Comparison of transverse and spin-lattice relaxation based electron paramagnetic resonance oxygen images

Recent experiments have shown that transverse relaxation (TR) T₂-based in vivo oxygen electron paramagnetic resonance imaging results are confounded by the effects of additional relaxation mechanisms. On the contrary, spin-lattice relaxation (SLR) T1-based oxymetry is more precise and nearly free from those interfering mechanisms. In this article we study the differences between TR and SLR in vivo images by varying the spin probe concentration in an animal. We demonstrate that the dominant mechanism that differentiates TR and SLR images is the spin probe intermolecular interaction. The concentration dependence of TR observed in vivo is up to factor of three stronger than that in phantoms. We hypothesize that this difference is due to spin probe occupying only a small portion of the overall volume of an animal - the extracellular space. This leads to underestimation of the spin probe concentration and, hence, overestimation of concentration dependence coefficient. On the other hand, the imaging of the concentration dependence TR enhancement in vivo may allow investigation of the ratio of extra- and intra- cellular volumes, which is of interest for cancer biology and biomedical applications.