Imaging gas-exchange lung function and brain tissue uptake of hyperpolarized 129 Xe using sampling density-weighted MRSI.

Abstract

Imaging of the different resonances of hyperpolarized 129 Xe in the brain and lungs was performed using a 3D sampling density-weighted MRSI technique in healthy volunteers. Four volunteers underwent dissolved-phase hyperpolarized 129 Xe imaging in the lung with the MRSI technique, which was designed to improve the point-spread function while preserving SNR (1799 phase-encoding steps, 14-s breath hold, 2.1-cm isotropic resolution). A frequency-tailored RF excitation pulse was implemented to reliably excite both the 129 Xe gas and dissolved phase (tissue/blood signal) with 0.1° and 10° flip angles, respectively. Images of xenon gas in the lung airspaces and xenon dissolved in lung tissue/blood were used to generate quantitative signal ratio maps. The method was also optimized and used for imaging dissolved resonances of 129 Xe in the brain in 2 additional volunteers. High-quality regional spectra of hyperpolarized 129 Xe were achieved in both the lung and the brain. Ratio maps of the different xenon resonances were obtained in the lung with sufficient SNR (> 10) at both 1.5 T and 3T, making a triple Lorentzian fit possible and enabling the measurement of relaxation times and xenon frequency shifts on a voxel-wise basis. The imaging technique was successfully adapted for brain imaging, resulting in the first demonstration of 3D xenon brain images with a 2-cm isotropic resolution. Density-weighted MRSI is an SNR and encoding-efficient way to image 129 Xe resonances in the lung and the brain, providing a valuable tool to quantify regional spectroscopic information.