The Olfactory System Revealed: Non-Invasive Mapping by using Constrained Spherical Deconvolution Tractography in Healthy Humans

Abstract

Although the olfactory sense has always been considered with less interest than the visual, auditive or somatic senses, it does plays a major role in our ordinary life, with important implication in dangerous situations or in social and emotional behaviors. Traditional Diffusion Tensor signal model and related tractography have been used in the past years to reconstruct the cranial nerves, including the olfactory nerve (ON). However, no supplementary information with regard to the pathways of the olfactory network have been provided. Here, by using the more advanced Constrained Spherical Deconvolution (CSD) diffusion model, we show for the first time in vivo and non-invasively that, in healthy humans, the olfactory system has a widely distributed anatomical network to several cortical regions as well as to many subcortical structures. Although the present study focuses on an healthy sample size, a similar approach could be applied in the near future to gain important insights with regard to the early involvement of olfaction in several neurodegenerative disorders.

Keywords: CSD; lateral stria; olfactory system; olfactory tract; tractography.

Figure 1

Coronal T1 weighted image shows the olfactory tracts (OTs). (A) Note the slight positional asymmetry on the cranio-caudal plane. On anisotropic map (B) OTs appears green due to their main antero-posterior direction. A good matching between T1 weighted scan and diffusion maps (C) is necessary to obtain consistent and robust results for of such small anatomic structures.

Figure 2

Olfactory lateral stria of a representative subject. (A) Medio-lateral sagittal views show the course of OT connections with amygdala, entorhinal and pyriform cortices. Axial (B) and coronal (C) views of the OT, as indicated by the crosshair and red voxels.

Figure 3

Olfactory direct pathway. (A) Latero-medial sagittal views of the connections between the primary olfactory cortices—amygdala, entorhinal and piriform cortices—with the medial orbitofrontal cortex (OFC). Axial (B) and coronal (C) representative views of the same pathways.

Figure 4

Olfactory indirect pathway. (A) Latero-medial sagittal views of the connections between the pyriform lobe and the MDTN (in red), and between medio-dorsal nucleus of the thalamus (MDNT) and the medial OFC. Axial (B) and coronal (C) views of the indirect pathway at the level of the medial OFC.