Unspecified olfactory disorders, dizziness, facial pain and paralysis – these are typical symptoms that require a neuroradiological examination of the cranial nerves. Just like many other nerves, the cranial nerves are superfine, minute structures running through and around the brain. When the soft tissue of the brain is next to bones or cavities, it is often difficult for the MRI to identify and visualize cranial nerves.
New techniques, better image quality
The signal-to-noise ratio (SNR) and spatial resolution are two major challenges in MR imaging of cranial nerves. The more caudal the cranial nerves, the more difficult it is to identify them unambiguously. This holds particularly true for nerves XI and XII which are both extremely fine. The good news is: state-of-the-art MR techniques generate much better images due to high resolution, stable image quality, optimized sequences and artefact reduction.
1,5, 3, and 7 Tesla
In low-tesla systems, such as 1.5T, CISS sequences used to be the sequence of choice due to its high CSF/tissue contrast which offered good quality images. In 3T and 7T scanners however this advantage is lost as CSF is emphasized which causes artefacts. This is where MP2RAGE comes in, a T2-weighted sequence which allows 3D post-processing and is less volatile than CISS. “3T MRI scanners have a lower SNR and thus provide better resolution of the cranial nerves”, says Dr Gizewski. So-called parallel acquisition techniques (PAT) and modern multi-channel coils enhance the results even further.
In 7T systems resolution can become a problem, as the neuroradiologist explains: “Particularly in those areas where bones and cavities meet, the high field strength tends to cause more artefacts. In short: higher field strength does not necessarily mean more diagnostic information.” Thus a combination of high-resolution MRI and CT is preferred for cases when bony structures and cavities are concerned, for example the petrous part of the temporal bone and the inner ear.
Pain and dizziness
In addition to patients who present with olfactory disorders and require imaging of the olfactory bulb, patients who complain about pain and dizziness are usually referred for imaging of the cranial nerves. Severe dizziness can be caused by nerve-vessel contact, i.e. vessels extend into the internal acoustic pore. Since not the nerve but another structure such as an MS lesion may be involved, high-resolution images of the brain stem have to be acquired. The radiologist’s expertise is also required for cancer patients, for example in meningeal carcinomatosis, where a solid tumor diffusely spreads into the CSF, often with the involvement of the cranial nerves. Patients who present with retrobulbar neuritis or unspecified vision disorders undergo examination of the optical nerve which fortunately is fairly easy to imagine due to its length and thickness.
Neuroradiological examinations take between 20 and 30 minutes and – depending on the diagnostic complexity of the case – are performed either by a neuroradiologist or a radiologist. Patients are usually referred by ETN specialists, neurologists, neurosurgeons or ophthalmologists. Certain diagnostic issues require imaging with contrast agents, for example when meningeal carcinomatosis or a tumor affecting the vestibulocochlear nerve (vestibular schwannoma) is suspected. With regard to pathologies of the optical nerve, contrast MRI can exclude neurinoma. Typical facial pain is usually assessed without contrast agents.
CT to treat trigeminal neuralgia
CT is used for therapy purposes, for example in CT-guided radiofrequency thermocoagulation of the trigeminal nerve. This common therapy to reduce facial pain caused by trigeminal neuralgia is also routinely applied in Innsbruck as Dr Gizewski confirms.
Professor Dr Elke R Gizewski was appointed Director of the University Clinic for Neuroradiology at the Medical University Innsbruck, Austria, in 2012. A radiologist by training, she focuses on diagnostic and interventional neuroradiology, in addition, she holds a certificate in psychotherapy and a Master in Health Business Administration. In Innsbruck she expanded the multimodal MRI capabilities and established fMRI paradigms as well as special applications, including MR spectroscopy. Dr Gizewski heads the 3T MRI scanner of the Core Facility Neuroscience Imaging Research at the Medical University Innsbruck; she is member of DFG research groups looking at placebo effects and the effects of learning in pain processes. She cooperates with many national and international groups, inter alia to evaluate new – mostly endovascular – therapies.