MRF based B1+ mapping for 7T Magnetic Resonance Electrical Properties Tomography and RF pulse design

While the first clinical magnetic resonance imaging (MRI) scanners introduced in the early 1980s still operated at a magnetic field strength of 0.2-0.5 Tesla (T), today's clinical MRI scanners typically operate at a field strength of 1.5 and 3 T. In addition, so-called ultra high field (UHF) MRI devices operating at 7 T and more are increasingly being investigated for clinical use and scientific purposes. The advantage of the high field strengths is especially the higher signal-to-noise ratio (SNR), which allows a higher spatial and temporal resolution of the images and which can accelerate the image acquisition process. However, a major challenge of UHF MRI is the spatially inhomogeneous contrast and the inhomogeneous signal of the image. These are caused by the spatially inhomogeneous distribution of the radio frequency (RF) fields, especially their magnetic component (B1+), which is responsible for the excitation of the nuclear spins. Although this inhomogeneous distribution can be compensated by using several transmitters (the "RF coils") and special RF pulses, this approach requires a very precise mapping of the spatial three-dimensional B1+ fields of the individual transmitters. Besides the possibility to compensate this inhomogeneity, B1+ mapping also offers high potential to obtain information about the electrical properties of the human tissue, i.e. conductivity (as a measure of the possibility of conducting electric currents) and permittivity (as a measure of the electric polarizability). These are of interest because they vary greatly as a function of tissue type and cell composition, and thus tumor tissue can possibly be detected and differentiated. Despite its potential, however, 'electrical properties tomography' (EPT) has not been widely used so far. Among the reasons for this fact is the demand for high SNR in the B1+ field maps, and also the lack of large patient studies with sufficient statistical significance.The aim of this proposal is to develop an accurate and precise method for mapping the B1+ fields in the human head and the body that will benefit from the high SNR available at 7 T. For this purpose, the principle of "Magnetic Resonance Fingerprinting" (MRF) will be used, which is becoming increasingly important in quantitative MRI as it promises high encoding efficiency. The obtained maps will be compared to an existing but biased method and the impact of the novel technique i) on the derived electric properties values and ii) on the achieved spatial homogeneity in UHF body imaging will be analyzed. In the long term, the developed technique enables the investigation of novel methods for UHF body imaging and serves for future EPT studies with tumor patients.