Extended RF shimming: Sequence‐level parallel transmission optimization applied to steady‐state free precession MRI of the heart. (14th February 2017)

• Record Type: Journal Article
• Authors: Beqiri, Arian; Price, Anthony N.; Padormo, Francesco; Hajnal, Joseph V.; Malik, Shaihan J.
• Is Part Of: NMR in biomedicine. Volume 30:Number 6(2017:Jun.)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2017-02-14
• Subjects: cardiac -- parallel transmission MRI -- RF shimming -- SAR; Nuclear magnetic resonance -- Periodicals; Magnetic Resonance Spectroscopy -- Periodicals; 574
• DOI: 10.1002/nbm.3701
• Languages: English
• ISSNs: 0952-3480

Abstract

Abstract : Cardiac magnetic resonance imaging (MRI) at high field presents challenges because of the high specific absorption rate and significant transmit field ( B 1 + ) 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. Abstract : We present a 'sequence‐level' RF shimming optimization for minimization of TR for balanced SSFP 3 T cardiac MRI using an 8‐channel body‐coil. We optimize RF shims alongside sequence variables while respecting hardware and safety constraints. Two optimizations were compared to quadrature mode operation; one seeking to achieve the correct flip angle ('minimum bias') and one seeking to improve homogeneity ('mean‐square‐error'). The latter produced 32 ± 3% reduction in imaging duration for 7 male subjects with 32 ± 8% improvement in flip angle homogeneity.