Purpose: Our goal was to develop a rapid, simple, near-real-time method of functional MRI (fMRI) to measure brain activation in response to olfactory stimuli, to use it to identify patients with smell loss (hyposmia), and to differentiate their types of hyposmia.
Method: fMRI was obtained in 16 patients with Type I hyposmia (who could detect but not recognize odors), 5 patients with Type II hyposmia (who could both detect and recognize odors, albeit with less than normal acuity), and 2 volunteers with normal olfactory acuity by use of a rapid echo planar imaging technique in which one coronal brain section from the anterior cortical region was studied and a single olfactory stimulus was used. Actual scanning time performed by a variation of methods previously published required 26 s. Three patients with Type I hyposmia were treated with theophylline 250-500 mg for 4-6 months and were studied before and after treatment.
Results: Brain activation in response to olfactory stimuli was demonstrated using a new, rapid, and simple fMRI technique. Patients with Type I hyposmia had less activation than patients with Type II hyposmia. Both patient groups had less activation than normal volunteers. Activation in patients with Type I hyposmia was essentially absent from regions of the middle frontal, orbitofrontal, and temporal cortex and was totally absent in regions of inferior frontal, insular, and cingulate cortex. Activation in patients with Type II hyposmia was greatest in the middle frontal cortex and the orbitofrontal cortex bilaterally and was present in regions of inferior frontal, temporal, and cingulate cortex. Each patient with Type I hyposmia treated with theophylline had improved smell function to Type II hyposmia and after treatment demonstrated activation in inferior frontal and cingulate cortex bilaterally, whereas before treatment, no activation in these regions was apparent.
Conclusion: We describe a simple, rapid technique that can be used in a practical clinical setting to identify patients with hyposmia and to differentiate patients with different types of olfactory loss. These studies confirm the presence and classification of patients with Type I and Type II hyposmia. Results of this study suggest that regions of the frontal cortex may act to guide or direct olfactory signals to other brain areas such as temporal and cingulate regions. Although these latter regions are involved with olfactory recognition, their role in olfactory memory, olfactory meaning, and attention needs to be considered.