Todd C. Soesbe1, Osamu Togao1, Masaya Takahashi1, A. Dean Sherry1,2
1Advanced Imaging Research Center, The University of Texas Southwestern Medical Center, Dallas, TX, United States; 2Department of Chemistry, The University of Texas at Dallas, Dallas, TX, United States
Paramagnetic chemical exchange saturation transfer (PARACEST) agents use water molecule exchange with lanthanide ions and radiofrequency (RF) spin saturation to create negative contrast in MRI. One advantage that PARACEST agents have over Gd-based T1 agents is that image contrast can be turned off and on via the RF saturation pulse frequency. We have recently shown that chemical exchange of water molecules between Eu3+based PARACEST agents and bulk water can also facilitate T2 exchange (T2exch). T2exch causes a significant reduction in the bulk water T2 (i.e. negative contrast) for agents with high local concentrations, intermediate exchange rates, and large chemical shifts. The negative contrast is present even when the RF saturation pulse is omitted, causing the PARACEST agents to behave like susceptibility or T2 agents. We hypothesized that the ultra-short TE (<10 μs) used in the SWIFT sequence could reclaim the loss in signal due to T2exch and enable fast and sensitive in vivo PARECEST imaging.