The quantification of Delta R-2(*) under brain activation: dependence on relaxation rate at rest and significance threshold

Transverse relaxation rates R-2* were measured in subjects performing a mot or task using a segmented EPI double gradient echo sequence (TE = 23/70 ms) with five different voxel sizes between 1.8 mm(3) and 41.7 mm(3). An analy sis of the errors involved in the calculation of the change of the transver se relaxation rate -DeltaR(2)* and of the consequences of defining an arbit rary threshold of statistical significance in the data analysis was perform ed. Correlations between the magnitude of the BOLD effect and the significa nce level on one hand and between the transverse relaxation time at rest an d its change under activation on the other, both referenced in the literatu re, can be understood as a consequence of this procedure. Analysing histogr ams of parameter changes rather than average values alone allows for an est imate of the contribution of false positive voxels. Furthermore, while the averaged signal change increases in proportion to the selection threshold t he histograms of activated voxels remain insensitive to the latter. (C) 200 1 Elsevier Science Inc. All rights reserved.