Modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) are widely accepted measures of digital radiographic system performance. However the implementation of these measurement methodologies has been limited to a handful of researchers using an assortment of techniques. A prototype edge tool and easy-to-use software program, which can generate MTF, NPS, and DQE results quickly and easily in the field, have been developed. The edge tool consists of 1mm or 250 μ thick tungsten with two polished edges. Edge and NPS data were obtained and analyzed by 3 investigators using three analysis methods: Method A, the software under development for this report; Method B, code available on the web site of one of the investigators [Saunders and Samei, Med. Phys. 33, 308-319 (2006)]; and Method C, code developed by two other of the investigators [Samei and Flynn, Med Phy. 30, 608-622, (2003)]. In all cases the differences between the results using Method B and Method A were less than 1%. The differences between Method A and Method C were larger, up to 5.26%. NPS were calculated using Method A and B. The results were very close, with average errors less than 2.5% for exposures of 27.3, 9.3, and 2.7 μGy. Analysis of data for a 10 cm misalignment shows no significant error for either the 250 μ or 1mm edge. The method developed gives results that correlate closely with results obtained from established methods. The software is easy-to-use and flexible in its application. The Edge Tool developed has the necessary precision to accurately determine the MTF values of the system. Further validation of NPS and DQE is ongoing.
The risk of radiation exposure is greatest for pediatric patients and, thus, there is a great incentive to reduce the radiation dose used in diagnostic procedures for children to "as low as reasonably achievable" (ALARA). Testing of low-dose protocols presents a dilemma, as it is unethical to repeatedly expose patients to ionizing radiation in order to determine optimum protocols. To overcome this problem, we have developed a computed-radiography (CR) dose-reduction simulation tool that takes existing images and adds synthetic noise to create realistic images that correspond to images generated with lower doses. The objective of our study was to determine the extent to which simulated, low-dose images corresponded with original (non-simulated) low-dose images. To make this determination, we created pneumothoraces of known volumes in five neonate cadavers and obtained images of the neonates at 10 mR, 1 mR and 0.1 mR (as measured at the cassette plate). The 10-mR exposures were considered "relatively-noise-free" images. We used these 10 mR-images and our simulation tool to create simulated 0.1- and 1-mR images. For the simulated and original images, we identified regions of interest (ROI) of the entire chest, free-in-air region, and liver. We compared the means and standard deviations of the ROI grey-scale values of the simulated and original images with paired t tests. We also had observers rate simulated and original images for image quality and for the presence or absence of pneumothoraces. There was no statistically significant difference in grey-scale-value means nor standard deviations between simulated and original entire chest ROI regions. The observer performance suggests that an exposure ≥0.2 mR is required to detect the presence or absence of pneumothoraces. These preliminary results indicate that the use of the simulation tool is promising for achieving ALARA exposures in children.