Noise, sampling, and the number of projections in cone-beam CT with a flat-panel detector

Purpose: To investigate the effect of the number of projection views on image noise in cone-beam CT (CBCT) with a flat-panel detector. Methods: This fairly fundamental consideration in CBCT system design and operation was addressed experimentally (using a phantom presenting a uniform medium as well as statistically motivated "clutter") and theoretically (using a cascaded systems model describing CBCT noise) to elucidate the contributing factors of quantum noise (sigma(Q)), electronic noise (sigma(E)), and view aliasing (sigma(view)). Analysis included investigation of the noise, noise-power spectrum, and modulation transfer function as a function of the number of projections (N-proj), dose (D-tot), and voxel size (b(vox)). Results: The results reveal a nonmonotonic relationship between image noise and N-proj at fixed total dose: for the CBCT system considered, noise decreased with increasing N-proj due to reduction of view sampling effects in the regime N-proj <similar to 200, above which noise increased with N-proj due to increased electronic noise. View sampling effects were shown to depend on the heterogeneity of the object in a direct analytical relationship to power-law anatomical clutter of the form kappa/f(beta)-and a general model of individual noise components (sigma(Q), sigma(E), and sigma(view)) demonstrated agreement with measurements over a broad range in N-proj, D-tot, and b(vox). Conclusions: The work elucidates fairly basic elements of CBCT noise in a manner that demonstrates the role of distinct noise components (viz., quantum, electronic, and view sampling noise). For configurations fairly typical of CBCT with a flat-panel detector (FPD), the analysis reveals a "sweet spot" (i.e., minimum noise) in the range N-proj similar to 250-350, nearly an order of magnitude lower in N-proj than typical of multidetector CT, owing to the relatively high electronic noise in FPDs. The analysis explicitly relates view aliasing and quantum noise in a manner that includes aspects of the object ("clutter") and imaging chain (including nonidealities of detector blur and electronic noise) to provide a more rigorous basis for commonly held intuition and heurism in CBCT system design and operation. (C) 2014 American Association of Physicists in Medicine.