Fig. 6. Model I3 rings and parameter extraction from high-spatial-resolution MRIs. (A) Model I3 rings. The maximum width of the lumen is 2a, the height of the lumen above the maximum width is b1, and the height of the lumen below the maximum width is b2. The radius of the semi-circular cross section of the outer half-ring at the top and bottom and of the inner half-ring surrounding the lumen is r. The width added between the outer and inner half-rings so that the medio-lateral width matches that of the muscle at its widest extent, is q. The total height h of the model ring is equal to 4r+b1+b2, and its total width w is equal to 4r+2q+2a. (B) High-spatial-resolution MRI of the I3 muscle in axial section from an isolated buccal mass. The maximum lumen width, a, and of the heights below and above this maximum width (b1 and b2, respectively), are shown on the image. In addition, the measurement of the maximum width, w, and maximum height, h, is illustrated. The parameter r is calculated from (h-b1-b2)/4, and the parameter q is calculated from (w-2a-4r)/2. The top and bottom borders of the lumen used in measuring b1 and b2 are measured from the dorsal and ventral extremes of the cartilage of the lumen, which appears black in the MRI because the lumen is partially closed. Measurements were made in pixels and then scaled to arbitrary model units. Note that, although lumen width 2a and muscle width w are not measured at the same dorso-ventral height, the calculation of q is performed as if they were at the same height. This follows from the model approximation (A), which assumes that the maximum lumen width and the maximum I3 ring width are at the same dorso-ventral height. Magnetic resonance acquisition parameters for the slice shown: fast spin echo, TE (time to echo)=120 ms, TR (time to repeat)=3000 ms, ETL (echo train length)=16, FOV (field of view)=5 cmx5 cm, SW (slice width)=1.5 mm, AM (acquisition matrix)=512x512, NEX (number of excitations)=4.