Influence of proton T₁ on oxymetry using Overhauser enhanced magnetic resonance imaging

Abstract

In Overhauser enhanced magnetic resonance imaging (OMRI) for in vivo measurement of oxygen partial pressure (pO₂), a paramagnetic contrast agent is introduced to enhance the proton signal through dynamic nuclear polarization. A uniform proton T1 is generally assumed for the entire region of interest for the computation of pO₂ using OMRI. It is demonstrated here, by both phantom and in vivo (mice) imaging, that such an assumption may cause erroneous estimate of pO₂. A direct estimate of pixel-wise T₁ is hampered by the poor native MR intensities, owing to the very low Zeeman field (15-20 mT) in OMRI. To circumvent this problem, a simple method for the pixel-wise mapping of proton T₁ using the OMRI scanner is described. A proton T₁ image of a slice through the center of an SCC tumor in a mouse clearly shows a range of T₁ distribution (0.2~1.6 s). Computation of pO₂ images using pixel-wise T₁ values promises oximetry with minimal artifacts by OMRI.