Introduction Contrast media are widely used in multidetector- row computed tomography (CT) to improve visu- alisation of the vascular system and renal tract and to increase lesion-to-tissue contrast. Nevertheless, for patients within the first 6 h of acute stroke, the diagnostic and prognostic ability of conventional CT remains poor. Similarly, despite conventional contrast-enhanced techniques, mass lesions on CT may remain hard to characterize as benign or ma- lignant, both at diagnosis and following cancer therapy. Furthermore, visual assessment of tumor enhancement rarely provides useful prognostic in- formation beyond conventional staging. This pa- per describes how functional CT techniques can maximize the benefits of administering contrast media and so improve the assessment of patients suffering acute stroke or cancer. Technical Considerations Quantification for Contrast Enhancement Table 1 compares the information that can be ob- tained using contrast media for anatomical and functional purposes. The additional information from functional CT is obtained by quantifying the amount of contrast medium within a given region or volume element (voxel), usually during a time sequence of CT images [1]. Each CT image displays the X-ray attenuation values within each voxel of the anatomical slice studied as an X-ray attenua- tion map. Following intravenous administration of contrast medium, the iodine component of the con- trast medium causes a local increase in the X-ray attenuation that is linearly proportional to the io- dine concentration. The amount of attenuation change for a given concentration of contrast medi- um depends upon a range of factors including the CT system used, the tube voltage (kVp), and the body region examined (e.g., chest or abdomen) and can be ascertained with a simple phantom (Fig. 1). A greater change in attenuation is observed with a lower tube voltage and functional CT protocols may advocate a tube voltage as low as 80 kVp [2]. The concentration of contrast medium at cer- tain time-points following injection can be used to calculate a range of physiological parameters, in- cluding cardiac output and glomerular filtration per gram of renal tissue. At tissue level, it is possible to measure blood flow, blood volume, blood vessel permeability and the size of the extracellular com- partment within each voxel. Absolute quantifica- tion of physiological parameters requires knowl- edge of contrast enhancement within the vascular system as well as the tissue of interest. However, a simple measurement of peak tissue concentration of contrast medium, when combined with the dose of contrast medium administered per kilogram body weight, can be used to calculate the ratio of tissue perfusion to average whole-body perfusion, also known as the Standardized Perfusion Value IV.2 Functional CT Imaging in Stroke and Oncology Kenneth A. Miles Table 1. Comparison of anatomical and functional applications of contrast media Anatomical Functional Visualise blood vessels Determine cardiac output Visualise renal tract Assess renal function Improve lesion-to-tissue contrast Assess physiology of tissue microcirculation, e.g., perfusion, vascular permeability