| Surface coils are used in Magnetic Resonance Imaging (MRI) for their high signal-to-noise ratio (S/N) when placed near the region to be imaged. However, their optimization for high field MRI systems is hampered by the lack of understanding of the electromagnetic effects taking place, at high frequencies, with a coil near the human body. The aim of this dissertation was to study the high frequency effects and parameters determining the fields created by dipoles precessing in the body and also to calculate the S/N of surface coils using complete solutions to Maxell's equations.;The human body is modelled as an infinitely long homogeneous dielectric cylinder exhibiting both conductive and dielectric losses.;Considering a dipole oriented perpendicular to the cylinder axis, we develop expressions for the electric and magnetic fields set up. This yields an insight into the different sources (quasi-static, eddy currents, displacement currents, ...) contributing to these fields and into their relative importance at increasing frequencies. Using average tissue properties and body size we show quasi-static models describe dipole field accurately below 10 MHz but above 41 MHz all frequency contributions of a full solution to Maxwell's equations are required to describe the electromagnetic effects taking place.;The S/N computation for a coil facing the cylinder is approached using Dyadic Green's functions. We derive complete solutions for the fields of a dipolar source located arbitrarily in the cylinder and, applying the reciprocity principle, we deduce the fields created, at the dipole position, by a coil excited with a unit radiofrequency current. These yield the expressions for the power dissipated in the cylinder, for its reciprocal, the noise picked up by the coil, and also for the signal received.;The effects of coil size and position, tissue properties and source location, on the S/N are demonstrated for a 40 cm diameter body at frequencies between 1 and 170 MHz. The benefits of high field MRI systems for imaging superficial structures are presented.;The theoretical predictions made from our model are confirmed by experiments carried out, in the laboratory, on a saline-filled plexiglas cylinder. |
