Project 2: Overhauser enhanced Magnetic Resonance Imaging. (OMRI)This methodology provides anatomically co-registered pO2 images by combining the sensitivity of EPR detection at low fields and the dynamic nuclear polarization (DNP) to enhance the MR image intensity significantly at low magnetic fields. Therefore, the scanner is a combination of EPR Imaging and MR imaging and we use a contrast agent whose relaxation properties are linearly dependent on pO2. Earlier studies demonstrated the quantitative pO2 imaging capabilities of this technique. However, two aspects needed to be addressed for this technique to progress for in vivo imaging applications with minimal artifacts.pO2 images normalized for changes in proton T1. The enhancement of the water 1H based MR images from OMRI studies depends upon: a) concentration of the contrast agent; b) RF power deposited to excite the contrast agent; c) tissue pO2; and d) intrinsic 1H T1 relaxation times. Of these, the T1 of the protons in tissue varies with the regions examined and hence may lead to significant errors in estimating pO2. We have developed image data collection strategies to correct for this artifact by using novel pulse sequences from which the changes in the tissue based T1 values can be independently estimated.Use of Inductive loop for RF Focusing in OMRI: One unique feature in OMRI imaging is that the irradiation of the unpaired spin to generate the required Ovehauser enhancement uses high RF power at a duty cycle of nearly 50% of the total measurement time. This leads to a high SAR (Specific Absorption Rate) well above the one prescribed for human use by the FDA. Any prospective human application therefore necessitates efforts to reduce SAR in OMRI. Among several strategies that we are planning, the one we have recently successfully tested is the use of local inductive loop, which is a passive tuned resonator (such as a surface loop) that is tuned to the same frequency as the main volume resonator, kept close to the region of interest (ROI). Such a passive tuned element draws RF flux close to itself at the expense of RF flux elsewhere, and produces enhanced image intensity compared to locations which are outside the loop. This strategy has been tested both on phantoms and in vivo, and produces RF focusing leading to improved image sensitivity at the localized region. This will allow the use of significantly lower RF power than normal, and yet produce enhanced image intensity at the ROI. Such a focusing effect of RF will allow effective OMRI imaging and oxymetry of topical tumors in animal models at much reduced SAR. We are also experimenting on the use of quadrature transmit and receive coils to reduce the power requirements further by factor of 2, and bring the effective SAR well under FDA limits.
