Dixon Robert L, Ballard Adam C
Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, North Carolina 27157-1088, USA.
Med Phys. 2007 Aug;34(8):3399-413. doi: 10.1118/1.2757084.
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 CTDI100 correctly predicts the accumulated dose only for a total scan length L 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 L (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 L (up to the available phantom length) including the limiting equilibrium dose (symbolically CTDIinfinity), 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 l=100 and 150 mm confirm that the two methodologies provide the same dose values at the corresponding scan lengths L=l. The measured equilibrium doses obtained for GE MDCT scanners at 120 kVp are CTDIinfinity = 1.75 CTDI100 on the central axis and 1.22 CTDI100 on the peripheral axes, illustrating a nontrivial shortfall of CTDI100 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. 34, 1364-1371 (2007)].
本文是对计算机断层扫描剂量测定新方法的实验演示和验证[R. L. 迪克森，《医学物理》30，1272 - 1280(2003)]，该方法使用的是一个短的传统电离室而非笔形电离室，并且比后者用途更广泛。CTDI100值仅在总扫描长度L等于100毫米时能正确预测累积剂量，而对于更长的、临床相关的身体扫描长度，会低估接近的极限平衡剂量[R. L. 迪克森，《医学物理》30，1272 - 1280(2003); K. D. 纳科内奇尼、B. G. 法隆和S. 拉西，《医学物理》32，98 - 109(2005); S. 森、M. 远藤、K. 西泽、T. 津野、T. 青山、H. 藤原和K. 村濑，《医学物理》32，1061 - 1069(2005); R. L. 迪克森、M. T. 芒利和E. 贝拉姆，《医学物理》32，3712 - 3728(2005); R. L. 迪克森，《医学物理》33，3973 - 3976(2006)]。迪克森[《医学物理》30，1272 - 1280(2003)]最初提出了一种替代方法，使用短电离室和螺旋扫描采集，以在任何扫描长度L(不限于100毫米)下收集相同的积分。这项工作的主要目的是通过实验证明这种小电离室方法在测量身体模体中任意所需扫描长度L(直至可用模体长度)的累积剂量时的实施情况、稳健性和通用性，包括极限平衡剂量(符号为CTDI无穷大)，并针对笔形电离室方法对该方法进行验证。此外，还描述了一种简单且稳健的独立验证笔形电离室有效长度的方法。使用一个小型 Farmer 型电离室和两个长度分别为l = 100毫米和150毫米的笔形电离室，在一个400毫米长、32厘米直径的聚甲基丙烯酸甲酯身体模体中进行测量的结果表明，两种方法在相应扫描长度L = l时提供相同的剂量值。在GE MDCT扫描仪上120 kVp条件下测得的平衡剂量为，中心轴上CTDI无穷大 = 1.75 CTDI100，外周轴上为1.22 CTDI100，这表明CTDI100在这方面存在显著不足，且与可比数据[S. 森、M. 远藤、K. 西泽、T. 津野、T. 青山、H. 藤原和K. 村濑，《医学物理》32，1061 - 1069(2005); J. M. 布恩，《医学物理》34，1364 - 1371(2007)]吻合良好。
