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Longitudinal dose distribution and energy absorption in PMMA and water cylinders undergoing CT scans
Author(s) -
Li Xinhua,
Zhang Da,
Liu Bob
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 - 10.1118/1.4895825
Subject(s) - full width at half maximum , imaging phantom , materials science , dosimetry , absorbed dose , absorption (acoustics) , optics , beam (structure) , percentage depth dose curve , irradiation , nuclear medicine , attenuation coefficient , centimeter , beam diameter , physics , ionization chamber , nuclear physics , medicine , ion , quantum mechanics , ionization , laser , astronomy , laser beams
Purpose: The knowledge of longitudinal dose distribution provides the most direct view of the accumulated dose in computed tomography (CT) scanning. The purpose of this work was to perform a comprehensive study of dose distribution width and energy absorption with a wide range of subject sizes and beam irradiated lengths.Methods: Cumulative dose distribution along the z ‐axis was calculated based on the previously published CT dose equilibration data by Li, Zhang, and Liu [Med. Phys. 40 , 031903 (10pp.) (2013)] and a mechanism for computing dose on axial lines by Li, Zhang, and Liu [Med. Phys. 39 , 5347–5352 (2012)]. Full width at half maximum (FWHM), full width at tenth maximum (FWTM), the total energy ( E ) absorbed in a small cylinder of unit mass per centimeter square about the central or peripheral axis, and the energy ( E in ) absorbed inside irradiated length ( L ) were subsequently extracted from the dose distribution.Results: Extensive results of FWHM, FWTM, and E in / E were presented on the central and peripheral axes of infinitely long PMMA (diameters 6–50 cm) and water (diameters 6–55 cm) cylinders with L < 100 cm. FWHM was greater than the primary beam width only on the central axes of large phantoms and also with L ranging from a few centimeter to about 33 cm. FWTM generally increased with phantom diameter, and could be up to 32 cm longer than irradiated length, depending on L , phantom diameter and axis, but was insensitive to phantom material (PMMA or water). E in / E increased with L and asymptotically approached unity for large L . As phantom diameter increased, E in / E generally decreased, but asymptotically approached constant levels on the peripheral axes of large phantoms. A heuristic explanation of dose distribution width results was presented.Conclusions: This study enables the reader to gain a comprehensive view of dose distribution width and energy absorption and provides useful data for estimating doses to organs inside or beyond the irradiated region. The dose length product (DLP) presented by CT scanners is equal to neither E nor E in . Both E and E in can be evaluated using the equations and results presented in this paper and are robust with both constant and variable tube current scanning techniques.