EPR-based oximetric imaging: a combination of single point-based spatial encoding and T weighting.

Purpose: Spin-lattice relaxation rate (R )-based time-domain EPR oximetry is reported for in vivo applications using a paramagnetic probe, a trityl-based Oxo71.

Methods: The R dependence of the trityl probe Oxo71 on partial oxygen pressure (pO ) was assessed using single-point imaging mode of spatial encoding combined with rapid repetition, similar to T -weighted MRI, for which R was determined from 22 repetition times ranging from 2.1 to 40.0 μs at 300 MHz. The pO maps of a phantom with 3 tubes containing 2 mM Oxo71 solutions equilibrated at 0%, 2%, and 5% oxygen were determined by R and apparent spin-spin relaxation rate ( R2*) simultaneously.

Results: The pO maps derived from R and R2* agreed with the known pO levels in the tubes of Oxo71. However, the histograms of pO revealed that R offers better pO resolution than R2* in low pO regions. The SDs of pixels at 2% pO (15.2 mmHg) were about 5 times lower in R -based estimation than R2*-based estimation (mean ± SD: 13.9 ± 1.77 mmHg and 18.3 ± 8.70 mmHg, respectively). The in vivo pO map obtained from R -based assessment displayed a homogeneous profile in low pO regions in tumor xenografts, consistent with previous reports on R2*-based oximetric imaging. The scan time to obtain the R map can be significantly reduced using 3 repetition times ranging from 4.0 to 12.0 μs.

Conclusion: Using the single-point imaging modality, R -based oximetry imaging with useful spatial and oxygen resolutions for small animals was demonstrated.