The aim of this study was to evaluate visual fatigue objectively by measuring accommodation time and critical fusion frequency (CFF) before and after reading posteroanterior chest radiographs displayed on medical-grade liquid-crystal displays (LCDs) under different monitor conditions. A color LCD (500, 170 cd/m2) and a monochrome LCD (500 cd/m2) were used in this study. Six observers independently kept reading the radiographs for two hours to understand various lung nodules in the “Fatigue Session”. Objective visual fatigue was measured by using the accommodation device and the CFF meter before and after the Fatigue Session. The ambient lighting of the laboratory was set at 35 lux. Both the accommodation time and the CFF between before and after the Fatigue Session indicated statistically significant differences (p<0.05). Our results on accommodation time and CFF before and after reading the radiographs on medical-grade LCDs indicated that visual fatigue could be evaluated objectively.
The purposes of this study were to evaluate the image quality of five types of liquid-crystal display (LCD) monitors by utilizing the normalized-rank approach and to investigate the effect of LCD monitor specifications, such as display colors, luminance, and resolution, on the evaluators’ ranking. The LCD monitors used in this study were 2, 3 and 5 mega-pixel monochrome LCD monitors, and 2 and 3 mega-pixel color LCD monitors (Eizo Nanao Corporation). All LCD monitors were calibrated to the grayscale standard display function (GSDF) with different maximum luminance (recommended luminance) settings. Also, four kinds of radiographs were used for observer study based on the normalized-rank approach: three adult chest radiographs, three pediatric chest radiographs, three ankle joint radiographs, and four double-contrasted upper gastrointestinal radiographs. Ten radiological technologists participated in the observer study. Monochrome LCD monitors exhibited superior ranking with statistically significant differences (p<0.05) compared to color LCD monitors in all kinds of radiographs. The major difference between monochrome and color monitors was luminance. Therefore, it is considered that the luminance of LCD monitors affects observers’ evaluations based on image quality. Moreover, in the case of radiographs that include high frequency image components, the monitor resolution also affects the evaluation. In clinical practice, it is necessary to optimize the luminance and choose appropriate LCD monitors for diagnostic images.
Instantaneous detectability was introduced in a method for the subjective assessment of fluoroscopic images, particularly in the interventional radiology (IVR) procedure. Quantitatively, instantaneous detectability was obtained by measuring the time required for observers to detect the tip of a linear pattern, such as a guide wire, just after the image is displayed. Dynamic images used in this measurement, which mimic degraded fluoroscopic images, were created in the computer simulation software by adding a low-contrast linear pattern to a noisy background image. Radiological technologists and students in the faculty of computer engineering participated in the assessment, and all measurements were performed using a personal computer system. Even if the contrast-to-noise ratio was identical, instantaneous detectability was remarkably increased when the background noise was dominated by higher frequency components. Also, the sign test suggested that a frame rate of 30 f/s significantly improved detectability compared to a frame rate of 15 f/s. These results enable us to discuss new possibilities for image processing and the optimization of system performance. Although the standard deviation of the measured inter- and intra-observer data was large, statistical significance should be suitably examined by a paired-comparison like the sign test, which will be one of the important analyzers in experiments investigating human performance.
Flat-panel detector (FPD) digital radiography systems have direct and indirect conversion systems, and the 2 conversion systems provide different imaging performances. We measured some imaging performances [input-output characteristic, presampled modulation transfer function (presampled MTF), noise power spectrum (NPS)] of direct and indirect FPD systems. Moreover, some image samples of the NPSs were visually evaluated by the pair comparison method. As a result, the presampled MTF of the direct FPD system was substantially higher than that of the indirect FPD system. The NPS of the direct FPD system had a high value for all spatial frequencies. In contrast, the NPS of the indirect FPD system had a lower value as the frequency became higher. The results of visual evaluations showed the same tendency as that found for NPSs. We elucidated the cause of the difference in NPSs in a simulation study, and we determined that the cause of the difference in the noise components of the direct and indirect FPD systems was closely related to the presampled MTF.
With the screen/film X-rays imaging system, Wakamatsu et al. reported that there was a close relationship between the square root of spectral signal-to-noise ratio area and sensitivity measure d’ in receiver operating characteristic (ROC) analysis. In this study, we investigated the relationship between image quality and signal detectability in two digital X-ray imaging systems using computed radiography (CR) and a flat panel detector (FPD). We used urethane resin balls with a diameter of 2 mm as a signal for cases samples in ROC analysis. In this experiment, the square root of the spectral signal-to-noise ratio area was closely related to d’ in ROC analysis in both digital X-ray imaging systems. In addition, when the exposure dose increased, signal detectability improved, but then saturated at one level. These results suggest that the exposure dose can be reduced when the optimal dose setting can be determined.
The purpose of this study was to examine the correlation between the basic imaging properties of two digital radiographic X-ray systems with a direct conversion flat-panel detector and their image qualities, which were evaluated by the observer in hard copy and soft copy studies. The subjective image quality was evaluated and compared in terms of the low-contrast detectability and image sharpness in the two digital radiographic X-ray systems. We applied the radiographs of a contrast detail phantom to the evaluation of low-contrast detectability and analyzed the contrast detail diagrams. Finally, low-contrast detectability was evaluated by the image quality figure (IQF) calculated from the contrast detail diagrams. Also, the subjective image sharpness of human dry bones of two systems was examined and evaluated by the normalized-rank method. The results indicated that System A tended to provide superior subjective image quality compared to System B in both observer studies. We also found high correlations between IQFs and basic imaging properties, such as the noise power spectrum (NPS) and the noise equivalent quantum (NEQ). In conclusion, the low-contrast detectability of the two digital radiographic X-ray systems with a direct conversion flat-panel detector corresponded to the NPS and the NEQ in both outputs (soft copy and hard copy). On the other hand, the subjective image sharpness of human dry bones was affected by their noise properties.
Selection of the signal is one of the major factors in a receiver operating characteristic (ROC) study for evaluating the diagnostic accuracy of medical imaging systems. We investigated the effect of signal selections in ROC studies which were applied to evaluate a difference between the two systems. Each of 50 positive and 50 negative images obtained with and without two different types of phantom signals, but with the same background noise and two different digital imaging systems, were used as case samples. We assumed that two different types of phantom signal could be related to image resolution and graininess, respectively. We employed two systems which were assumed to have advantages in resolution and graininess, respectively. Twelve observers participated in this ROC study, which aimed to compare two parameter settings in computed radiography systems. A statistical significance test considering case and reader variations was conducted for each of the ROC data sets with two different signals. As results, p values obtained in the statistical significance test were varied by changing the type of signal even if the same observer group participated and the same two different systems were compared (p value = 0.0003 and 0.0944). In conclusion, it was suggested that the selection of phantom signal for an ROC study could lead to different conclusions if the type of phantom signal was not matched to the purpose of the ROC study.
Gadoxetate Sodium (Gd-EOB-DTPA) is a relatively new contrast agent used for liver examination by magnetic resonance (MR) imaging. The setting of the flip angle (FA) is very important to obtain images with high contrast in hepatocellular MR imaging. The aim of this study was to determine the optimal FA for hepatocellular MR imaging. Optimal FA was estimated using a visual evaluation by observers. We made the visual evaluation with a paired comparison test. When the overall characteristic showed a significant difference, we then evaluated the optimal FA using a yardstick analysis. The visual evaluation by observers indicated that the FA was best at 22 degrees for the overall characteristic, the contrast characteristic, and the sharpness characteristic, and for the noise characteristic, the best FA was 18 degrees. All characteristics showed a significant difference between 18 and 22 degrees. Based on our results, the most important factor was the contrast characteristic for the visual evaluation, and we concluded that the optimal FA in hepatocelluar MR imaging was 22 degrees.
The purpose of this study was to discuss the optimal X-ray beam quality for detection of simulated lung nodule on a computed radiography system. We set up four types of X-ray beam quality (90 kV, 120 kV, 150 kV, and 120 kV + gadolinium filter), and kept the incident dose on the patient at 0.3 mGy. A receiver operating characteristic (ROC) analysis and a granularity measurement were used to evaluate the relationship between the detection of a low-contrast object and X-ray beam quality. As a result, the areas under the ROC curve (AUC) of 90 kV and 120 kV + gadolinium filter were significantly superior to those of 120 kV and 150 kV (p<0.05). However, a significant difference was not observed between 90 kV and 120 kV + gadolinium filter, 120 kV and 150 kV. The order of the granularity values gave good agreement with the results of visual evaluation. In conclusion, we considered that the optimal X-ray beam quality was 90 kV or 120 kV + gadolinium filter.
Understanding inter-observer variability in clinical diagnosis is crucial for reliability studies. As the statistical measurements of reliability, the kappa statistic and its extensions have been widely adopted in medical research, but it has been discussed that kappa is vulnerable to prevalence and presence of bias. As an alternative robust statistic, AC1 has attracted recent statistical attentions. This article describes fundamental ideas and quantitative features of AC1. The reliability of infrared thermoscanner as an application in detecting febrile patients of pandemic influenza is discussed by means of Monte Carlo simulation. AC1 adjusts chance agreement more appropriately than kappa and is regarded as a more useful measurement for assessing inter-observer agreement, especially when prevalence is small.
To improve the reliability of statistically significant test results between two imaging systems in receiver operating characteristic (ROC) analysis, it was important to evaluate the variances among readers. In this study, we tested statistically significant differences with actual experimental data from ROC studies that were performed to evaluate the detectability of 4 kinds of image intensifier fluorographic films. The DBM MRMC—i.e., the software for analyze of statistically significant differences by the Jackknife method—was used. The relationship between p values obtained from analysis that treated only readers as a random sample in DBM MRMC, and the variances among readers, were investigated. The testing results showed that, when there were slight differences in the mean area under the corve (AUC) between imaging systems, variances among readers were estimated by p values, because p values indicated the degree of variance among readers. Furthermore, we found that p values obtained from analysis that treated only readers as a random sample in DBM MRMC were equal to p values of the two-tailed paired t-test. It was considered that variances among readers could be analyzed by investigating the relationship between the differences in the mean AUC of two imaging systems and the p values obtained in a two-tailed paired t-test.