Feasibility of in vivo three-dimensional T2∗ mapping using dicarboxy-PROXYL and CW-EPR-based single-point imaging

Abstract

Objectives

The aim of this study was to demonstrate the feasibility of in vivo three-dimensional (3D) relaxation time ${\mathrm{T}}_2^{\ast }$ mapping of a dicarboxy-PROXYL radical using continuous-wave electron paramagnetic resonance (CW-EPR) imaging.

Materials and methods

Isotopically substituted dicarboxy-PROXYL radicals, 3,4-dicarboxy-2,2,5,5-tetra( ²H ₃)methylpyrrolidin-(3,4- ²H ₂)-(1- ¹⁵N)-1-oxyl ( ²H, ¹⁵N-DCP) and 3,4-dicarboxy-2,2,5,5-tetra( ²H ₃)methylpyrrolidin-(3,4- ²H ₂)-1-oxyl ( ²H-DCP), were used in the study. A clonogenic cell survival assay was performed with the ²H-DCP radical using squamous cell carcinoma (SCC VII) cells. The time course of EPR signal intensities of intravenously injected ²H, ¹⁵N-DCP and ²H-DCP radicals were determined in tumor-bearing hind legs of mice (C3H/HeJ, male, n = 5). CW-EPR-based single-point imaging (SPI) was performed for 3D ${\mathrm{T}}_2^{\ast }$ mapping.

Results

²H-DCP radical did not exhibit cytotoxicity at concentrations below 10 mM. The in vivo half-life of ²H, ¹⁵N-DCP in tumor tissues was 24.7 ± 2.9 min (mean ± standard deviation [SD], n = 5). The in vivo time course of the EPR signal intensity of the ²H, ¹⁵N-DCP radical showed a plateau of 10.2 ± 1.2 min (mean ± SD) where the EPR signal intensity remained at more than 90% of the maximum intensity. During the plateau, in vivo 3D maps with ²H, ¹⁵N-DCP were obtained from tumor-bearing hind legs, with a total acquisition time of 7.5 min.

Conclusion

EPR signals of ²H, ¹⁵N-DCP persisted long enough after bolus intravenous injection to conduct in vivo 3D ${\mathrm{T}}_2^{\ast }$ mapping with CW-EPR-based SPI.