OPTIMIZING PARACEST AGENTS FOR HUMAN MR IMAGING

Ut Southwestern Medical Center, Dallas TX

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. MRI has excellent soft tissue contrast, is non-invasive, very safe, and can provide detailed images of both anatomical structure and function. These attributes are particularly appealing for human studies. However, MRI is relatively insensitive as compared to radionuclide imaging methods or optical imaging techniques. The major platforms for the development of MR "molecular imaging" contrast agents have been either Gadolinium based chelates or small iron oxide particles. Moreover, the effects of gadolinium cannot be modulated externally after the agent has been injected. The recent reports of the paramagnetic chemical exchange saturation transfer (PARACEST) effect, based on the lanthanide shift reagents, show promise that these agents offer alternatives to gadolinium based agents. Although the members of the lanthanide series have very similar chemical and toxicological properties, they have very different properties as MR contrast agents that may give them significant advantages over gadolinium based agents. These advantages are: + The ability to switch each agent on or off through the use of selective RF irradiation applied at the frequency of its bound water resonance can be exploited in a variety of ways. Multiple PARACEST agents can be injected and their effects imaged either sequentially or simultaneously. In their "off'state, i.e. without selective irradiation, conventional MR imaging sequences can be employed, with or without gadolinium contrast agents, because unlike gadolinium based agents, these PARACEST agents have little effect on relaxation times, T1 or T2. The ability to externally modulate the PARACEST effect can potentially improve their detection through optimized acquisition schemes described in this proposal. + Using chemical modifications to the chelate structures, the PARACEST agents with defined water exchange rates can be prepared based on rational chemical principles. Since the water exchange rate determines the required strength of the applied irradiation field, W1 and the RF power defined as the specific absorption rate, SAR for each PARACEST agent, the ideal PARACEST agent can be tailored to each application, i.e. body part, RF coil, and MR observation field. + The ability to rationally modify the water exchange rate makes PARACEST an attractive platform for the development of responsive agents. A knowledge of lanthanide chemistry, NMR exchange theory, and practical MR considerations is necessary to fully realize and exploit these unique advantages in vivo. Theoretical and practical considerations of the PARACEST effect indicate that there will be trade-offs between water exchange rates, B1, SAR, RF coils, irradiation strategies and field strength. This project will make use of the whole body 1.5T, 3T, and 7T MR scanners available to the Resource to systematically investigate some of these effects. We have also become aware that there can be significant interference of the effects of magnetization transfer (MT) when PARACEST agents are used in vivo. We have developed three approaches to minimize these MT effects on PARACEST studies in vivo. These three approaches, which are detailed below, will be explored in collaboration with the Research Resource. Approach 1. Chemical approach. Our chemistry group has made a very important recent discovery: water exchange in these complexes can be slowed dramatically (into the msec range) by creating DOTA-tetraamide complexes with extended phosphonate diester side chains. We already know that the Eu3+ complex of this same ligand has a very long bound water lifetime and, consequently, a very sharp PARACEST exchange peak. This will allow activation of PARACEST in the Eu3+ ligand using a much weaker RF pulse, perhaps as weak as 20-40 Hz. This in turn would be too weak for significant MT activation, and consequently the PARACEST effect would be magnified and the MT effect dramatically reduced. The Tb3+ complex has a highly shifted water peak near -500 ppm, well outside the tissue MT window. This is attractive because this agent can be activated without any confounding effects from tissue MT. Approach 2. Positive Contrast PARACEST (PARAPCEST). Saturation of the exchanging pool during relaxation measurements results in shortening of the apparent longitudinal relaxation time, T1(app). This effect has been described historically in MR studies of chemical exchange. We have investigated the utility of this approach in both diamagnetic CEST (DIACEST) and PARACEST applications. Alternate acquisitions are carried out with the frequency of the RF saturation pulse set to the resonance of the bound water and the same frequency offset on the opposite side of water as a control. The optimization of positive PARACEST are currently being carried out as part of Elena Vinogradov's R03 (EB008183: Frequency Shifting Paramagnetic Agents: Quantitative MRI of Exchange Effects) and R21 (EB009425: Optimization of DIACEST and PARACEST methodology for quantitative in-vivo imaging). The value of TI is chosen to null out the background signal. As a result the effects of MT are almost completely suppressed. We will compare the results obtained using conventional PARACEST and PARAPCEST on EuDOTA in agar and in rats at 1.5T, 3T, and 7T in the RR. Approach 3. Omega Plot. The Omega Plot approach is conceptually similar to the QUESP method described by McMahon et al. who used the modified Bloch equations to derive expressions relating the CEST effect to the B1 of the CEST pulse. We can extend this approach to derive an expression that can be used to generate plots which we have called Omega-plots. These Omega-plots can be used to determine k, the bound water exchange rate, without having to know the concentration of the PARACEST agent present. We will determine whether the Omega Plot approach can be employed to separate the effects of MT from the PARACEST effect in samples of EuDOTA in agar at 1.5T, 3T, and 7T in the Research Resource.