Broadband Radio Frequency Transmitter for Magnetic Resonance Imaging

Abstract

In Magnetic Resonance Imaging (MRI) application, radio frequency (RF) coils play an important role to excite the nuclei inside the imaging subject and receive MR generated signals. To achieve optimal transmit efficiency/receive sensitivity, it is necessary to re-tune the RF coils to its resonant frequency for different scanning samples. And the re-tune procedure is time-consuming. In this study, we proposed a novel RF volume transmission method which takes advantage of parallel plate waveguide (PPW). Due to the frequency-independent nature of the PPW, the designed volume transmit coils are able to work in broadband and are insensitive the loading changes so that the tedious re-tune procedure for different loadings can be avoided. In order to interface the PPW coils with the scanner, RF front-end circuits at both transmit end and receive end are designed, optimized and fabricated as discussed in Chapter 2. In Chapter 3, a linear PPW coil is designed and fabricated for human forearm proton imaging and phosphorous spectroscopy analysis. The calibrated transmit efficiency of PPW coil measured is close to that of the un-shielded Birdcage coil with same coil dimensions. Both high SNR proton images and strong phosphorous signals are obtained from both buffer solution phantom and human forearm. In Chapter 4, to meet with requirements of large volume transmission for MRI, a quadrature PPW coil’s design, fabrication and testing procedure is discussed. The transmit efficiencies of linear excitation and quadrature excitation are measured and compared. An approximate 40% increase in transmit efficiency is achieved by using quadrature excitation. To boost the local SNR of the images, a conformal four-channel “clover” shape detonable receive-only array is designed for MR signal reception. Both overlap and capacitive decoupling methods are employed to eliminate the mutual coupling between neighboring coils. The obtained uncombined images are compared with simulated receive coil profile to demonstrate the effectiveness of the decoupling strategies employed.