Abstract
Purpose::
To investigate the contractility within the horizontal extraocular muscles during horizontal eye movement using high resolution magnetic resonance imaging (MRI).
Methods::
Seven healthy subjects gazed at a horizontal sinusoidal moving target (2s period, peak velocity 64°/s, amplitude ±20°) presented with a typical functional MRI setup. A microscopic coil (47mm diameter) at 1.5T was placed in front of the right orbit to acquire 2 dimensional complementary spatial modulation of magnetization images (scan time 4.5min, 15 time frames of 70ms, resolution 0.5x0.5x4mm3, tag line distance 3mm). The optic nerve and the horizontal rectus muscles were tracked. Five longitudinal marker chains were depicted within each muscle and averaged. For interindividual comparison, the lengths of the horizontal extraocular muscles were normalized and divided into three equal portions.
Results::
The contracting and relaxing horizontal rectus muscles, and the non contracting optic nerve were reliably tracked. Their contractility could be separated statistically significantly from the 4th time frame on (= 210 ms from the beginning of the right to left movement of the right eye; p<0.01, ANOVA). The muscle belly portion could statistically significantly be sorted from the posterior and the anterior portion from the 6th time frame on (= 350ms) in the contracting medial rectus muscle and from the 8th time frame on (= 490ms) in the relaxing lateral rectus muscle (both p<0.01).
Conclusions::
The MRI motion encoded technique resolved the differential contraction of extraocular muscle portions in healthy subjects. This is the first study on muscle contraction pattern that was actually conducted on moving eyes.
Keywords: eye movements • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • strabismus