Ultra High Magnetic Field (UHF) MR scanners (7 Tesla and higher) have strong potential to unveil anatomical, functional and chemical information that is not accessible at lower field strengths and could allow for earlier, non invasive diagnoses. Important obstacles, however, must be solved to obtain high quality data at UHF. Arguably, the most challenging problems relate to Radio Frequency (RF) propagation in large biological samples at UHF. First, the reduced RF penetration leads to increased tissue heating, which limits the types of scans that can be performed within safety limits. Second, transmit B1 fields are severely distorted and inhomogeneous, corrupting images and spectral acquisitions. It has been shown that RF issues can be addressed with new technologies such as B1 Shim and Parallel Excitation, that both require multiple transmit RF coils. This, in turn, requires magnitude B1 mapping for each RF coil, with two main subsequent issues: long scan times and high RF peak power. Indeed, conventional B1 mapping cannot be used with numerous coils as it typically requires several minutes of scan time per coil. Furthermore, conventional approaches necessitate large flip angles, which is not generally feasible because generating a large flip angle over a large region (head or torso) from a single transmit coil element would require levels of RF power beyond hardware and/or patient safety limits. Recent approaches allow for faster B1 mapping but are still problematic at high field with multiple channels. In this proposal we introduce novel techniques and algorithms aiming at mapping transmit B1 profiles for multiple transmit coils in a short time and with limited RF peak power. The first mapping approach relies on merging information from small flip angle (linear regime) and large flip angle (sinusoidal regime) data. A new category of B1 shim algorithms will be optimized for minimizing the needed amount of large flip angle data. Assuming that in many clinical applications approximate B1 estimation will provide sufficient improvement in image quality, an additional technique is proposed, allowing for even faster B1 approximation in low flip angle regime. This technique relies on a new formalism utilizing transmit and receive B1 information in transceiver arrays at UHF. All experiments in this proposal will be conducted on human scanners operating at 7T and 9.4T equipped with multi transmit capability. It is expected that those novel B1 mapping methods will greatly facilitate clinical use of B1 shim and Parallel Excitation at UHF.
In this proposal we introduce new methods to calibrate the transmit profile of multiple radiofrequency coils in human scanners operating at 7Tesla and 9.4Tesla. Those new methods are expected to be significantly faster and to require less RF peak power than conventional approaches. This should greatly facilitate the development of clinical applications on ultra high magnetic field MR scanners.
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