Abstract
In hyperpolarized xenon magnetic resonance imaging (HP 129Xe MRI), the inhaled spin-1/2 isotope of xenon gas is used to generate the MR signal. Because hyperpolarized xenon is an MR signal source with properties very different from those generated from water-protons, HP 129Xe MRI may yield structural and functional information not detectable by conventional proton-based MRI methods. Here we demonstrate the differential distribution of HP 129Xe in the cerebral cortex of the rat following a pain stimulus evoked in the animal's forepaw. Areas of higher HP 129Xe signal corresponded to those areas previously demonstrated by conventional functional MRI (fMRI) methods as being activated by a forepaw pain stimulus. The percent increase in HP 129Xe signal over baseline was 13–28%, and was detectable with a single set of pre and post stimulus images. Recent innovations in the production of highly polarized 129Xe should make feasible the emergence of HP 129Xe MRI as a viable adjunct method to conventional MRI for the study of brain function and disease.
Highlights
Not inherent to biological tissue, the spin K nucleus of the isotope of xenon (129Xe) is made detectable by magnetic resonance spectroscopy (MRS) and MRI in animals and humans by prior ex-vivo hyperpolarization of 129Xe through spin-exchange optical pumping which increases its magnetization by up to five orders of magnitude [1,2]
Our results show that the HP 129Xe signal was increased in many areas of the brain following a pain stimulus and that these areas coincide with those previously found to be activated using conventional BOLD and perfusion based MRI methods
Increases in HP 129Xe signal were observed in the primary somatosensory cortex and cingulated cortex contralateral to the forepaw injected, consistent with the activation pattern seen using conventional proton functional MRI (fMRI) [19]
Summary
Not inherent to biological tissue, the spin K nucleus of the isotope of xenon (129Xe) is made detectable by magnetic resonance spectroscopy (MRS) and MRI in animals and humans by prior ex-vivo hyperpolarization of 129Xe through spin-exchange optical pumping which increases its magnetization by up to five orders of magnitude [1,2]. The resulting in vivo signal to noise ratio (SNR) of the HP 129Xe signal is not as great as the signal produced by protons in conventional MRI, HP 129Xe has several unique characteristics which may endow it with advantages in some imaging applications [3], including brain imaging [4]. The nuclear magnetic resonance frequency range (chemical shift) of HP 129Xe in vivo is large compared to protons (200 ppm vs 5 ppm respectively) and is substantially affected by the local chemical environment, providing a means to detect localized physiological changes and biochemical binding events [3,4,5]. Because xenon is not intrinsic to biological tissue, HP 129Xe produces virtually no background signal, which, in turn, results in high contrast HP 129Xe MR images [10]. HP 129Xe MRI may be beneficial for imaging patients with brain disease or trauma as evidenced by recent findings showing xenon exerts neuroprotective effects against neurotoxic and ischemic damage [11]
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