title: Three-dimensional mapping of the B1 field using an optimized phase-based method: application to hyperpolarized 3He in lungs.
abstract: A novel method is presented for the three-dimensional mapping of the B(1) -field of a transmit radio-frequency MR coil. The method is based on the acquisition of phase images, where the effective flip angle is encoded in the phase of the nonselective hard pulse excitation. The method involves the application of a rectangular composite pulse as excitation in a three-dimensional gradient recall echo to produce measurable phase angle variation. However, such a pulse may significantly increase the radio-frequency power deposition in excess of the standard acceptable SAR limits, imposing extremely long TRs (>100 msec), which would result in acquisition times significantly greater than a single breath-hold. In this study, the phases of the radio-frequency excitation are modified, resulting in a different pulse sequence scheme. It is shown that the new method increases sensitivity with respect to radio-frequency inhomogeneities by up to 10 times, and reduces the total duration of the pulse so that three-dimensional B(1) mapping is possible with (3) He in lungs within a single breath-hold. Computer simulations demonstrate the increase in sensitivity. Phantom results with (1) H MRI are used for validation. In vivo results are presented with hyperpolarized (3) He in human lungs at 1.5T.
authors.affiliation: Section of Medical Physics, Department of Diagnostic and Interventional Radiology, Johannes Gutenberg University Medical Centre, Mainz, Germany. santoro@uni-mainz.de
authors.forename: Davide
authors.initials: D
authors.lastname: Santoro
authors.forename: Julien
authors.initials: J
authors.lastname: Rivoire
authors.forename: Florian
authors.initials: F
authors.lastname: Meise
authors.forename: Maxim
authors.initials: M
authors.lastname: Terekhov
authors.forename: Zahir
authors.initials: Z
authors.lastname: Salhi
authors.forename: Klaus
authors.initials: K
authors.lastname: Gast
authors.forename: Laura M
authors.initials: LM
authors.lastname: Schreiber
mesh_terms: D000465:Algorithms; D003287:Contrast Media; D006371:Helium; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D007554:Isotopes; D008168:Lung / Q000033:anatomy & histology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019275:Radiopharmaceuticals; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D003287:Contrast Media; D007554:Isotopes; D019275:Radiopharmaceuticals; D006371:Helium