Figure 2.

Four-detector, four-voxel PET system (schematic drawing). Three events k = 1, 2, 3 are detected during data acquisition, yielding incomplete data ${\mathit{n}}^{*}=\left\{n_1^{*}=1,n_2^{*}=2,n_3^{*}=\cdots =n_6^{*}=0\right\}.$ Perfect collinearity of annihilation photons is assumed, hence α₁₅ = α₂₆ = α₃₁ = α₄₃ = 0. Consequently, there are 48 possible OEs, and 24 sets of potential complete data. For example, the actual, unobservable emission counts n = {n₁₁ = 1, n₁₂ = 1, n₁₃ = ··· = n₄₁ = 0, n₄₂ = 1, n₄₃ = ··· = n₄₆ = 0} are represented by the OEs ω₁ = {(1, 1), (1, 2), (4, 2)} and ω₂ = {(1, 1), (4, 2), (1, 2)}.