Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2015 / S10 / 1 / 4 / Solution

In the ideal unscattered coincidence model, let a positron annihilation occur at position $s$ on a coincidence line of response connecting detectors $A$ and $B$. Its two oppositely directed photons must both survive. Their independent attenuation factors multiply:The source position cancels because the complementary path integrals span the whole chord. In a uniform slab of chord length $L$, the same argument is $e^{-\mu s}{e}^{-\mu (L-s)}=e^{-\mu L}$. Therefore attenuation correction requires the full linear attenuation coefficient map along the chord, but not the depth of the particular annihilation along that chord. This argument addresses attenuation of true PET coincidences; scatter, random coincidences and detector efficiency need their own corrections.