Figure 1.

(a) Schematic representation of the time-density curves (TDCs), after the injection of a bolus of contrast material, in the three compartments (artery, tissue or compartment, and vein): a(t), c(t), and v(t). The arterial a(t) and tissue c(t) density curves are related to each other through a convolution (see (2)) of the artery curve with the impulse response function (IRF) shown in (b). (b) Scheme of the residual impulse response function (IRF) and perfusion parameters obtained through the deconvolution method (GE Medical Systems. User Guide. Milwaukee, WI: GE Medical Systems, 2002; 240-255). A measure of the IRF can be obtained by deconvolving the density curve a(t) from c(t) measured with the CTp. The CTp parameter values thus fully characterised the IRF shape as shown. In more detail, the PS is related to the diffusion coefficient of the contrast agent through the pores of the capillary endothelium into the interstitial space. In the tissue IRF, the contrast agent diffusion is related to the extraction fraction F ₀/BF (or the fraction of contrast agent), which remains in the intravascular space after the initial IRF response and which then diffuses exponentially into the interstitial space. The extraction fraction is thus related to the PS in the following way: F ₀/BF = 1 − exp⁡(PS/BF), where the parameter F ₀ is the blood flux measured after one mean transit time, MTT (i.e., when the contrast bolus is passed).