Apparent pO2 from R1 (0) and R2 (•) relaxation rates in a mouse as a function of spin probe concentration for different experiments as the spin probe is infused at different rates.
We discovered that the self-broadening of trityls is much larger in the presence of salt than had been measured in pure water. This effect introduces additional enhancement of R2 and diminishes the absolute pO2 resolution of the measurements. The rate of energy dissipation to the lattice of the detected magnetization (R1) is far less affected by spin exchange between spin probes because the energy is transferred to another member of the magnetization spin system. Thus, T2 (or R2) related processes are affected far more by spin exchange processes than are T1 (or R1) processes. We implemented R1imaging by using inversion recovery (IR), sampling the inversion recovery time with a fixed echo time ESE readout (IRESE). Figure shows the in vitro reduction of the concentration effect on R1 relative to R2. In this experiment, a mouse was imaged repetitively as the rate of trityl infusion was increased. In vitro, the effect is a factor of 5 reduction of self-broadening, whereas in vivo, there is an order of magnitude reduction in self-broadening when R1-sensitive pulse sequences are used. This establishes the R1 EPROI as virtually an absolute oxygen image.