Definition: What is a Blood Pool Contrast Agent?
CE-MRA is currently performed exclusively with extracellular contrast media (ECCM). After injection these agents rapidly diffuse out of the vasculature into the interstitial space. The initial distribution half-life of the agents that are currently commercially available is only about 15 minutes. Since the MR image acquisition process is slow this leads to several practical constraints. First, the ‘acquisition window’ is very limited (usually to 1-3 minutes). In order to obtain the best possible vessel-to-background contrast, the CE-MRA acquisition needs to be performed during peak arterial enhancement. Image acquisition starting either before or after peak arterial enhancement leads to significant artifacts which may render images uninterpretable. Second, vessel-to-background contrast is limited because of the relatively low relaxivity rate of currently available ECCM. In addition, with the current generation of ultrafast MR imaging hardware, the vessel-to-background signal is reduced even further, for instance, when ultrashort TR values are used. This is an especially pertinent concern when imaging small-caliber diseased vessels with high-spatial resolution imaging sequences.
The first blood pool or intravascular contrast agent has recently received marketing approval. Due to their inherent characteristics, blood pool agents offer many advantages. To begin with, they have a much higher relaxivity than extracellular agents, yielding higher vessel-to-background signal. Second, they also have a much longer intravascular half-life, which effectively extends the imaging window from about 1 minute to about 1 hour. The advantage of this is that multiple arterial territories can be imaged after a single contrast injection, or, alternatively, a specific region of interest can be evaluated at a higher spatial resolution. A third advantage of using blood pool agents is that they make it possible to measure tissue blood volume and perfusion. Finally, blood pool agents allow evaluation of capillary membrane integrity, which is useful for the detection of slow bleeding.
Blood pool-enhanced MRA in the equilibrium phase or steady state, as opposed to imaging during initial arterial passage of the contrast agent, does not only lead to the enhancement of arteries, but also of veins. This might seem to be tedious. However, the spatial resolution of steady-state images is typically much higher which ensures that separation of arteries and veins is actually quite easy when source images are used. Furthermore, the simultaneous depiction of veins could actually assist pre-operative planning by providing the vascular surgeon with information about the suitability of autologous veins for use as bypass grafts.