Simultaneous B0‐ and B1+‐Map acquisition for fast localized shim, frequency, and RF power determination in the heart at 3 T

Abstract

High‐field (≥3 T) cardiac MRI is challenged by inhomogeneities of both the static magnetic field (B₀) and the transmit radiofrequency field (B₁+). The inhomogeneous B fields not only demand improved shimming methods but also impede the correct determination of the zero‐order terms, i.e., the local resonance frequency f₀ and the radiofrequency power to generate the intended local B₁+ field. In this work, dual echo time B₀‐map and dual flip angle B₁+‐map acquisition methods are combined to acquire multislice B₀‐ and B₁+‐maps simultaneously covering the entire heart in a single breath hold of 18 heartbeats. A previously proposed excitation pulse shape dependent slice profile correction is tested and applied to reduce systematic errors of the multislice B₁+‐map. Localized higher‐order shim correction values including the zero‐order terms for frequency f₀ and radiofrequency power can be determined based on the acquired B₀‐ and B₁+‐maps. This method has been tested in 7 healthy adult human subjects at 3 T and improved the B₀ field homogeneity (standard deviation) from 60 Hz to 35 Hz and the average B₁+ field from 77% to 100% of the desired B₁+ field when compared to more commonly used preparation methods. Magn Reson Med, 2010.