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Hyperpolarized Xenon MR Imaging of the Brain

Exciting prospects for developing hyperpolarized xenon magnetic resonance imaging (HP 129Xe MRI) for the clinical assessment of brain anatomy and function are now on the horizon. The ultimate goal is to develop a new method that is practical for clinical use that can be used in conjunction with proton MRI to diagnose and evaluate treatment of brain diseases, and to ascertain information about normal human brain anatomy and function. Several characteristics make HP 129Xe MRI exquisitely well suited for studies of brain anatomy and function.

HP Xenon acts as a biochemical sensor. Different chemical environments affect the resonance frequency (chemical shift) of the HP 129Xe spin, resulting in separated spectral peaks detectable by MR spectroscopy. Because HP 129Xe presents one of the largest NMR chemical shift ranges of all nuclei that demonstrate magnetic resonance, it is extremely sensitive to its chemical environment, which may allow differentiation of diseased and normal tissue. Such shifts in resonance frequency can be exploited in MR chemical shift imaging (CSI) to produce spatial images of the different frequencies. Thus, CSI allows an anatomical map to be made of the separate spectral components, and thus the distinct brain tissue that they originate from.

HP Xenon can detect oxygen concentration. Both the relaxation time (T1 ) and chemical shift of HP 129Xe are highly sensitive to the oxygen saturation of blood, making HP129Xe MRI potentially useful for studies of functional brain activity and for determining areas of hypoxia following brain injury or stroke. In addition, HP129Xe 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.

HP Xenon can yield absolute, quantitative measures of cerebral blood flow. Because xenon is considered an ideal perfusion tracer, HP129Xe MRI is ideally suited for studies of cerebral blood flow. The element xenon has proven its usefulness as a perfusion tracer; xenon enhanced computed tomography (CT) and single positron emission CT (SPECT) using 133Xe have long been used to measure brain blood flow and perfusion. These methods have set the standard for measures of cerebral blood flow because they yield both an absolute and quantitative measurement of perfusion.

HP Xenon brain images are not degraded by background signal. HP 129Xe gas is not inherent in biological tissue, and so produces no background signal, which in turn results in high contrast HP 129Xe MR images of the brain.


 

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