title: PET imaging of thin objects: measuring the effects of positron range and partial-volume averaging in the leaf of Nicotiana tabacum.
abstract: BACKGROUND
  PET imaging in plants is receiving increased interest as a new strategy to measure plant responses to environmental stimuli and as a tool for phenotyping genetically engineered plants. PET imaging in plants, however, poses new challenges. In particular, the leaves of most plants are so thin that a large fraction of positrons emitted from PET isotopes ((18)F, (11)C, (13)N) escape while even state-of-the-art PET cameras have significant partial-volume errors for such thin objects. Although these limitations are acknowledged by researchers, little data have been published on them.
  
  
  METHODS
  Here we measured the magnitude and distribution of escaping positrons from the leaf of Nicotiana tabacum for the radionuclides (18)F, (11)C and (13)N using a commercial small-animal PET scanner. Imaging results were compared to radionuclide concentrations measured from dissection and counting and to a Monte Carlo simulation using GATE (Geant4 Application for Tomographic Emission).
  
  
  RESULTS
  Simulated and experimentally determined escape fractions were consistent. The fractions of positrons (mean±S.D.) escaping the leaf parenchyma were measured to be 59±1.1%, 64±4.4% and 67±1.9% for (18)F, (11)C and (13)N, respectively. Escape fractions were lower in thicker leaf areas like the midrib. Partial-volume averaging underestimated activity concentrations in the leaf blade by a factor of 10 to 15.
  
  
  CONCLUSIONS
  The foregoing effects combine to yield PET images whose contrast does not reflect the actual activity concentrations. These errors can be largely corrected by integrating activity along the PET axis perpendicular to the leaf surface, including detection of escaped positrons, and calculating concentration using a measured leaf thickness.
authors.affiliation: Medical Department, Brookhaven National Laboratory, Upton, New York, USA. alexoff@bnl.gov
authors.forename: David L
authors.initials: DL
authors.lastname: Alexoff
authors.forename: Stephen L
authors.initials: SL
authors.lastname: Dewey
authors.forename: Paul
authors.initials: P
authors.lastname: Vaska
authors.forename: Srilalan
authors.initials: S
authors.lastname: Krishnamoorthy
authors.forename: Richard
authors.initials: R
authors.lastname: Ferrieri
authors.forename: Michael
authors.initials: M
authors.lastname: Schueller
authors.forename: David J
authors.initials: DJ
authors.lastname: Schlyer
authors.forename: Joanna S
authors.initials: JS
authors.lastname: Fowler
mesh_terms: D016477:Artifacts; D001692:Biological Transport; D004583:Electrons; D018515:Plant Leaves / Q000378:metabolism; D049268:Positron-Emission Tomography / Q000379:methods*; D011849:Radioactive Tracers; D014026:Nicotiana / Q000378:metabolism
chemical_list: D011849:Radioactive Tracers