Introduction

Whenever non-standard devices are introduced into the MRI environment, testing to ensure that the MRI environment will remain safe for both subjects and equipment is important. There are numerous studies combining EEG and fMRI at 3T and lower fields. These experiments demonstrate that with appropriate caution, EEG-fMRI studies can be performed safely at these field strengths. Moving to higher magnetic fields requires further safety testing. Jorge et al. reported that heating of the electrodes in the EEG cap were minimal, but significant heating of the RF amp was observed.[1] In this study we explore the impact of the slew rate of the EPI read-out gradient pulses, on heating induced in the EEG amps at 7T.

Methods

A phantom EEG-MRI experiment was conducted on a 7 Tesla MRI system (Signa 7T, GE HealthCare). The Brian Products EEG cap was placed on a spherical water phantom, which was place in the 32-channel head coil (Nova Medical). The EEG amplifiers (BrainAmp MR Plus and BrainAmp ExG MR; Brain ProductsGmbH) were placed behind the RF coil with the EEG cap attached. A fiber optic temperature probe (Rugged Monitoring model L201), was placed between the EEG and ExG amps, as illustrated in Figure 1. Previous measurements had shown this to be the location of maximum heating. EPI images were acquired for at least 5 minutes, with read-out gradient slew rates of 120, 88, 61, and 46 T/m/s, while the temperature between the EEG amps was continually monitored with the fiber optic temperature probe.

In addition, to evaluate the impact of changing the slew rate on image quality, EPI images were acquired with a human subject (30 yr. old male). Data sets were acquired with slew rates of 120 and 61 T/m/s. The data sets were acquired with an isotropic resolution of 1.5mm³, TE=28ms, multi-band factor=6, and TR = 800, 900ms respectively.

Results

Figure 2 shows the measured temperature changes, for the EPI scans with each slew rate. Both the slew rates of 120 and 80 T/m/s showed substantial heating of the EEG amps over the 5 min. test periods. For the slew rate of 61 T/m/s, only very modest heating was observed, with a maximum of 0.25 °C. For the slew rate of 46 T/m/s, the observed temperature change, never rose above baseline.

Reducing the slew rate for 120 to 61 T/m/s, does not have a pronounced impact on image quality, as demonstrated by the images in Figure 3. Since the EPI sequence employed in this study uses ramp sampling, reducing the slew rate has only a minimal impact on the overall read-out window, therefore there is not a dramatic loss of image quality.

Conclusions

Previous studies have observed heating of the EEG amps during simultaneous EEG-fMRI at 7T. The results presented here replicated that observation. In addition, we demonstrated that by reducing the slew rate of the EPI read-out gradient, the heating of the EEG amps can be dramatically reduced or even eliminated. Further we demonstrated the use of the reduced slew rates do not lead to a noticeable loss of image quality. The results of the experiments presented here demonstrate that by reducing the slew rate of the read-out gradient, we can significantly reduce or even eliminate heating of the EEG amps. This demonstrates that EEG-fMRI can be done safely at 7T, without excessive wear on the EEG amps.

Acknowledgements

No acknowledgement found.

References

1. Jorge J, Grouiller F, Ipek Ö, et al. Simultaneous EEG–fMRI at ultra-high field: Artifact prevention and safety assessment. NeuroImage 2015;105:132-144.