Functional Activities Detected in the Olfactory Bulb and Associated Olfactory Regions in the Human Brain Using T2-Prepared BOLD Functional MRI at 7T

Functional magnetic resonance imaging in coronavirus disease 2019 induced olfactory dysfunction

OBJECTIVE

To evaluate the functional magnetic resonance imaging changes in the olfactory structures of coronavirus disease 2019 patients experiencing olfactory dysfunction.

METHODS

This study included patients aged 25-65 years who presented with a sudden loss of smell, confirmed coronavirus disease 2019 infection, and persistent olfactory dysfunction for a minimum of 2 months without any treatment.

RESULTS

Irrespective of the side of brain activation, the analysis of the cumulative maximum diameter of the activation zones revealed significantly lower activation in the upper frontal lobe (p = 0.037) and basal ganglia (p = 0.023) in olfactory dysfunction patients. Irrespective of the side of activation, the analysis of the number of activation points demonstrated significantly lower activation in the upper frontal lobe (p = 0.036) and basal ganglia (p = 0.009) in olfactory dysfunction patients.

CONCLUSION

Patients with coronavirus-triggered olfactory dysfunction exhibited lower activity in their basal ganglia and upper frontal lobe.

What Does the Human Olfactory System Do, and How Does It Do It?

Historically, the human sense of smell has been regarded as the odd stepchild of the senses, especially compared to the sensory bravado of seeing, touching, and hearing. The idea that the human olfaction has little to contribute to our experience of the world is commonplace, though with the emergence of COVID-19 there has rather been a sea change in this understanding. An ever increasing body of work has convincingly highlighted the keen capabilities of the human nose and the sophistication of the human olfactory system. Here, we provide a concise overview of the neuroscience of human olfaction spanning the last 10-15 years, with focus on the peripheral and central mechanisms that underlie how odor information is processed, packaged, parceled, predicted, and perturbed to serve odor-guided behaviors. We conclude by offering some guideposts for harnessing the next decade of olfactory research in all its shapes and forms.

Leveraging Multi-echo EPI to Enhance BOLD Sensitivity in Task-based Olfactory fMRI

Functional magnetic resonance imaging (fMRI) using blood-oxygenation-level-dependent (BOLD) contrast relies on gradient echo echo-planar imaging (GE-EPI) to quantify dynamic susceptibility changes associated with the hemodynamic response to neural activity. However, acquiring BOLD fMRI in human olfactory regions is particularly challenging due to their proximity to the sinuses where large susceptibility gradients induce magnetic field distortions. BOLD fMRI of the human olfactory system is further complicated by respiratory artifacts that are highly correlated with event onsets in olfactory tasks. Multi-Echo EPI (ME-EPI) acquires gradient echo data at multiple echo times (TEs) during a single acquisition and can leverage signal evolution over the multiple echo times to enhance BOLD sensitivity and reduce artifactual signal contributions. In the current study, we developed a ME-EPI acquisition protocol for olfactory task-based fMRI and demonstrated significant improvement in BOLD signal sensitivity over conventional single-echo EPI (1E-EPI). The observed improvement arose from both an increase in BOLD signal changes through a T 2 * -weighted echo combination and a reduction in non-BOLD artifacts through the application of the Multi-Echo Independent Components Analysis (ME-ICA) denoising method. This study represents one of the first direct comparisons between 1E-EPI and ME-EPI in high-susceptibility regions and provides compelling evidence in favor of using ME-EPI for future task-based fMRI studies.

Integrating the patient's voice into the research agenda for treatment of chemosensory disorders

World-wide some 658 million people were infected with coronavirus disease 2019 (COVID-19) and millions suffer from chemosensory impairment associated with long COVID. Current treatments for taste and smell disorders are limited. Involving patients has the potential to catalyze the dynamic exchange and development of new ideas and approaches to facilitate biomedical research and therapeutics. We assessed patients' perceptions of the efficacy of treatments for chemosensory impairment using an online questionnaire completed by 5,815 people in the US Logistic regression determined variables predictive of reported treatment efficacy for patients aged 18 to 24, 25 to 39, 40 to 60, and 60+ yrs. who were treated with nasal steroids, oral steroids, zinc, nasal rinse, smell training, theophylline, platelet-rich plasma, and Omega 3. The most consistent predictor was age, with the majority of those 40 to 60 and 60+ reporting that nasal steroids, oral steroids, zinc, nasal rinse, and smell training were only slightly effective or not effective at all. Many of these treatment strategies target regeneration and immune response, processes compromised by age. Only those under 40 reported more than slight efficacy of steroids or smell training. Findings emphasize the need to include patients of all ages in clinical trials. Older adults with olfactory impairment are at increased risk for Alzheimer's disease (AD). We speculate that olfactory impairment associated with long COVID introduces the potential for a significant rise in AD. Long COVID-associated chemosensory impairment increases the urgency for translational and clinical research on novel treatment strategies. Suggestions for high-priority areas for epidemiological, basic, and clinical research on chemosensory impairment follow.

Olfactory brain activations in patients with Major Depressive Disorder

Depression is associated with reduced olfactory function. This relationship is assumed to be based on either a reduced olfactory bulb volume or diminished functioning of higher cortical areas. As previous results are controversial, we aimed to re-evaluate central olfactory processing in depression. We recorded the BOLD signal of 21 patients with Major Depressive Disorder and 21 age and gender matched healthy controls during odor presentation. In addition, we measured the individual olfactory bulb volume, tested odor identification and odor threshold, and asked for hedonic odor perception. In both groups, odor presentation led to a pronounced activation of primary olfactory areas. However, secondary olfactory areas were significantly less activated in depressed individuals. The two groups did not differ in olfactory bulb volume. Our results point towards altered olfactory processing in patients in those regions that relate to sensory integration and attention allocation. Difficulties in cognitive processing could impact olfactory function in depression. We are therefore in favor of a top-down mechanism originating in higher cortical areas explaining parts of the relation between depression and olfaction.

Evaluation of T2-prepared blood oxygenation level dependent functional magnetic resonance imaging with an event-related task: Hemodynamic response function and reproducibility

T2-prepared (T2prep) blood oxygenation level dependent (BOLD) functional MRI (fMRI) is an alternative fMRI approach developed to mitigate the susceptibility artifacts that are typically observed in brain regions near air-filled cavities, bleeding and calcification, and metallic objects in echo-planar-imaging (EPI) based fMRI images. Here, T2prep BOLD fMRI was evaluated in an event-related paradigm for the first time. Functional experiments were performed using gradient-echo (GRE) EPI, spin-echo (SE) EPI, and T2prep BOLD fMRI during an event-related visual task in 10 healthy human subjects. Each fMRI method was performed with a low (3.4 × 3.4 × 4 mm3) and a high (1.5 mm isotropic) spatial resolution on 3T and a high resolution (1.5 mm isotropic) on 7T. Robust activation were detected in the visual cortex with all three fMRI methods. In each group of fMRI scans (3T low resolution, 3T high resolution, and 7T high resolution), GRE EPI showed the highest signal change (ΔS/S), largest full-width-at-half-maximum (FWHM) and longest time-to-peak (TTP) extracted from the hemodynamic response functions (HRF), indicating substantial signal contribution from large draining veins which have longer response times than microvessels. In contrast, T2prep BOLD showed the lowest ΔS/S, smallest FWHM, and shortest TTP, suggesting that T2prep BOLD may have a purer T2-weighted BOLD contrast that is more sensitive to microvessels compared to GRE/SE EPI BOLD. This trend was more obvious in fMRI scans performed with a lower spatial resolution on a lower field (3T with a 3.4 × 3.4 × 4 mm3 voxel). Scan-rescan reproducibility in the same subjects was comparable among the three fMRI methods. The results from the current study are expected to be useful to establish T2prep BOLD as a useful alternative fMRI approach for event-related fMRI in brain regions with large susceptibility artifacts.

Olfactory system measurements in COVID-19: a systematic review and meta-analysis

PURPOSE

The neurotropism of SARS-CoV-2 and the consequential damage to the olfactory system have been proposed as one of the possible underlying causes of olfactory dysfunction in COVID-19. We aimed to aggregate the results of the studies which reported imaging of the olfactory system of patients with COVID-19 versus controls.

METHODS

PubMed and EMBASE were searched to identify relevant literature reporting the structural imaging characteristics of the olfactory bulb (OB), olfactory cleft, olfactory sulcus (OS), or olfactory tract in COVID-19 patients. Hedge's g and weighted mean difference were used as a measure of effect size. Quality assessment, subgroup analyses, meta-regression, and sensitivity analysis were also conducted.

RESULTS

Ten studies were included in the qualitative synthesis, out of which seven studies with 183 cases with COVID-19 and 308 controls without COVID-19 were enrolled in the quantitative synthesis. No significant differences were detected in analyses of right OB volume and left OB volume. Likewise, right OS depth and left OS depth were also not significantly different in COVID-19 cases compared to non-COVID-19 controls. Also, we performed subgroup analysis, meta-regression, and sensitivity analysis to investigate the potential effect of confounding moderators.

CONCLUSION

The findings of this review did not confirm alterations in structural imaging of the olfactory system, including OB volume and OS depth by Covid-19 which is consistent with the results of recent histopathological evaluations.

Brain Activation During Processing of Depression Emotion in College Students With Premenstrual Syndrome in China: Preliminary Findings

BACKGROUND

This study aimed to investigate the neural substrates of processing depression emotion in premenstrual syndrome (PMS) and healthy subjects of college students using blood oxygenation level-dependent functional magnetic resonance imaging (BOLD-fMRI).

METHODS

During BOLD-fMRI scanning, 13 PMS patients and 15 healthy controls (HC) performed a picture visual stimulation task during luteal and follicular phases, in which participants and HC were asked to see pictures containing depression and non-depression emotions. Simultaneously, self-rating depression scales (SDS) were employed to evaluate the emotional status of participants.

RESULTS

Compared to HC, right inferior occipital gyrus, right middle occipital gyrus, right lingual gyrus, right fusiform gyrus, right inferior temporal gyrus, cerebelum_crus1_R, cerebelum_6_R, culmen, the cerebellum anterior lobe, tuber, and cerebellar tonsil of PMS patients showed enhanced activation. In contrast, sub-lobar, sub-gyral, extra-nuclear, right orbit part of superior frontal gyrus, right middle temporal gyrus, right orbit part of inferior frontal gyrus, limbic lobe, right insula, bilateral anterior and adjacent cingulate gyrus, bilateral caudate, caudate head, bilateral putamen, and left globus pallidus exhibited decreased activation.

CONCLUSION

The findings indicate that abnormal functional regulation of brain regions such as occipital lobe and cerebellum leads to abnormal changes in emotional regulation, cognitive ability, and attention distribution in PMS patients, implying significant central pathogenesis.