9.6 Conclusions: Perfusion MRI is a valuable and flexible clinical tool enabling the assessment of regional cerebral haemodynamics using a variety of techniques and playing a relevant role in treatment strategies (e.g. in deciding which patient should undergo thrombolysis). The most frequently applied technique uses rapid T2- or T2-weighted EPI sequences to monitor the first pass of a bolus of gadolinium chelate to calculate semi-quantitative maps of relative blood flow, blood volume and transit time. The arterial spin labelling technique uses magnetically tagged blood as an endogenous tracer, allowing absolute CBF measurement using the same model as PET. Both techniques benefit from high-field imaging. Despite disadvantages related to increased susceptibility to field inhomogeneities, use of high-field imaging, especially in combination with parallel imaging, affords higher SNR, greater sensitivity to the signal drop produced by exogenous tracers (thus permitting use of a smaller dose of contrast agent) and better performances of ASL methods, including greater sensitivity to labelled blood and lower sensitivity to uncertainties in arterial arrival time. Finally, research is in progress to study and develop new endogenous tracers, such as H2 15O and hyperpolarized nucleus-based imaging.
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