Experimental validation of a versatile system of CT dosimetry using a conventional ion chamber: Beyond

This article is an experimental demonstration and authentication of a new method of computed tomography dosimetry [R. L. Dixon, Med. Phys. 30, 1272–1280 (2003)], which utilizes a short, conventional ion chamber rather than a pencil chamber, and which is more versatile than the latter. The value of correctly predicts the accumulated dose only for a total scan length equal to 100 mm and underestimates the limiting equilibrium dose approached for longer, clinically relevant body scan lengths [R. L. Dixon, Med. Phys. 30, 1272–1280 (2003); K. D. Nakonechny, B. G. Fallone, and S. Rathee, Med. Phys. 32, 98–109 (2005); S. Mori, M. Endo, K. Nishizawa, T. Tsunoo, T. Aoyama, H. Fujiwara, and K. Murase, Med. Phys. 32, 1061–1069 (2005); R. L. Dixon, M. T. Munley, and E. Bayram, Med. Phys. 32, 3712–3728 (2005); R. L. Dixon, Med. Phys. 33, 3973–3976 (2006)]. Dixon [Med. Phys. 30, 1272–1280 (2003)] originally proposed an alternative using a short ion chamber and a helical scan acquisition to collect the same integral for any scan length (and not limited 100 mm). The primary purpose of this work is to demonstrate experimentally the implementation, robustness, and versatility of this small ion chamber method in measuring the accumulated dose in the body phantom for any desired scan length (up to the available phantom length) including the limiting equilibrium dose (symbolically ), and validation of the method against the pencil chamber methodology. Additionally, a simple and robust method for independently verifying the active length of a pencil chamber is described. The results of measurements made in a 400 mm long, 32 cm diameter polymethylmethacrylate body phantom using a small Farmer‐type ion chamber and two pencil chambers of lengths and 150 mm confirm that the two methodologies provide the same dose values at the corresponding scan lengths . The measured equilibrium doses obtained for GE MDCT scanners at 120 kVp are on the central axis and 1.22 on the peripheral axes, illustrating a nontrivial shortfall of in that regard and in good agreement with comparable data [S. Mori, M. Endo, K. Nishizawa, T. Tsunoo, T. Aoyama, H. Fujiwara, and K. Murase, Med. Phys. 32, 1061–1069 (2005); J. M. Boone, Med. Phys., 1364–1371 (2007)].