Initial evaluation of a four-dimensional Computed Tomography system using a programmable motor

Abstract

The geometrical reconstruction accuracy of a commercial four-dimensional Computed Tomography (4D-CT) system has been investigated using a dynamic lung phantom tumor. A ball filled with resin embedded in a cork cube was placed on a moving platform. Different realistic antero-posterior (AP) motions were programmed to reproduce the respiratory motion of a lung tumour. Several 3D-CT and 4D-CT images of this moving object were acquired and compared using different acquisition parameters. Apparent volume and diameter of the ball were measured and compared to the real values. Position of two points (AP limits of the ball) during the motion in the coordinate system of the CT scanner were also compared to the expected values. Volume error was shown to increase with the object speed. However, although the volume error was associated with intra-slice artefacts, it did not reflect large inter-slices inconstancies in the object position, and should not be used as an indicator of image accuracy. 3D-CT gave a random position of the tumor along the phantom excursion while accuracy in the assessment of the position by 4D-CT ranged from 0.4 to 2.6 mm during extreme phases of breathing. An Internal Target Volume (ITV) was created by merging Gross Tumor Volume (GTV) images at different respiratory phases using Average (AVE) and Maximum Intensity Projection (MIP) algorithms available on the commercial software. These ITV were compared to a theoretical value computed from the programmed ball excursion. ITV created by MIP algorithms was closer to the theoretical value (within 12%) than AVE (40%).