High-resolution 3D MR imaging of the trochlear nerve

Abstract

Background and purpose: The cisternal segment of the trochlear nerve is difficult to identify reliably by routine MR imaging. We investigated the visibility and anatomic features of the trochlear nerve by using high-resolution 3D-bTFE imaging in healthy subjects.

Materials and methods: This study was conducted with 32 healthy subjects without ocular movement disorders. For us to visualize the cisternal segment of the trochlear nerve, all subjects underwent 3D-bTFE imaging at 3T with 2 different resolutions: conventional resolution (voxel size, 0.67 x 0.45 x 1.4 mm) and high resolution (voxel size, 0.3 x 0.3 x 0.25 mm). Visibility of the trochlear nerve was graded with the use of a qualitative scale of certainty as follows: definite, probable, and indeterminate. The diameter of the trochlear nerve was measured.

Results: On conventional-resolution images, the visibility of the trochlear nerve was definite in 3 nerves, probable in 12 nerves, and indeterminate in 49 nerves. On high-resolution images, visibility was definite in 63 nerves and probable in 1 nerve. The mean diameter of the trochlear nerve was 0.54 mm (range, 0.35-0.96 mm).

Conclusions: The trochlear nerve was visualized 100% of the time on high-resolution imaging with a voxel smaller than the nerve diameter. High-resolution imaging should have an important role in investigating the pathogenic mechanism of neuropathic strabismus, such as congenital superior oblique palsy.

Fig 1.

Concordant visibility of the trochlear nerves on conventional- and high-resolution 3D-bTFE images in a 48-year-old man. Conventional- (A) and high-resolution (B) images show clearly the cisternal segment of the right trochlear nerve (arrows) with definite visibility from the root exit point at the posterior aspect of the pontomesencephalic junction to the ipsilateral tentorium.

Fig 2.

Concordant visibility of the trochlear nerves on conventional- and high-resolution 3D-bTFE images in a 34-year-old man. A, Conventional image with a fusion of different right and left levels shows curvilinear nonbranching structures (arrows) with “probable” visibility, coursing in an anterolateral direction toward the ipsilateral tentorium. B, Reformatted high-resolution image clearly shows the trochlear nerves bilaterally (arrows) from the root exit points with “definite” visibility.

Fig 3.

False-positive case of the trochlear nerve on a conventional-resolution 3D-bTFE image in a 47-year-old man. A, Conventional image shows the root exit point and its cisternal course of the left trochlear nerve (arrows) with “definite” visibility. B, Reformatted high-resolution image clearly separates the left trochlear nerve (arrows) from the vessel (arrowheads), which is misinterpreted as the trochlear nerve in A.

Fig 4.

False-positive case of the trochlear nerve on conventional-resolution 3D-bTFE image in a 3-year-old girl. A, Conventional image shows the cisternal segment of the left trochlear nerve (arrows) with “probable” visibility. B, Reformatted high-resolution image clearly separates the left trochlear nerve (arrows) from the adjacent vessel (arrowheads) running parallel, which is misinterpreted as the trochlear nerve in A.