Neural activation in human brain detected with hyperpolarized 129 XE and MRI

Introduction

Hyperpolarized 129Xe (HpXe) ¹ has the potential to be an NMR tracer with a decay time of 10–25 s in blood and in tissue. In this study we apply it to detect neural activation in human.

Method

Chemical shift imaging (CSI) was performed on a 1. 5 T MRI (GE Signa) with a birdcage coil tuned to the xenon resonance. Scans were performed with no slice selection. CSI spectra were obtained in either a 3×3 or 5×5 matrix, corresponding to voxel sizes of 6 cm or 3.5 cm respectively. HpXe gas was produced by a commercial polarizing system (Toyoko-Kagaku Ltd, Tokyo). 500 ml of HpXe gas (polarization ∼8%) was supplied into a 1-L bag and the bag was connected to a manual inhalation unit. The subject inhaled HpXe from the bag and held their breath for 30 sec. The MRI scan started after the subject had indicated that they had inhaled all of the gas. Beginning 1 min before gas inhalation, the subject was instructed to watch a computer-generated pattern projected onto the backside of a translucent screen placed at the foot of the gantry bed. The screen was observed via a mirror attached to the birdcage coil. Measurements were carried out with an 8 Hz-reversing checkerboard (CHK) and a fixed-point pattern used as a control (FIX). Five pairs of measurements (CHK and FIX) were carried out on three normal volunteers.

Results

Figure 1 shows CSI spectra superimposed on a proton T1-weighted image. All voxels revealed spectra of HpXe with multiple peaks. The main and second peaks are at 195 and 192 ppm with respect to the gas peak. We normalized the spectra by the height of main peak (195 ppm) in the frontal cortex voxel and the CHK/FIX ratios at the visual cortex were then calculated. We found a relative increase of 1.37+/−0.17 for the main peak and 1.26+/−0.23 for the second peak. Assuming that the main and second peaks correspond to gray and white matter, respectively, and that the peak height mainly reflects increased delivery of HpXe due to an increase in blood flow, the relative increase in evoked CBF is approximately the same in both gray and white matter.

Figure 1

Discussion

This is the first study applying HpXe to the investigation of neural activation in humans. Fractional increases in CBF were found in both gray and white matter. An improvement in sensitivity to the HpXe signals promises to provide further useful information such as an excess production of lactate.