Other
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........44a&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF FLORIDA, 1993.Source: Dissertation Abstracts International, Volume: 55-07, Section: B, page: 2817
Other
1
Scientific paper
The EGS4 Monte Carlo system was used to study the factors that affect dose distributions and the energy imparted to dosimetry phantoms in computed tomography (CT). The energy imparted is a useful quantity since it is directly correlated with the stochastic risk to the patient. Also, since the energy imparted per slice is independent of both the precise anatomic location of the scanned slice and the total number of slices imaged, the total energy imparted from an exam can be easily found. The parameters that were investigated were incident energy, beam shaping filter, slice thickness, phantom size, phantom composition, and source-to-axis-distance (SAD). These parameters were evaluated for irradiation by both monoenergetic x-ray beams and typical x-ray spectra incident upon acrylic cylinders modeled on standardized Computed Tomography Dose Index (CTDI) phantoms. In addition, two phantoms similar to the anthropomorphic phantoms used in previous Monte Carlo investigations of patient dosimetry were studied, and the results obtained from these phantoms were compared to those obtained from the CTDI phantoms. In all cases the results obtained were normalized to the dose to acrylic determined "in-air" at the isocenter of the scanner in the absence of any other phantom. The results quantitatively demonstrate the effects of modifying key CT parameters on energy deposition patterns in CT dosimetry phantoms. Also, a method is presented that allows estimation of the total energy imparted per slice to a CTDI phantom. In general, the factors most affecting the quantitative values of energy deposited in CT dosimetry phantoms are beam filtration, the size and composition of the object being scanned, and the incident energy. The results also suggest that the energy imparted is only moderately dependent on the characteristics of the incident x-ray spectrum. The results of this work may be used as a basis for estimating the energy imparted to a phantom or a patient for non-standard or newly developed CT exams or scanners. The values of the energy imparted may also be used to estimate other quantities such as the effective dose. These estimates may then be used as indices of stochastic risk for comparisons between scanners or imaging modalities, exam optimization, or for population-based studies of radiation detriment.
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