Analysis of the electromagnetic field inside the gradient coils and investigation of the nerve and cardiac stimulation risk for the patients during MRI

Abstract

During magnetic resonance imaging (MRI), the static magnetic field, the radio frequency field and the gradient fields are utilized. In the literature, there are several studies with the aim of understanding the bioeffects of these fields. Due to the time varying gradient fields, an electric field is induced in the body which may cause nerve and cardiac stimulation. In order to investigate this risk, researchers have been working on solving the induced electric field both analytically and by using computational methods. The field distribution inside the body has also been investigated. It is vital to verify the risk of MRI to patients with an implant. In order to improve the understandability of the field pattern, the field should be expressed as simple as possible. In this thesis, the simplified expressions of induced electric field are derived inside and outside of the cylindrical, homogenous volume, which is taken as the human body model. For this derivation, low frequency based assumptions are used and gradient field is assumed to be perfectly uniform. The obtained results satisfy the expected conditions for the electric and magnetic fields. The field patterns obtained with these simplified expressions are compared with the former studies and the maximum electric field values obtained for a different gradient field and slew rates are used to investigate the stimulation risk. Moreover, by using these electric field values, the worst position for an implant lead and the length of the lead is determined. We believe that with these simplified expressions, the understandability of the field distribution is enhanced and to comment on the risk of MRI to a patient with an implant becomes easier.