. Moreover, MRI produces undistorted images of the entire lens surface. As previously presented 1 , both the ciliary muscle ring diameter and the circumlental space decrease with age, the decrease in circumlental space is primarily the result of an age dependent decrease in the ciliary muscle ring diameter since the length of the lens equator is not significantly correlated with age. Recently, investigators 2 found evidence that the lens is not symmetrically positioned in the ciliary ring of the rhesus monkey. Specifically, Croft and colleagues 2 reported that during resting accommodation, the temporal ciliary processes are 0.21 mm closer to the lens equator than to the nasal ciliary processes. Here,we present MRI based on data that demonstrate this asymmetry also occurs in the position of the human lens. METHODS A 1.5 Tesla MR imager (1.5T-Signa, General Electric Medical Systems, Waukesha, WI) was used to obtain images from 47 subjects, ages 21 through 83. The soft tissue image contrast available with MRI is superior to that obtained with any other imaging modality. Moreover, by varying MR pulse sequences and timing parameters, desired contrast differences can be maximized. We used a T1 weighted spin-echo pulse sequence to differentiate between muscle and other soft tissue, while clearly delineating lens shape. A series of contiguous axial slices through the lens were acquired simultaneously, requiring 5 minutes. These images were collected with a 4 cm field-of-view and 256 x 256 pixel matrix, resulting in 0.156 mm in-plane resolution. The contiguous 3 mm thick axial slices ensured that the entire lens was captured and allowed the desired “center” slice as well as its flanking slices to be evaluated in order to determine the lens dimensions, as previously described. 3 A custom eye coil consisting of a three-turn solenoid with a 2.5 cm diameter, posi