Modeling and design of a connected birdcage coil for ultra-high field MRI

Magnetic Resonance Imaging (MRI) is a non-invasive and versatile medical diagnostic tool capable of providing images from inside bodies. MRI is based on the hydrogen atoms that are polarized in the presence of a strong static magnetic field and uniformly (in the deal case) tipped by a radio frequency (RF) magnetic field generated by an RF coil. The birdcage coil is the most used RF coil in clinical applications as it produces a highly homogeneous magnetic field. It has been shown that high-resolution images resulting in the early-stage diagnosis are accessible by increasing the resonance frequency of the birdcage coil. However, increasing the resonance frequency is accompanied by the shortened wavelength inside the body, which corresponds to the creation of bright and dark spots and reduces the quality of MRI images. This thesis aims to propose new designs for the birdcage coil to improve the homogeneity of the RF magnetic field at High-Field (3T) and Ultra-High Field (7T) MRI. The new design, called connected birdcage, has electrical connections between the birdcage and shield, and improves the coil's efficiency. Also, the connection types in the connected birdcage provide more degrees of freedom to optimize the current distributions on the birdcage legs, resulting in an improved magnetic field homogeneity. A 3T connected birdcage coil has been fabricated and encouraging results have been obtained. For a 7T MRI, a new design based on the floating loops is also proposed, and it has been shown that the floating loops between the consecutive legs act as additional legs and improve the magnetic field homogeneity.