Using difference detail curves to assess image quality of low contrast abdominal CT scans

Purpose: The desire for dose- and image quality co-optimization in Computer Tomography (CT) scans presumes the ability to measure radiation exposure and image quality as realistically as possible, in phantoms or preferably in patients. In this realm of CT image quality, object detectability is a pivotal quantity, especially when iterative reconstruction algorithms are involved: Since these algorithms behave in a non-linear way, the total modulation transfer function (MTF) does not correspond to the product of the partial MTFs and thus the MTF is an inadequate figure of merit for image quality. In this work, we propose an alternative measurement method for assessing dose-related image-quality of CT scans based on the difference detail curve (DDC), and we showcase its use in a low contrast setting. Materials and Method: Our method is based on a phantom consisting of elliptical slices of different sizes into which contrast object modules can be inserted. These modules contain contrast objects based on (synthetic) resin mixtures with sucrose or sodium iodine. We provide mixing ratios to achieve a range of clinically relevant CT-numbers with these materials and verify contrast accuracy and energy dependency. We also describe a custom-made software used by up to 5 observers to evaluate image quality of 655 scans of the phantom acquired at 30 different clinical institutions and with 16 different CT scanner models from 4 manufacturers. For the scans, the tube voltage is set to 100 and 120 kV and the tube current is chosen such that the CTDIVol calculated by the machine is on a level in the clinically relevant range of 3 mGy to 20 mGy. Results: Phantom verification: The contrast objects meet the targeted CT-numbers accurately, the energy dependency is similar to that of water or soft tissue and no drift is observed over the course of a year. Image evaluation: The relationship between the contrast object size and the lowest contrast of that size seen by an observer is logarit