Hemodynamic scaling of fMRI-BOLD signal amplitude in normal aging

Introduction: Vascular reactivity and resting cerebral blood flow (CBF) levels lead to inter-subject variability in functional magnetic resonance imaging (fMRI) studies using the Blood Oxygen Level Dependent (BOLD) contrast to a neural task. This variability is exacerbated in an aging population [1]. Hemodynamic scaling using Resting State Fluctuation Amplitude (RSFA) or Breath hold (BH) can mitigate intra and inter-subject BOLD signal variability to a large extent by minimizing vascular variability [2]. In this study, we investigated the BOLD signal variability during a motor and a cognitive task in young and old subjects. BOLD signal change was hemodynamically scaled using RSFA or BH, which led to a reduction in BOLD variability in both the young and old groups. The extent of reduction in BOLD signal variability in the older subjects was significantly larger than the young. This indicates that vascular variability in the elderly exacerbates age-related BOLD signal contrast variability in neural task-induced hemodynamic activity. Methods: Twelve younger healthy human subjects (6M and 6F; mean age: 24 years; range: 19-27 years) and twelve older healthy subjects (5M and 7F; mean age: 58 years; range: 55-71 years) with no history of head trauma and neurological disease were scanned in a 3T PHILIPS MR-scanner. The Institutional Review Board of the University of Texas at Dallas approved all experimental procedures. Each subject performed a breath hold (BH), bilateral fingertapping (FTAP) and Digit-Symbol Substitution task (DSST). The MR scanner was equipped with a fixed asymmetric head gradient coil and a quadrature transmit/receive birdcage radio-frequency coil. Foam padding and a pillow were used to minimize subject head motion. High-resolution T1 weighted anatomical images were obtained from all subjects. Gradient echo-EPI images were subsequently obtained during rest, BH, FTAP and the DSST runs. 32 slices were obtained in the axial plane covering the entire brain. Imaging parameters were: FOV of 22 cm, matrix size of 64x64, TR/TE = 2000/30 msec and slice thickness of 4mm. 110 EPI images were obtained during each of rest, BH, DSST and FTAP tasks. Hemodynamic amplitude scaling was accomplished by dividing the BOLD signal response amplitude during the task (FTAP or DSST) with the BH-induced BOLD signal change or RSFA in the corresponding voxels [2]. The resting state data from one young subject was corrupted and data from all runs except the DSST task in one elderly subject was not considered for analysis due to excess motion. Results and Discussion: Table 1 shows the BOLD signal amplitude change (percent change from pre-task baseline) for all tasks prior to and after hemodynamic