Abstract
Detection of weak magnetic fields induced by electrical currents using magnetic resonance imaging (MRI) is necessary for mapping neuronal activities in the brain. However, such detection is dependent on the signal-to-noise ratio and sensitivity limit of MRI. Evaluation of these factors requires calculation of radiofrequency (RF) electromagnetic fields in the brain. To determine the feasibility of detecting the magnetic fields induced by neuronal activities, we computed brain eddy current distributions induced by RF magnetic fields from a birdcage coil in MRI by simulations based on the finite element method (FEM), and calculated Johnson noise from the head. With commonly used parameters, the noise was 43.2 nV. On the basis of a theory of the signal-to-noise ratio in MRI, we obtained an equation for the theoretical sensitivity limit for detecting weak magnetic fields by using gradient echo phase images. The theoretical sensitivity limits for gray matter and white matter were 26.1 nT and 21.7 nT, respectively.
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