“…The product of D and n is usually called the sensitivity of the detection system. Among the factors that impact the sensitivity are: [1][2][3][4][5][6] 1) the absorption and scattering of photons, 2) collimator efficiency, 3) detector detection efficiency and window fraction, 4) counting rate losses, 5) volume-of-interest (VOI) definition and the partial volume effect, 6) counts detected within the VOI which originated from outside of it, 7) kinetics of the radiopharmaceutical during the period of imaging, 8) reconstruction parameters and implementation in the case of SPECT, and 9) noise. In this review we will focus on compensating for the absorption and scattering of photons.…”