Application of a tandem system for HVL evaluation in computed tomography

Abstract

Computed Tomography (CT) is a method of imaging used for diagnosis and diseases treatment. In CT equipment due to its geometry, the determination of the HVL is a difficult task and is usually determined only by the manufacturer. By definition, the energy of a beam is determined by the value of HVL. When HVL values are not easily determined, as in the case of CT, it is possible to evaluate the effective energy of the beam through a system consisting of the use of different energy dependent dosimeters, where the ratio between the calibration curve responses in Energy can provide the effective energy of the beam (Tandem System). The application of this system was proposed by Kenney and Cameron1 and Gorbics and Attix2 that used thermoluminescent materials to determine energy of gamma and X radiation. In 2004, Maia3 studied the application of a Tandem System obtained through a set formed by an ionization chamber of the Type pencils and cylindrical absorber sleeves made of aluminum, PMMA and copper, as a non-invasive method for the determination of HVL values in computed tomography beams. Although the proposed Tandem System initially consists of two dosimeters with different energetic dependencies, the sets formed by the ionization chamber and the cylindrical absorber layers of different materials can also be considered a Tandem System. Taking as a reference the System built by Maia3, a Tandem System was developed at the Institute of Energy and Nuclear Research (IPEN), formed by cylindrical absorber layers of aluminum and PMMA. The thickness of the aluminum layers was chosen from the HVL values of the standard RQT beams, implemented in 2010 by Dias4 in IPEN's Instrument Calibration Laboratory (LCI).