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

Subject Area Medical Physics, Biomedical Technology
Term from 2021 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 464387898
 
Final Report Year 2025

Final Report Abstract

A key component of MRI is the coherent excitation of nuclear spins by an external radiofrequency (RF) field generated by the MR coil. In particular, the magnetic RF field component, denoted B1⁺, is critical for image generation, as it induces the necessary spin excitation. In ultra-high-field (UHF) MRI - i.e., systems operating at field strengths of 7 Tesla (T) or higher - significant challenges arise due to the spatial inhomogeneity of the B1⁺-field. In the body, this effect can lead to local signal voids at UHF, rendering images diagnostically unusable. To mitigate these effects, the technique of parallel transmission (pTx) has been successfully employed. This approach uses RF transmit coils with multiple independent transmit (Tx) channels and custom-tailored RF pulses that are individually optimized for each transmit element. However, this method requires highly accurate and precise spatial mapping of the B1⁺ field for each Tx channel, which poses substantial challenges at UHF. To achieve high precision and accuracy in B1⁺ mapping with existing techniques, the spins must be flipped by flip angles (FA) of at least 20° into the transverse plane. However, large FAs demand high RF power, which is typically available in head imaging but not in body imaging at 7T. In this work, we demonstrated that B1+ mapping based on 'Magnetic Resonance Fingerprinting' (MRF) can achieve an error of less than 10%, even when using very small FA as low as 6°. Furthermore, the dynamic range of the B1⁺ maps was significantly extended compared to all previously established methods - doubled in many cases and in some cases tripled. It became evident that this approach holds substantial potential for various applications, for example we could show their benefit for correcting quantitative parameters. For the first time, we acquired accurate B1⁺ maps in the body at a magnetic field strength of 10.5 Tesla. We are now routinely using this method to acquire a library of reference maps in the body that will be made available as open-source data. Additionally, the technique is employed for validation of electromagnetic simulations and for testing prototype RF coil elements. Moreover, it is used for B1⁺ correction of imaging data that would otherwise be biased by spatial variations of B1⁺. Overall, the results provide an important contribution to future ultra-high-field (UHF) studies, particularly in the body, and enable a wide range of applications. This work has led to five publications and multiple conference contributions. In addition, one manuscript is presently under review and the method is currently being applied in three further publications that are in preparation.

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