Safety and data quality of simultaneous EEG-fMRI using multi-band fMRI imaging

Purpose Simultaneously recorded electroencephalography and functional magnetic resonance imaging (EEG-fMRI) is highly informative yet technically challenging. Until recently, there has been little information about the data quality and safety when used with newer multi-band (MB) fMRI sequences. Here, we assessed heating-related safety of a MB protocol on a phantom, then evaluated EEG quality recorded concurrently with the MB protocol on humans. Materials and Methods We compared radiofrequency (RF)-related heating and magnetic field magnitude () of a fast MB fMRI sequence with whole-brain coverage (TR=440ms, MB factor=4) against a previously recommended, safe single-band (SB) sequence using a phantom outfitted with a 64-channel EEG cap. Temperatures were recorded at the ECG and TP7 electrodes using a fluoroptic thermometer. Next, 6 human subjects underwent eyes-closed resting state EEG-fMRI with the MB sequence. EEG data quality was assessed by the ability to remove gradient and cardioballistic artifacts and a clean spectrogram. Results RF induced heating was lower at both electrodes in the MB sequence compared to the SB sequence at ratios of 0.7 and 0.8, respectively. These ratios are slightly greater than the ratio of RF power deposition of the sequences, which is 0.64. However, our results are consistent with the use of RF power deposition, characterized by , in predicting less heating in the MB sequence than the SB sequence. In the resting state EEG data, gradient and cardioballistic artifacts were successfully removed using traditional template subtraction. All subjects showed an individual alpha peak in the spectrogram with a posterior topography characteristic of eyes-closed EEG. Conclusions Our study shows that is a useful indication of the relative heating of fMRI protocols. This observation indicates that simultaneous EEG-fMRI recordings using this MB sequence can be safe in terms of RF-related heating, and that EEG data recorded using this sequence is of acceptable quality.

underwent eyes-closed resting state EEG-fMRI with the MB sequence. EEG data quality was assessed by 23 the ability to remove gradient and cardioballistic artifacts and a clean spectrogram.

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RF induced heating was lower at both electrodes in the MB sequence compared to the SB sequence at 26 ratios of 0.7 and 0.8, respectively. These ratios are slightly greater than the ratio of RF power deposition 27 of the sequences, which is 0.64. However, our results are consistent with the use of RF power 28 deposition, characterized by + 1 , in predicting less heating in the MB sequence than the SB 29 sequence. In the resting state EEG data, gradient and cardioballistic artifacts were successfully removed 30 using traditional template subtraction. All subjects showed an individual alpha peak in the spectrogram 31 with a posterior topography characteristic of eyes-closed EEG.

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Our study shows that + exceeding an MB factor of 4, while sparing the EEG alpha frequency band (~8-12Hz) from residual RF 84 excitation repetition artifacts (appearing at 15.9Hz for our sequence, see methods).

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More important than data quality when considering a new MB EEG-fMRI sequence is ensuring subject 86 safety. For EEG-fMRI the key safety concern is the deposition of radiofrequency ( The temperature of the TP7 electrode also steadily increased over the course of the experiment, as 307 shown in Fig 4A. The temperature increase persisted during the rest periods during which no scanning 308 was performed, indicating temperature drift, likely due to changes in the temperature of the 309 environment. Superposition of the temperature changes during each of the scans revealed similar 310 heating for the two sequences, except for one of the SB-RF scans which showed greater heating, as 311 shown in Fig. 4B. The average rate of heating for the MB protocol was lower than that of the SB-RF 312 protocol by a factor of 0.8, as shown in  eyes-closed resting state in a single subject (Subject 2). We demonstrate that each successive step substantially improves data quality, first showing the raw data without GA or BCG artifact subtraction 328 (Fig 5A), then with only the GA artifact cleaned (Fig 5B), and finally the fully cleaned data with both GA 329 and BCG artifacts removed (Fig 5C).The cleaned spectrogram showed a clear power peak in the alpha 330 range (~10Hz) as expected during relaxed eyes-closed states, while displaying only a minimal residual 331 power increase related to the GA at the RF excitation repetition frequency (Fig 5D). Fig 6a shows  374 Therefore, the heating associated with the MB sequence was less than that of the SB-RF, as expected.

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Our measured heating ratios are slightly greater than the expected ratio. This is likely due to the 376 presence of experimental noise, which tends to cause the measured heating ratio to approach unity.

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Experimental noise arises from limitations in the precision of the fluoroptic thermometer, and from 378 variability between scans.

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In absolute terms, the heating rates we measured for all conditions were greater than 0.01 degrees 380 C/min (see Table 1). can be maintained at acceptable levels even when using a MB factor of 4, 397 a low TR of 440 ms, and 28 slices for whole-cerebrum coverage. This was achieved by using a relatively 398 low flip angle of 40°, and a moderately long pulse duration of 5300 µs.

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In addition to verifying heating safety, in our second experiment we conducted preliminary 400 investigations into the success of GA and BCG artifact rejection for the EEG data acquired concurrently 401 with our MB sequence. To this end, we assessed the spectral dominance and topography of the alpha 402 frequency band due to its uniquely high and easily identifiable power during eyes-closed resting state.