title: Mitigating transmit B 1 inhomogeneity in the liver at 7T using multi-spoke parallel transmit RF pulse design.
abstract: In this work, the use of multi-spoke slice-selective parallel transmit (pTX) RF pulse was explored to address B 1+ inhomogeneity in the largest transverse section of the liver at 7T. The impact of the number of spokes was specifically investigated, considering RF pulses consisting of 2, 3 and 4 spokes, as well as single-spoke RF pulses corresponding to static B 1 shimming. Healthy volunteers were imaged on a whole body MR scanner equipped with an eight-channel transmit system. A robust and fast transmit B 1 (B 1+) estimation method was employed to obtain the eight-channel B 1+ maps within a single breath hold. Gradient echo (GRE) images of the liver were acquired using the four different RF pulses and the results were compared. The use of static B 1 shimming (i.e., 1-spoke RF pulse) resulted in partial improvement but significant signal dropouts were still observed in the target region. By comparison, the use of multi-spoke pTX RF pulse design gave rise to much improved excitation homogeneity without signal dropouts. These results demonstrate the effectiveness of multi-spoke pTX RF pulse design in B 1+ homogenization for liver magnetic resonance imaging (MRI) at 7T. The current findings at 7T may have implications for body imaging applications in clinical settings at 3T where B 1+ inhomogeneities are also known for degrading image quality in the torso.
authors.affiliation: Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Wu
authors.affiliation: Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Schmitter
authors.affiliation: Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Auerbach
authors.affiliation: Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
authors.forename: Kâmil
authors.initials: K
authors.lastname: Uğurbil
authors.affiliation: Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota Medical School, Minneapolis, Minnesota, USA.
authors.forename: Pierre-François
authors.initials: PF
authors.lastname: Van de Moortele
keywords: Parallel transmit (pTX); RF pulse design; transmit B1 homogenization; ultrahigh field magnetic resonance imaging (MRI)