Development of Head Phantom for Dosimetry and Protocol Optimization in Computed Tomography

Abstract

Computed tomography (CT) scans promote higher patient exposure to radiation than a conventional radiography examination. These tests contribute to increasing the absorbed dose by the patient and the collective dose of the population, being a public health concern. Therefore, there is important to improve image acquisition protocols, aiming to reduce doses, without decreasing the diagnostic quality of the image. Head and neck CT scans are among those that deposit the most dose in patients and, consequently, increase the risk of developing cancers, especially in babies, children, and adolescents. In addition, the risks of stochastic effects are higher for children, due to tissue radiosensitivity combined with longer life expectancy and the fact that smaller volumes receive higher doses in CT. The development of human phantoms becomes possible to know the values of absorbed doses in patients, when they adequately represent the individual, as in terms of composition as in volume. For the development of this work, four cylindrical phantoms were made of polymethylmethacrylate (PMMA) that represented different head volumes of children and adults, with diameters of 11, 12, 13 and 16 cm. These phantoms were submitted to CT scans in four different radiodiagnostic services in Belo Horizonte using the routine acquisition protocol of each service. The central slice of the phantoms was successively irradiated, and the measurements were performed using a pencil-type ionization chamber at five different points of each head phantom. Absorbed dose values were obtained for each phantom in order to compare the dose variation between phantoms and between radiodiagnostic services. From these data, different acquisition protocols were tested using different values of X-ray tube supply voltage (70, 80, 100 and 120 kV) and load (mA.s). Among the acquisition protocols tested, those that presented the lowest value of absorbed dose, defined by the Volumetric Dose Index (CTDI_vol), were selected as optimal. As a control parameter for testing new protocols, it was defined that the noise in the central slice image should be limited to 1% in order to guarantee the image diagnostic quality. Absorbed dose values (CTDI_vol) using routine protocols for head scans ranged from 23.67 to 50.91 mGy. Proposed optimized protocols reduced the absorbed dose by up to 91,04% in the smallest phantom, with 11 cm in diameter that represents a head of newborn pacient.