The sense of smell directs the intake of airborne agents into the human respiratory system and determines the flavor and palatableness of foods and beverages. Olfaction enhances the quality of life and yet olfactory disorders are neglected by the medical community .
Many studies showed a statistically significant discrepancy in the OB volume between patients with bilateral sinonasal polyposis and healthy controls, in keeping with the findings revealed in experimental studies on animals, in which decreased inputs from the olfactory epithelium resulted in reduction of OB/T volume [30,31,32,33].
DTI is a non-invasive imaging technique that analyzes the diffusion of water molecules within the tissues. The size, arrangement, and myelination of the axons of the white matter of the brain influence the diffusion of water molecules within them. Fractional anisotropy (FA) reflects the diffusion in various directions while ADC reflects the whole diffusion of water molecules, so an intact white matter tract has higher FA and lower ADC values than a structurally disturbed one (Fig. 1b) .
In this study, we integrate 3D DRIVE sequences and DTI to evaluate the effect of olfactory deprivation caused by sinonasal polyposis on OB/T volume and integrity.
In many studies, age proved to be the strongest correlate of smell disorder in healthy persons, having a larger impact than cigarette smoking. Although marked individual differences are present, age-related smell disorder is more severe in men than women [10, 19, 29]. We could not find a correlation between age, sex, and smoking with OB/T volume, FA, and ADC values in our study. This may be attributed to the small sample of patients, small number of female patients, and small numbers of smokers in the study. Future studies with large representative samples are required to further study the effect of these factors.
We did not perform olfactory tests in this study because our patients had nasal obstruction which led to decreased delivery of particles to the olfactory area. Also, we tried to omit the subjective factors in the study.
In our study, the difference in OB/T volume between patient and control groups was highly statistically significant (p < 0.001). This was in accordance with Islam et al.  who reported that there is a significant reduction in OB volume in patients with bilateral sinonasal polyposis when compared with healthy controls. This result supports the findings in experimental animals, in which decreased inputs from the olfactory epithelium lead to reduction of OB volume [17, 30]. Also, Veyseller et al.  reported that diminished OB volume in total laryngectomy patients may be due to the inability to sniff normally, the lack of naso-pulmonary airflow, and complex neural interruptions caused by surgical denervation of the larynx.
Rombaux et al.  found that patients with post-traumatic or post-infectious olfactory loss have smaller OB volume which correlates with poorer recovery of olfactory function. Hence, OB/T volume reduction may not only impair the sense of smell in sinonasal polyposis patients but may also be responsible of hindering olfactory recovery post operatively. On the other hand, Gudziol et al.  reported that the OT volume in patients with chronic rhinosinusitis had increased significantly after 3 months of medical treatment.
Another study found a positive correlation between olfactory deficit and the reduction in OT volume both in patients with Parkinson’s disease (PD) and controls  which is contrary to what Hummel et al., Huissman et al., and Wattendorf et al. [38,39,40] concluded in their study where little or no difference was found between PD patients with anosmia/hyposmia and healthy controls in terms of OT volume.
Similarly, there were statistically significant differences in FA and ADC values between patients with sinonasal polyps and control group (p values 0.2114 and 0.00438, respectively). Also, by using cutoff criterion ≤ 0.05 cm3, ≤ 0.35, and ≥ 1.417 (× 10-3 mm2/s) for OB/T volume, FA, and ADC values, respectively, discrimination between cases and controls could be obtained.
There were several studies that pointed out the capability of the analysis of the ADC and FA computed from DTI to reveal brain tissue damage in early clinical stages of neurodegenerative diseases, particularly PD. These studies found that PD patients could be completely differentiated from the control group based on reduced FA values [26, 40, 41].
Previous studies suggested that head trauma is the most common cause of olfactory dysfunction, accounting for approximately 17% of all cases. Complete anosmia occurs in 5–10% of all head traumas, while an element of olfactory dysfunction in 20–44% of mild traumatic brain injury (TBIs) and 49–56% of moderate to severe TBIs. Of these cases, only 30–40% achieve partial recovery. DTI was used to furnish physical evidence of post-traumatic anosmia. It revealed dis-integrity of olfactory tracts [42,43,44].
The clinical value of imaging in patients with olfactory dysfunction has been controversial. Busaba  concluded that imaging was not needed in a patient with isolated olfactory dysfunction if clinical examination was normal. Mueller et al. , however, concluded that computed tomography (CT) scans are useful to diagnose conductive/inflammatory olfactory dysfunction in patients suspected of non-sinonasal disease. Imaging studies of the brain and paranasal sinuses are often obtained to determine the site and nature of the underlying pathology. Sinonasal CT is the technique of choice for the study of sinonasal structures .
Our study was the first to address the use of DTI as an objective measure of olfactory dysfunction in patients with nasal polyps. In the absence of obvious physical derangement, the diagnosis of olfactory dysfunction relied upon the patients’ subjective reports. DTI can differentiate hyposmic/anosmic patients from controls and has shown to be useful in the evaluation of the central causes of an olfactory dysfunction.
The potential of this methodology thrilled the DTI community; however, it is critical to note that there is no established gold standard to assess the measurement limits and errors in DTI to confirm that the DTI interpretation is correct. So, introduction of several innovative readout strategies will provide better quality of DTI images and results.
There are few limitations in performing DTI of the olfactory region that need to be kept in mind such as susceptibility artifacts, which result from air spaces within sinonasal spaces or dental fillings with metallic materials. Due to the small number of patients, larger studies in different centers are required to confirm the results. Quantitative olfactory tests are recommended to study the correlation between subjective olfactory derangement and objective quantitative measurements. Also, study of the OB/T volumes and DTI parameters in patients with chronic sinonasal polyposis who underwent surgery to correlate between the subsequent expected increase in volume and recovery of olfactory tract integrity would be interesting to study.
OB/T volumetry and DTI (FA and ADC) could be used as objective measurements to evaluate olfactory dysfunction in patients with sinonasal polyposis.