Abstract
The resting-state fMRI (rs-fMRI) signal is affected by a variety of low-frequency physiological phenomena, including variations in cardiac-rate (CRV), respiratory-volume (RVT), and end-tidal CO2 (PETCO2). While these effects have become better understood in recent years, the impact that their correction has on the quality of rs-fMRI measurements has yet to be clarified. The objective of this paper is to investigate the effect of correcting for CRV, RVT and PETCO2 on the rs-fMRI measurements. Nine healthy subjects underwent a test-retest rs-fMRI acquisition using repetition times (TRs) of 2 s (long-TR) and 0.323 s (short-TR), and the data were processed using eight different physiological correction strategies. Subsequently, regional homogeneity (ReHo), amplitude of low-frequency fluctuation (ALFF), and resting-state connectivity of the motor and default-mode networks are calculated for each strategy. Reproducibility is calculated using intra-class correlation and the Dice Coefficient, while the accuracy of functional-connectivity measures is assessed through network separability, sensitivity and specificity. We found that: (1) the reproducibility of the rs-fMRI measures improved significantly after correction for PETCO2; (2) separability of functional networks increased after PETCO2 correction but was not affected by RVT and CRV correction; (3) the effect of physiological correction does not depend on the data sampling-rate; (4) the effect of physiological processes and correction strategies is network-specific. Our findings highlight limitations in our understanding of rs-fMRI quality measures, and underscore the importance of using multiple quality measures to determine the optimal physiological correction strategy.
Highlights
Resting-state fMRI is typically measured through blood oxygenation level dependent (BOLD) contrast, which indirectly measures brain function through blood oxygenation changes following neuronal activity
We investigate the effect of a number of correction strategies involving three low-frequency physiological signal sources (CRV, respiratory volume per unit time (RVT), and pressure of end-tidal CO2 fluctuations (PETCO2)) on the rs-fMRI measurements
For the purposes of rsfMRI, we found amplitude of low-frequency fluctuation (ALFF) and regional homogeneity (ReHo) maps to be insensitive to the choice of physiological correction method, as ALFF and ReHo maps for different physiological corrections are nearly identical
Summary
Resting-state fMRI is typically measured through blood oxygenation level dependent (BOLD) contrast, which indirectly measures brain function through blood oxygenation changes following neuronal activity. Respiration causes susceptibility changes in the lungs that interfere with the static magnetic field and induce shifts in the MR image, mainly in the phase-encoding direction (Hu et al, 1995; Raj et al, 2001; Pfeuffer et al, 2002; Murphy et al, 2013)—a major concern for GE-EPI. The most common examples of these include cardiac rate variation (CRV), respiratory volume per unit time (RVT) and pressure of end-tidal CO2 fluctuations (PETCO2). Fluctuations in arterial pressure of CO2, which can be indirectly measured through PETCO2, alter the BOLD signal through vasodilatory and constrictive action. Like the RVT effect, the PETCO2 effect is dominant in the gray matter (Wise et al, 2004; Chang and Glover, 2009; Golestani et al, 2015)
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