ALTHOUGH PACS (picture archiving and communication systems) and teleradiology have changed the practice of radiology quite dramatically over the past number of years, there are still many changes taking place in the digital reading room1, 2, 3
. One aspect that seems to evolve continuously is the display.4,5
Although early investigations tended to focus on how many monitors were required in the digital reading environment and what resolution was required,5
a critical issue today is whether flat-panel liquid crystal displays (LCDs) are more suitable for primary diagnostic reading than the more traditional CRT (cathode ray tube) display.6,7
In many respects, the LCD has performance characteristics that exceed those of the CRT.8,9
The MTF (modulation transfer function) of an LCD is essentially isotropic, whereas the MTF of a CRT is non-isotropic. At the highest digitally addressable spatial frequency along the CRT scanline in the horizontal direction, the value is usually only 10% to 20%. In the vertical direction the MTF at twice the line frequency is closer to 30% to 40%.8,9
The result is that, for the CRT, more than half the contrast modulation is lost at the highest spatial frequencies, but this degradation does not occur with the LCD. Practically speaking, this could mean degradation of high-frequency lesions in radiographic images such as microcalcifications in mammograms, resulting in reduced detection performance.
Veiling glare, defined as the diffuse spreading of light or scattering within various parts of a display device,9,10
is also significantly different for the LCD compared to the CRT, In the CRT monitor, the thick faceplate of the vacuum bulb, back-scattering of electrons and light leakage through aluminum-layer non-uniformities, all contribute to veiling glare. This diffuse spreading of light results in a significant degradation of the maximum contrast capabilities of the CRT monitor. The decreased contrast capabilities will especially affect the detection of low-contrast objects in images such as masses in mammograms. LCDs (liquid crystal displays), on the other hand, tend to have much less (almost negligible in fact) veiling glare than CRTs because there is no need for the vacuum barrier glass faceplate. The protective layer for the LCD TFTs (thin-film transistors) may result in some veiling glare, but not nearly as much as with the CRT display. The question that arises is whether the relatively low veiling glare and isotropic MTF of an LCD display affects observer performance less than the generally higher veiling glare and non-isotropic MTF of a CRT display. The goal of the present investigation was to compare performance on an LCD to a CRT display using both human and model observers.
We have demonstrated the utility of model observers to predict observer performance in a number of previous experiments aimed at defining the optimal display characteristics for viewing radiographic images in the digital environment.11, 12, 13
The eventual goal of investigating the validity of such models for predicting human observer performance is to eliminate, or at least reduce, the number of receiver operating characteristic (ROC) studies conducted. ROC studies are the gold standard for measuring observer performance in radiology, but they are time-consuming and require a significant amount of effort on the part of both the investigator and observer. If we could use model observers to at least narrow the relevant number parameters to study, we could reduce this experimental burden on observers. Additionally, model observers may help us understand better how the human visual system encodes and processes information during the diagnostic interpretation process.