Magnetization recovery for signal enhancement: a fast imaging DEFT-based technique

Abstract

This paper describes the development and application of a new fast MRI technique based on the DEFT principle. The sequence named MAgnetization RecoverY for Signal Enhancement (MARYSE) is composed of two completely symmetric gradient echoes separated by a 180° refocusing pulse. The RF pulse scheme, 90° x –180° y –90° − x enables restoration of the transverse magnetization along the longitudinal axis, and consequently artificially increases R 1 relaxation rate. In this sequence, the period between the excitation pulse and the restoring pulse (Tem: transverse magnetization evolution time) is very short (<10 ms). This makes possible a significant increase in signal-to-noise ratio, even with a relatively short repetition time (20 ms). Simulations were performed for different values of Tem and TR at definite T 1 and T 2 and for different values of T 1 and T 2 at constant Tem and TR. Relevant signal enhancement for species with long relaxation time constants as compared to classical gradient echo and fast spin-echo imaging was expected. In vitro studies on a fat/water phantom confirmed this simulation. Application of MARYSE to mouse brain imaging permitted to visualize almost completely cerebrospinal fluid of the ventricles, a signal usually partially saturated in fast gradient echo imaging.