In Vivo Application of Proton-Electron Double-Resonance Imaging

Significance:
Proton-electron double resonance imaging (PEDRI) employs electron paramagnetic resonance irradiation with low field magnetic resonance imaging (MRI) so that the electron spin polarization is transferred to nearby protons, resulting in higher signal. PEDRI provides information about free radical distribution and, indirectly, about the local microenvironment such as pO2, tissue permeability, redox status, and acid-base balance.
Recent Advances:
Local acid-base balance can be imaged by exploiting the different resonance frequency of radical probes between R and RH+ forms. Redox status can also be imaged using the loss of radical-related signal after reduction. These methods require optimized radical probes and pulse sequences.
Critical Issues:
High power radiofrequency irradiation is needed for optimum signal enhancement, which may be harmful to living tissue by unwanted heat deposition. Free radical probes differ depending on the purpose of PEDRI. Some probes are less effective for enhancing signal than others, which can reduce image quality. It is so far not possible to image endogenous radicals by PEDRI because low concentrations and broad line widths of the radicals lead to negligible signal enhancement.
Future Directions:
PEDRI has similarities with electron paramagnetic resonance imaging (EPRI) because both otechniques bserve the EPR signal; directly in the case of EPRI and indirectly with PEDRI. PEDRI provides information vital to research on homeostasis, development of diseases or treatment responses in vivo. It is expected that the development of new EPR techniques will give insights into novel PEDRI applications and vice versa.