Effect of CT Simulator Tube Voltages on CT Number and Relative Electron Density of CT Images

Abstract

The radiotherapy treatment planning for the cancer patient begins with the imaging of the patient using a Computed Tomography (CT) simulator. The CT simulator determines the locations and densities of patient organs using pixels. Each pixel has a unit called a CT number, or Hounsfield Unit (HU), to express the quantity of radiation attenuation in patient tissues. The density of human tissue is presented in Relative Electron Density (RED). The conversion relationship between RED and CT numbers is required to be determined and fed to the Treatment Planning System (TPS). The TPS calculates the radiotherapy dose for the patient using this CT-RED conversion curve. Most TPSs contain a single CT-RED curve defined at a specific CT tube voltage.

In this research, we studied the effect of CT tube voltages using a CT simulator (GE; CT-RED curve defined at 140kV) at Teaching Hospital Karapitiya, which provides step voltages of 80 kV, 120 kV, and 140 kV, XIO TPS, and Thorax Phantom (CIRS) containing 5 tissue types. According to the measured results, CT numbers decrease with increasing CT Tube voltage. Also, there were no significant errors in RED values of tissues at 120 kV, with respect to the 140 kV.  But there were 5.55%, 2.10%, 0.99%, 0.99%, 0.95%, and 1.34% errors in RED for lung, adipose, water, T1420, muscle, and bone tissues respectively at 80kV with respect to 140 kV.

Key Words: Radiotherapy, CT imaging, CT Number, CT tube voltage, Relative Electron Density