Development of a computed tomography phantom to evaluate radiation dose and image quality

One of the major challenges with the use of computed tomography (CT) is to establish the proper tradeoff between radiation dose and image quality. Imaging phantoms are a primary tool that can be used to optimize acquisition protocols without exposing the patients to any unnecessary risk. The purpose of this work was to develop a flexible and low cost imaging phantom to be used for CT optimization. A phantom container made of acrylic and with an elliptical shape was built. A scaffold to insert up to 9 samples with materials of radiological relevance was included within the phantom. Samples of iodinated contrast agent and bone-like material (calcium chloride) were inserted to the phantom. For experimentation two 64-slice systems were used: a dual-source and a conventional single-source CT systems. Imaging protocols with tube potential ranging from 80 to 140 kVp were used. Image noise, contrast, contrast-to-noise ratio (CNR) and figure of merit were calculated in order to assess the performance of the systems at the various acquisition schemes. A strong linear relationship (R²>0.95 in all cases) was found between iodine and calcium materials at different concentrations relative to the CT numbers. This was the case at all tube potentials evaluated and independent of the CT system. At lower tube potential values image contrast increased but image noise also increased. When evaluating a dose-weighted CNR, it was found that the lower kVp provided the most CNR per dose. In conclusion, the development of a low cost imaging phantom which can be used for CT protocols was demonstrated. Experimental evaluations of the phantom across CT protocols with several acquisition parameters and different vendors were performed. It was also demonstrated that this phantom allows to evaluate these systems (and corresponding protocol) performance by relating both achievable CNR and scanner radiation output.