X-Ray Phantom Development For Observer Performance Studies

AbstractThe requirements for radiographic imaging phantoms for observer performance testing include realistic tasks which mimic at least some portion of the diagnostic examination presented in a setting which approximates clinically derived images. This study describes efforts to simulate chest and vascular diseases for evaluation of conventional and digital radiographic systems. Images of lung nodules, pulmonary infiltrates, as well as hilar and mediastinal masses are generated with a conventional chest phantom to make up chest disease test series. Vascular images are simulated by hollow tubes embedded in tissue density plastic with widening and narrowing added to mimic aneurysms and stenoses. Both sets of phantoms produce images which allow simultaneous determination of true positive and false positive rates as well as complete ROC curves.IntroductionThe ability to realistically evaluate new medical imaging systems as they are introduced is essential. Simple resolution tests are not sufficient. Rossmann et al.l demonstrated that the optimum film screen combination depends on the imaging task. The film screen combination superior in imaging high contrast small objects did not produce superior images of low contrast moderately large objects such as those encountered in a gallbladder examination. Shea et al.2 reported that there is an optimum viewing distance for each diagnostic task. Their results imply that factors such as viewing distance which alter information display should change depending on the diagnostic task. An ideal test phantom should allow evaluation of system performance under realistic conditions. The correct answer must be unambiguously known and the difficulty of the diagnostic task should vary from clearly obvious to fiendishly subtle. In practice it has not yet been possible to develop a phantom which mimics all portions of the diagnostic task. We have chosen to develop two types of phantoms to evaluate the response of different imaging systems to selected disease signs in chest and vascular radiographic examinations.An ideal phantom would be constructed of tissue eouivalent material whose average is equivalent to muscle so the KVP dependence is the same as tissue. Cur current phantoms are constructed from a polyester resin which has a mass density nearly equal to tissue. An improved material would be required to use the phantoms for dual KVP systems evaluation. Regardless of the system under evaluation, both time positive and false positive rates must be determined because both are important in clinical practice.Phantom developmentWe have chosen to develop phantoms of chest disease signs which can be used in conjunction with a commercially available phantom (Humanoid Systems, Inc.)3 and a vascular phantom which can be used to evaluate both neurovascular and cardiovascular systems. Chest and vascular radiographic examinations make up almost three-quarters of conventional radiographic examinations. Thus, these two phantom systems should provide the ability to evaluate imaging systems covering the majority of examinations in conventional diagnostic practice.Phantoms for chest diseasesWe have attempted to develop phantoms which mimic chest nodules, lung infiltrates, and hilar or mediastinal masses. The use of lunp nodule images for observer performance testing is well documented in the literature, Kelsey et al.^, Kundel et al.5> and Sorensen et al.o. The density difference between the nodule and the surrounding material should be about 9.7 grams per cc to mimic the density difference between a tissue nodule and surrounding lung tissue. We have used wooden spheres ranging in size from 8 to 15 mm on the posterior surface of the commercial lung phantom, to obtain nodule images.Lung infiltrate phantomsIn a lung infiltrate the alveolar air spaces fill with fluid, blood or cells, and the overall density of the lung increases. In extreme cases of pneumonia the lung can have