Feasibility study of using detection of direct positrons in plant imaging research

While nuclear methods find application in plant biology there are very limited amount of instrumentation that is geared to the unique challenges that plant biology presents. Mostly researchers are utilizing devices created for clinical and preclinical imaging. The majority of the biologically significant elements involved in plant formation have positron emitting isotopes, making positron emission tomography a naturally fitting imaging modality. One particular caveat in using PET instrumentation in plant imaging is extremely small thickness of the leaf as an annihilation medium for emitted positrons. For positron energies around 1 MeV (as in case of C-11), about 1 mm of water equivalent material is required for positron conversion, so most of the positrons would not annihilate inside the leaf, therefore limiting both sensitivity and spatial resolution of the imaging system. In this work we made an attempt of determining whether direct imaging of positrons could be beneficial in plant biology studies. We compared performance of direct positron detector with a planar PET system. Detection efficiency ratio of ~650 in favor of direct positron detection system was measured. Direct comparison of spatial resolutions yielded comparable numbers for a setup, when a phantom was placed directly onto the entrance window of the positron detector. We also determined quick deterioration of spatial resolution as a function of source to detector distance. The last observation makes it necessary to maintain close proximity of the object to the detector, but this condition can be met in plant biology study.