Transient hemodynamics during a breath hold challenge in a two part functional imaging study with simultaneous near-infrared spectroscopy in adult humans.

During a breath hold (BH) challenge, functional MR imaging using flow-sensitive alternating inversion recovery (FAIR) and blood oxygenation level dependent (BOLD) contrast was performed with simultaneous near-infrared spectroscopy (NIRS). Time courses for the BOLD signal, cerebral blood flow (CBF), absolute deoxyhemoglobin (Hb) concentration, and relative concentration changes for total hemoglobin (HbT) were generated to (1) characterize the relationship between transient BOLD responses and the transient hemodynamic response, and (2) compare results from previous empirical animal experiments. During this mild hypercapnia task, the increase in the BOLD signal during the task indicated that an increase in CBF outweighed the competing effect of a volume-induced increase in Hb. After the task, the increase in the concentration of Hb mirrored the posttask undershoot in the BOLD and CBF data. Finally, we found a strong linear relationship between R(2)(*) and absolute Hb, except for outlier points in the Hb time series corresponding to the task cessation, which suggests there are differences in measurement sensitivity between BOLD and NIRS.