Dynamic imaging with MRI contrast agents: quantitative considerations
Time-resolved MRI has had enormous impact in cognitive science and may become a significant tool in basic biological research with the application of new molecular imaging agents. In this paper, we examine the temporal characteristics of MRI contrast agents that could be used in dynamic studies. We consider "smart" T1 contrast agents, T2 agents based on reversible aggregation of superparamagnetic nanoparticles and sensors that produce changes in saturation transfer effects (chemical exchange saturation transfer, CEST). We discuss response properties of several agents with reference to available experimental data, and we develop a new theoretical model that predicts the response rates and relaxivity changes of aggregation-based sensors. We also perform calculations to define the extent to which constraints on temporal resolution are imposed by the imaging methods themselves. Our analysis confirms that some small T1 agents may be compatible with MRI temporal resolution on the order of 100 ms. Nanoparticle aggregation T2 sensors are applicable at much lower concentrations, but are likely to respond on a single second or slower timescale. CEST agents work at high concentrations and temporal resolutions of 1–10 s, limited by a requirement for long presaturation periods in the MRI pulse sequence.
© 2006 Elsevier Inc. Received 2 December 2005; accepted 2 December 2005. The authors gratefully acknowledge comments from G. Odriozola and P. Gillis about the proposed models for nanoparticle aggregation and SPIO relaxivity changes, respectively. David Cory is thanked for helpful remarks on the manuscript. MGS was supported by a Hertz Fellowship and a Soros Fellowship, HF was supported by award DAMD17-03-1-0413 from the Department of the Army, and GGW and AJ were supported by a grant from the Raymond and Beverley Sackler Foundation.