Kruger R L, McCollough C H, Zink F E
Department of Diagnostic Radiology, Mayo Clinic, Rochester, Minnesota 55905, USA.
Med Phys. 2000 Aug;27(8):1915-9. doi: 10.1118/1.1287440.
The purpose of this paper is to determine the accuracy and reproducibility of two noninvasive methods of measuring half-value layer (HVL), ring and localization, compared with an invasive technique (suspending tube rotation). The ring method uses concentric aluminum rings about a CTDI ionization chamber at isocenter. Data were acquired using axial CT protocols (rotating x-ray tube, stationary patient table). The localization technique uses square aluminum sheets secured to the gantry shroud to filter the radiation beam, and a CTDI chamber suspended externally at isocenter. Data were acquired using localization image protocols (stationary x-ray tube, moving patient table). The invasive technique was similar to the localization technique except that the ion chamber was placed on the patient table and the tube rotation disabled using service software. Data for all techniques were collected on the same CT system. Independent data sets were collected to determine reproducibility. Sensitivity to ionization chamber lateral displacement from isocenter was investigated. Measured HVLs (mm aluminum, mean+/-std, n=4) were 7.19+/-0.03 (ring); 7.17+/-0.04 (localization); and 7.24+/-0.02 (service mode), which were not significantly different (p = 0.05). Displacing the chamber from isocenter changes the HVL, depending on the bow-tie filter, by as much as 5 mm aluminum. Aluminum filter to ion chamber distances of 25-35 cm provided accurate results. Both noninvasive techniques were accurate and reproducible at isocenter. However, the measured HVL was dependent upon the bow-tie filter and the lateral displacement of the ionization chamber with respect to isocenter. Greater than 2 cm off of isocenter, the ring technique did not provide accurate HVL measurements.
本文的目的是确定两种非侵入性测量半值层（HVL）的方法，即环形法和定位法，与一种侵入性技术（悬吊管旋转）相比的准确性和可重复性。环形法在等中心处围绕CTDI电离室使用同心铝环。使用轴向CT协议（旋转X射线管，固定患者检查床）采集数据。定位技术使用固定在机架护罩上的方形铝板来过滤辐射束，并在等中心处外部悬吊一个CTDI室。使用定位图像协议（固定X射线管，移动患者检查床）采集数据。侵入性技术与定位技术相似，只是将电离室放置在患者检查床上，并使用服务软件禁用管旋转。所有技术的数据均在同一CT系统上收集。收集独立数据集以确定可重复性。研究了电离室相对于等中心横向位移的敏感性。测量的HVL（铝毫米数，平均值±标准差，n = 4）为7.19±0.03（环形法）；7.17±0.04（定位法）；和7.24±0.02（服务模式），差异无统计学意义（p = 0.05）。将电离室从等中心处移开会根据蝴蝶结滤波器使HVL变化多达5毫米铝。铝滤波器到电离室的距离为25 - 35厘米时可提供准确结果。两种非侵入性技术在等中心处均准确且可重复。然而，测量的HVL取决于蝴蝶结滤波器以及电离室相对于等中心的横向位移。距离等中心大于2厘米时，环形法无法提供准确的HVL测量值。
