Coil-to-coil physiological noise correlations and their impact on functional MRI time-series signal-to-noise ratio.

Purpose: Physiological nuisance fluctuations ("physiological noise") are a major contribution to the time-series signal-to-noise ratio (tSNR) of functional imaging. While thermal noise correlations between array coil elements have a well-characterized effect on the image Signal to Noise Ratio (SNR0), the element-to-element covariance matrix of the time-series fluctuations has not yet been analyzed. We examine this effect with a goal of ultimately improving the combination of multichannel array data. Theory and Methods: We extend the theoretical relationship between tSNR and SNR0 to include a time-series noise covariance matrix Psi(t), distinct from the thermal noise covariance matrix Psi(0), and compare its structure to Psi(0) and the signal coupling matrix SSH formed from the signal intensity vectors S. Results: Inclusion of the measured time-series noise covariance matrix into the model relating tSNR and SNR0 improves the fit of experimental multichannel data and is shown to be distinct from Psi(0) or SSH. Conclusion: Time-series noise covariances in array coils are found to differ from Psi(0) and more surprisingly, from the signal coupling matrix SSH. Correct characterization of the time-series noise has implications for the analysis of time-series data and for improving the coil element combination process. (C) 2016 International Society for Magnetic Resonance in Medicine