Investigating the Axonal Magnetic Fields Corresponding to Delta and Theta Waves in the Human Brain Using Direct Detection MRI

Abstract

This study showed an alternative and non-invasive method for measuring brainwaves using Magnetic Resonance Imaging (MRI) with a gradient echo - echo planar imaging (GE-EPI) sequence. An attempt was made to measure the axonal magnetic fields of delta and theta waves using direct detection with MRI. Time-varying brainwaves produce an axonal current which may induce a magnetic field according to the Biot-Savart law. The MR scanner can detect inhomogeneous magnetic fields caused by weak currents generated in a subject that interact with the main magnetic field, $B_{o}$ , of the scanner. Fifteen healthy volunteers were scanned with closed eyes in a dark imaging room. The GE-EPI sequence was used to acquire 1500 time frame images in an axial plane on a 3.0 T Philips scanner. A Butterworth bandstop filter was applied to filter out physiological signals before the detection of brainwave signals. Fast Fourier Transform (FFT) was used to produce frequency spectra where the brainwave frequencies could be detected. Our study measured an axonal magnetic field of 1.5 ± 0.2 nT for the delta waves and 1.5 ± 0.3 nT for the theta waves. Delta waves were found in the range 1.5-4.0 Hz and theta waves in the range 4.0-6.5 Hz. The waves were found on both sides of the occipital lobe, temporal lobe, and hippocampus. We detected more theta waves (2.1% of the brain slice with 5 mm thickness) than delta waves (1.5% of the brain slice with 5 mm thickness). PLCC for the % $\vert \Delta S/S_{eq}\vert $ between the delta and theta waves was 0.7584 at $p =0.001$ significance level and 95% confidence level. We also applied Short Time Fourier Transform (STFT) with epoch lengths of 3.85 s, 7.7 s, and 30.8 s. But the appearance of brainwave signals was not as clear as using FFT over the entire imaging duration.