The development of rotating radiofrequency techniques for ultra-high field magnetic resonance imaging

Abstract

Ultra-high field (≥7 T) magnetic resonance imaging (MRI) offers improved image quality to facilitate clinical diagnosis. However, the wavelength of radiofrequency (RF) electromagnetic (EM) waves becomes a fraction of the size of the human body at ultra-high fields; therefore, constructive and destructive interference of RF magnetic field (B 1 field) forms in the human body, causing bright and dark regions in the output images. These artefacts seriously affect the accuracy and diagnostic value of the images. The RF power deposited in the human body is dramatically increased at ultrahigh fields and the resultant overheating could potentially damage tissues. The long scan duration is also an issue which is reported as the most unpleasant factor during an MRI scan at 7 T. It can cause patient discomfort, both psychologically and physically; consequently, inevitable motion of the patient could induce motion artefacts in the images that seriously degrade image quality for clinical use. In terms of hardware, phased array coils are considered to be the best choice to implement parallel transmission and parallel imaging for solving ultra-high field issues. Parallel transmission and parallel imaging use multiple coils to transmit and receive signals. Theoretically, increasing the number of coils in an array can improve the performance of parallel transmission and parallel imaging. However, this raises RF coil related issues, such as reduced B 1 penetration and increased mutual coupling, which undermine the efficiency of parallel transmission/imaging. Alternatively, a physical rotating RF coil is capable of emulating a large number of coils without these RF coil related issues, so it could be employed as a solution. However, with only one coil element, the rotating RF coil (RRFC) is limited in its ability to reduce the scan duration. The specific aim of this project is to develop RF techniques that strategically combine the array structure with the rotating concept, to provide better solutions for ultra-high field issues. By introducing the rotation, the coil has the capability of acquiring a large number of transmit/receive sensitivity profiles, and thus fewer coils are needed for parallel transmission and parallel imaging. This offers the opportunity of constructing a coil array with lower coupling and bigger coils that have higher efficiency and B 1 penetration. Compared to a stationary coil array, the rotating array will also introduce an extra degree of freedom in the spatial domain to facilitate B 1 field homogenising (B 1 shimming) and Specific Absorption Rate (SAR) control. iii The geometry of a 4-element rotating RF coil array (RRFCA) was optimised to achieve natural decoupling and reasonable B 1 penetration, and then the RRFCA prototype was built for imaging a head-size homogenous phantom at 7 T. In order to achieve an image reconstruction without significant 1 B inhomogeneity, a time interleaved, spatial varying B 1 inhomogeneity mitigation strategy for the RRFAC was developed. A dedicated rotating-SENSE algorith...