Podgorsak E B, Gosselin M, Evans M D
Department of Medical Physics, Montreal General Hospital, Québec, Canada.
Med Phys. 1998 Jul;25(7 Pt 1):1206-11. doi: 10.1118/1.598309.
Output of superficial and orthovoltage x-ray units may be measured with cylindrical or end-window parallel-plate ionization chambers. The air-kerma calibration factors for these chambers are usually determined free in air, and the x-ray machine output is stated as the air-kerma rate free in air, which, when multiplied with the appropriate backscatter factor, gives the air-kerma rate on the surface of a phantom or patient. For end-window chambers, especially when they are used for measurements of small fields or low x-ray energies, the air-kerma calibration factors may also be determined with the chamber embedded in a tissue-equivalent phantom. This results in field size dependent air-kerma in-air calibration factors but obviates the requirement for knowledge of back-scatter factors when determining the air-kerma rate on the surface of a phantom. Since there still is considerable uncertainty in tabulated backscatter factors as a function of field size and x-ray beam energy, the output measurement technique which determines the air-kerma rate on phantom surface with a phantom-embedded end-window ionization chamber offers a clear advantage over the in-air calibration method.
浅表和正交电压X射线装置的输出可以用圆柱形或端窗平行板电离室进行测量。这些电离室的空气比释动能校准因子通常在空气中自由确定，X射线机的输出表示为空气中的空气比释动能率，将其乘以适当的反向散射因子，即可得到模体或患者表面的空气比释动能率。对于端窗电离室，特别是当它们用于小射野或低X射线能量的测量时，空气比释动能校准因子也可以在电离室嵌入组织等效模体的情况下确定。这会导致空气比释动能在空气中的校准因子与射野大小有关，但在确定模体表面的空气比释动能率时无需了解反向散射因子。由于作为射野大小和X射线束能量函数的表格化反向散射因子仍存在相当大的不确定性，使用嵌入模体的端窗电离室确定模体表面空气比释动能率的输出测量技术比空气中校准方法具有明显优势。
