Distribution dose profile of Computed Tomography (CT) along the z-axis with pitch variation: in-house phantom study

Abstract

Computed Tomography (CT) is an x-ray modality for scanning organ in three dimensional images. Image acquisition is performed by rotating x-ray tube that match with table movements, there by the tube can cover patient body in a spiral scan. Patient table movements by gantry rotation divided by the width of the collimator on the isocenter is known as a pitch, which affects the image quality and radiation dose in the patient. A dose distribution profile has been observed along the z-axis of the in-house thorax phantom simulation in an elliptical cylinder form with the size of 28 cm x 21 cm and 22 cm length. Phantom was equipped with a lung simulation object using a cork with a Hounsfield Unit (123.1 HU), a spine using Teflon material (918.6 HU), and 9 bar and a cylinder to place 1 cm x 25 cm gafchromic films. The position of the film was marked with point position 1 – 9 for the series of coordinates (0,0), (5, 0), (10, 0), (-5, 0), (-10, 0), (0, 4), (0, 8), (0, -4), (0, -8) cm. The phantom images were performed with an exposure condition by 120 kV, 100 mAs and pitch variations (0.8, 1.0 and 1.5). The minimum dose occured at the beginning and end of the scan for all profiles and pitch values. The average dose of lung material (1, 2, 4, and 5) in the range 2.32-2.88 mGy for pitch 0.8 and 1.0 respectively, and 2.25 -2.68 mGy for pitch 1.5. The dose in lung was relatively lower compared to the dose in soft tissue and bone. The maximum dose does not always occur in the middle of the z-axis, but it was fluctuated as it was influenced by the pitch values in the range of 10 - 12 cm. It can be concluded that the use of pitch 0.8 and 1.0 did not provide a significant dose difference and reduced the average dose on pitch 1.5. Moreover, the maximum dose does not always occur in the middle of the z-axis due to an isotropic material.