MRI measurements of normal extraocular muscles and other orbital structures

Background: Studies of extraocular muscle (EOM) by magnetic resonance imagi ng (MRI) need to be extended to normal subjects of different ages to obtain data on the muscle thickness, cross-sectional area, and the volume of EOM and other orbital tissues. Methods: Forty-two orbits of 21 normal subjects in three age groups with an age range of 19-70 years were examined with sur face-coil MRT. The transverse and sagittal images were used to measure the thickness of the four rectus muscles during fixation in different gaze posi tions. The coronal images with eyes in the primary position were used to ca lculate the cross-sectional areas. The volumes of all six EOM, orbital fatt y tissue, the optic nerve and the eyeball were measured in the coronal plan e and in either the transverse or the sagittal plane. Results: The horizont al muscles were thinner than vertical muscles. Muscle volume was larger in SR (superior rectus) than in IR (inferior rectus), larger in SO (superior o blique) than in IO (inferior oblique), and the same in LR (lateral rectus) as in MR (medial rectus). No significant differences were found in the valu es of the cross-sectional area in any image plane between the three age gro ups. There were no significant differences in muscle thickness and size and fatty tissue volume between age groups. The muscle thickness was linearly correlated to the angle of the eye deviation for all four rectus muscles, b oth in the "on" and "off" directions of the muscles. Conclusions: The study provides quantitative data, in normal subjects of different ages, on the t hickness and size of EOM and the volume of other orbital tissues by MRI, to serve as a basis for further studies on the morphological changes of EOM i n various orbital diseases.