A combined PET-EPR system: initial testing

Objectives: Electron paramagnetic resonance (EPR) enables interrogation of electron spins of free radicals to detect relatively stable compounds. EPR-based techniques in combination with paramagnetic contrast agents are accurate methods for measuring characteristics of the extracellular physiologic environment (oxygen saturation and pH, for example). Whereas, PET imaging is used mostly to interrogate intracellular physiology (glucose metabolism and amino acid incorporation, for example). The combination of these two methods has the potential to enable unique investigations studying the dynamics between cellular physiology and tissue microenvironment. Our objective in this investigation is to explore the interactions between PET and EPR systems when they are combined in to a single scanner.

Methods: The PET scanner used in this study is a portable ring of twelve detector modules, each consisting of an array of LYSO detector elements (1.5mmx1.5mmx10mm) coupled to an array of SiPMs. The EPR system is comprised of a dipole electromagnet (field strength up to 400G), an RF bridge/resonator (resonant frequency=720MHz) and a rapid scan (RS coil) magnetic field modulation unit (operating at up to 100 kHz and peak-to-peak field modulation=40G). The RS coil and resonator were mounted inside the bore of the PET scanner. The combined system was then inserted into the magnet. A 3ml sample solution containing a mixture of EPR contrast agent (3mM water solution of stable trityl radical) and 70uCi of FDG was placed in a conical vial and positioned at the center of the PET-EPR scanner. Simultaneous PET image data and EPR spectra were then acquired.

Results: PET energy and EPR spectra acquired during the scan showed no effects from simultaneous operation of the systems. Specifically, no artifacts or energy shifts were apparent in the PET spectra. Likewise, the EPR spectra demonstrated no apparent artifacts or anomalous source of noise from the operation of the PET scanner. PET images and EPR spectra of the combined PET-EPR tracer also showed no apparent artifacts.

Conclusion: A combined PET-EPR system has the promise to enable unique and potentially important studies exploring the relationship between intracellular function and the extracellular microenvironment in cancer and cardiac tissues, potentially leading to new insights into the physiologic dynamics of these cells. This initial investigation demonstrated that there are no significant impediments to the melding of these two techniques. The next step is to add gradient coils to the EPR system to permit imaging, and modification of the PET scanner to increase imaging performance (resolution and detection sensitivity).