# 3D virtual biopsy of in vivo pH and metabolism using PRESS and semi‐LASER MRS of hyperpolarized 13C nuclei

## TL;DR

The paper introduces new MRI techniques to study pH and metabolism in living organisms using hyperpolarized molecules.

## Contribution

The novel contribution is the first use of multi-voxel PRESS and semi-LASER sequences with hyperpolarized 13C molecules for in vivo MRS.

## Key findings

- Multi-voxel PRESS and semi-LASER sequences achieved comparable SNR to 2D FID-CSI for hyperpolarized 13C nuclei.
- Both sequences enabled high-resolution MRS in mouse kidneys and pancreatic tumors.
- Semi-LASER reduced RF saturation and allowed flexible voxel refocusing.

## Abstract

To develop and evaluate sequences for multi‐voxel magnetic resonance spectroscopy using hyperpolarized molecules.

A standard single voxel PRESS sequence was extended to acquire multiple voxels consecutively. Its SNR was compared against a 2D FID‐CSI with both 1H and hyperpolarized 13C nuclei in phantoms and in a healthy mouse at 7T. This sequence was also used to determine tumor pH and metabolic activity in an endogenous murine pancreatic ductal adenocarcinoma model. Furthermore, a semi‐LASER sequence, using adiabatic full passage RF pulses for refocusing, was implemented. Multi‐voxel PRESS and semi‐LASER were then compared in healthy mice for measuring metabolic activity and pH using hyperpolarized [1‐13C]pyruvate and [1,5‐13C2]Z‐OMPD, respectively.

Multi‐voxel PRESS and semi‐LASER detected 13C metabolites in mouse kidneys and endogenous pancreatic ductal adenocarcinoma (PDAC) tumors with SNR comparable to that of standard 2D FID‐CSI. They enable fast MRS with a high spectral resolution that is highly customizable to recover spectra from regions not coverable by a single CSI slice.

For the first time, we show hyperpolarized MRS using multi‐voxel PRESS and semi‐LASER sequences for hyperpolarized 13C‐labeled molecules. By implementing a semi‐LASER sequence using adiabatic full passage refocusing pulses, RF saturation was reduced. Semi‐LASER allows flexible overlapping of voxel refocusing planes, while for PRESS, signal from these regions is attenuated.