ABSTRACT

The procedures of the radiation treatment of the cancer patient start with imaging of patient by CT machine. CT machine determine the locations and densities of patient organs in specific coordinate called pixels. Each pixel has a unit called CT number or Hounsfield Unit (HU) describing the amount of radiation attenuation in patient organ. The density of organ tissue is expressed in relative electron density (RED). The conversion relation between RED and CT number need to be measured and then transferred to the treatment planning system (TPS). The TPS calculates the radio-therapeutic dose to the patient depending on this RED-CT number conversion curve. In this research, the effect of CT x-ray tube voltage and current variations on the RED-CT number relationship are studied. Siemens CT machine of the type Somatom definition As, was used. It provides step voltages 70, 80, 100, 120, and 140 kV, with wide range of currents from 30 to 800 mA. CT RED phantom of the type CIRS 062M containing 10 tissue densities equivalent inserts was used. It was found that both CT x-ray tube voltages and currents variations have considerable effects on the RED-CT number relationships. For different CT current values, the differences between RED-CT number curves decrease when the CT voltages increase from 70 to 100 kV. At CT voltages 120 and 140 kV the differences are negligible whatever is the CT current value.

摘要

癌症患者的放射治疗程序始于通过CT机对患者进行成像。CT机在称为像素的特定坐标中确定患者器官的位置和密度。每个像素都有一个称为CT数或Hounsfield单位（HU）的单位，描述患者器官中的辐射衰减量。器官组织的密度以相对电子密度（RED）表示。需要测量RED和CT编号之间的转换关系，然后将其转换到治疗计划系统（TPS）。在TPS计算取决于这个无线电治疗剂量给患者RED-CT数字转换曲线。在这项研究中，研究了CT X射线管电压和电流变化对RED-CT数关系的影响。使用了Somatom定义为As的Siemens CT机器。它提供阶跃电压70、80、100、120和140 kV，电流范围从30到800 mA。使用CIRS 062M类型的CT RED幻像，其中包含10个组织密度等效的插入物。已经发现，CT X射线管的电压和电流变化都对RED-CT数关系有相当大的影响。对于不同的CT电流值，RED-CT号之间的差异当CT电压从70 kV增加到100 kV时，曲线会减小。在CT电压为120 kV和140 kV时，无论CT电流值如何，其差异都可以忽略不计。