The quantification of ΔR∗2 under brain activation: dependence on relaxation rate at rest and significance threshold

Transverse relaxation rates R∗2 were measured in subjects performing a motor task using a segmented EPI double gradient echo sequence (TE = 23/70 ms) with five different voxel sizes between 1.8 mm3 and 41.7 mm3. An analysis of the errors involved in the calculation of the change of the transverse relaxation rate −ΔR∗2 and of the consequences of defining an arbitrary threshold of statistical significance in the data analysis was performed. Correlations between the magnitude of the BOLD effect and the significance level on one hand and between the transverse relaxation time at rest and its change under activation on the other, both referenced in the literature, can be understood as a consequence of this procedure. Analysing histograms of parameter changes rather than average values alone allows for an estimate of the contribution of false positive voxels. Furthermore, while the averaged signal change increases in proportion to the selection threshold the histograms of activated voxels remain insensitive to the latter.