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Noise, sampling, and the number of projections in cone‐beam CT with a flat‐panel detector
Author(s) -
Zhao Z.,
Gang G. J.,
Siewerdsen J. H.
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4875688
Subject(s) - noise (video) , quantum noise , optical transfer function , gradient noise , image noise , value noise , sampling (signal processing) , noise power , clutter , flat panel detector , detector , imaging phantom , noise reduction , cone beam computed tomography , optics , aliasing , physics , noise measurement , mathematics , noise floor , computer science , acoustics , computer vision , filter (signal processing) , power (physics) , telecommunications , quantum , image (mathematics) , radar , medicine , computed tomography , quantum mechanics , radiology
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 ( σ Q ), electronic noise ( σ E ), and view aliasing ( σ 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 <∼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 κ / f β —and a general model of individual noise components ( σ Q , σ E , and σ 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 ∼ 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.