Abstract
Determination of the half‐value layer (HVL) and quarter‐value layer (QVL) values is not an easy task in X‐ray computed tomography (CT), because a nonrotating X‐ray tube must be used, which requires the assistance of service engineers. Therefore, in this study, we determined the accuracy of the lead‐covered case method, which uses X‐rays from a rotating X‐ray tube, for measuring the HVL and QVL in CT. The lead‐covered case was manufactured from polystyrene foam and a 4 mm thick lead plate. The ionizing chamber was placed in the center of the case and aluminum filters were placed 15 cm above the aperture surface. Aperture widths of 1.0, 2.0, and 3.0 cm for a tube voltage of 110 kV and an aperture width of 2.0 cm for the tube voltages of 80 and 130 kV were used to measure exposure doses. The results of the HVL and QVL were compared with those of the conventional nonrotating method. A 2.0 cm aperture was believed to be adequate, because of its small differences in the HVL and QVL in the nonrotating method and its reasonable exposure dose level. When the 2.0 cm aperture was used, the lead‐covered case method demonstrated slightly larger HVLs and QVLs (0.03‐0.06 mm for the HVL and 0.2‐0.4 mm for the QVL) at all the tube voltage settings. However, the differences in the effective energy were 0.1‐0.3 keV; therefore, it could be negligible in an organ‐absorbed dose evaluation and a quality assurance test for CT.PACS numbers: 87.57.‐s; 87.57.Q‐; 87.57.uq
Highlights
X-rays used in radiographic systems are polyenergetic bremsstrahlung, and the type and total amount of filtration, together with the type of rectification, are the major factors affecting beam quality
We investigated the accuracy of this method for the half-value layer (HVL) and quarter-value layer (QVL) measurements and determined the appropriate aperture width for accurate measurement
HVL and QVL at different aperture widths The HVLs and QVLs obtained with the tube voltage of 110 kV at 1.0, 2.0, and 3.0 cm apertures by the lead-covered case and nonrotation methods are shown in Fig. 3 and Table 1
Summary
X-rays used in radiographic systems are polyenergetic bremsstrahlung, and the type and total amount of filtration, together with the type of rectification, are the major factors affecting beam quality. Maia and Caldas[2] developed tandem systems that were formed by a pencil ionization chamber with and without a specific covering, and they showed that the systems could possibly be used to measure the HVLs instead of the conventional method in CT. Their systems could measure the HVLs while in a rotating exposure mode in CT, they would require chamber-specific covering forms, which are difficult or time-consuming to prepare
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