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 Ψ_t , distinct from the thermal noise covariance matrix Ψ₀ , and compare its structure to Ψ₀ and the signal coupling matrix SS^H 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 Ψ₀ or SS^H .

Conclusion: Time-series noise covariances in array coils are found to differ from Ψ₀ and more surprisingly, from the signal coupling matrix SS^H . Correct characterization of the time-series noise has implications for the analysis of time-series data and for improving the coil element combination process. Magn Reson Med 76:1708-1719, 2016. © 2016 International Society for Magnetic Resonance in Medicine.