Journal Article EPR‐based oximetric imaging: a combination of single point‐based spatial encoding and T1 weighting

Ken-ichiro, Matsumoto  ,  Kishimoto, Shun  ,  Devasahayam, Nallathamby  ,  V. R. Chandramouli, Gadisetti  ,  Ogawa, Yukihiro  ,  Matsumoto, Shingo  ,  C. Krishna, Murali  ,  Subramanian, Sankaran

2018-03 , Wiley-Blackwell
Purpose: Spin‐lattice relaxation rate (R1)‐based time‐domain EPR oximetry is reported for in vivo applications using a paramagnetic probe, a trityl‐based Oxo71. Methods: The R1 dependence of the trityl probe Oxo71 on partial oxygen pressure (pO2) was assessed using single‐point imaging mode of spatial encoding combined with rapid repetition, similar to T1‐weighted MRI, for which R1 was determined from 22 repetition times ranging from 2.1 to 40.0 μs at 300 MHz. The pO2 maps of a phantom with 3 tubes containing 2 mM Oxo71 solutions equilibrated at 0%, 2%, and 5% oxygen were determined by R1 and apparent spin–spin relaxation rate (R2*) simultaneously. Results: The pO2 maps derived from R1 and R2* agreed with the known pO2 levels in the tubes of Oxo71. However, the histograms of pO2 revealed that R1 offers better pO2 resolution than R2* in low pO2 regions. The SDs of pixels at 2% pO2 (15.2 mmHg) were about 5 times lower in R1‐based estimation than R2*‐based estimation (mean ± SD: 13.9 ± 1.77 mmHg and 18.3 ± 8.70 mmHg, respectively). The in vivo pO2 map obtained from R1‐based assessment displayed a homogeneous profile in low pO2 regions in tumor xenografts, consistent with previous reports on R2*‐based oximetric imaging. The scan time to obtain the R1 map can be significantly reduced using 3 repetition times ranging from 4.0 to 12.0 μs. Conclusion: Using the single‐point imaging modality, R1‐based oximetry imaging with useful spatial and oxygen resolutions for small animals was demonstrated.

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