Volume measurement of horizontal extraocular muscles with magnetic resonance imaging

The horizontal extraocular muscle volume of 11 normal adults and 3 ophthalmoplegic patients was measured with magnetic resonance imaging (MRI), using a 1.5 Tesla superconductive system which can provide T1 weighted images of 3 mm gapless slices of the orbit with the spin echo technique. The MRI film was projected and magnified on Kent paper with an overhead projector; muscle shapes were traced and cut from the paper. Muscle volume was defined as the total weight of the Kent paper shapes representing the muscles from all MRI slices. The average volumes of the medial and lateral rectus muscles (MRM, LRM) of the 11 subjects were 690 +/- 87 mm3 and 734 +/- 77 mm3. In two patients with peripheral nerve palsy, the small muscle volume was classified as atrophic; in the one patient with orbital myositis, the large muscle volume was classified as hypertrophic. This measurement technique is useful for evaluation of the extraocular muscles, especially in ophthalmoplegia.