MR gradient response modeling to ensure excitation coherence

Gradient system response has a significant effect on the shape and dispersion of complex k-space trajectories, as used in echo-planar magnetic resonance imaging and designed excitation. The authors have developed a method that characterizes the gradient response directly by placing k-space "landmarks" in the raw data. The method produces a clear delineation of the k-space trajectory, while providing information about related factors such as magnetic field homogeneity and temporal coherence of the radio-frequency (RF) and gradient waveforms. By using parameters derived from data collected under varying conditions, gradient response is modeled as a linear system consisting of a response delay function with a frequency-dependent slope. The results allow corrections that can be applied to the RF waveform or to the k-space trajectory. Application of this correction to designed excitation with the sinusoidal k-space trajectory is demonstrated and discussed.