Thin-section imaging with high-spatial resolution is required to directly image the CNs or their course. Slice thickness should be calculated for optimal spatial resolution without introducing partial-volume effect. The primary plane of imaging varies with the CN of interest and should include orthogonal views. The CN nuclei and fascicular nerve segments (brainstem segments) are not well seen on MRI, but their location can be deduced by recognizing surrounding anatomy and are best imaged with various T2-weighted, multi-echo, and fluid-attenuated inversion- recovery (FLAIR) sequences [15]. Three-dimensional isotropic heavily T2-weighted sequences with low CSF artifact provide submillimeter high-spatial and contrast resolution to image the cisternal and dural cave segments of the CNs and can be reformatted into multiple planes [5,21-23]. Intradural and foraminal CN segments, surrounded by a vascular plexus, are best imaged with contrast-enhanced 2- D or 3-D T1-weighted images, contrast-enhanced MR angiography (MRA) or contrast-enhanced modified balanced steady-state free precession (SSFP) sequences in which the nerves appear dark while surrounded by the vascular plexus [5,15,23,24]. The peripheral or extraforaminal segments of the CNs begin as they exit the skull base where they are initially surrounded by fat and are well imaged with high-resolution axial and coronal T1-weighted and T2-weighted sequences and with SSFP sequences as they course within the face and neck [5,15]. Contrast-enhanced fat-suppressed T1-weighted techniques may emphasize abnormal nerves or lesions but can mask subtle pathology if the fat-suppression is nonuniform. Because CN examinations tend to be lengthy, strategies such as parallel imaging may improve patient compliance and image quality [15]. DWI and fiber tract imaging techniques have shown some improvement in visualization of CN fascicles and fiber tracts but are without sufficient reliability for routine clinical use [5,25,26].
Cranial Neuropathy. Thin-section imaging with high-spatial resolution is required to directly image the CNs or their course. Slice thickness should be calculated for optimal spatial resolution without introducing partial-volume effect. The primary plane of imaging varies with the CN of interest and should include orthogonal views. The CN nuclei and fascicular nerve segments (brainstem segments) are not well seen on MRI, but their location can be deduced by recognizing surrounding anatomy and are best imaged with various T2-weighted, multi-echo, and fluid-attenuated inversion- recovery (FLAIR) sequences [15]. Three-dimensional isotropic heavily T2-weighted sequences with low CSF artifact provide submillimeter high-spatial and contrast resolution to image the cisternal and dural cave segments of the CNs and can be reformatted into multiple planes [5,21-23]. Intradural and foraminal CN segments, surrounded by a vascular plexus, are best imaged with contrast-enhanced 2- D or 3-D T1-weighted images, contrast-enhanced MR angiography (MRA) or contrast-enhanced modified balanced steady-state free precession (SSFP) sequences in which the nerves appear dark while surrounded by the vascular plexus [5,15,23,24]. The peripheral or extraforaminal segments of the CNs begin as they exit the skull base where they are initially surrounded by fat and are well imaged with high-resolution axial and coronal T1-weighted and T2-weighted sequences and with SSFP sequences as they course within the face and neck [5,15]. Contrast-enhanced fat-suppressed T1-weighted techniques may emphasize abnormal nerves or lesions but can mask subtle pathology if the fat-suppression is nonuniform. Because CN examinations tend to be lengthy, strategies such as parallel imaging may improve patient compliance and image quality [15]. DWI and fiber tract imaging techniques have shown some improvement in visualization of CN fascicles and fiber tracts but are without sufficient reliability for routine clinical use [5,25,26].