Flexible coil arrays for magnetic resonance imaging – performance comparison of coaxial transmission line resonators and stranded wire elements

Abstract

Magnetic resonance imaging (MRI) is a non-ionizing medical imaging technique, based on the ability of certain atomic nuclei to interact with externally applied magnetic fields. Image quality is strongly determined by the receive sensitivity of the radiofrequency coils, used to pick up the MR signal emitted from the measured body region, among other factors like strength of the static magnetic field. Coil sensitivity can be greatly enhanced by proximity to the body, which can be achieved using flexible receive coils, enabling close form-fitting to arbitrary body shapes.In this thesis a comparative study between flexible receive-only stranded wire coils and coaxial coils for magnetic resonance imaging at 3T is presented. The focus lies on the development process of a 4-channel stranded wire coil array. Stranded wire coils (SWC) were chosen due to their similar behavior to standard copper loop coils (SC). Their resonance frequency is determined by the inductance determined by the conductor geometry and capacitors along the conductor. Coaxial coils (CC) are self-resonant transmission line resonators, which are tuned by their geometry and cable characteristics.Single channel coils and individual interfacing circuitries were constructed with rigid copper wire, flexible stranded wire, and flexible coaxial cables. Bench tests using a vector network analyzer were conducted to verify tuning, impedance matching and the efficiency of the active detuning network. Lower unloaded Q-factors were found for the coaxial coil, indicating higher coil losses. Nevertheless, all coils were clearly sample noise dominated. Bench measurements of the three coils showed robustness against bending in terms of frequency shifting, which was below 3 %. The MR measurements showed that the flexible SWC and CC only had slight signal-to-noise ratio (SNR) disadvantages against the rigid SC in a circular ROI on a flat phantom.A 4-channel stranded wire coil was constructed, tested, and compared to a 4-channel coaxial coil. Similar geometric and preamplifier decoupling was found between the arrays. The MR measurements of the 4-channel coils included gradient echo, flip angle and noise scans. Considering that different preamplifiers were used, similar SNR was found in the defined region of interest for the two arrays. Therefore, other criteria than SNR performance should be considered for the choice between the two coil designs. These include the mechanical robustness against bending, where the CC outperforms the SWC as no additional solder joints along the conductor are required.