CONTRAST-ENHANCED PERFUSION-SENSITIVE MR IMAGING IN THE DIAGNOSIS OF CEREBROVASCULAR DISORDERS

''CEREBRAL PERFUSION'' is a functional concept that describes the rate of delivery of essential metabolic substrates from blood to neurons and glia. In vivo assessment of tissue perfusion is important in the diagnosis and treatment of brain and spinal cord injury because of the demonstrated close relationship among cerebral blood flow, metabolism, physiologic function, and pathologic disturbance (1). Over the past 20 years, a number of noninvasive techniques have been developed to map cerebral blood flow as a first approximation to measuring cerebral perfusion. Dynamic computed tomographic (CT) scanning is widely available for evaluating blood flow within the entire brain or a region of the brain; however, this technique primarily provides a means of assessing the blood pool. Functional perfusion images can be generated with single photon emission CT imaging by using specially designed radiopharmaceuticals with a high rate of first-pass extraction. positron emission tomographic (PET) imaging with diffusible tracers, and CT imaging with both diffusible and intravascular agents. In each case, however, the imaging method requires exposure to ionizing radiation and can be imprecise because of poor temporal and spatial resolution. A major problem in measuring flow with magnetic resonance (MR) angiography is that capillaries are orders of magnitude smaller than the volume elements in MR imaging, a feature that leads inescapably to an overall decrease in sensitivity and spatial resolution. A much higher sensitivity to capillary flow and, hence. tissue perfusion can be obtained by using MR imaging in combination with magnetic susceptibility contrast enhancement (2). Intravascular compartmentalization of a contrast agent with a high magnetic moment. such as a lanthanide chelate of dysprosium or gadolinium, induces a field gradient between the capillary space and the surrounding parenchyma. A transient reduction in signal intensity on T2*-weighted images is observed in normally perfused brain parenchyma, thereby delineating the boundaries of hypoperfused tissues. Precise quantification of cerebral blood flow and perfusion is nevertheless complicated by the fact that an ideal bolus of contrast agent is difficult to achieve and the measured time dependence of signal intensity changes is a complex paradigm of organ transit function superimposed on the arterial input function (3). An increasing number of clinical and basic research studies, however. now utilize tissue and blood concentration-time data to evaluate regional blood flow. blood volume, and tissue perfusion status. Calculations of the signal intensity changes over time for each voxel can be used to produce high-resolution images of regional cerebral blood volume (Fig 1). If an arterial input function is added, deconvolution analysis can then be used to find the true brain clearance, the mean transit time through the capillary network, and ultimately the regional cerebral blood flow (2). The feasibility of this approach in human subjects is currently being studied with Dy-DTPA-BMA. The remainder of this article will be a review of some of the current and projected future clinical applications of contrast agent-enhanced perfusion-sensitive MR imaging in the diagnosis of cerebrovascular disorders.