A New Pulse Sequence for "Fast Recovery" Fast-Scan NMR Imaging

Abstract

This paper describes the "fast recovery" (FR) method for fast NMR imaging. The FR method combines a sequence of four RF pulses-alternating selective 90° nutation pulses and nonselective 180° pulses-with a gradient field pulse sequence which includes "spoiler" pulses to destroy the coherence between successive sequence cycles. We use the 2-D backprojection method of image reconstruction, but other imaging methods could be applied. The paper analyzes the behavior of the macroscopic magnetization-compares the FR method with other methods and proposes "figure of merit" expressions for relative signal-to-noise (S/N) ratios, scan time reduction ratios, and image contrast-and presents experimental results, including backprojection image reconstruction 2-D images and computed T1 and T2 images. For the FR method, in theory and practice, we find that, after each scan sequence cycle, magnetization is restored to equilibrium quickly and exactly; scan time can consequently be less than a tenth that for the saturation recovery method without any penalty in signal-to-noise ratio. Image contrast is even higher than that of the SR method, and compromise "optimum" sequence (interpulse timing) parameters give high image contrast for a wide range of tissue T1 and T2 (spin-lattice and spin-spin relaxation time) values.