Imaging the olfactory tract (Cranial Nerve #1)
Introduction
The introduction of magnetic resonance imaging (MRI) into clinical practice in the early 80s has greatly improved the value of radiological approach to olfactory disorders by allowing precise depiction of the olfactory bulb (OB) and olfactory tract (OT), and very sensitive detection of even very little damage to the central projection areas of the sense of smell. In the mid-90s the pioneering works by Yousem et al. demonstrated the ability of MRI to yield accurate volumetric measurements of the OB in various pathological conditions [1], [2], [3]. This had major clinical relevance because the OB is a unique central nervous organ in which size and function closely correlate. OB measurements have demonstrated high diagnostic and even prognostic value in the evaluation of olfactory disturbances [4], [5], [6]. Concomitantly, major technical improvements in CT technology such as multi-row, multi-slice helical CT and improved matrix sizes with isotropic voxels also enhanced the value of CT for traumatic and tumoral conditions by allowing very accurate and multiplanar analysis of bone structures surrounding olfactory organs. Lastly, innovative methods of psychophysical and electrophysiological investigations have been recently implemented into the clinical work-up of smell disorders [7]. Thus far, the up-to-date evaluation of smell disturbances tightly associates clinical, psychophysical, and electrophysiological testing, together with morphological, quantitative, and emerging functional data yielded by MRI. What the radiologist needs to know about imaging work-up of smell disorders is hereby described.
Section snippets
Radiological/functional anatomy (Figs. 1–3)
The olfactory neuroepithelium containing primary olfactory receptor neurons is located in the upper part of the nasal cavities and covers the cribriform plates of the ethmoid bones and the cranial part of the superior turbinates (Fig. 2a). Multiple olfactory receptor neurons are clumped together within ‘bundles’ interleaved with glial cells being called ‘olfactory ensheating cells’ from which schwannomas can arise. Receptor neurons have a bipolar morphology. Dendrites of olfactory neurons are
X-ray conventional radiology (CR)
X-ray plain films have obviously become obsolete. Absence of information on soft tissue and only raw information on bone status render CR improper for OT imaging. The only credible alternative to CT and mainly MRI in severely claustrophobic patients is the Cone-Beam-CT technique.
X-ray Cone-Beam Computed Tomography (CB-CT)
2D flat panel receptors used for cone-beam imaging with 3D ‘CT-like’ back-projections are a recently available tremendous improvement for the tomographic work-up of sinonasal, temporomandibular, and dental disorders. By
Congenital anosmia
The definition of congenital anosmia has combined clinical, paraclinical, and radiological bases. Patients who say that they have never had any sense of smell and in whom olfactory dysfunction is assessed by functional tests can be reputed as ‘congenitally anosmics’ after ruling out acquired causes for olfactory dysfunction. MR examination may demonstrate severely hypoplastic or absent OB together with flattening or even the absence of OS. Congenital anosmia may be isolated or associated to
Conclusions
The olfactory tract is a central nervous organ with unique features of lifelong supply of newly generated neurons (neurogenesis) and of continuing synaptogenesis responsible for the plasticity of the sense of smell. By allowing precise and accurate measurements of the olfactory bulb and tracts together with the sensitive depiction of parenchymal damage, MRI has allowed crucial insights into the pathophysiology of olfaction and has fully integrated the modern clinical diagnostic armamentarium of
References (27)
- et al.
Correlation between olfactory bulb volume and olfactory function
Neuroimage
(2008) - et al.
Cross-modal integration of intranasal stimuli: a functional magnetic resonance imaging study
Neuroscience
(2007) - et al.
Odour discrimination and identification are improved in early blindness
Neuropsychologia
(2009) - et al.
Posttraumatic smell loss: relationship of psychophysical test and volumes of olfactory bulbs and tracts and the temporal lobes
Acad. Radiol.
(1999) - et al.
Post-traumatic olfactory dysfunction: MR and clinical evaluation
Am. J. Neuroradiol. (AJNR)
(1996) - et al.
MR evaluation of patients with congenital hyposmia of anosmia
Am. J. Radiol. (AJR)
(1996) - et al.
Olfactory bulb and tract and temporal lobes volumes. Normative data across decades
Ann. N. Y. Acad. Sci.
(1998) - et al.
Reduced olfactory bulb volume in post-traumatic and post-infectious olfactory dysfunction
Neuroreport
(2005) - et al.
Olfactory function assessed with orthonasal and retronasal testing, olfactory bulb volume, and chemosensory event-related potentials
Arch. Otolaryngol. Head Neck Surg.
(2006) - Rombaux Ph. Thesis: assessment of olfactory function using orthonasal and retronasal testing, magnetic resonance...
The olfactory bulb coding and processing of odor molecule information
Science
The gustatory, visceral afferent, and olfactory systems
Neuroanatomy, basic and clinical
Cited by (75)
The Olfactory Nerve: Anatomy and Pathology
2022, Seminars in Ultrasound, CT and MRICitation Excerpt :The unmyelinated axons of these cells form the first-order olfactory nerves called fila olfactoria that ascend intracranially through the multiple perforations in the cribriform plate and reach the olfactory bulb (OB) at the olfactory groove (Fig. 1A and 1B). At the olfactory bulb, these bundles merge to form spherical structures called olfactory glomeruli, that make synapses with second-order neurons, the mitral cells.4,5 Myelinated axons of mitral cells, located within the olfactory tract (OT), courses posteriorly through the olfactory groove, below and between the gyrus rectus and the medial orbital gyrus (Fig. 1C).
Peripheral and central smell regions in children with epilepsy: An MRI evaluation
2022, Journal of Clinical NeuroscienceCitation Excerpt :From anterior to posterior screening, in the image, OB was seen clearly. The surface of the OB was measured manually (mm2) using an electronic cursor, and the volume was calculated by multiplying this value by the slice thickness [10–13]. The resulting data were recorded in cubic millimeters.
Peripheric smell regions in patients with semicircular canal dehiscence: An MRI evaluation
2021, Journal of Clinical NeurosciencePeripheric smell regions in patients with temporal and frontal lobe epilepsies: An MRI evaluation
2021, Journal of Clinical NeuroscienceFunctional magnetic resonance imaging in coronavirus disease 2019 induced olfactory dysfunction
2024, Journal of Laryngology and Otology