Positron range-free and multi-isotope tomography of positron emitters
Freek J. Beekman, Chris Kamphuis, Sofia Koustoulidou, Ruud M. Ramakers, Marlies C Goorden
Abstract
Abstract Despite improvements in small animal PET instruments, many tracers cannot be imaged at sufficiently high resolutions due to positron range, while multi-tracer PET is hampered by the fact that all annihilation photons have equal energies. Here we realize multi-isotope and sub-mm resolution PET of isotopes with several mm positron range by utilizing prompt gamma photons that are commonly neglected. A PET-SPECT-CT scanner (VECTor/CT, MILabs, The Netherlands) equipped with a high-energy cluster-pinhole collimator was used to image 124 I and a mix of 124 I and 18 F in phantoms and mice. In addition to positrons (mean range 3.4 mm) 124 I emits large amounts of 603 keV prompt gammas that—aided by excellent energy discrimination of NaI—were selected to reconstruct 124 I images that are unaffected by positron range. Photons detected in the 511 keV window were used to reconstruct 18 F images. Images were reconstructed iteratively using an energy dependent matrix for each isotope. Correction of 18 F images for contamination with 124 I annihilation photons was performed by Monte Carlo based range modelling and scaling of the 124 I prompt gamma image before subtracting it from the 18 F image. Additionally, prompt gamma imaging was tested for 89 Zr that emits very high-energy prompts (909 keV). In Derenzo resolution phantoms 0.75 mm rods were clearly discernable for 124 I, 89 Zr and for simultaneously acquired 124 I and 18 F imaging. Image quantification in phantoms with reservoirs filled with both 124 I and 18 F showed excellent separation of isotopes and high quantitative accuracy. Mouse imaging showed uptake of 124 I in tiny thyroid parts and simultaneously injected 18 F-NaF in bone structures. The ability to obtain PET images at sub-mm resolution both for isotopes with several mm positron range and for multi-isotope PET adds to many other unique capabilities of VECTor’s clustered pinhole imaging, including simultaneous sub-mm PET-SPECT and theranostic high energy SPECT.