Design of radiofrequency coils for Magnetic Resonance applications

Magnetic Resonance Imaging (MRI) is a non-invasive imaging technique that plays an important role in the medical community. Radiofrequency (RF) coils are key components in MRI systems. The purpose of the transmitter coil is to produce an highly homogeneous alternating field in a wide field of view (FOV) while the receiver coil has to maximize signal detection while minimizing the noise, mainly generated from patient’s tissue. The use of receiver coil array permits to provide a large region of sensitivity, similarly to that obtained with volume coils, and a high Signal-to-Noise Ratio (SNR), usually associated with surface coils. This thesis deals with the design and simulation of MR coils, both in single loop and phased-array configurations, using two different approaches. The first approach uses the equivalent circuit method, which can be employed for analysis and design of coils whose size is a small fraction of the wavelength. Using this home-made software is possible to calculate the magnetic field pattern and the inductance of the simulated coil. The developed software was validated simulating array coils composed of circular or rectangular loops. Finally, a novel structure with two butterfly coils was designed by the simulator and tested with electronic instrumentation. The second coil simulation approach employes Finite-Difference Time-Domain (FDTD) algorithm: this technique is very attractive because complex structures such as part of human body can be incorporated in the computational space, permitting to study the electromagnetic interaction between the coil and the biological phantom. In particular, the knowledge of a sample-coil interaction model is very useful for the design of a system strictly coupled to the sample, in such a way minimizing the coil noise with respect to the sample noise. With FDTD algorithm, we designed and tested a novel array constituted by two decoupled elliptical loops, which guarantees the desired FOV and provides high SNR. In this thesis, both these simulation methods (magnetostatic and FDTD algorithm) were also used for the sample resistance estimation of a real MR experiment, whose accuracies were validated with experimental measurements using home-made coils.