Dynamic Contrast Enhanced MRI (DCE-MRI): From the Simplest to the Most Advanced DCE-Model

1. Tracer Kinetic Modeling • Extracellular agents (such as Gd-DTPA) accumulate in the interstitium of many tissues (e.g. muscle) but do not cross the healthy blood-brain-barrier. • Functional information may be obtained by treating Gd-DTPA as a tracer and following its kinetics (as opposed to the conventional ‘static’ approach, inject wait – image once). • After intravenous administration (normally in the form of a bolus) Gd-DTPA transits the venous and arterial system leaking from capillary beds wherever the endothelial cell junctions allow. It is excreted via the kidneys [1]. • The choice of imaging method is crucially dependent on what is to be measured and the rate of tracer delivery and uptake. • When imaging tissues with leaky capillaries (e.g. tumors), measurements are typically made using rapid T1-weighted gradient echo sequences (often FLASH or turboFLASH). o Gd-DTPA in the plasma has a relatively small influence on the signal except where the vascular volume is large (e.g. in the kidney, heart or certain tumors). o Gd-DTPA in the interstitial space has a significant effect since this often represents a significant fraction of the tissue volume. • The temporal sampling requirements vary enormously. For perfusion studies a sampling interval of ~ 1 s is required to measure the rapid transit of the Gd-DTPA bolus. For so-called permeability studies the uptake of Gd-DTPA may take several minutes and sampling rates can be reduced to tens of seconds or even minutes. • Transport of Gd-DTPA can be described using a number of parameters: • Perfusion, F (or plasma perfusion, Fp); capillary permeability-surface area product, PS (a measure of vessel leakiness); plasma volume, vp and interstitial volume, ve. • When put together in a physiologic model (e.g. Fig. 1) and formulated in mathematical terms the tracer kinetic parameters provide a powerful tool for data analysis.