Comparing rapid direct neural evoked responses using non-selective MRI and simultaneous EEG

2438 

Abstract Submission 

Peter Molfese¹, A. Morgan¹, J Derbyshire¹, Laurentius (Renzo) Huber¹, Peter Bandettini¹

¹National Institute of Mental Health, Bethesda, MD

Peter Molfese, PhD  
National Institute of Mental Health
Bethesda, MD

A. Morgan  
National Institute of Mental Health
Bethesda, MD

J Derbyshire, PhD  
National Institute of Mental Health
Bethesda, MD

Laurentius (Renzo) Huber, PhD  
National Institute of Mental Health
Bethesda, MD

Peter Bandettini, Ph.D.  
National Institute of Mental Health
Bethesda, MD

Several groups have reported attempts to directly measure neural responses using MRI sequences (DIANA) in mice^1,2 and humans^3,4,5. Measuring brain responses with DIANA presents great potential for impact on the field of functional MRI, as it would circumvent the necessity for indirect inference of neural activity based on vascular signals^1,6. The feasibility of measuring DIANA responses is still an open question, as these responses are reportedly very small¹, requiring many trial to averages to measure useful signal². Furthermore, the structure of 2D line-scanning sequences used to image DIANA responses necessitates trials be repeated for every acquired k-space line⁵, inflating the required number of trials. We measured DIANA responses from the center of k-space using a fast (3 ms), non-selective MRI sequence without phase or frequency encoding, negating the need to repeat trials for multiple k-space lines. For comparison, we simultaneously acquired EEG during evoked auditory and visual stimulation experiments.

We tested a fast, non-selective, balanced steady-state free precession (bSSFP) sequence⁷ without gradient encoding to record the center of k-space (Fig. 1). MRI scan parameters: TR=3 ms; TEs=0.75 to 2.25 ms; RF: hard pulse length=0.5 ms, FA=12º; ADC length=1.5 ms, BW=21370 Hz, vector size=64; RF and ADC phase progressed by 180º with every TR; an SSFP preparation preceded the first TR (FA=6º; TR=1.5 ms).

Two participants underwent testing on a Siemens 3T Skyra with a Siemens 64-receiver coil. The first participant viewed a flashing strobe light (10 us; Grass Instruments PS33) via a fiber optic cable with an interstimulus interval (ISI) of 834 ms. The second participant heard two auditory tones (1000 and 2000 Hz; duration=300 ms), with random presentation order and pseudo-random ISIs (800-2000 ms).

MRI time series were formed by Fourier transforming every ADC and taking the magnitude of the spectral peak response. Data were bandpass filtered between 0.5 and 50 Hz. MRI receiver coil locations were estimated by computing the center of mass of sensitivity maps within a skull ROI. We cut epochs between -100 and 800 ms from stimulus time and averaged within conditions.

EEG data were recorded using a MR-conditional 256-channel EGI GES 400 (EGI; Eugene, OR). To reduce the impact of the BCG, QRS were estimated from the EEG channels, and subtracted using optimal basis sets⁸, bandpass filtered 0.1-30Hz, segmented -100 to 700 ms from stimulus onset, baseline corrected, average referenced, and averaged.

Evoked responses (MRI) and potentials (EEG) to visual flash stimulation are shown in Figure 2. We observe response dynamics in both MRI and EEG at 100 ms as well as correspondence between MRI and EEG topographical features. However, a prominent feature of the MRI time series for the visual stimulation paradigm is a substantial positive and negative peak at 200 ms. This feature is likely caused by a blink artifact, and is less visible in the EEG potentials due to semi-automated removal of epochs containing a blink.

Responses and potentials to auditory stimulation are shown in Figure 3. These responses are more lateralized than the visual responses, and we see increased dynamics between conditions in the MRI signal, with an initial negative response for the 1000 Hz tone (100 ms) and a delayed positive response (500 ms). There is some correspondence between MRI and EEG topographies, though MRI responses are comparatively delayed. The initial negative response was not visible with the 2000 Hz tone and the response peak occurred earlier (250 ms).

We provide strong yet still preliminary evidence for evoked MRI response dynamics on the order of tens to hundreds of milliseconds, and compare the shape and topography of and responses to simultaneously acquired EEG measurements.

EEG ²

Non-BOLD fMRI ¹

Neurophysiology of Imaging Signals

Electroencephaolography (EEG)

MRI

NORMAL HUMAN