Imaging the olfactory tract (Cranial Nerve #1)

doi:10.1016/j.ejrad.2009.05.065

European Journal of Radiology

Volume 74, Issue 2, May 2010, Pages 288-298

https://doi.org/10.1016/j.ejrad.2009.05.065 Get rights and content

Abstract

This review paper browses pros and cons of the different radiological modalities for imaging the olfactory tract and highlights the potential benefits and limitation of more recent advances in MR and CT technology. A systematic pictorial overview of pathological conditions affecting olfactory sense is given. Techniques for collecting quantitative data on olfactory bulb volume and on olfactory sulcus depth are described. At last, insights into functional imaging of olfactory sense are shown.

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

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The Olfactory Nerve: Anatomy and Pathology
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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).
The human sense of smell is the unique sense through which the olfactory system can identify aromatic molecules within the air and provide a taste sensation. Still, also it plays an essential role in several other functions, warning about environmental safety and even impacts our emotional lives. Recently, olfactory impairment has become an issue of interest due to the COVID-19 pandemic. The dysfunction may vary from only reduced smell detection (hyposmia) to complete loss of it (anosmia) but also includes changes in the normal perception of odors (parosmia). Computed tomography and magnetic imaging resonance are the modalities of choice to evaluate the olfactory pathways. Computed tomography is the initial imaging modality for olfactory disturbances, allowing recognition of sinonasal pathologies, inflammatory processes, or bone-related tumors. Magnetic imaging resonance with dedicated protocols for olfactory disorders enables a detailed assessment of the sinonasal compartment and the anterior cranial fossa. Provides a better depiction of olfactory bulb volume, morphology and signal intensity, as well the status of signal intensity of the central olfactory projection areas. Several diseases can affect the olfactory nerve, such as congenital disorders, trauma, inflammatory or infectious diseases, neoplasms, and even post-operative involvement. This article aims to review the normal anatomy of the olfactory nerve pathway and highlight the spectrum of conditions that most commonly affect it.
Peripheral and central smell regions in children with epilepsy: An MRI evaluation
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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.
We investigated the peripheral and central olfactory regions in children with epilepsy using cranial MRI.
In this retrospective study, cranial MRI images were obtained from 23 children with epilepsy and 23 healthy controls. Olfactory bulb (OB) volume and olfactory sulcus (OS) depth in the peripheral olfactory region and insular cortex and corpus amygdala areas in the central olfactory region were measured.
There was no significant difference in the OB volume and OS depth in the peripheral olfactory regions in the two groups (p > 0.05). In the central olfactory region, the insular cortex and corpus amygdala areas in the epilepsy group were significantly smaller than those in the control group (p < 0.05). In both groups, the OS depth on the right side was significantly higher than that on the left side (p < 0.05). In the epilepsy group, there were positive correlations between each of the OB volumes, OS depths, insular cortex areas, and corpus amygdala areas bilaterally (p < 0.05). In both groups, there were positive correlations between the OB volume and OS depth, OS depth and insular cortex area and insular cortex area and corpus amygdala areas (p < 0.05).
A change in the central olfactory region in epileptic patients may be related to central tissue damage due to epilepsy. This finding has important implications for epilepsy patients, with early diagnosis and treatment potentially preventing a reduction in the volumes/depths of components of the central olfactory region in the pediatric population.
Peripheric smell regions in patients with semicircular canal dehiscence: An MRI evaluation
2021, Journal of Clinical Neuroscience
We investigated the peripheric smell regions in patients with semicircular canal dehiscence (SCD) by cranial MRI.
In this retrospective study, cranial MRI images of 186 adult patients were included. Group 1 consisted of 83 adult patients with SCD. The control group (Group 2) consisted of 83 healthy subjects without central vertigo. Olfactory bulb (OB) volume and olfactory sulcus (OS) depth were measured in all groups.
In group 1, SCD was detected on the right (33.7%), left (26.5%) sides and bilateral (39.8%). Localization of dehiscence was at superior SC (75.9%), posterior SC (21.7%), lateral SC (1.2%), and posterior + superior SCs (1.2%). OB volumes of the SCD group were significantly lower than the control group bilaterally (p < 0.05). There were no significant differences between OS depths of groups 1 and 2 (p > 0.05). In SCD group, there were positive correlations between OB volumes; OS depths; and OB volumes and OS depths (p < 0.05). In older patients, bilateral OS depth values got lower (p < 0.05). In females, left OB volume values were lower than males (p < 0.05). In right SCD (+) patients, left OS depth values got lower (p < 0.05).
We concluded that possible changes in CSF dynamics may cause the development of SCD at thin bone segments; and a decrease in the OB volume. CSF leaks into the perineural sheet of the olfactory bulb (OB) maybe responsible for the decrease in the OB volume. In addition, minor trauma, infection, and inflammation may also be responsible for both coexistences of SCD development and OB volume decrease.
Peripheric smell regions in patients with temporal and frontal lobe epilepsies: An MRI evaluation
2021, Journal of Clinical Neuroscience
We investigated peripheric smell regions of olfactory bulb (OB) volume and olfactory sulcus (OS) depth in temporal and frontal lobe epilepsy patients by cranial magnetic resonance imaging (MRI).
In this retrospective study, cranial MRI images of 150 adult patients were included. Group 1 was consisted of 50 adult patients with temporal lobe epilepsy (TLE). Group 2 was consisted of 50 adult patients with frontal lobe epilepsy (FLE). The control group (Group 3) was consisted of 50 healthy subjects without epilepsy. OB volume and OS depth were measured in all groups.
OB volumes of the temporal and frontal epilepsy groups were significantly lower than those of the control group (p_adjusted < 0.0175). However, OS depths were not different between groups 1–3 (p > 0.05). In the temporal and frontal epilepsy groups, there were positive correlations between OB volumes; OS depths; left OB volume and bilateral OS depths p < 0.05). There were no significant correlations between OB volume and OS depth; and age and gender of the epilepsy group (p > 0.05).
We concluded that temporal and frontal epilepsy maybe related to decrease in OB volume and may cause olfactory impairment. Olfactory deficit maybe related to central epileptic focus. Therefore, early diagnosis and appropriate treatment of epilepsy are important to prevent olfactory impairment.
Functional magnetic resonance imaging in coronavirus disease 2019 induced olfactory dysfunction
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Imaging the olfactory tract (Cranial Nerve #1)

Abstract

Introduction

Section snippets

Radiological/functional anatomy (Figs. 1–3)

X-ray conventional radiology (CR)

X-ray Cone-Beam Computed Tomography (CB-CT)

Congenital anosmia

Conclusions

Neuroimage

Neuroscience

Neuropsychologia

Acad. Radiol.

Post-traumatic olfactory dysfunction: MR and clinical evaluation

Am. J. Neuroradiol. (AJNR)

MR evaluation of patients with congenital hyposmia of anosmia

Am. J. Radiol. (AJR)

Olfactory bulb and tract and temporal lobes volumes. Normative data across decades

Ann. N. Y. Acad. Sci.

Reduced olfactory bulb volume in post-traumatic and post-infectious olfactory dysfunction

Neuroreport

Olfactory function assessed with orthonasal and retronasal testing, olfactory bulb volume, and chemosensory event-related potentials

Arch. Otolaryngol. Head Neck Surg.

The olfactory bulb coding and processing of odor molecule information

Science

The gustatory, visceral afferent, and olfactory systems

Neuroanatomy, basic and clinical