Reproducibility of BOLD, perfusion, and CMRO2 measurements with calibrated-BOLD fMRI

Abstract

The coupling of changes in cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO(2)) during brain activation can be characterized by an empirical index, n, defined as the ratio between fractional CBF change and fractional CMRO(2) change. The combination of blood oxygenation level dependent (BOLD) imaging with CBF measurements from arterial spin labeling (ASL) provides a potentially powerful experimental approach for measuring n, but the reproducibility of the technique previously has not been assessed. In this study, inter-subject variance and intra-subject reproducibility of the method were determined. Block design %BOLD and %CBF responses to visual stimulation and mild hypercapnia (5% CO(2)) were measured, and these data were used to compute the BOLD scaling factor M, %CMRO(2) change with activation, and the coupling index n. Reproducibility was determined for three approaches to defining regions-of-interest (ROIs): 1) Visual area V1 determined from prior retinotopic maps, 2) BOLD-activated voxels from a separate functional localizer, and 3) CBF-activated voxels from a separate functional localizer. For estimates of %BOLD, %CMRO(2) and n, intra-subject reproducibility was found to be best for regions selected according to CBF activation. Among all fMRI measurements, estimates of n were the most robust and were substantially more stable within individual subjects (coefficient of variation, CV=7.4%) than across the subject pool (CV=36.9%). The stability of n across days, despite wider variability of CBF and CMRO(2) responses, suggests that the reproducibility of blood flow changes is limited by variation in the oxidative metabolic demand. We conclude that the calibrated BOLD approach provides a highly reproducible measurement of n that can serve as a useful quantitative probe of the coupling of blood flow and energy metabolism in the brain.

Figure 1

Visual cortex regions-of-interest generated for a typical subject. The top row of images depicts 4 slices of visual area V1 overlaid onto an oblique representation of a high resolution anatomical scan (FSPGR). Retinotopic mapping was performed in a preliminary scan session on a different day. The second and third rows of images depict voxels with significant CBF and BOLD activation (r=0.5) overlaid onto their respective average perfusion and BOLD images.

Figure 2

Average BOLD and CBF responses for a typical subject under activation (Aand B) and hypercapnia (C and D) conditions for the CBF localizer. Evidence of CMRO₂ increase with activation is given by the observation that a larger BOLD signal is elicited by hypercapnia than activation for similar flow changes.

Figure 3

Scatter plots of day 1 versus day 2 measurements for six physiological quantities available with calibrated BOLD fMRI. Red and black data points correspond to average measurements for individual subjects for the BOLD and CBF localizer, respectively. Different shapes and objects are used to identify individual subject data. The red line of slope one represents the case of perfect reproducibility. Data from subjects 6 and 7 is complicated by a CO₂ leak on day 2 as described in the results section. It is evident that the reproducibility of calculated measurements (M and n) are more stable across days than direct measurements (%BOLD, %CBF and CRC).

Figure 4

Bar graph depicting the coefficient of variation (CV) for inter-subject variance (purple) and single subject reproducibility (blue-gray) of calibrated-BOLD fMRI measurements. An asterisk next to a measurement denotes a significantly different (p<0.05) CV_inter or CV_intra value compared to the BOLD response. This figure demonstrates that discrimination of calibrated BOLD measurements between healthy individuals is not limited by poor reproducibility.

Figure 5

Intra-subject reproducibility (CV_intra) for the CBF localizer (r=0.5), the BOLD localizer (r=0.5) and the threshold-adjusted BOLD localizer (BOLD localizer*). The threshold-adjusted BOLD localizer was constructed to contain the same number of voxels as the CBF localizer. It is evident that BOLD-activation regions, whether constructed at the same level of significance of activation or the same volume of activation as perfusion-activated regions, are associated with poorer reproducibility.