CASP3 and neoplasm: The absolute tumor uptake was too low to be visualized in vivo using SPECT imaging, but a significant tracer uptake was detected ex vivo in the tumors of treated mice, with 1.5%ID/g compared to 0.4%ID/g in the control group, 60 min p.i. These results highlight the advantage of PET imaging and the requirements of a highly sensitive imaging technique to detect small changes in caspase-3 activity.