PET imaging with the non-pure positron emitters: (55)Co, (86)Y and (124)I.

PET/CT with non-pure positron emitters is a highly valuable tool in immuno-PET and for pretherapeutic dosimetry. However, imaging is complicated by prompt gamma coincidences (PGCs) that add an undesired background activity to the images. Time-of-flight (TOF) reconstruction improves lesion detectability in (18)F-PET and can potentially also improve the signal-to-noise ratio in images acquired with non-pure positron emitters. Using the GE Discovery 690 PET/CT system, we evaluated the image quality with (55)Co, (86)Y and (124)I, and the effect of PGC-correction and TOF-reconstruction on image quality and quantitation in a series of phantom studies. PET image quality and quantitation for all isotopes were significantly affected by PGCs. The effect was most severe with (86)Y, and less, but comparable, with (55)Co and (124)I. PGC-correction improved the image quality and the quantitation accuracy dramatically for all isotopes, especially when the activity was limited to a few hot lesions in a warm background. In imaging situations, where high levels of activity were present in the background, activity concentrations were overestimated. TOF-reconstruction improved image quality in isolated lesions but worsened the accuracy of quantitation and uniformity in homogeneous activity distributions. Better modelling of PGCs in the scatter correction can potentially improve the situation.