Abstract
Purpose: To compare 2D and 3D echo-planar imaging (EPI) in a higher cognitive level fMRI paradigm. In particular, to study the link between the presence of task-correlated physiological fluctuations and motion and the fMRI contrast estimates from either 2D EPI or 3D EPI datasets, with and without adding nuisance regressors to the model. A signal model in the presence of partly task-correlated fluctuations is derived, and predictions for contrast estimates with and without nuisance regressors are made.Materials and Methods: Thirty-one healthy volunteers were scanned using 2D EPI and 3D EPI during a virtual environmental learning paradigm. In a subgroup of 7 subjects, heart rate and respiration were logged, and the correlation with the paradigm was evaluated. FMRI analysis was performed using models with and without nuisance regressors. Differences in the mean contrast estimates were investigated by analysis-of-variance using Subject, Sequence, Day, and Run as factors. The distributions of group level contrast estimates were compared.Results: Partially task-correlated fluctuations in respiration, heart rate and motion were observed. Statistically significant differences were found in the mean contrast estimates between the 2D EPI and 3D EPI when using a model without nuisance regressors. The inclusion of nuisance regressors for cardiorespiratory effects and motion reduced the difference to a statistically non-significant level. Furthermore, the contrast estimate values shifted more when including nuisance regressors for 3D EPI compared to 2D EPI.Conclusion: The results are consistent with 3D EPI having a higher sensitivity to fluctuations compared to 2D EPI. In the presence partially task-correlated physiological fluctuations or motion, proper correction is necessary to get expectation correct contrast estimates when using 3D EPI. As such task-correlated physiological fluctuations or motion is difficult to avoid in paradigms exploring higher cognitive functions, 2D EPI seems to be the preferred choice for higher cognitive level fMRI paradigms.
Highlights
BOLD sensitivity is usually defined as the temporal signalto-noise ratio per square root of the volume repetition time (TRvol )
The correlation results for 2D EPI and 3D EPI had overlapping confidence intervals except for two cases, these were the motion regressor m3 correlated with the Learning contrast and the respiratory phase regressor rc2 correlated with the Cross fixation contrast
The results presented in this study illustrate that the use of 2D EPI or 3D EPI for BOLD fMRI in a higher cognitive level paradigm can have an effect on the contrast estimate results
Summary
BOLD sensitivity is usually defined as the temporal signalto-noise ratio (tSNR) per square root of the volume repetition time (TRvol ). It has been shown that 3D EPI yields comparable or higher BOLD sensitivity than 2D EPI for a range of resolutions at 7T (Poser et al, 2010), as well as at 3T for low resolution (Goerke et al, 2005) and recently for high resolution (Lutti et al, 2013) Signal instabilities such as physiological noise behave differently and have a more adverse effect on tSNR in 3D EPI than in 2D EPI (Goerke et al, 2005; Poser et al, 2010; Kristoffersen and Goa, 2011; van der Zwaag et al, 2012; Jorge et al, 2013). This difference in vulnerability to physiological noises is linked to the differences in data acquisition schemes between 2D EPI and 3D EPI, as described above
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