  Precise quantitative delineation of tumor hypoxia is essential in radiation
therapy treatment planning to improve the treatment efficacy by targeting
hypoxic sub-volumes. We developed a combined imaging system of positron
emission tomography (PET) and electron para-magnetic resonance imaging (EPRI)
of molecular oxygen to investigate the accuracy of PET imaging in assessing
tumor hypoxia. The PET/EPRI combined imaging system aims to use EPRI to
precisely measure the oxygen partial pressure in tissues. This will evaluate
the validity of PET hypoxic tumor imaging by (near) simultaneously acquired
EPRI as ground truth. The combined imaging system was constructed by
integrating a small animal PET scanner (inner ring diameter 62 mm and axial
field of view 25.6 mm) and an EPRI subsystem (field strength 25 mT and resonant
frequency 700 MHz). The compatibility between the PET and EPRI subsystems were
tested with both phantom and animal imaging. Hypoxic imaging on a tumor mouse
model using $^{18}$F-fluoromisonidazole radio-tracer was conducted with the
developed PET/EPRI system. We report the development and initial imaging
results obtained from the PET/EPRI combined imaging system.
