title: Extended RF shimming: Sequence-level parallel transmission optimization applied to steady-state free precession MRI of the heart.
abstract: Cardiac magnetic resonance imaging (MRI) at high field presents challenges because of the high specific absorption rate and significant transmit field (B1+) inhomogeneities. Parallel transmission MRI offers the ability to correct for both issues at the level of individual radiofrequency (RF) pulses, but must operate within strict hardware and safety constraints. The constraints are themselves affected by sequence parameters, such as the RF pulse duration and TR, meaning that an overall optimal operating point exists for a given sequence. This work seeks to obtain optimal performance by performing a 'sequence-level' optimization in which pulse sequence parameters are included as part of an RF shimming calculation. The method is applied to balanced steady-state free precession cardiac MRI with the objective of minimizing TR, hence reducing the imaging duration. Results are demonstrated using an eight-channel parallel transmit system operating at 3 T, with an in vivo study carried out on seven male subjects of varying body mass index (BMI). Compared with single-channel operation, a mean-squared-error shimming approach leads to reduced imaging durations of 32 ± 3% with simultaneous improvement in flip angle homogeneity of 32 ± 8% within the myocardium.
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, UK.
authors.forename: Arian
authors.identifier: http://orcid.org/0000-0002-9169-1788
authors.initials: A
authors.lastname: Beqiri
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, UK.|Centre for the Developing Brain, King's College London, London, UK.
authors.forename: Anthony N
authors.initials: AN
authors.lastname: Price
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, UK.
authors.forename: Francesco
authors.initials: F
authors.lastname: Padormo
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, UK.|Centre for the Developing Brain, King's College London, London, UK.
authors.forename: Joseph V
authors.initials: JV
authors.lastname: Hajnal
authors.affiliation: Division of Imaging Sciences and Biomedical Engineering, King's College London, London, UK.
authors.forename: Shaihan J
authors.initials: SJ
authors.lastname: Malik
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D055590:Electromagnetic Phenomena; D006321:Heart / Q000033:anatomy & histology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D011846:Radio Waves; D013997:Time Factors
keywords: RF shimming; SAR; cardiac; parallel transmission MRI