Introduction

Dynamic imaging, or tracking, of a “bolus” of intravascularly administered tracer can be interpreted to yield information both from the “first pass,” relating primarily to tissue perfusion, or from later “pseudo steady-state” phases, relating to transendothelial transport of the tracer between intravascular and extravascular compartments. Clearly these two processes are intimately related and not trivially resolved by temporal considerations alone. Integrated models of contrast media delivery and equilibration are emerging. Nonetheless, a practical and pragmatic distinction is often drawn between first-pass (perfusion) imaging (0–60 s) and subsequent imaging sensitive to microvascular permeability (typically over a few minutes). Different imaging strategies to track gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) are conventionally chosen: for pseudo steady-state or “permeability” imaging, an imaging sequence sensitive to the T1 shortening effect of Gd-DTPA is commonly employed, whereas first-pass imaging tends to use an imaging approach sensitive to the magnetic susceptibility effect of the tracer (manifest as transient T2 shortening). It must, however, be emphasized that this is a practical convention, and both T2*- and T1-weighted approaches can be used to study both first pass and subsequent stages of tracer transport; their relative advantages and disadvantages (especially in terms of temporal and spatial resolution) are discussed below.

The theory of dynamic contrast-enhanced MRI (DCE-MRI), and derived permeability estimation, is related to the dynamic assessment of signal changes in a tissue voxel fed by an arterial tree and drained by the venous system. Simple kinetic models relate the volume of tracer “out” to be equal to the volume “in” (conservation of matter). Two-compartment models incorporate two environments (capillary and interstitium), the exchange of water between which is governed by rate constraints. A simple exploration of the mathematics reveals how parameters of the microvasculature, namely fractional blood volume (fBV), microvascular permeability and extravascular, extracellular space (EES), can be extracted from a single dynamic scan.