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Quantitative ionization chamber alignment to a water surface: Performance of multiple chambers
Author(s) -
Ververs James D.,
McEwen Malcolm R.,
Siebers Jeffrey V.
Publication year - 2017
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 - 10.1002/mp.12315
Subject(s) - ionization chamber , materials science , ionization , optics , radius , image resolution , resolution (logic) , physics , ion , computer security , quantum mechanics , artificial intelligence , computer science
Purpose The purpose of this study was to experimentally examine the reliability of the gradient chamber alignment point ( gCAP ) determination method for accurately identifying water surface location with a range of ionization chambers ( IC s). Materials and methods Twelve cylindrical IC s were scanned from depth through a water surface into air using a customized high‐accuracy scanning system which allows for accurate alignment of the IC with respect to the true water surface. Thirteen other cylindrical IC s and five parallel‐plate IC s were scanned using a standard commercially available scanning system. The thirty different IC s used in this study represent 22 different IC models. Measurements were taken with different radiation field parameters such as incident photon beam energies and field sizes. The effects of scan direction and water surface tension were also investigated. The depth at which the gradient of the relative ionization was maximized and discontinuous, the gCAP , was found for each curve. Each measured gCAP depth was compared with the theoretically expected gCAP location, the depth at which the submerged IC outer radius ( OR ) coincides with the water surface. Results When scanning an IC from in water to air, the only parameter that affects the gCAP location is the IC OR . The gCAP location corresponds with the IC central axis positioned at a depth equal to the IC OR within the 0.1 mm measurement scan resolution for all eighteen IC s studied with the commercially available system. Using the customized scanning system, all but three IC s were identified exhibiting a gCAP within the scan resolution, with the other three within 0.25 mm of the expected location. This discrepancy was not observed in the same IC model when using the conventional scanning system. Altering the beam energy from 6 to 25 MV did not alter the gCAP location, nor did variations in the radiation field size or scan parameters. In‐air IC response is proportional to the IC wall thickness. Conclusion The water‐to‐air scanning method coupled with gCAP analysis identifies the alignment of the IC OR to the water surface within the scanning resolution for all IC s studied. The gCAP method can precisely and reproducibly align the physical center of a given cylindrical IC with the water surface, be applied prospectively or retrospectively, and provides the prospect for automated water surface identification for scanning systems. The gCAP method eliminates the visual subjectivity inherent to current IC ‐to‐water surface alignment techniques, has been validated with a wide variety of commercially available IC s, and should be independent of the scanning system used for data acquisition.