Figure 4.

CT images reconstructed with different noise levels and object structures using 66 projections. (a) I₀ = 5 × 10⁵; (b) I₀ = 5 × 10⁴; (c) I₀ = 5 × 10⁴ with the geometry modified on the Shepp–Logan phantom. All units are in photon counts. First row of each subfigure shows the reconstructed images (display window: [−100, 100] HU), while second row contains the difference images (display window: [−20, 20] HU). Column (1): Using ABOCS, with a constant μ as 1. Column (3): Using the TV regularization method, with a comparable image quality to that of (1) by a manually tuned λ. Columns (2) and (4): Using the TV regularization method with λ about 1/4 and 4 times of that in (3), respectively.