9 Yet it provides only local measurements and is sufficiently invasive to discourage widespread use. 8 The development in the late 1980s of the polarographic needle electrode system has allowed accurate measurements of pO 2 in vivo. Unfortunately, no in vivo method currently fulfills all these requirements. Important criteria for a useful oxygenation imaging method include noninvasiveness, adequate spatial and temporal resolution, ability to quantify oxygenation levels, low radiation exposure, good safety profile, and widespread clinical availability. 4 In addition, many other disorders of the brain, including Alzheimer disease, Parkinson disease, Huntington disease, and multisystem atrophy, also appear to be associated with alterations in cerebral oxygen metabolism. 1, 2 In acute stroke, a parameter that could reflect the metabolic state of the ischemic brain may improve the ability to identify tissue at risk of infarction, 3 to select patients for reperfusion therapies, or to avoid thrombolytic therapy in futile situations. In tumors, it has long been recognized that the lack of oxygen in tissues (hypoxia) influences the response to therapies, including aggressiveness, local recurrence, and metastasis, as well as overall prognosis. The development of a reliable brain oxygenation mapping technique would not only enable a better understanding of normal physiology during rest, sleep, or functional brain tasks but also be of great help in managing pathophysiologic conditions in which oxygen supply is disturbed. The aim of this review is to give an overview of the theoretic basis of these methods as well as their application to measure oxygenation in both healthy subjects and those with disease.ĪBBREVIATIONS: BOLD blood oxygen level–dependent CMRO 2 cerebral metabolic rate of oxygen consumption OEF oxygen-extraction fraction pO 2 partial pressure of oxygen qBOLD quantitative BOLD QUIXOTIC QUantitative Imaging of eXtraction of Oxygen and TIssue Consumption SaO 2 arterial oxygen saturation SO 2 blood oxygen saturation SvO 2 venous oxygen saturation TRUST T2-relaxation under spin-tagging These include qBOLD, phase- and susceptibility-based imaging, and intravascular T2-based approaches. While the complex relationship between the MR imaging signal and tissue oxygenation hinders a direct approach, many different methods have been developed during the past decade to obtain specific oxygenation measurements. However, the high spatial and temporal resolution afforded by BOLD imaging does not need to be limited to the study of healthy brains. The unique sensitivity of the BOLD effect to the presence of deoxyhemoglobin has led to its widespread use in the field of cognitive neurosciences. Furthermore, oxygenation changes following a stress challenge, such as with carbogen gas or acetazolamide, can yield information about cerebrovascular reactivity. SUMMARY: In many pathophysiologic situations, including brain neoplasms, neurodegenerative disease, and chronic and acute ischemia, an imbalance exists between oxygen tissue consumption and delivery.
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