Mimicking the Chest Radiograph: Patient Equivalence of the CDRH Chest Phantom with Added Insert

Abstract

Objective: To evaluate medical X-ray doses and image quality, so called phantoms that mimic particular aspects of the patient are used. The Centre for Devices and Radiological Health (CDRH) chest phantom has been used for studies of radiation exposure to the lung fields under automatic exposure (AEC) conditions. Recently, a quasi-anatomical insert was introduced to create contrast between chest organs. Direct comparison of the phantom performance with clinical chest images has not been reported previously. This study applies the phantom to conventional radiography imaging of the chest to establish its patient equivalence. Methods: Entrance doses with backscatter and chest radiographs of 77 patients were mirrored in the phantom at the same exposure factors. Optical density (OD) as well as beam transmission through the different regions of both media were also compared. A 2-sample t-test was used to test for differences while the Pearson's correlation coefficient (r) was used to test the strength of (any) linear relationship between the measured parameters in both media. Results: Results show a 13.5% difference in entrance surface doses (ESD). Beam transmission through both media showed statistically significant differences (P = 0.05). ODlung was higher in CXR than the phantom images. OD(lung + ribs) on the CXR was not statistically different (P = 0.09) from the phantom ODlung. Mean OD for the mediastinum varied by 28%. Differences are statistically significant (P < 0.05) in all areas except the diaphragm (P = 0.8). There is good +ve correlation in ESD and beam transmission for all regions. A weaker +ve correlation was found for OD in all areas. In both cases correlation is significant (P < 0.05). The phantom and CXR parameters vary linearly together, but are not of equal value. Conclusion: A linear relationship was found between measurements made with the phantom and CXR for beam transmission and for optical density. Thus the phantom can provide a useful test tool for both perceptual studies and quality assurance (QA) in chest radiography