Effectiveness of the single-shot dual-energy subtraction technique for portal images.

The aim of the present study was to evaluate the clinical efficacy of the single-shot dual-energy subtraction technique for obtaining portal images. We prepared two storage phosphor plates for this study. A 1 mm thick tungsten sheet was placed between the two storage phosphor plates. A single use of the double-exposure technique provides two portal images simultaneously (i.e., a standard image and a low-contrast image), using the same patient position and with no additional radiation delivered to the patient. A bone-enhanced image is created by image subtraction between these two images. For evaluation of clinical efficacy, three treatment sites--the brain, lung, and pelvis--were imaged. Ten sets of images were obtained for each site, and five landmarks were selected for each treatment site. The visibility of each landmark and the ease of overall verification for the selected treatment sites were assessed separately for the standard and bone-enhanced images. Four observers consisting of one radiation oncologist and three radiation therapists participated in the present study. For most of the landmarks studied, the bone-enhanced images were significantly superior to the standard images. Regarding the ease of overall verification, the bone-enhanced images were significantly superior to the standard images at all sites. The p-values of mean rating for the brain, lung, and pelvis were 0.002, 0.012, and 0.003, respectively. The bone-enhanced images obtained using our technique increased the image quality in terms of bone visibility, and are considered useful for routine clinical practice.

Investigation of optimal viewing size for detecting nodular ground-glass opacity on high-resolution computed tomography with cine-mode display.

We evaluated the effect of the displayed image sizes on observers' ability to detect nodular ground-glass opacity (n-GGO) on CT and investigated the optimal viewing size for soft-copy reading at CT screening for lung cancer. A total of 46 patients' high-resolution computed tomography (HRCT) images (22 patients with one GGO; 24 without GGO) were displayed on a monochromatic liquid crystal display monitor at a resolution of 1,200 × 1,600. HRCT was presented on the screen with cine-mode display. We compared two viewing sizes (original size, i.e., the image displayed with a zoom factor of 1 in which each pixel value in the image is displayed as one pixel on the display: 13 cm × 13 cm; fit size, i.e., by zooming the captured image until it occupies the entire screen: 30 cm × 30 cm) in terms of radiologists' performance for detecting n-GGO on HRCT and the viewing times required for soft-copy reading decisions. Observer performance was analyzed in terms of the receiver operating characteristic (ROC) curve. A statistically significant improvement was found with the original size in the average area-under-the-ROC curve values for the accuracy of diagnosis and the viewing times compared to the fit size (P < 0.05). The original size with cine-mode display leads to increased lung GGO detection at CT screening for lung cancer, and the reduced time spent performing the diagnosis offers cost savings.

Investigation of optimal display size for detecting ground-glass opacity on high resolution computed tomography using a new digital contrast-detail phantom.

The purpose of this study was to clarify the relationship between display sizes of high resolution computed tomography (HRCT) images for detecting ground-glass opacity (GGO) and observer performance using a digital contrast-detail (d-CD) phantom. A structure of the d-CD phantom was determined on the basis of the actual images of GGOs and background noises of 22 patients who were diagnosed as GGO by chest HRCT. The d-CD phantom has a 512×512 matrix in size and has total of 100 holes: the diameter of these holes increases stepwise from 2 to 20 pixels with 2 pixels interval in a vertical direction and the CT value varies stepwise from 2 to 200 HU in a horizontal direction. The observer performance study was carried out for three different display sizes (30 cm×30 cm as an enlarged size, 13 cm×13 cm as an original size, and 7 cm×7 cm as a reduced size) using a 2-megapixels LCD monitor, and it was analyzed using Friedman and Wilcoxon statistical tests. As a result, the observer performance for the original display and the reduced display sizes was superior to that for the enlarged size (P=0.006 and 0.037 for the original display and the reduced display sizes, respectively), whereas there was no significant difference between the original display and reduced display sizes (P=0.77). The d-CD phantom enables a short-term evaluation of observer performance and is useful in analyzing relationship between display size and observer performance.

Patient setup verification procedure for a portal image in a computed radiography system with a high-resolution liquid-crystal display monitor.

In our conventional visual inspection for setup verification, we have routinely used a console monitor of a CR system and a monitor of a treatment-planning system (i.e., the separate-monitor method) in order to avoid the need for CR portal-film generation. However, the separate-monitor method provided insufficient precision in detecting setup errors. We devised a setup verification procedure that uses a high-resolution liquid-crystal display monitor (i.e., the single-monitor method). Our objective in the present study was to evaluate the precision of the single-monitor method. These two methods were compared in terms of the precision of visual inspection. The single-monitor method was significantly superior to the separate-monitor method in sensitivity and in the magnitude of the discrepancy that could not be detected. The single-monitor method provides higher precision in visual inspection than does the separate-monitor method, and is a useful verification procedure.

Usefulness of reduced image display size in softcopy reading: evaluation of lung nodules in chest screening.

RATIONALE AND OBJECTIVES: The purposes of the present study were to investigate the impact of viewing size on soft-copy diagnosis for detecting abnormalities on digital chest radiographs and to verify the usefulness of reduced digital chest radiography. MATERIALS AND METHODS: Receiver-operating characteristic (ROC) and localized ROC (LROC) analysis of clinical images was performed using the standard digital image database of the Japanese Radiation Technology Society. A total of 30 images with and 20 images without nodule samples were extracted randomly from the database and used for ROC analysis. A total of 100 images were prepared for observation of reduced and nonreduced images. Observers' viewing images were adjusted as nonreduced size (large, 30 x 30 cm) and reduced size (small, 15 x 15 cm). RESULTS: The estimated mean areas under the LROC curves were 0.665 +/- 0.071 for large-size images and 0.669 +/- 0.087 for small-size images. However, no statistically significant difference was found between the two groups (P = .823). The viewing time of small-size images (1201.4 seconds) was significantly shorter than that of large-size images (1719.7 seconds). A statistically significant difference was also found in viewing times between reduced and nonreduced digital images with Wilcoxon's signed-rank test (P < .05). CONCLUSION: This study provided important information that there was a statistically significant difference in viewing times between reduced and nonreduced digital chest radiographic images, whereas no significant difference was found in areas under the LROC curves for the accuracy of diagnosis between the two groups. However, the double-check method for chest x-ray screening would be done efficiently by changing the viewing size to improve the specificity of diagnosis.

Psychophysical evaluation of calibration curve for diagnostic LCD monitor.

PURPOSE: In 1998, Digital Imaging Communications in Medicine (DICOM) proposed a calibration tool, the grayscale standard display function (GSDF), to obtain output consistency of radiographs. To our knowledge, there have been no previous reports of investigating the relation between perceptual linearity and detectability on a calibration curve. MATERIALS AND METHODS: To determine a suitable calibration curve for diagnostic liquid crystal display (LCD) monitors, the GSDF and Commission Internationale de l'Eclairage (CIE) curves were compared using psychophysical gradient delta and receiver operating characteristic (ROC) analysis for clinical images. RESULTS: We succeeded in expressing visually recognized contrast directly using delta instead of the just noticeable difference (JND) index of the DICOM standard. As a result, we found that the visually recognized contrast at low luminance areas on the LCD monitor calibrated by the CIE curve is higher than that calibrated by the GSDF curve. On the ROC analysis, there was no significant difference in tumor detectability between GSDF and CIE curves for clinical thoracic images. However, the area parameter Az of the CIE curve is superior to that of the GSDF curve. The detectability of tumor shadows in the thoracic region on clinical images using the CIE curve was superior to that using the GSDF curve owing to the high absolute value of delta in the low luminance range. CONCLUSION: We conclude that the CIE curve is the most suitable tool for calibrating diagnostic LCD monitors, rather than the GSDF curve.

Comparison of gradation characteristics between three typical medical imaging display devices using psychophysical analysis.

Although there are remarkable differences in maximum luminance in cathode ray tube (CRT) and liquid crystal display (LCD) monitors and film/viewer systems, these differences cannot be recognized in our perception of them. To clarify the reason for this conflict, we analyzed the psychophysical gradient (delta), which is based on the minimum perceptible luminance difference (DeltaL(min)) and can express contrast visually recognized by observers. In this study, we first confirmed the compatibility of the psychophysical analysis to the CRT and the LCD monitors by using their threshold contrasts (C(t)s). Second, we calculated and compared the delta's of the above output devices. The C(t)s values of each device were in good agreement. Moreover, the Moon & Spencer model, which expressed the perceptibility of luminance change, was well suited to the measured C(t)s over the whole luminance range. The psychophysical analysis is therefore available not only for the film/viewer system, but also for the CRT and LCD systems. The difference of physical gradient G of the luminance characteristics curve among the output devices was larger than 20 times, whereas that of d was within 3 times. The display devices listed in the order of decreasing delta were film/viewer>LCD>CRT. These results corresponded to the visual contrast sensation and our clinical experience, which cannot recognize remarkable differences in perception. By using the psychophysical analysis, we clarified the reason for the conflict between the results of physical evaluation and the contrast visually recognized by observers.

Verification imaging using a computed radiography system for high-energy electron beam therapy.

PURPOSE: The aim of this study was to examine a method to more conveniently acquire verification images. A computed radiography (CR) system was employed using a photostimulable phosphor plate and diagnostic cassette used for taking diagnostic X-ray photographs. MATERIALS AND METHODS: Verification images were acquired using the diagnostic cassette and therapy cassette. Image processing parameters were adjusted to improve the image quality. Verification images were printed using optimum parameters, and 10 radiation technicians visually evaluated the images. The sign test was used, and image quality was evaluated using a two-sided test at a 5% level of significance. To assess its clinical value, a radiation oncologist evaluated patient verification images. RESULTS: Verification image quality was improved by the adjustment of image processing parameters. As a result of the image quality evaluation, there were no significant differences between the two types of cassette (p>0.05). In most of the clinical cases (98%), the verification images were useful. CONCLUSION: We found that good quality verification images were acquired by a CR system with a diagnostic cassette. This system is suitable for practical use to acquire daily verification images, and it is considered useful for maintaining quality assurance (QA) in high-energy electron beam therapy.