6 MR perfusion imaging of the brain

Measurements of cerebral perfusion have become an important part of diagnostic imaging and therapy monitoring in a variety of brain diseases. Magnetic resonance perfusion imaging provides high-resolution images of various perfusion parameters in addition to the important morphological information acquired during the same imaging session. In conjunction with magnetic resonance imaging (MRI) diffusion-weighted imaging MR perfusion imaging has developed to a mainstay in early stroke imaging. The delineation of the so-called “tissue at risk’’ of ischemic damage characterized as a mismatch between alterations in diffusion-weighted and perfusion images is a key information if thrombolysis is a treatment option. Current ongoing clinical trials will show whether this information may allow to further extent the time window for thrombolytic therapy in stroke. MR perfusion imaging allows the investigation of the well-established relationship between cerebral activity, metabolism, and regional hemodynamics. Under a wide range of physiological conditions the cerebral blood flow (CBF) is maintained at a constant level to provide a sufficient oxygen and glucose supply to the brain. This autoregulatory control is affected by the small cerebral arterioles, which are able to reduce the vascular resistance by widening of arterial sphincters and consecutive dilatation of small veins to maintain normal cerebral flow even under conditions where the systemic arterial blood pressure is as low as 70 mm Hg. A further drop of the perfusion pressure results in a CBF decrease, which is partly compensated with a higher oxygen extraction fraction and a reduction of cellular metabolism and energy demand. Irreversible tissue damage occurs if CBF drops below 10–12 mL/100 g of brain tissue per minute. The complexity of these CBF patterns and autoregulatory control in various cerebrovascular diseases require the assessment of different parameters of cerebral perfusion, including the CBF, the cerebral blood volume, and the mean transit time (MTT). Considering these various parameters of cerebral perfusion, different stages of impaired brain perfusion can be identified. The dilatation of small arteries, as the initial reaction to compensate for a reduced perfusion pressure, would increase the cerebral blood volume and to a certain extent the MTT while maintaining CBF at a constant level. This pattern is known as stage-one cerebral ischemia. With a decreasing perfusion pressure, the cerebral blood volume and the MTT will further increase; however, the CBF can no longer be kept at a constant level and will progressively decrease. This latter perfusion pattern is described as stage-two cerebral ischemia. If CBF falls beneath the critical level (stage three), irreversible damage to brain parenchyma will occur. Positron emission tomography (PET) and 133-xenon computed tomography (CT) can visualize and absolutely quantify these changes. However, both methods remain largely impractical for routine and emergency clinical use. Single photon emission tomography based on the detection of the distribution of 99mtechnetium labeled hexamethylpropyleneamine oxime is a widely used method, although provided data are only relative indicators, and model-based calculation of CBF, cerebral blood volume, and MTT is imprecise. The potential of MRI to delineate the different perfusion parameters has been extensively evaluated in the recent years and the advantages of a multimodal MRI approach to various cerebrovascular disorders and cerebral tumors have been underlined. In general, MRI offers two generic approaches to determine cerebral perfusion parameters. First, the blood flowing into an imaging slice can be marked either by magnetically tagging the selected slice or by the blood flowing into it. These techniques of arterial spin labeling are time-consuming, and measurable signal changes in states of reduced flow are usually small. However, if time is not a constraint, these techniques are promising especially if hemodynamic responses to various stimuli are studied during the same MRI study or breakdown of the blood–brain barrier compromises contrast-agent-based methods.