The effects of CT x-ray tube voltage and current variations on the relative electron density (RED) and CT number conversion curves

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.