Shielded coaxial cable coils for whole head and neck imaging at 7T MRI

At ultra-high field (UHF) magnetic resonance imaging (MRI), nuclei have higher Larmor frequency than in the traditional MRI systems. Therefore, electromagnetic waves have shorter wavelengths, and electromagnetic interferences become more prominent. Moreover, higher tissue loss at UHF leads to shorter penetration depth inside the human body and shrinks the field of view of acquired images. Moreover, local energy depositions become noteworthy, leading to higher local specific absorption rate (SAR) levels and higher B⁺₁ -field inhomogeneities. Therefore, radio-frequency (RF) coil design at UHF MRI is multidimensional and challenging.
The birdcage coil is a robust and convenient design to image the brain at UHF MRI, specifically at 7T. However, it has a limited field-of-view (FOV) due to the shorter wavelength and suffers from signal loss in the lower brain regions and neck. Consequently, a convenient and adaptable coil design for whole head and neck imaging is still under investigation in the RF coil community. Among different alternatives, combining a birdcage coil with surface coil elements with active shimming capabilities placed around the neck can be a convenient solution.
Shielded coaxial cable coils (SCCs) were recently used for building a neck array, and they have taken attention in the MRI community with their flexible body designs and self-decoupled properties. However, researchers working with high-impedance coils (HICs), which share the same physical geometry as SCCs, could not confirm their self-decoupling properties. Although there have been several attempts to understand their working mechanism, the understanding how they differ has yet to be specified more precisely. 
The work presented in this thesis aimed to combine SCCs with a birdcage coil to extend the FOV to achieve a whole head and neck coverage at 7T MRI. Due to the ambiguity in the field, the mode of operation of SCCs was first investigated comparatively with HICs. A specific focus was given to the loops to elaborate on their coupling mechanism. The difference between SCCs and HICs, specifically in their resonance modes, was shown with simulation and experimental results. Secondly, a method different from the conventional literature to improve signal-to-noise ratio (SNR) in SCC transmit arrays was presented. Finally, SCCs were combined with a commercial birdcage coil to extend the coverage in the head and neck. The accuracy of simulations was confirmed with phantom measurements, and subject images were acquired using a Philips Achieva 7T scanner.