Modelling hemodynamic response function in epilepsy

• We investigated the impact of a subject-specific hemodynamic response function (HRF) to describe the hemodynamic differences in fMRI data analysis across epileptic patients versus the use of a standard model.
• Simulations assess how the use of different HRF models affect the results of the statistical analysis and suggest the importance of using an individual-based HRF model.
• The individual-based HRF model parsimoniously describes the fMRI time course for epileptic patients and possibly provides reliable BOLD activation maps.

ObjectiveElectroencephalography and functional magnetic resonance imaging (fMRI) can be combined to noninvasively map abnormal brain activation elicited by epileptic processes. A major aim was to investigate the impact of a subject-specific hemodynamic response function (HRF) to describe the differences across patients versus the use of a standard model.MethodsWe developed and applied on simulated and real data a method designed to choose optimum HRF model for identifying fMRI activation maps. In simulation, the ability of five models to reproduce data was assessed: four standard and an individual-based HRF model (ibHRF). In clinical data, drug-resistant epileptic patients underwent fMRI to investigate hemodynamic responses evoked by interictal activity.ResultsWhen data are simulated with models different from the standard ones, the results obtained with ibHRF are superior to those obtained with the standard HRFs. Results on real data indicate an increase in extent and degree of activation with the ibHRF in comparison of the results obtainable using standard HRFs.ConclusionsThe use of the same HRF in all patients is inappropriate and resolves in biased extension of the activation maps.SignificanceThe new method could represent an useful diagnostic tool for other clinical studies that may be biased because of misspecification of HRF.