Contrast agents in functional MR imaging.

Contrast agents have greatly expanded the role of MR imaging (MRI) to allow assessment of physiologic, or "functional," parameters. Although activation mapping generally does not require contrast agents, other forms of functional MRI, including mapping of cerebral hemodynamics (eg, perfusion imaging), are best done with the use of contrast agents. Serial echo planar images are obtained after bolus injection of lanthanide chelates. Application of susceptibility contrast physics and standard tracer kinetic principles permits generation of relative cerebral blood volume maps. Deconvolution of cerebral blood flow and mean transit time parameters is also possible within technical limitations. By using diffusion and perfusion pulse sequences, an imaging correlate to the ischemic penumbra can be identified. Functional MRI perfusion imaging of intraaxial tumors is analogous to positron emission tomography for delineation of metabolic activity, yet may be even more sensitive to neovascularity and possesses improved image quality. Clinical applications include biopsy site selection and postirradiation follow-up. Further improvements in data analysis and map generation techniques may improve diagnostic accuracy and utility.