In principle, the signal-to-noise ratio of magnetic resonance imaging (MRI) increases with the strength of the static magnetic field, B0. This increase can be used to detect smaller image features and subtler signal changes, for example in earlier stages of disease. However, due to practical limitations in image quality, high field MRI has found only limited application in both research and clinical studies. The most severe limitation is due to inhomogeneities in the radio frequency (RF) magnetic field, B1, required for spin excitation. These inhomogeneities are caused by shorter wavelengths and increased conductive losses at the higher frequencies required for spin resonance in higher static fields. When the wavelength of the RF field is not large compared to the region of interest, it is no longer possible to generate a uniform B1 field over that region. This typically leads to strong variations in signal and contrast over the field of view and hence relatively low overall image quality. Although progress has been made to mitigate these effects, e.g., with new pulse designs and/or multiple transmit coils, there is still no general solution for arbitrary field strengths, excitation flip angles, and volumes of interest. This project will develop and evaluate a new approach to the problem. The electromagnetic fields transmitted by a coil array and generated within the body can be described in terms of discrete modes. The coupling between transmitted and internal modes will be determined and used to solve the inverse problem, ?what combination of transmitted field modes is required to generate a desired internal target field?? The RF coil array is designed to provide independent control over all modes of the transmitted field (through 6th order modes). This provides control over the internal field through the same maximum order. In dielectric samples, a candidate target field is a traveling plane wave, which subjects all spins in the sample to the same magnitude RF magnetic field. Hence the new method is called Traveling Internal Plane-wave Synthesis (TIPS). For dielectric conducting samples (such as tissues), a focused wave compensates for conductive losses and can provide excellent uniformity over the sample volume. The project has two specific aims. The first is to build the RF system for full control of the transmitted field (through 6th order), using a dense array of steerable magnetic dipoles. This provides the maximum possible control over internal fields (to the same order).
The second aim i s to evaluate the system?s performance in human volunteers in a 7 Tesla human scanner. The new method will be compared to conventional RF shimming, using the uniformity of the magnitude of B1 and specific absorption rate (SAR) over the volumes of interest as figures of merit. The outcome of the project will be a radically new approach to the design of RF pulses and hardware and the removal of a critical barrier to progress in magnetic resonance imaging.
The project aims to improve the quality of images generated by high field MRI scanners. This may lead to improved diagnosis of medical conditions and to new information on the structure and function of the brain.
