# Rapid RASER MRI

## TL;DR

This paper introduces a new MRI technique called RASER that eliminates the need for high-power radio waves, enabling faster and safer imaging.

## Contribution

The novel RASER MRI method enables rapid imaging without RF excitation, using hyperpolarization for self-excitation of nuclear spins.

## Key findings

- RASER MRI images of pyrazine were acquired in 78 ms with a 128x128 matrix at 500 MHz.
- Dynamic tracking was demonstrated using hyperpolarized pyrazine in a time-series of images.
- The method eliminates the need for transmit-receive electronics, suitable for portable and hyperpolarized MRI.

## Abstract

Conventional Magnetic Resonance Imaging (MRI) relies on high‐power Radio‐Frequency (RF) pulses to excite nuclear spins and in turn generate NMR signals. These pulses require large high‐power RF‐amplifiers and cause heat deposition in the tissue, which must be minimized for safety, presenting a growing problem when moving toward ever‐higher field MRI. An alternative to RF‐pulse excitation is self‐excitation of nuclear spins using Radiofrequency Amplification by Stimulated Emission of Radiation (RASER), where the nuclear spins undergo spontaneous transition, without RF excitation, from an over‐populated state to a ground state. Here, the feasibility of recording rapid proton RASER MRI images of pyrazine at low concentration (120 mM) with large matrix (128x128 pixels) in as little as 78 ms is demonstrated at 500 MHz (11.7 T). We also recorded a time‐series of images using a single bolus hyperpolarized pyrazine highlighting the feasibility of dynamic tracking. The demonstrated approach allows recording MRI scans without transmit‐receive electronics of the MRI scanner, which is highly desirable for portable MRI as well as the emerging field of hyperpolarized MRI using, e.g., HP protons, 129Xe gas or HP 13C labeled biomolecules as molecular tracers and imaging agents.

Rapid RASER MRI allows for image acquisition without RF excitation. Instead, a population inversion on the nuclear spins is generated by SABRE hyperpolarization, which allows for self‐excitation through the RASER approach. When the desired transverse magnetization is built‐up, conventional EPI encoding is triggered to record the image.