The doses received by the patients during the computed tomography (CT) exams are the highest when compared to other diagnostic radiology procedures. This fact makes it so important to keep the quality control using the correct instrument. For dosimetry in CT, a pencil type ionization chamber is usually utilized with a sensitive volume length of 10 cm. A comparison of the performance of an extrapolation chamber in two different laboratories, at the Instrument Calibration Laboratory (LCI) from Instituto de Pesquisas Energéticas e Nucleares (IPEN, Brazil) and at the National Physical Laboratory (NPL, UK) was undertaken. Two chamber characteristics were determined (zero depth and extrapolation curve), and the main recommended characterization tests were performed (stabilization time, saturation curve, polarity effect, ion collection efficiency, linearity of response, and variation of response with source-detector distance). This extrapolation chamber was calibrated using the standard system of NPL laboratory, and the energy dependence was determined. The results obtained were within the international recommendations of IEC 61674, except for the energy dependence, as already expected. This study also established a reference system for the LCI of the IPEN, using a homemade extrapolation chamber for X-ray CT beams. Using the extrapolation curves obtained in the characterization tests and the correction factors of ionic recombination, and scattering and fluorescence, the air kerma rates were obtained, as a reference system in standard laboratory computed tomography beams. The last correction factor was obtained using computer simulations by the MCNP5 code (Monte Carlo). Moreover, two pencil type ionization chambers were calibrated using the replacement method in relation to the extrapolation chamber.

与其他放射诊断程序相比，患者在计算机断层扫描 (CT) 检查期间接受的剂量最高。这一事实使得使用正确的仪器保持质量控制变得非常重要。对于 CT 中的剂量测定，通常使用敏感体积长度为 10 cm 的铅笔型电离室。在 Instituto de Pesquisas Energéticas e Nucleares（IPEN，巴西）的仪器校准实验室 (LCI) 和国家物理实验室（NPL，英国），对两个不同实验室的外推室性能进行了比较。确定了两个腔室特性（零深度和外推曲线），并进行了主要推荐的特性测试（稳定时间、饱和曲线、极性效应、离子收集效率、响应的线性以及响应随源-探测器距离的变化）。该外推室使用 NPL 实验室的标准系统进行校准，并确定了能量依赖性。获得的结果在 IEC 61674 的国际建议范围内，除了能量依赖性，正如已经预期的那样。本研究还为 IPEN 的 LCI 建立了参考系统，使用自制的 X 射线 CT 光束外推室。使用表征测试中获得的外推曲线以及离子复合、散射和荧光的校正因子，获得空气比释动能率，作为标准实验室计算机断层扫描光束中的参考系统。最后一个校正因子是通过 MCNP5 代码（蒙特卡罗）使用计算机模拟获得的。此外，