Table 3.

Overview of vascular parameters that are often assessed using dynamic contrast-enhanced (μ)CT and MRI and their biological relevance

Parameter	Biological Relevance
amplitude A [a.u.]	Two Compartment Model (Brix) Maximal signal intensity Strong dependency on perfusion and rBV Rapidly decreasing during initial phase of anti-angiogenic therapy due to tumor blood vessel regression
area under curve (AUC) [a.u.]	Is the integral of a curve resulting from the signal intensity as a function of time Is a mixed parameter of perfusion and rBV Is initially increased in tumors due to a high MVD Is decreased under anti-angiogenic therapy because of regression of tumor blood vessels
BF [ml min−1]	Indicates how much blood (ml) flows per tissue mass (g) per time (min) Is decreased under anti-angiogenic therapy because of tumor blood vessel regression
Kep [min−1]	Two Compartment Model (Brix) Efflux volume transfer constant of a contrast agent from the interstitial compartment to the vascular compartment in a pharmacokinetic two-compartment model Major dependency on vessel permeability and perfusion Variable change under anti-angiogenic therapy
Ktrans [min−1]	Two Compartment Model (Tofts) Influx volume transfer constant of a contrast agent from the vascular compartment to the interstitial compartment Major dependency on perfusion, minor dependency on permeability Is decreased under anti-angiogenic therapy because of tumor blood vessel regression
mean transit time (MTT) [s]	Indicates how long a contrast agent bolus needs to pass from a feeding artery through the tissue of interest (e.g. tumor) in a venous/draining vessel Correlates with tissue perfusion Is decreased under anti-angiogenic therapy because of tumor blood vessel regression
perfusion [ml−1 min−1 (100 g)−1]	Is characterized as the blood supply of various tissues including organs or tumors Is increased in tumors due to a high arterial feeding, high vessel density and high vessel permeability Is decreased under anti-angiogenic therapy because of tumor blood vessel regression
rBV [%]	Percentage of the blood volume to the total tumor volume Correlates with the amount of functional blood vessels within a tumor Is decreased under anti-angiogenic therapy because of tumor blood vessel regression
vascular permeability [μl min−1 ml−1]	Depends on the contrast agent/ molecule properties, vessel surface area, vessel leakiness and the size of the interstitial distribution space Is also used synonymously as capillary permeability Vascular permeability is high in tumors with mainly immature, leaky blood vessels Decreases in tumors with mainly immature, leaky vessels under anti-angiogenic therapy because of vessel regression/increasing vessel maturity
Ve [%]	Two Compartment Model (Tofts) Is the extravascular, extracellular leakage space volume as a fraction of the whole tumor volume Strong dependency on both interstitial and plasma volume