Spatial and temporal characteristics of physiological noise in fMRI at 3T

Rationale and Objectives. Physiological noise in blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) has been shown to have characteristics similar to the BOLD signal itself, suggesting that it may have a vascular dependence. In this study, we evaluated the influence of physiological noise in fMRI as revealed by the differences in vasculature sensitivity of gradient-echo echo-planar imaging (GE-EPI) and spin-echo EPI (SE-EPI). Materials and Methods. The contribution of physiological noise to the fMRI signal during activation of the visual cortex was assessed by comparing its temporal characteristics with respect to echo time (TE), using both GE-EPI and SE-EPI. The correlation of the noise in fMRI with apparent diffusion coefficient (ADC) and the number of components required to describe its variance, as determined by principal-component analysis (PCA), were also assessed. Results. The SE-EPI data were less affected by a TE-dependence of noise, in contrast to the apparent physiological noise in GE-EPI. Voxel-wise analysis revealed that total apparent noise increased as ADC values increased, and the relationship was different for GE-EPI and SE-EPI. PCA revealed that while the number of components characterizing the noise in SE-EPI data increased in a TE-dependent manner, approaching that of white noise at long echo time, the number of components from GE-EPI data was TE-independent. Conclusions. The difference in sensitivities to physiological noise between SE-EPI and GE-EPI suggests that extravascular BOLD processes around draining veins contribute significantly to physiological noise in BOLD fMRI, and the suppression of this noise component may enhance SE-EPI BOLD sensitivity at higher fields.