Abstract
Objective: This paper presents a systematic review of diffusion MRI (dMRI) and tractography of cranial nerves within the posterior fossa. We assess the effectiveness of the diffusion imaging methods used and examine their clinical applications.Methods: The Pubmed, Web of Science and EMBASE databases were searched from January 1st 1997 to December 11th 2017 to identify relevant publications. Any study reporting the use of diffusion imaging and/or tractography in patients with confirmed cranial nerve pathology was eligible for selection. Study quality was assessed using the Methodological Index for Non-Randomized Studies (MINORS) tool.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. Most acquisition protocols used single-shot echo planar imaging (88%) with a single b-value of 1,000 s/mm2 (78%) but there was significant variation in the number of gradient directions, in-plane resolution, and slice thickness between studies. dMRI of the trigeminal nerve generated interpretable data in all cases. 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. 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. Fiber tractography was performed in 30 studies (73%). 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. 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.Conclusions: dMRI has the potential to inform our understanding of the microstructural changes that occur within the cranial nerves in various pathologies. Cranial nerve tractography is a promising technique but new avenues of using dMRI should be explored to optimize and improve its reliability.
Highlights
Diffusion magnetic resonance imaging is a non-invasive magnetic resonance imaging (MRI) technique that is able to provide a quantitative assessment of a tissue’s microstructure. 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)
This paper provides a systematic review of the clinical applications of dMRI and tractography of the cranial nerves within the posterior fossa (CN III-XII)
Probabilistic tractography aims to address the problem of uncertainty of directional information by creating multiple streamlines from a selected distribution of possible fiber orientations with the results presented in the form of a probability distribution, rather than a single “best fit” (Behrens et al, 2007; Tournier et al, 2011)
Summary
Diffusion 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. 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). 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 an essential tool in neuroimaging analysis and diagnosis (Assaf and Pasternak, 2008; Ciccarelli et al, 2008). 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. 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)
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