The effect of scatter on quantitation in positron volume imaging of the thorax

In positron volume imaging (PVI) a high percentage of detected coincidences are scatter events. The authors performed a series of phantom and simulation studies of scatter contamination in thorax imaging. Scatter fractions of 50 to 70% were measured for a torso phantom in a General Electric Advance positron emission tomograph with the septa retracted. The authors also measured scatter contribution from outside the field-of-view (FOV) using a small cylindrical source (1 cm long, 0.5 cm diameter) at various axial and transaxial positions in a solid Lucite cylinder (43 cm long, 20 cm diameter). A source 9 cm outside the FOV contributed approximately 30% the scatter that the same source would contribute if placed near the center of the FOV. To isolate the effect of scatter from outside the FOV in realistic imaging situations the authors performed Monte Carlo simulations of PVI for heart studies. Organs outside the FOV can contribute significantly to detected scatter. For instance, with equal activity concentrations the liver and heart contributions to scatter were virtually identical even though 90% of the liver was outside the FOV