RASER MRI: Magnetic Resonance Images formed Spontaneously exploiting Cooperative Nonlinear Interaction

TL;DR

This paper introduces RASER MRI, a novel imaging contrast mechanism that exploits cooperative nonlinear interactions and population inversion to produce spontaneous, RF pulse-free images with unique contrast and artifacts.

Contribution

The paper presents the first demonstration and theoretical explanation of RASER MRI, a new contrast method based on stimulated emission and nonlinear spin interactions.

Findings

RASER MRI produces images without RF pulses.

Contrast depends on initial population inversion density.

Artifacts include amplitude distortions and side lobes.

Abstract

The spatial resolution of magnetic resonance imaging (MRI) is fundamentally limited by the width of Lorentzian point spread functions (PSF) associated with the exponential decay rate of transverse magnetization (1/T2*). Here we show a different contrast mechanism in MRI by establishing RASER (Radio-frequency Amplification by Stimulated Emission of Radiation) in imaged media. RASER imaging bursts emerge out of noise and without applying (Radio Frequency) RF pulses when placing spins with sufficient population inversion in a weak magnetic field gradient. A small difference in initial population inversion density creates a stronger image contrast than conventional MRI. This contrast is based on the cooperative nonlinear interaction between all slices. On the other hand, the cooperative nonlinear interaction gives rise to imaging artifacts, such as amplitude distortions and side lobes outside of the imaging domain. Both the contrast and the artifacts are demonstrated experimentally and predicted by simulations based on a proposed theory. This theory of RASER MRI is strongly connected to many other distinct fields related to synergetics and non-linear dynamics.