Computer Tomography Tube Voltage and Phantom Dimensions Influence on the Number of Hounsfield Units

Abstract

Hounsfield units (HU) are a dimensionless units universally used in computed tomography (CT) scanning to express CT numbers in a standardized and convenient form. Calibration curve (CC) implemented in radiotherapy treatment planning system (TPS) shows the dependence of HU on relative electron density (RED). Radiotherapy treatment planning process is based on data from the CC and linear accelerator (Linac) data set. RED is constant and specified for each material, but the HU can change depending on several factors. Dosimetric phantom in radiotherapy properly represents human anatomy and enable thorough analysis of both the imaging and dosimetry system. For the purpose of CC checking, two phantoms were used: Catphan 504 (The Phantom Laboratory) and CIRS 062MA (Computerized Imaging reference Systems, Inc.). The aim of the paper was to check whether the HU depends on the dimensions of the phantom. Catphan 504 phantom was scanned first, in order to verify the HU determination methodology. After that, CIRS 062MA phantom representing both abdomen and head configuration was scanned. By analyzing Catphan 504 phantom obtained scanning results, it was concluded that the HU values are correctly determined, since the results are in accordance with the manufacturer’s recommendations. Based on results of CIRS 062MA phantom scan, the HU dependence on the CT simulator tube voltage was confirmed. Significant difference in HU for high density materials (up to 135 HU) due to different phantom sizes was observed.