The overall goal of this research is to predict and minimize the radio frequency (RF) losses affecting the success and safety of human head imaging at very high magnetic field strengths. This goal will be pursued by proving the hypothesis: """"""""RF losses in high field MR head imaging can be minimized by appropriate coil design."""""""" The rationale for this study lies in the need to know and control RF dependent transmit power, tissue heating, field penetration, and signal-to-noise for high field (1.5-7.0)T) human head studies. Increased signal-to-noise (SNR) and its dependent spatial resolution, temporal resolution and contrast are among the chief reasons given for human imaging at higher magnetic field strengths. RF dependent losses, defined as energy dissipated or signal reduced, diminish the theoretical maximum SNR in real RF coils and in lossy human tissues. A reduction in the quality (or quantity) of head images results. Additionally, much of the coil generated RF energy is dissipated in the head tissues as thermal energy. The potential safety risk of excessive heating is increasingly present at higher fields. Understanding, accounting for and minimizing RF losses is therefore critical to the quality and safety of high field MR head imaging. To address these needs the investigators propose three specific aims. 1) develop better understanding of RF losses through modeling, 2) verify and account for RF losses by measurement, and 3) minimize these RF losses by appropriate RF coil design and application. In the first aim, the RF losses in finite element models of the coil adjacent to the head anatomy will be numerically calculated. The consequences of these RF losses, to the coil's RF field, to RF safety, and to the SNR will also be predicted by solving the time-dependent electromagnetic and thermodynamic equations for these models.
The second aim will be achieved by verifying the model predictions by bench measurement methods. The modeled coils will be built and tested, with and without phantom, animal, and human loads. A comprehensive experiment to correlate RF transmit power loss with tissue temperature elevations in live pig heads will be conducted for head and surface coils at very high field proton frequencies. For the third aim, the theoretical predictions and empirical verifications will be tested in targeted applications. Very high field RF coils will be dedicated to each of four applications targeting deep localized structure, whole brain, superficial local, or superficial global cortex. By minimizing RF losses for each application they will demonstrate improved image quality at safe RF power levels in very high field head imaging.
