Noise power properties of a cone-beam CT scanner with unconventional scanning geometry

Purpose: This work aims at investigating, via in-silico evaluations, the noise properties of an innovative scanning geometry in cone-beam CT (CBCT): eCT. This scanning geometry substitutes each of the projections in CBCT with a series of collimated projections acquired over an oscillating scanning trajectory. The analysis focused on the impact of the number of the projections per period (PP) on the noise characteristics. Methods: In-silico eCT scanner was simulated with a GPU based Monte Carlo software. We employed two homogeneous PMMA phantoms with a diameter of 12 cm and 16 cm whose tomographic images were reconstructed via an in-house developed software. Noise properties of the reconstructed volumes were evaluated in terms of coefficient of variation (COV), non-uniformity index, noise power spectrum (NPS), and null-cone over the 3D NPS. Results: The beam narrowing at higher PP led to a significant reduction of cupping artifacts, with a non-uniformity index reducing of about 33% going from conventional CBCT to PP = 10. Oscillating scan orbits almost fully recovered missing data in conventional CBCT, with a narrowing of the null-cone in 3D NPS to below 2.5% for PP ≥ 5 compared to 11.0% in conventional CBCT at 6.5 cm from the orbit plane Conclusions: The work characterizes the noise in reconstructed 3D images in eCT, with particular focus on the NPS. The impact of the beam collimation on cupping artifacts reduction is outlined. Similarly, the missing data outlined by the null-cone is considerably narrowed in comparison to conventional CBCT, especially for portions of the FOV far from the middle-reconstructed plane.