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1 All authors: Department of Radiology, Rinku General Medical Center, 2-23 Rinku-orai-kita, Izumisano, Osaka 598-8577, Japan.
Received February 12, 2001;
accepted after revision May 29, 2001.
Address correspondence to K. Awai.
Abstract
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MATERIALS AND METHODS. One hundred twenty-four patients underwent selenium-based digital radiography, high-resolution storage phosphor radiography, and chest CT for evaluation of pulmonary nodules. Thirty-one patients with pulmonary nodules smaller than 3 cm in diameter and 40 patients with normal lungs were selected for receiver operating characteristic curve analysis. Five board-certified radiologists who were unaware of the CT results independently reviewed each of the hard copies of selenium-based digital radiography and storage phosphor radiography, identified pulmonary nodules, and graded their confidence for the presence of each nodule. For each radiologist, we calculated the areas under the receiver operating characteristic curve (AUC) for selenium-based digital radiography and storage phosphor radiography.
RESULTS. The average performance of selenium-based digital radiography (AUC = 0.72) was higher than that of high-resolution storage phosphor radiography (AUC = 0.64), which is statistically significant (p <0.05).
CONCLUSION. Our results indicate that selenium-based digital radiography is superior to high-resolution storage phosphor radiography for detecting solitary pulmonary nodules without calcification.
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The selenium-based digital radiography system (Thoravision; Philips Medical Systems, Hamburg, Germany) was introduced by Phillips Medical Systems in 1993 [2] and is a widely used digital chest radiography system. It provides digitized images with a nominal matrix size of 2000 x 2000 pixels, that is, 0.2-mm pixel size and 12-bit depth [2,3,4]. In observer preference studies, radiologists rated the quality of selenium-based digital radiography superior to that of conventional film radiography for the visualization of normal structures and equivalent for visualization of the remaining chest features [5, 6]. An evaluation of radiologists' ability to detect simulated pulmonary lesions also suggested that selenium-based images and conventional images yield equivalent diagnostic performance [7]. Selenium-based digital radiography systems have been recently applied to imaging of the pelvis, hand, and foot [8, 9].
Another digital radiography technique in wide clinical use is storage phosphor radiography. This technique was introduced in 1983 (Fuji Computed Radiography [FCR]; Fuji Medical systems, Tokyo, Japan) and originally yielded digitized images with a nominal matrix size of 2000 x 2000 pixels (2K systems), each pixel having a 0.2-mm size and 10-bit depth [10]. Currently, a higher-resolution storage phosphor radiography system is available, which has a nominal matrix size of 4000 x 4000 pixels (4K systems), each pixel having a 0.1-mm size [11, 12]. In the literature, 2K storage phosphor systems have been reported to be equivalent or superior to conventional film radiography in detection and evaluation of pulmonary nodules and opacities [13, 14]. When compared with conventional film radiography in inspection of pneumothorax or interstitial disease, 4K storage phosphor systems have been reported to show equivalent reviewer performance, although 2K storage phosphor systems do not do as well in such cases [11, 12].
The objective of this study was to compare selenium-based digital radiography with high-resolution storage phosphor radiography for the detection of solitary pulmonary nodules without calcification.
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The CT scans and radiographs were reviewed by board-certified radiologists and interpretation was obtained by consensus. Of the 124 patients studied, 40 patients whose findings were normal and 31 patients with solitary pulmonary nodules smaller than 3 cm in diameter were selected for receiver operating characteristic curve analysis. Fifty-three patients were excluded because they had pulmonary nodules containing calcification (n = 10), nodules larger than 3 cm in diameter (n = 5), multiple nodules more than two in number (n = 3), severe diffuse pulmonary diseases (e.g., idiopathic pulmonary fibrosis, chronic pulmonary emphysema, or diffuse bronchiectasis) (n = 28), or nodules smaller than 5 mm in diameter visible on CT scans (n = 7). The 71 patients selected included 29 men and 42 women, with a mean age of 60.5 years (range, 32-80 years).
CT Scanning
Chest CT examinations were performed on either a HiSpeed Advantage SG or a
LightSpeed QX/i scanner (GE Medical Systems, Milwaukee, WI). Helical CT scans
of the whole lung were obtained at 7.0-mm collimation with a helical pitch of
1.4, 0.8-sec rotation time, 7.0-mm image intervals, 120 kVp and 200 mAs on the
HiSpeed Advantage SG, and at 7.5-mm collimation with a helical pitch of 3,
7.5-mm image intervals, 120 kVp and 160 mAs on the LightSpeed QX/i. All
examinations were performed without IV administration of contrast media. All
images were reconstructed with a standard algorithm and displayed with a
soft-tissue window (width, 350 H; level, 30-50 H) and lung window (width, 1500
H; level, 600 H).
In patients with pulmonary nodules, thin-section helical CT scans through the nodules were obtained in addition to the routine scans. Thin-section helical scans were obtained at 1.0-mm collimation with a helical pitch of 1.4, 1.0-mm image intervals, 120 kVp and 230 mAs on HiSpeed Advantage SG and at 1.25-mm collimation with a helical pitch of 3, 1.25-mm image intervals, 120 kVp and 200 mAs on LightSpeed QX/i. Voxel readout through the nodules was performed to confirm that the nodules did not include calcification.
Selenium-Based Digital Radiography
Standard posteroanterior chest radiographs were obtained using a
selenium-based digital chest radiographic system (Thoravision; Philips Medical
Systems) with the patients in an erect position at a source-to-image distance
of 200 cm. Studies were phototimed at 120 kVp using a 0.1-mm copper filter
plus a 1.0-mm aluminum filter. For all examinations, a 15-cm air gap and fixed
antiscatter grid (10:1, with 40 lines per centimeter) was used. Parameters for
image processing with our Thoravision unit were set as follows: pulmonary
optical density, 1.50; abdominal optical density, 0.40; lower gamma value,
1.70; upper gamma value, 4.00; detail contrast enhancement, 0.80; and noise
suppression, 0.75.
The processed image data were compressed to 8 bits per pixel and were sent to a laser printer (Fuji CPLPD; Fuji Medical Systems) for printing at 98% magnification on 14 x 17-inch films (Fuji CR 780F; Fuji Medical Systems).
High-Resolution Storage Phosphor Radiography
Standard posteroanterior chest radiographs were obtained by using a
FCR9000HQ system (Fuji Medical Systems, Tokyo, Japan) and automatic
radiographic systems (Radiotex; Shimazu Medical Systems, Kyoto, Japan) with
the patients in an erect position at a source-to-image distance of 200 cm.
Studies were phototimed at 120 kVp using a 0.2-mm copper filter plus a 1.0-mm
aluminum filter. For all examinations, a Buck-Blende (10:1, with 40 lines per
centimeter) was used to reduce scatter.
Parameters for image processing with our storage phosphor radiography units were set as follows: gradient rotation amount, 1.1; gradient curve type, E; gradation rotation center, 1.6; gradation shift, -0.2; frequency rank, 2; frequency type, R; frequent enhancement, 0.2; dynamic range rank, 4; dynamic range type, B; dynamic range enhancement, 0.6; lower gamma value, 1.5; upper gamma value, 3.85; contrast, 0.85; and noise suppression, 0.70. The output images were printed on the same film and with the same laser printer as the selenium-based digital radiography at 98% magnification.
Image Scoring
Five board-certified radiologists who were unaware of the CT results
independently assessed each of the hard copies of selenium-based digital
radiography and storage phosphor radiography. They were asked to rank their
level of confidence in the presence or absence of a lesion by using a
continuous rating scale (0 = definitely no lesion, 100 = lesion definitely
present) [15] and to record
the rating of each patient on a questionnaire. The reviewers were also asked
to put a mark on the nodule on each radiograph, using a dermatograph, to
verify that the nodule identified by the reviewer corresponded to the location
of the actual nodule confirmed on CT. The image scoring was done in two
sessions, one for selenium-based digital radiography and the other for storage
phosphor radiography, with at least 1 week between sessions.
Data Analysis
The diagnostic value of both techniques for the detection of pulmonary
nodules was analyzed using receiver operating characteristics methods. For
each radiologist, we calculated the area under the receiver operating
characteristics curve and 95% confidence limits for selenium-based digital
radiography and for storage phosphor radiography
(Fig. 1).
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As can be predicted from physical properties, selenium-based digital radiography has a lower noise level and an improved low contrast resolution, and this may contribute to its greater ability to detect pulmonary nodules without calcification. Woodard et al. [16] compared selenium-based digital radiography with film-screen radiography and found no overall statistically significant difference in nodule detection between the two. However, they found trends of greater visibility on selenium-based digital radiography of nodules in obscured lungs and of nodules smaller than 1.0 cm in diameter. Our study did not compare selenium-based digital radiography and storage phosphor radiography for the detection of nodules smaller than 1.0 cm in diameter because our sample did not include enough nodules of that size to yield subsets of statistically significant power.
Although the low-contrast-resolution of selenium-based digital radiography is superior to that of high-resolution storage phosphor radiography (Fig. 2A,2B,2C), the spatial resolution of high-resolution storage phosphor radiography is superior to that of selenium-based digital radiography. Concretely, high-resolution storage-phosphor radiography provides a 0.1 mm pixel size, which is half of the pixel size of selenium-based digital radiography. This high spatial-resolution of high-resolution storage-phosphor radiography is indispensable for the diagnosis of subtle interstitial lung abnormality [12]. However, in our study, selenium-based digital radiography was superior to high-resolution storage phosphor radiography for the detection of pulmonary nodules. This result suggests that a lower noise level and an improved low-contrast-resolution may be more important factors than high-spatial resolution for the detection of the pulmonary nodules.
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The current selenium-based digital radiography system is the first radiography system to use amorphous selenium as a detector. One problem with the current selenium units is the system configuration, which requires a large detector unit including a rotating drum, and examination must be performed only in the erect position. In recent years, flat-panel detectors using amorphous selenium combined with direct electronic readout systems have been developed. This type of detector has a high detective quantum efficiency and signal-to-noise ratio comparable with that of a selenium-based digital radiography system. This detector is also compact and light-weight, enabling examinations to be performed in various patient positions. In the near future, flat-panel detector systems may expand the potential application of selenium-based digital radiography.
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