Radio Frequency Systems for NMR Imaging: Coil Development and Studies of Non-Thermal Biological Effects

Sammanfattning: This thesis concerns both design and optimization of NMR imaging systems and studies on non-thermal biological effects of electromagnetic fields. These two research fields are linked together by the fact that humans are subject to static and pulsed magnetic fields as well as to magnetic radio frequency fields (RF) during an NMR imaging investigation. This work bridges the gap between technology, physics and medicine and is in this respect highly interdisciplinary. The design of receiving coil antennas and low noise amplifiers for NMR imaging and spectroscopy applications is described. The objective of this work was to improve the performance of NMR imaging systems by development of RF detection coils for regions where the coils provided by the manufacturer do not give optimal performance. Three coil antennas are described two of which are designed and implemented for use in clinical routine investigations of the spine, at the level of the shoulders, at a vertical magnetic field of 0.3 tesla (12.7 MHz). One coil antenna is designed for imaging in adults and the other for imaging in small children. The results indicate an improvement in SNR by a factor of 2 to 3 in the thoracic region. The third coil antenna described is a circularly polarized surface coil, optimized by numerical modelling of the wire structure and designed for imaging of the superficial parts of the human brain at a horizontal magnetic field of 1.5 tesla (63.6 MHz). A low noise preamplifier with high input reflection coefficient when matched for optimal noise performance is described. The high input reflection coefficient, when connected to the coil antennas, is utilized to reduce the currents in the coils that cause the inductive coupling between coils when closely packed. The design procedure as well as the evaluation of the final amplifier are presented. Studies are presented on the permeability of the blood-brain barrier (BBB) in rats after exposure to static magnetic fields, low frequency pulsed magnetic fields, magnetic radio frequency (RF) fields (3.5 MHz) and transverse electromagnetic RF fields (915 MHz). For good interpretation of the biological experimental results three-dimensional electromagnetic calculations for loaded transverse electromagnetic (TEM) transmission cells are presented, using the finite difference time domain (FDTD) method. Design, construction and test of an anechoic chamber for both near and far field exposures at frequencies above 1.5 GHz for biomedical research is presented. The results from an initial study performed in the anechoic chamber on biological effects of electromagnetic fields at a frequency of 1.8 GHz are presented. Both continuous wave (CW) and GSM-modulated electromagnetic exposure fields were used.