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Concurrent NIRS-fMRI activation studies by using a new method for BOLD signal analysis

We propose a new method for BOLD signal calculation that is more meaningful for comparison with NIRS data. We provide evidence that BOLD signal can be highly localized within the region of activation not only in terms of the amplitude of activation but also for the nuances of the temporal trend. Therefore, in order to take into account of the spatial dependence of BOLD signal we propose to introduce a weighting function given by a photon hitting density function calculated for a given optical source-detector pair. In one case study, after we define this new method of BOLD calculation and extract a BOLD trend for each optical source-detector pairs we show that there is an high correlation between this BOLD signal and the changes of oxy and deoxy hemoglobin calculated at the same source-detector pair; however the correlation becomes poorer when the newly defined BOLD signal is compared with the changes of oxy and deoxy hemoglobin occurring in different channels. This results is consistent with the fact that for this experiment (which used a 3T MRI machine) the BOLD and NIRS signals were sensitive to changes occurring in the same locations and in similar vascular compartments.