Full Length ArticleBallistocardiogram artifact correction taking into account physiological signal preservation in simultaneous EEG-fMRI
Introduction
The complementarity between the high temporal resolution of electroencephalography (EEG) and the millimeter spatial resolution of functional magnetic resonance imaging (fMRI) has strongly motivated the integration of these two neuroimaging techniques (for reviews, please refer to Jorge, J., et al., 2013, Murta, T., et al., 2015). Simultaneous EEG-fMRI acquisitions, however, induce two major artifacts on EEG data. Firstly, the rapidly-changing, high-amplitude MR-related artifacts are induced by the switching of magnetic fields during the fMRI acquisition. The gradient-related artifact occurrences are practically time-invariant and the use of standard average artifact subtraction (AAS) methods usually yields acceptable artifact reduction (Allen et al., 2000).
The correction for the ballistocardiogram (BCG) artifact represents a bigger challenge, mainly due to its non-stationary nature. As discussed in (Mullinger, K.J., et al., 2013, Yan, W.X., et al., 2010), several mechanisms contribute to the BCG artifact, the most plausible being: 1) the rotation of the head inside a strong, static magnetic field due to the cardiac pulse (Bonmassar et al., 2002), 2) the Hall effect of the pulsatile blood flow, which is an electrically conductive fluid, inducing changes of the voltage measured on the surface of the scalp (Tenforde et al., 1983) and 3) voltages generated by movement associated with pulse-driven expansion of the scalp (Debener et al., 2008). Studies aiming at a better characterization of the contribution of each of those mechanisms to the BCG artifact have found that most of the artifact variance is explained by flow-induced Hall voltage and pulse-driven head rotation (Mullinger et al., 2013). Additionally, a significant increase of the amplitude of the BCG artifact with the static magnetic field strength, B0, has also been shown (Debener, S., et al., 2008, Neuner, I., et al., 2013), with severely hampered visual inspection of typical EEG patterns at 7 T. The accurate removal of the BCG artifact while preserving the physiological EEG signal is of utmost importance in several applications, particularly when the quantification of EEG features allows the prediction of blood-oxygen-level dependent (BOLD-)fMRI signal changes (e.g., Rosa et al., 2010), including epilepsy applications (Leite et al., 2013).
The BCG artifact occurrences are known to be approximately time-locked with the cardiac cycle; therefore, an AAS algorithm can be employed whereby an artifact template is extracted by averaging across multiple cardiac cycles, and a time-domain subtraction is then performed for artifact correction (Allen et al., 1998). In the most common approach, a moving average template is computed from successive artifact occurrences, assuming that the BCG artifact occurrences change slowly over time. The BCG variability can be more accurately taken into account by computing the temporal Principal Component Analysis (tPCA) over all the time-locked occurrences of the artifact in order to build an optimal basis set (OBS), comprising a given number of principal components (PCs) that explain the BCG artifact variance to some extent (Niazy et al., 2005). This basis set is then fitted to, and subtracted from, each artifact occurrence. More direct approaches to correct for the BCG artifact have been proposed, which are based on the use of motion sensors (Bonmassar, G., et al., 2002, Chowdhury, M.E.H., et al., 2014, Jorge, J., et al., 2015a, Masterton, R.A.J., et al., 2007). However, currently these techniques require hardware-related changes of the experimental apparatus and may therefore not be applicable in general.
A third category of BCG artifact correction methods is based on blind source separation. Independent Component Analysis (ICA) is commonly used for this purpose. By removing the contribution of independent components (ICs) reflecting artifact-related processes in the back-reconstruction of the EEG signal, an artifact-corrected signal can be obtained (Bénar, C.-G., et al., 2003, Mantini, D., et al., 2007, Srivastava, G., et al., 2005). Most of the ICA algorithms, however, assume spatial stationarity of the sources, ignoring the spatio-temporal variability of the BCG artifact occurrences (Debener, S., et al., 2008, Vanderperren, K., et al., 2007). Many approaches nevertheless rely on ICA-based methods, since alternative methods present their own limitations as well (Vanderperren et al., 2010). When using ICA-based EEG de-noising methods, several questions arise regarding: 1) the implementation of the ICA decomposition of EEG data to be used; 2) the number of ICs to be estimated; and 3) the identification of the ICs to be classified as BCG-related. All three issues have been addressed in (Vanderperren et al., 2010); however, the objective and accurate classification of ICs remains a major concern and several criteria can be found in the literature for that purpose, which will be presented and discussed later in this paper.
Due to the relative advantages and disadvantages of OBS and ICA-based methods, the combination of both has already been proposed (Debener, S., et al., 2005, Debener, S., et al., 2007). The OBS method is used to remove most of the BCG artifact contribution, followed by an ICA decomposition of the OBS-corrected EEG data to further remove residual artifacts. Despite its effectiveness in removing the artifact, the application of OBS directly on the EEG signal may also induce undesirable physiological signal reductions, which would be exacerbated if an additional step of ICA-based correction is applied. Recently, it has also been proposed to use a modified version of OBS in the IC space instead (Liu et al., 2012). For that purpose, the EEG data is first decomposed into a set of ICs and the mutual information (MI) (Bell and Sejnowski, 1995) between them and the electrocardiography (ECG) data is computed. The number of ICs with the highest MI is then chosen by means of a modified method (Peng et al., 2005) that finds the optimal number of ICs yielding minimal error in leave-one-out cross-validation. The BCG-related ICs are removed from the back-reconstruction of the EEG signal and all the remaining ICs are singular value decomposition (SVD-)corrected before back-projecting them onto the EEG space, as it is hypothesized that the BCG artifact contributes to all ICs to varying degrees due to its non-stationary nature (Liu et al., 2012).
Regardless of the BCG artifact correction method used, the quality of the correction should be assessed (Freyer, F., et al., 2009, Grouiller, F., et al., 2007). Typically, event-related simultaneous EEG-fMRI studies are conducted and the performance of a given method can be computed based on features extracted from the event-related potentials (ERPs) of interest, such as: the inter-trial variability (Vanderperren et al., 2010), the signal-to-noise (SNR) ratio (Debener et al., 2007) or the difference between the ERPs extracted from the inside-MR EEG datasets and those that are obtained from the BCG artifact-free outside-MR EEG data (Mantini et al., 2007). When the frequency content of a task-modulated EEG signal is known, the power of the EEG within that frequency band can also be computed before and after BCG artifact correction (Xia et al., 2014). In resting-state fMRI (rs-fMRI) studies, the quality of the correction can be assessed by comparing the BCG artifact occurrences before and after correction, based on the root mean square (RMS) of the BCG waveform or their peak-to-peak (PTP) values (Chowdhury et al., 2014). Additionally, the total spectral power within windows around the cardiac fundamental frequency and its first harmonics (Liu et al., 2012) can be computed, and a ratio expressing the loss in normalized spectral power after the correction is used to quantify the amount of BCG artifact that was removed, measured by the Improvement in terms of Normalized Power Spectrum (INPS) (Tong et al., 2001).
Many studies continue to be dedicated to the development of more efficient methods for BCG artifact correction, but the associated, unwanted removal of true physiological signal in the background has not been systematically assessed. Here, we propose a novel method for the selection of BCG-related ICs (PROJection onto Independent Components, PROJIC), after which three ICA-based approaches for the removal of the BCG artifact are explored. First, the contribution of the BCG-related ICs is removed in the back-reconstruction step of the EEG signal (PROJIC). Alternatively, we propose the use of OBS (PROJIC-OBS) or AAS (PROJIC-AAS) for correcting the BCG-related ICs before back-projecting them, instead of simply removing their contribution. In parallel, a novel evaluation pipeline that assesses both artifact and background signal removal is presented, and used to compare the novel approaches with previous approaches, on data collected from a group of epilepsy patients (imaged at 3 T) and a group of healthy volunteers (imaged at 7 T). Data quality improvements on ERPs of interest collected with both setups (inter-ictal epileptiform discharges, IEDs, and visual-evoked potentials, VEPs, collected at, respectively, 3 T and 7 T) were also assessed by means of an inter-trial variability measure.
Section snippets
Materials and methods
The main steps of the processing pipeline proposed in this work for the optimal BCG artifact correction and subsequent evaluation, in terms of not only artifact removal and physiological signal preservation, but also data quality improvements on ERPs of interest collected with both datasets, are depicted in Fig. 1.
Results
The results obtained using the different algorithms tested on the data collected using Setups #1 and #2 are presented and discussed jointly in this section. The IC selection criterion underlying PROJIC was found to successfully select BCG-related ICs in both setups, as evidenced by the clear presence of artifact occurrences in the corresponding time-courses upon direct visual inspection. Moreover, the subsequent application of the three proposed approaches was also found to substantially reduce
Discussion
In this paper, a novel method for the selection of BCG-related ICs was proposed, following which three ICA-based approaches for BCG artifact removal were explored on EEG data recorded simultaneously with fMRI. These novel methods were compared with other ICA-based approaches in the literature, as well as with the commonly used AAS and OBS methods. We found that group-level optimization of the algorithms' parameters significantly improved performance of all methods, and that, when optimized,
Conclusion
We have proposed a novel method for the selection of BCG-related ICs, with three associated approaches for the correction of the BCG artifact in simultaneous EEG-fMRI, and have shown that they outperform both ICA-based and non-ICA-based state-of-the-art methods for most physiological background preservation weights. The PROJIC-OBS method returns the strongest attenuation of the BCG artifact, while the PROJIC-AAS method manages to preserve most of the physiological signal in the background with
Acknowledgments
We acknowledge the Portuguese Science Foundation (FCT) for financial support through Project PTDC/SAUENB/112294/2009, Project PTDC/EEIELC/3246/2012, Grant UID/EEA/50009/2013 and the Doctoral Grant PD/BD/105777/2014. We also acknowledge Centre d'Imagerie BioMédicale (CIBM) of the UNIL, UNIGE, HUG, CHUV, EPFL and the Leenaards and Jeantet Foundations for providing the data acquired at 7 T.
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