title: Modelling temporal stability of EPI time series using magnitude images acquired with multi-channel receiver coils.
abstract: In 2001, Krueger and Glover introduced a model describing the temporal SNR (tSNR) of an EPI time series as a function of image SNR (SNR(0)). This model has been used to study physiological noise in fMRI, to optimize fMRI acquisition parameters, and to estimate maximum attainable tSNR for a given set of MR image acquisition and processing parameters. In its current form, this noise model requires the accurate estimation of image SNR. For multi-channel receiver coils, this is not straightforward because it requires export and reconstruction of large amounts of k-space raw data and detailed, custom-made image reconstruction methods. Here we present a simple extension to the model that allows characterization of the temporal noise properties of EPI time series acquired with multi-channel receiver coils, and reconstructed with standard root-sum-of-squares combination, without the need for raw data or custom-made image reconstruction. The proposed extended model includes an additional parameter κ which reflects the impact of noise correlations between receiver channels on the data and scales an apparent image SNR (SNR'(0)) measured directly from root-sum-of-squares reconstructed magnitude images so that κ = SNR'(0)/SNR(0) (under the condition of SNR(0)>50 and number of channels ≤32). Using Monte Carlo simulations we show that the extended model parameters can be estimated with high accuracy. The estimation of the parameter κ was validated using an independent measure of the actual SNR(0) for non-accelerated phantom data acquired at 3T with a 32-channel receiver coil. We also demonstrate that compared to the original model the extended model results in an improved fit to human task-free non-accelerated fMRI data acquired at 7T with a 24-channel receiver coil. In particular, the extended model improves the prediction of low to medium tSNR values and so can play an important role in the optimization of high-resolution fMRI experiments at lower SNR levels.
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, London, United Kingdom. chuttonster@gmail.com
authors.forename: Chloe
authors.initials: C
authors.lastname: Hutton
authors.forename: Evelyne
authors.initials: E
authors.lastname: Balteau
authors.forename: Antoine
authors.initials: A
authors.lastname: Lutti
authors.forename: Oliver
authors.initials: O
authors.lastname: Josephs
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weiskopf
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D006801:Humans; D007089:Image Enhancement; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008962:Models, Theoretical; D009010:Monte Carlo Method; D059906:Neuroimaging; D019047:Phantoms, Imaging; D015203:Reproducibility of Results