Diffusion MRI and tractography of the cranial nerves in this region is technically challenging due to the nerves' small size (typically 1–5 mm in maximal diameter) and their anatomical location within cerebrospinal fluid (CSF) and close to tissue-air and tissue-bone interfaces ( Figure 1). The remaining ten cranial nerves (the oculomotor, trochlear, trigeminal, abducens, facial, vestibulocochlear, glossopharyngeal, vagus, spinal accessory, and hypoglossal ) emerge from the brainstem and course through the posterior fossa and fluid cisterns before exiting the skull (Matsuno et al., 1988). The first two cranial nerves-the olfactory and optic nerves are both sensory nerves, composed of afferent fibers relaying smell and vision, respectively, entering the brain within the anterior and middle cranial fossae. There are twelve sets of paired cranial nerves (CN I-XII) that typically relay information between the brain and regions of the head and neck and are numbered according to their rostral-caudal position when viewing the brain. More recently, there has been growing clinical interest in utilizing tractography of the cranial nerves in order to assist clinical diagnosis of various neurological pathologies and to inform the surgical planning of neurosurgical procedures such as brain tumor surgery. White matter fiber tractography is routinely used in preoperative surgical planning (Nimsky et al., 2005 Yogarajah et al., 2009 Duncan et al., 2016) and may also be incorporated into the neuronavigation system to guide surgery intraoperatively (Coenen et al., 2001 Nimsky et al., 2005 Nowell et al., 2015). Diffusion tensor imaging (DTI) was the initial model to describe the orientational information in dMRI data it has become a well-established method for imaging the brain's white matter tracts and is now an essential tool in neuroimaging analysis and diagnosis (Assaf and Pasternak, 2008 Ciccarelli et al., 2008). Fiber tractography is a three-dimensional reconstruction of the dMRI data enabling visualization of neural tracts and the brain's connectivity (Mori and van Zijl, 2002 Parker and Alexander, 2005 Gong et al., 2009). dMRI is sensitive to the displacement of water subject to thermally driven Brownian motion and can reveal a tissue's orientational organization (Le Bihan et al., 2001 Basser and Jones, 2002 Jellison et al., 2004). Cranial nerve tractography is a promising technique but new avenues of using dMRI should be explored to optimize and improve its reliability.ĭiffusion magnetic resonance imaging (dMRI) is a non-invasive magnetic resonance imaging (MRI) technique that is able to provide a quantitative assessment of a tissue's microstructure. Deterministic tractography using diffusion tensor imaging was performed in 93% of cases but the reported success rate and accuracy of generating fiber tracts from the acquired diffusion data varied considerably.Ĭonclusions: dMRI has the potential to inform our understanding of the microstructural changes that occur within the cranial nerves in various pathologies. Most studies evaluating fiber tractography involved patients with a vestibular schwannoma (82%) and focused on generating tractography of the facial nerve to assist with surgical planning. Fiber tractography was performed in 30 studies (73%). Diffusivity values within the trigeminal nerve correlate with the effectiveness of surgical treatment and there is some evidence that pre-operative measurements may be predictive of treatment outcome. Analysis of diffusivity measurements found significantly lower fractional anisotropy (FA) values within the root entry zone of nerves affected by TN and FA values were significantly lower in patients with multiple sclerosis. dMRI of the trigeminal nerve generated interpretable data in all cases. Most acquisition protocols used single-shot echo planar imaging (88%) with a single b-value of 1,000 s/mm 2 (78%) but there was significant variation in the number of gradient directions, in-plane resolution, and slice thickness between studies. Results: We included 41 studies comprising 16 studies of patients with trigeminal neuralgia (TN), 22 studies of patients with a posterior fossa tumor and three studies of patients with other pathologies.

Study quality was assessed using the Methodological Index for Non-Randomized Studies (MINORS) tool. Any study reporting the use of diffusion imaging and/or tractography in patients with confirmed cranial nerve pathology was eligible for selection. Methods: The Pubmed, Web of Science and EMBASE databases were searched from January 1st 1997 to December 11th 2017 to identify relevant publications. We assess the effectiveness of the diffusion imaging methods used and examine their clinical applications. Objective: This paper presents a systematic review of diffusion MRI (dMRI) and tractography of cranial nerves within the posterior fossa.

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