I think this is a wrong question
The question should be " why Compton scatter of gamma ray is important of the quality of the image
Answer is: Compton scatter happen above 1.012 MeV Gamma ray energy, as the gamma ray produced one electron and one positron of 0.56 MeV, so increasing the RESULTING positrons will provide better image
http://hyperphysics.phy-astr.gsu.edu...ua/compton.gif
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