A PET/EPR simultaneous imaging system for assessing tumor hypoxia: development and initial imaging results

We present a combined imaging system of positron emission tomography (PET) and electron paramagnetic resonance imaging (EPRI) developed for assessing tumor hypoxia. Tumor hypoxia has been known to be resistant for cancer treatment such as radiation therapy. Therefore, precise quantitative measurement of tumor hypoxia is important for effective cancer treatment planning in radiation therapy. In clinic, PET imaging by using
¹⁸
F-fluoromisonidazole (F-MISO) have been performed to delineate tumor hypoxia. However, the effectiveness of PET imaging based hypoxia targeting in radiation therapy was not yet firmly established. EPRI is a non-invasive imaging method capable of measuring the partial pressure of oxygen (pO
₂
) in tissue precisely. However, EPRI is not available for clinical use currently. The combined imaging system is to use EPRI as a gold standard to measure the pO
₂
in hypoxic tumor in animal, and aims to evaluate the validity of PET imaging in tumor hypoxia by simultaneously acquired EPRI. The PET component of the combined system, which was originally developed as an insert for MRI, consists of total 14 detector modules. Each PET detector module uses of 8×4 LYSO arrays (3×3×10 mm
³
) coupled to Hamamatsu S13361 MPPC arrays (3.2 mm pitch). The detector modules are encased in a cylindrical plastic holder with 60 mm inner diameter and 115 mm outer diameter in a ring configuration. The axial field of view of the PET is 25.6 mm. The EPRI component of the system is built with two permanent magnets (25 mT), three gradient field coils for spatial encoding (the maximum 15 mT/m) and a RF resonator. The PET is installed to fit into the two EPR magnets (12 cm distance apart) so that the PET detector center is aligned to the magnet center. The compatibility of two imaging modalities were tested by using a
²²
Na point source and imaging phantoms. Animal imaging with F-MISO and FDG were also performed by running the PET/EPR simultaneously. Initial imaging results are reported.