Detection of Olfactory Areas in Functional MRI - How Many Repetitions are Necessary?

Background and Purpose: A useful tool for the investigation of the human olfactory system is functional magnetic resonance tomography (fMRI). Since the length of the fMRI examination is a limiting factor for its usefulness in clinical routine in this study a fast, reliable paradigm should be found in order to reduce the overall examination time and the olfactory habituation. Methods: A MRI-compatible constant flow olfactometer was developed. 9 healthy normosmic subjects were stimulated 16 times with the odorant phenyl-ethyl-alcohol (PEA) using an event-related design. The statistical evaluation of the data was performed by the MATLAB based SPM5 software package. Results: The group analysis showed cerebral activations within the insula, the adjacent operculum and orbitofrontal cortex bilaterally. Furthermore, bilateral activations were measured in the cingulum, piriform cortex and the cerebellum. All mentioned areas could be already identified after 8 repetitions of odorant condition. Bilateral insular activations and activations in the left piriform cortex were even shown after 4 repetitions. In the single subject analysis, all 9 subjects exhibited insular activity and in 3 subjects showed activation of the piriform cortex after 4 repetitions. Conclusion: In olfactory fMRI, stimulus repetition more than 8 did not improve the quality and reliability of the results. Therefore, 4-8 stimuli are expected to be sufficient.