MR Angiography with a New Rapid Clearance Blood Pool Agent (P792): Initial Experience in Rabbits

Abstract

The T1-shortening effect of gadolinium chelates allows select enhancement of the arterial lumen without depending on time-of-flight effects. This assures maximal contrast between the arterial lumen and surrounding tissues and renders the technique less susceptible to flow dependent artifacts. The extracellular nature of the paramagnetic gadolinium chelates leads to rapid diffusion into the extracellular compartment, resulting in decreasing arterial signal intensity and concomitant enhancement of surrounding tissues. The short intravascular half-life requires optimal timing of the contrast bolus and provides only a very limited time window for data collection during the arterial phase of the intravenously administered contrast bolus. Timing errors may render the examination suboptimal or non-diagnostic. Thus high contrast doses and fast injection rates are required to induce adequate T1-shortening of the blood in the vascular territory under investigation. Contrast dose limitations limit most protocols to a single injection. Repeated imaging is possible only with low dose protocols which require time consuming image subtraction techniques to obtain adequate contrast to noise between vessels and surrounding tissues (1). The desirable concomitant assessment of organ perfusion as a complement to arterial imaging, possibly even prior to and following the application of pharmaceutical stressing agents, is limited by the leakage of the extracellular into the interstitial space. Blood pool contrast agents potentially overcome these limitations. On account of their larger molecular size, they are known to reveal a substantially higher relaxivity (2– 4). Even in the arterial phase they are available in a higher concentration due to less leakage into the pulmonary capillary bed. These factors further reduce the required dose. Furthermore, they allow for equilibrium phase imaging. Furthermore, lack of concomitant soft tissue enhancement permits better visualization of small distal intraparenchymal vessel branches (5–9). In this study we assessed the in vitro signal enhancement characteristics of P792 compared to gadoterate dimeglumine (Gd-DOTA), a representative, market introduced extracellular non-specific agent (NSA).