The feasibility of imaging human brain tissue-perfusion using inhaled hyperpolarized 129Xe magnetic resonance imaging is demonstrated. Enhancement of 129Xe gas polarization and a custom brain RF coil array have together enabled imaging of hyperpolarized 129Xe dissolved in the human brain at 1.5 T with a quality and signal-to-noise hitherto unseen. The images clearly demonstrate the uptake and washout of 129Xe in the brain with time and could provide novel insights into cerebral perfusion and blood brain barrier permeability without the use of intravenous contrast.
The time resolved imaging demonstrates the regional uptake of xenon by the brain tissue and the potential to quantify the washout of xenon from the brain tissues. For the assessment of brain tissue perfusion, the technique has a number of potential advantages over existing clinical methods because by imaging HP 129Xe dissolved in the extravascular tissue compartment, we are directly imaging the perfused tissue rather than imaging the intravascular compartment, e.g. using exogenous iodine or gadolinium-based contrast agents4.
Although previous studies have been conducted to image HP 129Xe dissolved in rat brain tissue, these studies were conducted in the anaesthetized state5,6. It has been observed earlier that cerebral blood flow does not change in rats under a stable-anaesthetized state7, unlike in humans8. Thus, it is neither possible to translate the findings of these earlier studies in rats5,6 into the context of human clinical studies, nor is it practical to perform human studies using the same protocols. In that respect, this study offers many practical advantages as it does not require the subject to be either anaesthetized or to be on respiratory apparatus.
Nevertheless, image resolution and SNR in the present study is limited by the quantity of HP 129Xe that is transported and delivered to the brain tissue and the polarization of 129Xe as it decays through longitudinal relaxation on its journey from the lungs to the brain by the processes of gas phase relaxation in the lungs and dissolved phase relaxation in blood and tissue.
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