Improved gradient‐echo 3D magnetic resonance imaging using pseudo‐echoes created by frequency‐swept pulses

Abstract

Frequency-swept pulses are not typically employed to excite spins in NMR. When used for selective excitation in MRI, such pulses do not produce a proper echo because the phase of the transverse magnetization varies in a quadratic manner across the slice or slab. Previously, frequency-swept pulses such as the chirp pulse have been shown to offer an approach to reduce the peak radiofrequency power required for excitation. It has also been shown that chirp excitation produces a unique type of echo (dubbed "pseudo-echo" here) and images can be generated from the resultant pseudo-echoes using a quadratic reconstruction method (J.G. Pipe, Magn Reson Med 1995;33:24-33). The present work describes a general theory and methods for exciting spins with other types of frequency-swept pulses (HSn pulses), which offer the advantage of delivering better excitation profiles than the chirp pulse. Here, pseudo-echoes are produced with HSn pulses in conventional gradient-echo 3D MRI, and high-quality images are reconstructed using standard fast Fourier transformation. An optional apodization procedure using a sliding window function is also introduced. When the dynamic range of the analog-to-digital converter is limiting, signal-to-noise ratio of pseudo-echo imaging is superior to that obtained with standard excitations.