Changes in the MR relaxation rate R(2)* induced by respiratory challenges at 3.0 T: a comparison of two quantification methods.

The consistent determination of changes in the transverse relaxation rate R(2)* (ΔR(2)*) is essential for the mapping of the effect of hyperoxic and hypercapnic respiratory challenges, which enables the noninvasive assessment of blood oxygenation changes and vasoreactivity by MRI. The purpose of this study was to compare the performance of two different methods of ΔR(2)* quantification from dynamic multigradient-echo data: (A) subtraction of R(2)* values calculated from monoexponential decay functions; and (B) computation of ΔR(2)* echo-wise from signal intensity ratios. A group of healthy volunteers (n = 12) was investigated at 3.0 T, and the brain tissue response to carbogen and CO(2)-air inhalation was registered using a dynamic multigradient-echo sequence with high temporal and spatial resolution. Results of the ΔR(2)* quantification obtained by the two methods were compared with respect to the quality of the voxel-wise ΔR(2)* response, the number of responding voxels and the behaviour of the 'global' response of all voxels with significant R(2)* changes. For the two ΔR(2)* quantification methods, we found no differences in the temporal variation of the voxel-wise ΔR(2)* responses or in the detection sensitivity. The maximum change in the 'global' response was slightly smaller when ΔR(2)* was derived from signal intensity ratios. In conclusion, this first methodological comparison shows that both ΔR(2)* quantifications, from monoexponential approximation as well as from signal intensity ratios, are applicable for the monitoring of R(2)* changes during respiratory challenges.