Reducing the gradient artefact in simultaneous EEG-fMRI by adjusting the subject's axial position

Abstract

Large artefacts that compromise EEG data quality are generated when electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) are carried out concurrently. The gradient artefact produced by the time-varying magnetic field gradients is the largest of these artefacts. Although average artefact correction (AAS) and related techniques can remove the majority of this artefact, the need to avoid amplifier saturation necessitates the use of a large dynamic range and strong low-pass filtering in EEG recording. Any intrinsic reduction in the gradient artefact amplitude would allow data with a higher bandwidth to be acquired without amplifier saturation, thus increasing the frequency range of neuronal activity that can be investigated using combined EEG-fMRI. Furthermore, gradient artefact correction methods assume a constant artefact morphology over time, so their performance is compromised by subject movement. Since the resulting, residual gradient artefacts can easily swamp signals from brain activity, any reduction in their amplitude would be highly advantageous for simultaneous EEG-fMRI studies. The aim of this work was to investigate whether adjustment of the subject's axial position in the MRI scanner can reduce the amplitude of the induced gradient artefact, before and after artefact correction using AAS. The variation in gradient artefact amplitude as a function of the subject's axial position was first investigated in six subjects by applying gradient pulses along the three Cartesian axes. The results of this study showed that a significant reduction in the gradient artefact magnitude can be achieved by shifting the subject axially by 4 cm towards the feet relative to the standard subject position (nasion at iso-centre). In a further study, the 4-cm shift was shown to produce a 40% reduction in the RMS amplitude (and a 31% reduction in the range) of the gradient artefact generated during the execution of a standard multi-slice, EPI sequence. By picking out signals occurring at harmonics of the slice acquisition frequency, it was also shown that the 4-cm shift led to a 36% reduction in the residual gradient artefact after AAS. Functional and anatomical MR data quality is not affected by the 4-cm shift, as the head remains in the homogeneous region of the static magnet field and gradients.

Figure 1

Illustration of the sequence used to characterise the artefacts generated by time-varying gradients along each Cartesian axis. (A) Schematic illustration of the gradient pulses applied in the right-left (RL), anterior-posterior (AP) and foot-head (FH) directions; (B) Average artefact voltages generated by these pulses on two example leads (FP1 and P4).

Figure 2

Variation of the average gradient artefact (RMS value (A) and range (B) over electrodes) with subject's axial position for gradients applied in RL, FH & AP directions (0 cm = nasion at iso-centre). Error bars show standard deviation across the six subjects studied.

Figure 3

Maps of the RMS (over time) of the gradient artefact produced by a multi-slice EPI acquisition with the nasion at: A) iso-centre; B) +4 cm. C) shows the difference, A-B. Data averaged over six subjects.

Figure 4

A: The RMS over channels of the average slice artefact before artefact correction averaged across subjects at the iso-centre (red dashed line) and optimal position (blue line). B: The standard deviation across slices after artefact correction using AAS averaged across subjects at the iso-centre (red dashed line) and optimal position (blue line). Only data acquired during time periods when the subject was stationary were used here.

Figure 5

Attenuation of signal power at optimal position compared to iso-centre after gradient artefact correction for first ten harmonics of the slice frequency, averaged over channels and subjects. Error bars: standard deviation over subjects. Asterisks indicate where a significant reduction was found: ** denotes p< 0.05 and * denotes p<0.1.

Figure 6

Standard anatomical image acquired at: A) iso-centre, B) +4 cm. C) shows the difference of two images acquired at iso-centre and D) shows the difference of A and B. In both cases the images were co-registered before subtraction. The colour bar shows the relationship of grey-scale to the percentage of the mean of the original images acquired at iso-centre.