Extracellular contrast media readily diffuse from the blood into the EES of tissues at a rate determined by tissue perfusion, permeability of the capillaries, and their surface area. Increases in 1 /T relaxation rate resulting from the leakage of contrast media yields increases in tissue signal intensity. Most DCE-MRI studies employ 2D/3D T1-weighted gradient-echo, saturation recovery/inversion recovery snapshot sequences (e.g., turboFLASH) or echoplanar sequences. Each of these techniques enable tissue T1 relaxation rates to be estimated in a reasonably short period of time, and this allows quantification of tissue contrast medium concentration (48-52). The choice of the sequence and parameters used is dependent on the intrinsic advantages and disadvantages of the sequences, taking into account T1 sensitivity, anatomical coverage, acquisition times, susceptibility to artefacts arising from magnetic field inhomogeneities, and accuracy for quantification. The amount of signal enhancement observed on T1-weighted images is dependent on a number of physiological and physical factors. Physiological factors include tissue perfusion, capillary surface area, permeability to contrast agent, and volume of the extracellular leakage space. Physical factors include the native (or precontrast) T1 relaxation rate of the tissue, contrast agent dose, rate of intracellular-extracellular water exchange, imaging sequence parameters used, and measurement gain and scaling factors.
T1-weighted kinetic enhancement curves have three distinct phases; the upslope, maximum enhancement, and washout (Figs. 6 and 7). It is generally recognized that the upslope is highly dependent on tissue perfusion and permeability, with perfusion predominating. Maximum enhancement is related to the total uptake concentration of the contrast medium in the interstitial space (with an additional vascular contribution), and washout rate is associated with tissue contrast agent concentration decrease and, thus, is strongly related to vascular permeability. If it is assumed that tissue enhancement has contributions from vascular and extravascular compartments (see two-compartment modelling below), then it is possible to separate these inputs mathematically using deconvolution techniques which is helpful for understanding the shape of kinetic curves (53,54). The dominant contribution of perfusion to the upslope of T1-weighted DCE-MRI enhancement curves can be verified empirically by correlating T1- and T2*-weighted DCE-MRI enhancement curves and corresponding kinetic pixel maps (Fig. 7) (26).
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