De-noising, phase ambiguity correction and visualization techniques for complex-valued ICA of group fMRI data

Pedro A. Rodriguez, Vince D. Calhoun, Tülay Adal

Research output: Contribution to journalArticlepeer-review

28 Scopus citations


Analysis of functional magnetic resonance imaging (fMRI) data in its native, complex form has been shown to increase the sensitivity and specificity both for data-driven techniques, such as independent component analysis (ICA), and for model-driven techniques. Therefore, the possibility of increasing the usability of fMRI data in clinical and group studies provides a powerful motivation for utilizing both the phase and magnitude data. However, the unknown and noisy nature of the phase requires the introduction of new de-noising, preprocessing and visualization techniques. In addition, many complex-valued analysis algorithms, such as ICA, suffer from an inherent phase ambiguity, which introduces additional difficulty for group analysis. We present solutions for these issues, which have been among the main reasons phase information has been traditionally discarded, and show their effectiveness when used as part of a complex-valued group ICA algorithm application. The methods we present thus allow the development of new fully complex data-driven and semi-blind methods to process, analyze, and visualize fMRI data. We first introduce a physiologically motivated de-noising method that uses phase quality maps to successfully identify and eliminate noisy areas in the fMRI data so they can be used in individual and group studies. We also introduce a phase correction scheme that can be either applied subsequent to ICA of fMRI data or can be incorporated into the ICA algorithm in the form of prior information to eliminate the need for further processing for phase correction. Finally, we present a Mahalanobis distance-based thresholding method, which incorporates both magnitude and phase information into a single threshold, that can be used to increase the sensitivity in the identification of voxels of interest. This method shows particular promise for identifying voxels with significant susceptibility changes but that are located in low magnitude (i.e., activation) areas. We demonstrate the performance gain of the introduced methods on actual fMRI data.

Original languageEnglish (US)
Pages (from-to)2050-2063
Number of pages14
JournalPattern Recognition
Issue number6
StatePublished - Jun 2012
Externally publishedYes


  • De-noising
  • Group analysis
  • ICA
  • Phase ambiguity
  • Visualization
  • fMRI

ASJC Scopus subject areas

  • Software
  • Signal Processing
  • Computer Vision and Pattern Recognition
  • Artificial Intelligence


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