title: Characterization of phase-based methods used for transmission field uniformity mapping: a magnetic resonance study at 3.0 T and 7.0 T.
abstract: Knowledge of the transmission field (B1(+)) of radio-frequency coils is crucial for high field (B0  = 3.0 T) and ultrahigh field (B0 ≥7.0 T) magnetic resonance applications to overcome constraints dictated by electrodynamics in the short wavelength regime with the ultimate goal to improve the image quality. For this purpose B1(+) mapping methods are used, which are commonly magnitude-based. In this study an analysis of five phase-based methods for three-dimensional mapping of the B1(+) field is presented. The five methods are implemented in a 3D gradient-echo technique. Each method makes use of different RF-pulses (composite or off-resonance pulses) to encode the effective intensity of the B1(+) field into the phase of the magnetization. The different RF-pulses result in different trajectories of the magnetization, different use of the transverse magnetization and different sensitivities to B1(+) inhomogeneities and frequency offsets, as demonstrated by numerical simulations. The characterization of the five methods also includes phantom experiments and in vivo studies of the human brain at 3.0 T and at 7.0 T. It is shown how the characteristics of each method affect the quality of the B1(+) maps. Implications for in vivo B1(+) mapping at 3.0 T and 7.0 T are discussed.
authors.affiliation: Berlin Ultrahigh Field Facility (BUFF), Max Delbrück Center for Molecular Medicine (MDC), Berlin, Germany.
authors.forename: Flavio
authors.initials: F
authors.lastname: Carinci
authors.forename: Davide
authors.initials: D
authors.lastname: Santoro
authors.forename: Federico
authors.initials: F
authors.lastname: von Samson-Himmelstjerna
authors.forename: Tomasz Dawid
authors.initials: TD
authors.lastname: Lindel
authors.forename: Matthias Alexander
authors.initials: MA
authors.lastname: Dieringer
authors.forename: Thoralf
authors.initials: T
authors.lastname: Niendorf
mesh_terms: D000465:Algorithms; D001921:Brain / Q000473:pathology; D001931:Brain Mapping / Q000379:methods; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D021621:Imaging, Three-Dimensional / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods*; D008962:Models, Theoretical; D019047:Phantoms, Imaging