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1
Department of Radiology, Seoul National University College of Medicine and
Institute of Radiation Medicine, SNUMRC, 28, Yongon-dong, Chongro-gu, Seoul
110-744, Korea.
2
Department of Radiology, Gil Medical Center, Gachon Medical School, 1198,
Kuwol-dong, Nambdong-gu, Inchon 405-220, Korea.
3
Department of Radiology, Samsung Medical Center, College of Medicine,
Sungkyunswan University, 50, Ilwon-Dong, Kangnam-Ku, Seoul 135-710,
Korea.
Received February 11, 2000;
accepted after revision March 23, 2000.
Address correspondence to J.-G. Im.
Abstract
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MATERIALS AND METHODS. Two chest radiologists and two residents analyzed 46 pairs of posteroanterior chest radiographs on high-resolution video monitors (2560 x 2048 x 8 bits). In each pair, one radiograph was obtained with a storage phosphor radiography system, and the other radiograph was obtained with a selenium-based flat-panel detector radiography system. Each pair of radiographs was obtained at the same exposure settings. The interpreter rated the visibility and radiographic quality of 11 different anatomic regions. Each pair of images was ranked on a five-point scale (1 = prefer image A, 3 = no preference, 5 = prefer image B) for preference of technique. Statistical significance of preference was determined using the Wilcoxon's signed rank test.
RESULTS. The interpreters had a statistically significant preference for the selenium-based radiography system in six (unobscured lung, hilum, rib, minor fissure, heart border, and overall appearance) of 11 anatomic regions (p < 0.001) and for the storage phosphor system in two regions (proximal airway and thoracic spine) (p < 0.05). Chest radiologists strongly preferred selenium-based images in eight regions, and they did not prefer storage phosphor images in any region.
CONCLUSION. The soft-copy images produced by the selenium-based radiography system were perceived as equal or superior to those produced by the storage phosphor system in most but not all anatomic regions.
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During recent years, with the rapid development of electronic and computer technology, digital radiologic detectors have undergone considerable investigation and development. A new technology, a flat-panel digital detector that uses active matrix readout of amorphous selenium, has been proposed [14,15,16,17,18]. The active matrix consists of a two-dimensional array of thin-film transistors. In comparison with film-screen and storage phosphor radiography systems, the selenium detector is characterized by a higher detective quantum efficiency [19]. Anticipating that the image quality of the selenium detector system exceeds that of film-screen and storage phosphor radiography systems, the new system could allow an improved detection of small, low-contrast lesions.
We compared the soft-copy images of a new digital chest radiography system that uses a flat-panel X-ray detector based on amorphous selenium with those of a storage phosphor radiography system for the visualization of anatomic regions of the chest.
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Image Acquisition and Display
Forty-six consecutive patients underwent imaging with the selenium-based
and the storage phosphor systems using an institutional review board-approved
protocol; patient consent was obtained in all cases. Patients with an opacity
occupying one third of their hemithorax on previous radiographs or with a
history of thoracic surgery were excluded from the study. The patient group
was primarily composed of inpatients, 43% of whom were women. The patients had
radiographs with normal findings (n = 15) or radiographs with one or
more abnormalities (n = 31).
Two Bucky stands were set up on opposite sides of the same room for each radiography system. Radiography was performed using the same tube generator. The same radiographer performed both examinations at the same exposure settings: 100 kV and 8.0 mAs using a 180-cm focus-detector distance. Both imaging systems included a moving 10:1 antiscatter grid (103 lines per inch).
The digital data were sent to a PACS server (Radmax; MaroTech, Seoul, Korea) and distributed to workstations. All images were downloaded onto a local hard drive of a display workstation before interpretation. Each storage phosphor image was 7.18 MB, and each selenium-based image was 15.0 MB. Two 21-inch video monitors with 2048 x 2560 x 8-bit pixels (DR110; Dataray, Denver, CO) were used in a darkened room for side-by-side image display. The monitor operated at 71 Hz in an interlaced mode. The monitor had a maximum brightness level of 100 footlamberts. Because the video monitors we used have a matrix of 2048 x 2560 pixels, only part of the selenium-based image data (2560 x 3072 matrix) was shown using these displays. The selenium-based image matrices were reduced by 20% for display on the workstation. Gray-scale digital images were modified by means of a 12- to 8-bit (selenium-based image) or 10- to 8-bit (storage phosphor image) look-up table. To eliminate the difference between the two monitors, the maximum brightness of the two monitors was adjusted to be equal. The soft-copy images were displayed without unsharp masking. Only the window width and window level of the images were automatically optimized by the customized program. Interpreters were allowed to adjust the brightness and contrast of the images. Magnification of the images was not allowed. For this interpreter preference study, patient identification was obscured on all images and replaced by a sequence number. Each pair of images was displayed side-by-side in a random manner.
Data Analysis
Four radiologists compared the paired images using a standardized protocol.
Two of the four interpreters were chest radiologists who had fellowship
training in thoracic imaging and whose clinical practice involved almost
exclusively chest imaging; the other two interpreters were senior residents.
All interpreters were accustomed to using PACS in daily practice. The images
were interpreted independently, and each interpreter was blinded to patient
history.
Eleven anatomic regions were evaluated in the posteroanterior views. The regions were the unobscured lung, hilum, minor fissure, retrocardiac lung, lung projected below the diaphragm (subdiaphragmatic lung), azygoesophageal recess, heart border, rib, proximal airway, thoracic spine, and overall appearance (Figs. 1A,1B and 2A,2B). Each pair of images was ranked on a scale from one to five (1 = strongly preferred A, 2 = somewhat preferred A, 3 = no preference, 4 = somewhat preferred B, and 5 = strongly preferred B [technique A = selenium-based radiography system]) for preference of technique. These responses were recorded and resorted to each system for statistical analysis.
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The difference caused by minor position changes (e.g., minor fissure) was ignored, and regions were analyzed as they appeared (Fig. 3A,3B).
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The interpreters' responses were evaluated in three groups: responses of chest radiologists, responses of residents, and responses of all interpreters. To determine interpreter preference for each imaging finding, statistical significance was calculated using the Wilcoxon's signed rank test at the conventional level (p < 0.05) for each of the 11 regions.
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Our results revealed that interpreters preferred selenium-based images in six of 11 anatomic regions. These regions are confined to nonmediastinal areas. Although the interpreters were not told which images were from the selenium-based system and which were from the storage phosphor system, the features of nonmediastinal areas, especially the unobscured lung, were so conspicuous in selenium-based images that the interpreters could distinguish between the two systems. This factor is a potential source of bias in our study. For example, bias could have influenced our results if the interpreter had a prejudiced opinion and preferred one radiography system over the other.
One of the possible causes for the difference in image quality is the difference in spatial resolution. Pixel size is an important parameter in digital radiography because it directly influences both the spatial resolution of the images and the cost of the imaging system. Other factors being equal, the smaller the pixel size, the better the image quality, particularly in the depiction of fine detail. Currently, most radiologists agree that a spatial sampling resolution of 5 pixels/mm (0.2-mm pixels) in an adult chest image is required for the depiction of fine detail such as septal lines or a subtle pneumothorax [13, 25, 26]. In our study, the pixel size of the storage phosphor system was 0.2 mm, and the pixel size of the selenium-based system was 0.139 mm. Theoretically, the spatial resolution of the selenium-based system was 1.4 times better than that of storage phosphor system. However, considering the limitation of the monitor in displaying images, the factor decreases to 1.2.
Within certain limits, a smaller pixel size allows improved spatial resolution in a digital image. Conversely, use of a larger pixel size (i.e., fewer data) facilitates storage, transmission, and processing of digital information. The optimal pixel size for a given application will be one that is just small enough to provide an acceptable level of diagnostic accuracy. Then adequate image quality will be maintained while data storage requirements are reduced to a practical minimum. Although currently a new generation of storage phosphor devices with higher resolution 4K image formats is available, the data size of the 4K system is too large to use in the PACS available at our hospital. This is an important factor limiting the clinical implementation of a PACS. The data size of selenium-based images (15.0 MB) was more than twice the size of the storage phosphor images (7.18 MB). However, recent research [27] suggests that data compression techniques that reduce the amount of digital data by as much as 25-fold can be used with a minimum loss of perceptible quality.
More pixels do not always mean higher spatial resolution because image blurring can result from the scatter of X-ray beams, scatter of light, or both in the detector. In a selenium-based flatpanel system, the light or electric charges are converted into digital data by arrays of semiconductor elements. This absorbs the X-ray photons and directly produces electric charges that are attracted to the plate electrodes by an applied potential difference. Therefore, the conversion of X-ray photons to electric charges is direct and does not use an intervening light stage as in an intensifying screen system (filmscreen radiography) or photostimulable phosphor (storage phosphor radiography system. The light is scattered in the intensifying screen or photostimulable phosphor producing a curved signal profile that blurs the image. The selenium-based system, without this intermediate light fluorescence, has a square profile signal, providing sharper image quality [28].
The number of gray levels in a digital system determines how well it reproduces subtle variations in attenuation in an accurate and visually acceptable way. Unlike analog images, a digital image has only a predetermined number of shades of gray. In practice, 256 shades of gray are the minimum number necessary for adequately displaying a digital chest radiograph; 1024 shades (10 bits) or, preferably, 4096 shades (12 bits) provide a visibly superior display [29]. In our study, interpreters preferred storage phosphor images in two regions (proximal airway and thoracic spine) that were located in the mediastinal area. Because the spatial resolution is superior in selenium-based images, this difference may be caused by the contrast scaling of an image. The contrast scaling refers to a series of lookup tables that can be applied to the digital image data to map pixel values to luminance of monitor display. The selenium-based image (12 bits) requires more contrast scaling than the storage phosphor image (10 bits), and the default setting on a monitor may not be adequate for displaying the mediastinum in a selenium-based image. Although this could be overcome by adjusting the contrast on a workstation, interpreters are not likely to adjust the contrast when comparing images. Another explanation for the difference is that these two regions are characterized by medium-sized low-contrast shadows in a highly attenuated area that does not require small pixels to preserve sharp edges. These areas have a relatively low signal-to-noise ratio because of the quantum mottles produced by the highly attenuated X-ray beam. When comparing the images of this area produced by the selenium-based system with those produced by the storage phosphor system, the quantum mottles were more notable on the selenium-based images. On the storage phosphor images, the quantum mottles might be blurred in the light scattering process, which does not occur in the selenium-based system. Therefore, the light scattering process in the storage phosphor system might have worked as an image filter that increased the apparent signal-to-noise ratio of the shadows in the two regions.
This study was a first step in the evaluation of the diagnostic use of selenium-based flat-panel chest radiography because interpreter preference is an important factor in its clinical acceptance. In general, chest radiologists preferred selenium-based images more than residents did. Chest radiologists had no statistically significant preference for storage phosphor images. Both groups strongly preferred selenium-based images in overall appearance. However, further receiver operating characteristic studies are necessary to compare the diagnostic accuracy of selenium-based images with that of storage phosphor images.
Further work is needed to optimize anatomy-specific exposure techniques. In our study, voltage and amperage were identical for the selenium-based and storage phosphor radiography systems. In our study, we used 100 kVp because this is the voltage used at our hospital. Further improvement in the quality of digital radiographic images and a possible reduction in X-ray dose may be possible by optimizing the voltage and amperage of flat-panel detectors. When we used a stationary grid, corduroy artifacts developed because of image aliasing; therefore, we used a moving grid to avoid these artifacts. Although the motor required for a grid may interfere with the electronics, there were no significant artifacts in our series.
In conclusion, the soft-copy images of a selenium-based flat-panel radiography system are perceived as equal to or superior to those of a storage phosphor radiography system in most but not all anatomic regions. This new digital radiography system could be an appropriate technique for the entry of images into a PACS.
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