Project 2: Overhauser enhanced Magnetic Resonance Imaging. (OMRI)Summary: This is a low field MRI modality which relies on the use of a contrast agent to provide images with enhanced intensity and resolution at low magnetic fields ( 10 mT) compared to the conventional MRI scanners which typically operate at fields 1 T or higher. We are now examining the fundamental factors in vivo which influence the image intensity, which is central to pO2 determination by this method. Since the Overhauser enhancement is dependent on concentration of the contrast media, the intrinsic proton T1, the microvessel density in tumors, we have taken mice implanted with three different tumors with different microvessel density and flow characteristics and examined the enhancement profile as well as determine the intrinsic T1 in addition to pO2 and concentration We are also exploring additional applications for OMRI in studying tumor physiology taking advantage of the inherent advantage of the process of dynamic nuclear polarization. Unlike Dynamic contrast Enhanced MRI which is used in tumor perfusion studies, where contrast in MRI intensity is probed, the OMRI method is fundamentally advantageous since it relies on enhancement via dynamic nuclear polarization rather than enhancement via contrast mechanisms. Early studies with MRI showed that, a with a steady state infusion of Gadolinium contrast agents, after an equilibrium of in-flow and out-flow of the contrast media is attained, the concentration under steady state conditions can be inversely correlated to the tumor interstitial fluid pressure. With OMRI and the contrast media used in OMRI, it is net necessary for the continuous infusion and therefore may have advantage in extracting such information non-invasively.One major instrumentation challenge is to minimize the RF deposition into the tissue while not significantly compromising the image intensity enhancement in OMRI. Towards this goal, we have been exploring several strategies including surface structures which can focus the RF to the region of interest while minimizing the RF deposition outside this. Another approach we are pursuing and have made some progress is the development of quadrature resonant structures which utilize the RF more efficiently than the previously used configurations. This involves using quadrature coils for two seprate RF frequencies which increase both sensitivity as well as utilizing the RF power more optimally. Such resonator configurations need two detection channels and the associated amplification chains. Both these aspects are completed and the resonator structures are completed in terms of design and assembly.
