  We developed a positron emission tomography (PET) system for multiple-isotope
imaging. Our PET system, named multiple-isotope PET (MI-PET), can distinguish
between different tracer nuclides using coincidence measurement of prompt
gamma-rays, which are emitted after positron emission. In MI-PET imaging with a
pure positron emitter and prompt-gamma emitter, because of the imperfectness of
prompt gamma-ray detection, an image for a pure positron emitter taken by
MI-PET is superposed by a positron-{\gamma} emitter. Therefore, in order to
make isolated images of the pure positron emitter, we developed image
reconstruction methods based on data subtraction specific to MI-PET. We tested
two methods, subtraction between reconstructed images and subtraction between
sinogram data. In both methods, normalization for position dependence of the
prompt {\gamma}-ray sensitivity is required in addition to detector sensitivity
normalization. For these normalizations, we performed normalization scans using
cylindrical phantoms of the positron-gamma emitters Sc-44m and Na-22. A long
period measurement using the activity decay of Sc-44m (Half-life 58.6 hours)
elucidated that the acquisition ratio between the prompt gamma-rays coincided
with PET event and pure PET event changes on the basis of object activities.
Therefore, we developed a correction method that involves subtraction
parameters dependent on the activities, i.e., the counting rate. From analysis
of dual-tracer phantom images, data subtraction in the sinogram data with
sensitivity correction gives a higher quality of isolated images for the pure
positron emitter than those from image subtractions. Furthermore, from
dual-isotope (F-18-FDG and Sc-44m) mouse imaging, we concluded that our
developed method can be used for practical imaging of a living organism.
