Scatter and beam hardening degrade image quality for X-ray computed tomography (CT). Applying beam hardening correction before scatter being fully removed may be needed in some scenarios. In this paper, we theoretically analyze the outcome of beam hardening correction with scatter uncorrected. Using the effective spectrum of CT system, beam hardening correction function and its derivatives can be explicitly derived and used, along with a Taylor expansion, to approximate the change of scatter-to-primary ratio (SPR) after beam hardening correction. Two types of SPR compensations are compared using cone- and fan-beam scans of a CatPhan phantom on a tabletop cone-beam CT system, validating our analysis to be accurate in practical applications.
Absorbed dose distributions in dental and maxillofacial cone beam computed tomography (dental CBCT) are essential to dental CBCT dose indices. Direct measurements by thermoluminescence detectors are laborious. We establish a valid GEANT4 based absorbed dose simulation program with mean deviation being 7.25% compared to experimental measurements. Dental CBCT absorbed dose distributions simulated by this program indicate : The distributions are not always symmetry; Non-symmetry cases are when phantom center is departured from isocenter, half-fan beam with 360° scan angle range and some case full-fan beam with less than 360° scan angle range; Dose index weights for circular symmetry absorbed dose distributions are quite different for fields of view diameter bigger and smaller than phantom diameter.
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