Quantitative SPECT imaging: influence of object size.

In positron emission tomography (PET) the measured radionuclide concentration or recovery coefficient (RC) in the transverse plane has been shown to be dependent on source size. The RC dependence on object size was therefore experimentally measured for gamma camera SPECT system to determine the influence of additional factors such as: (a) geometrical spatial resolution of each collimator type, (b) scattering effects at each photon energy due to surrounding background, (c) influence of choice of reconstruction filter and (d) photon penetration through the collimator septa. Data were acquired for different collimator types using 99Tcm (140 keV) and 131I (364 keV) to partly differentiate between photon scattering and collimator penetration effects. Results obtained demonstrated that thick septa collimators with low penetration fraction (theoretical leakage less than 3%) are required to minimize the measured response dependence on source size and to obtain a relatively unique relationship independent of photon energy and background activity. Measurements of this type are important for quantitative SPECT imaging in radioimmunoimaging or radioimmunotherapy.