Design and Development of Dual Tuned 19F and 1H RF Birdcage Coils for Small Animal MRI at 3T

Abstract

Conventional proton (1H) magnetic resonance imaging (MRI) is generally insensitive to the lung due to the low tissue density and other complicating factors. On the other hand, inhaled inert fluorinated gas MRI is a promising technique for functional lung imaging, since it can visualize the distribution of the inhaled gas. To better understand and develop this novel technique, a vast number of pre-clinical animal experiments are required for validating and optimizing the radio frequency (RF) coils that are used to acquire MRI data. The simplest approach would be to use single-tuned coils (i.e. RF coils each tuned to a single resonance frequency), such that a 1H coil is used to obtain anatomical information, while a separate 19F coil is used to obtain functional lung information. Unfortunately, this approach also requires image registration (i.e. co-alignment of separate images) in order to combine the information from the 19F and 1H coils. The purpose of this thesis is to eliminate the need for image registration by developing an optimized 1H/19F dual-tuned coil for rodent lung imaging and disease model investigation. Our initial design was a coil-inside-coil (CIC) approach with geometric decoupling of the 1H and 19F resonators. Using the CIC approach, two independent coils are positioned concentrically with each other. The inner coil is then rotated until the position inducing the minimum voltage level is found. This method is restricted to linear mode RF coils because geometrical decoupling cannot be performed in quadrature mode due to reflection of RF power resulting in a poor signal to noise ratio (SNR). Our next approach was to construct a single birdcage coil dual-tuned to 1H-19F frequencies. This was done by taking the advantage of the fact that birdcage coils inherently have two orthogonal channels that are electrically invisible to each other. Because the 1H and 19F nuclei have close resonant frequencies at 3T (127.74 MHz and 120.15 MHz), each channel can be tuned to be on resonance for one frequency. The coupling between the two channels was quantitatively measured and compared to geometrically decoupled coils demonstrating the differences in decoupling performances. The advantage of this coil is that it assures identical B1 field profiles for the two nuclei, and slightly increased the filling factor for 1H resulting in improved SNRs. On the other hand, the disadvantage is that because 1H and 19F channels are orthogonal to each other, neither can be built to operate in a quadrature mode. In our final approach, a switch-tuned quadrature coil was built which can be switched to resonate at either the 1H or 19F frequencies. PIN diodes are used to actively control the switching between the two frequencies. This method enables the combination of the benefits afforded by using a quadrature coil (i.e. factor of 2 increase in SNR) and those of the dual tuned coil (i.e. the ability to switch frequencies on the fly without having to physically change coils and move samples, animals or patients from their anatomically localized positions). Using this switch-tuned coil, quantitative lung ventilation imaging can take place investigating various new imaging pulse sequences and disease models. B1 field mapping, B1 homogeneity and the uniform distribution of the currents on both fluorine and proton channels were measured.