Figure 2.

M-lines and backprojection range. Reconstruction at x̱ can be achieved by computing and then inverting the HT on any M-line through x̱, but this procedure requires a different range of projections depending on the position of λ_M (the angle defining the M-line) relative to λ₁ and λ₂ (the angles defining the π-line through x̱). Three cases are possible: λ_M can be located before λ₁ (left diagram), after λ₂ (right diagram) or between λ₁ and λ₂ (middle diagram). In each case, the highlighted portion of the helix depicts the required backprojection range. Note also that the selection of λ_M is restricted to the interval [λ_i, λ_o] over which no interrupted illumination occurs.