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Original Research |
1 Department of Medical Physics and Radiation Protection, Ghent University,
Proeftuinstraat 86, Gent B9000, Belgium.
2 Department of Radiology, Ghent University Hospital, Gent, Belgium.
3 Department of Radiology, Heilig Hart Hospital, Roeselare, Belgium.
Received March 7, 2005;
accepted after revision July 20, 2005.
Address correspondence to K. Bacher
(klaus.bacher{at}ugent.be).
Abstract
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MATERIALS AND METHODS. In two groups of 100 patients each, digital chest radiographs were acquired with either an amorphous silicon or an amorphous selenium flat-panel system. The effective dose of the examination was measured using thermoluminescent dosimeters placed in an anthropomorphic Rando phantom. The image quality of the digital chest radiographs was assessed by five experienced radiologists using the European Guidelines on Quality Criteria for Diagnostic Radiographic Images. In addition, a contrast-detail phantom study was set up to assess the low-contrast performance of both systems at different radiation dose levels. Differences between the two groups were tested for significance using the two-tailed Mann-Whitney test.
RESULTS. The amorphous silicon flat-panel system allowed an important and significant reduction in effective dose in comparison with the amorphous selenium flat-panel system (p < 0.0001) for both the posteroanterior and lateral views. In addition, clinical image quality analysis showed that the dose reduction was not detrimental to image quality. Compared with the amorphous selenium flat-panel detector system, the amorphous silicon flat-panel detector system performed significantly better in the low-contrast phantom study, with phantom entrance dose values of up to 135 µGy.
CONCLUSION. Chest radiographs can be acquired with a significantly lower patient radiation dose using an amorphous silicon flat-panel system than using an amorphous selenium flat-panel system, thereby producing images that are equal or even superior in quality to those of the amorphous selenium flat-panel detector system.
Keywords: chest imaging • digital imaging • radiation dose • radiography • X-ray technology
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Storage phosphor plates, introduced approximately 20 years ago [6], were the first step toward full digitization of the radiology department. Nowadays, these computed radiography (CR) systems are widely used due to compatibility with existing radiographic equipment and they offer images comparable in quality to conventional screen-film combinations [1, 2].
Only recently, large-area full digital flat-panel radiography detectors that are based on active matrix thin-film transistor technology became commercially available. Depending on the detector type, conversion of X-rays into electric signals is either direct or indirect [1, 4, 5]. Direct X-ray conversion is performed in an amorphous selenium flat-panel system, whereas indirect conversion of X-rays is used in flat-panel detectors based on cesium iodide and amorphous silicon. Flat-panel systems combine all advantages of CR with a higher detective quantum efficiency (DQE) than both screen-film and CR systems [1, 4, 5]. In addition, these detectors allow an optimized working procedure because of instant image display and the elimination of the use of film cassettes.
Only a few studies have compared amorphous silicon and amorphous selenium flat-panel systems in clinical settings. The published results are mainly based on the measurement of noise characteristics, modulation transfer function (MTF), and DQE, and do not take into account patient acquisitions [7, 8].
Because chest radiography is the most commonly applied diagnostic radiography procedure [9], the aim of the present study was to compare the image quality and the patient dose in clinical chest imaging using either an amorphous silicon or an amorphous selenium flat-panel detector. Furthermore, a detailed analysis of contrast-detail was performed to compare the low-contrast performance of both systems at different dose settings.
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The amorphous silicon flat-panel detector system (Vertix FD, Siemens Medical Solutions; and Pixium 4600, Trixell) consisted of a ceiling-mounted X-ray tube (Opti 150/30/50 HC, Siemens; focal spot size, 0.6 mm), a high-voltage generator, and a motorized receptor wall stand with the flat-panel detector mounted behind a stationary antiscatter grid. In the amorphous silicon flat-panel detector, a needle-structured thallium-doped cesium iodide scintillator was used to convert the X-rays into visible light, which was deposited directly on the 43 x 43 cm2 amorphous silicon matrix. Subsequently, an amorphous silicon photodiode converted the light into electric charge, resulting in a 14-bit digital signal in a 3,001 x 3,001 pixel matrix.
The amorphous selenium flat-panel detector (Epex, Hologic; and DirectRay DR 1000, Direct Radiography Corporation) was used in combination with a ceiling-mounted X-ray tube (A192, Varian; focal spot size, 0.6 mm) and a high-voltage generator. The flat-panel detector was mounted behind a moving antiscatter grid. A detailed description of the selenium-based flat-panel detector can be found elsewhere [1, 4, 5]. Briefly, free electrons are released by the interaction of X-rays with the amorphous selenium semiconductor layer. Due to the application of an electric field across the selenium layer, the electric charges are drawn directly to the charge-collecting electrodes. The latter operation results in a 14-bit image stored in a 2,560 x 3,072 pixel matrix.
The automatic exposure control (AEC) of both flat-panel systems was used in the standard settings, as advised by the manufacturers. After acquisition, the digital data were sent to a PACS workstation (MasterPage, Eastman Kodak) for image quality assessment. All digital images were scored on a 21-inch (53-cm), high-contrast gray-scale monitor (MGD 2621P, Barco) with a resolution of 1,280 x 1,600 and a maximum luminance of 600 cd/m2. The DICOM header of all images was changed to remove system-specific information on the image display. In this way, no difference could be seen between the amorphous selenium and amorphous silicon soft-copy images, allowing a blind study.
Patient Study
Study group—Two hundred patients were referred to the
radiology department for both routine posteroanterior and lateral chest
radiography. At random, images were obtained with either the amorphous silicon
or the amorphous selenium flat-panel detector system. Pneumonectomy patients,
patients with extreme spinal deformity, and those who were unable to raise
their arms for the lateral view were excluded. The exposure for such patients
would be excessive and could therefore artificially increase the mean measured
radiation dose of a patient group.
The posteroanterior and lateral chest radiographs were obtained with the patients in an upright position at a focus-to-detector distance of 180 cm. All examinations were performed at 125 kVp using AEC. The displayed exposure values (in milliampere-seconds [mAs]) after each acquisition were recorded.
Scoring of patient images—Five experienced chest radiologists assessed all soft-copy posteroanterior chest radiographs and scored the image quality using a method adopted from the European Guidelines on Quality Criteria for Diagnostic Radiographic Images [10]. In that document, image quality criteria refer to characteristic features of imaged anatomic structures with a specific degree of visibility. In the Guidelines [10], three levels of visibility are defined: visualization (an anatomic feature is detectable but details are not fully reproduced), reproduction (details of anatomic structures are visible but not necessary clearly defined), and visually sharp reproduction (anatomic details are clearly defined). The images of both systems were presented in random order, and all images were interpreted independently. The radiologists were allowed to adjust the image brightness and contrast and to magnify the images to full resolution. The five radiologists had to decide whether a certain criterion (visualization, reproduction, or visually sharp reproduction) was fulfilled (score 1) or not (score 0) in a total of seven anatomic structures and four image details (Table 2). For low-contrast details (2 mm), the visualization of the normal bronchovascular structures up to the parietal pleura was considered. These peripheral structures are always smaller than the mentioned criteria (< 2 mm)— that is, a score of 1 was given when visible. When these bronchovascular structures could not be followed all the way up to the visceral pleura, the diameter of the smallest visible structure was measured. For high-contrast details, small calcifications, surgical material, or electrode leads were measured systematically.
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For each region or detail, the scores of all 100 chest radiographs were summed. In this way, the percentage of images that fulfilled the specific image criterion was calculated. An average percentage, using the scores from all five observers, was used in the data analysis.
Dose Measurements
The effective dose (E), representative of the risk of late
radiation-induced effects such as malignancies, is defined by the expression
[11]:
 | (1) |
Posteroanterior and lateral chest radiographs of the Rando phantom were obtained (125 kVp, 180-cm focus-to-detector distance) on both the amorphous silicon and the amorphous selenium systems. To obtain dose measurements well above the detection limit of the TLDs, the exposure was set to 300 mAs.
After the reading-out procedure of the TLDs, mean equivalent organ doses per milliampere-second could be calculated. Multiplying these results by the mean registered milliampere-second values of the two patient groups, the mean equivalent organ doses (in microSieverts [µSv]) for both imaging systems were derived. By combining these results with the corresponding tissue-weighting factors, wT, in equation 1, the effective dose (in microSieverts) could be calculated.
Contrast-Detail Study
Phantom acquisition—To achieve a more objective measure of
image quality, a contrast-detail phantom study (CDRAD 2.0, Artinis Medical
Systems) was set up [13,
14]. In this experiment, the
CDRAD 2.0 phantom was exposed at different entrance dose values. The CDRAD 2.0
phantom consists of an acrylic plastic plate with a 15 x 15 array of 1.5
x 1.5 cm2 cell regions in which holes are drilled. The holes
are logarithmically sized from 0.3 to 8.0 mm in both diameter and depth
(Fig. 1). For objects 4 mm or
smaller, the phantom contains an additional hole of matching diameter and
depth placed at random in one of the four cell corners. The presence of these
additional objects allows a four-alternative forced-choice experiment in which
the observer must select the location of the low-contrast objects among the
four possible corners [15]. In
this way, these extra holes help to minimize potential biases due to a priori
knowledge of the presence of objects in every square region
[16]. The phantom was used to
assess the minimum contrast required to visualize objects of different sizes
above the noise threshold.
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Image quality scoring of phantom images—As for the scoring of the patient images, the amorphous silicon and amorphous selenium images were displayed at random and analyzed by five independent observers. The observers were allowed to adjust the image brightness and contrast and to magnify the images to full resolution. The observers had to identify, in every square cell region, the locations of the corner holes. The results were entered on a score sheet for each image reviewed. After comparing the score forms to a reference form containing the correct locations of all corner holes, a correction scheme was used taking into account the nearest neighbors to get more accurate results [13-16]. Finally, for each different diameter (Di) the threshold contrast value (Ci,th) was determined as the minimum depth in regions of valid detection [16]. The obtained threshold contrast value results were averaged over 15 observations (3 image x 5 observers) and then plotted as the function of the object diameter to form the contrast-detail curves (Figs. 2A, 2B, and 2C). Holes above and to the right of this curve were visualized and holes below and to the left were not seen.
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The inverse image quality figure (IQFinv) was introduced for
quantitative comparison of the phantom images
[13]. The inverse image
quality figure is defined as follows:
 | (2) |
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In Table 3, the results of the exposure and dose measurements are summarized. For the posteroanterior and the lateral acquisitions, a significantly lower exposure and effective dose could be achieved with the amorphous silicon system when using the AEC settings of the manufacturers. For the posteroanterior view, the effective doses were 9.6 and 22.6 µSv for the amorphous silicon and amorphous selenium systems, respectively. For the lateral view, effective doses of 27.1 µSv (amorphous silicon) and 79.2 µSv (amorphous selenium) were obtained.
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For the image quality study, significant interobserver agreement was found
(p 
0.027), indicating that the individual scores of all
observers could be combined in an averaged value. In
Table 2, the mean scores for
the seven anatomic regions and the four image details in both systems are
indicated (maximum possible score, 100), together with the likelihood that
both systems gave the same result. The image quality scores of
Table 2 are based on
posteroanterior chest radiographs acquired with AEC. In general, low-contrast
anatomic structures were better visualized with the amorphous silicon
flat-panel detector. This difference reached significance only for the
thoracic spine (p = 0.0022). Low-contrast details were significantly
better scored on the amorphous silicon system (small round details, p
= 0.0425; linear and reticular details, p = 0.0041), whereas small
high-contrast details were slightly better visualized on the amorphous
selenium system. However, the latter difference was not significant.
In Figures 2A, 2B, and 2C, average experimental contrast-detail curves of both flat-panel detectors are presented. In Figure 2A, the phantom images were obtained with an entrance dose of 54 µGy. Figures 2B and 2C indicate the results obtained with AEC and with a phantom entrance dose of 135 µGy, respectively. Figure 2A and Figure 2C clearly indicate a (significant) better low-contrast detectability of the amorphous silicon system, whereas the detection of small high-contrast regions is comparable for both detectors. The phantom images taken with AEC (Fig. 2B) reflect in very good agreement the results of the patient chest images (also taken with AEC), on which a difference between systems could be observed for only low-contrast regions.
Figure 3 presents the mean inverse image quality figure values obtained from the phantom images as a function of the applied entrance dose. Overall, the contrast-detail study showed a significantly better low-contrast performance of the amorphous silicon system compared with the amorphous selenium detector system for phantom entrance doses up to 135 µGy (p < 0.032). For entrance doses of 218 µGy or higher, no significant difference in low-contrast detectability was found.
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Previous experimental and clinical studies showed excellent results concerning image quality of direct-readout detectors in thoracic radiography in comparison with conventional screen-film and CR systems [2, 3, 17-28]. Most of those studies, however, were performed with the amorphous silicon flat-panel detector type [2, 17-23] or refer to the selenium drum detector [24-27]. Only limited information about image quality can be found with respect to the amorphous selenium flat-panel detector [3, 28]. Moreover, only a very few studies have been performed to compare the diagnostic performance of direct (amorphous selenium) and indirect (amorphous silicon) flat-panel radiography units [7, 8]. The latter comparisons are mainly based on phantom measurements to compare noise characteristics, MTF, and DQE of both detector types.
In our study, two commercially available flat-panel systems based on amorphous silicon or amorphous selenium were compared with respect to the image quality and radiation dose of digital chest acquisitions. Both systems were used as advised by the manufacturer, using a specific grid (Table 1) and the standard AEC settings. Analysis of all patient images showed that both flat-panel systems produced images that were excellent in quality using the AEC settings of the manufacturer. In general, low-contrast details, such as the thoracic spine through the heart shadow and the normal bronchovascular structures up to the parietal pleura, were assigned significantly better scores on the amorphous silicon system, whereas small high-contrast details were slightly better visualized on the amorphous selenium system. The latter observation is in accordance with the higher MTF value of the amorphous selenium system at high frequencies [7, 8].
Using the AEC, the chest radiographs of the amorphous silicon flat-panel system could be acquired with a significantly lower patient effective dose than those obtained with the amorphous selenium detector. For the combination of a posteroanterior and a lateral acquisition—as is mostly the case in clinical chest radiography—the amorphous silicon system showed a reduction in effective dose of about 60% compared with the direct flat-panel system when using the AEC settings of the manufacturers. Similar results were found by Fischbach et al. [29], who reported that low-dose amorphous silicon phantom images scored better than amorphous selenium drum images. Samei and Flynn [7] and Borasi et al. [8] calculated DQE values of both direct and indirect flat-panel types. In both studies, the DQE of the amorphous silicon detector was significantly higher than the value for the amorphous selenium detector, explaining the dose reduction obtained with the indirect flat-panel detector. Previous measurements already showed the dose-saving effect of using amorphous silicon flat-panel detectors in chest imaging compared with CR and film-screen radiography [2, 16, 17, 20, 22, 23]. However, no significant dose reductions were reported using an amorphous selenium flat-panel detector.
Contrast-detail studies have been widely used for the objective analysis of the image quality performance of digital radiography systems [2, 8, 16, 19, 29]. The construction of the CDRAD 2.0 phantom allows a four-alternative forced-choice protocol for the analysis of contrast-detail perception [13, 15]. The latter procedure is more reliable and accurate than the image quality analysis with other contrast-detail objects without the four-alternative forced-choice possibility [15, 16].
The average experimental contrast-detail curves (Figs. 2A and 2C) of both flat-panel detectors clearly indicate low-contrast detectability is significantly better with the amorphous silicon system, whereas detection of small high-contrast regions is comparable for both detectors. This latter finding is in agreement with the contrast-detail curves calculated by Borasi et al. [8], in which no significant difference was observed in the detection of small objects. The phantom images obtained with AEC (Fig. 2B) reflect in very good agreement the results of the patient chest images on which a difference between the two systems could be proven only in low-contrast regions.
The overall contrast-detail performance (expressed in the quantity IQFinv [inverse image quality figure]) as a function of the radiation dose level showed a significantly better low-contrast performance with the amorphous silicon system than with the amorphous selenium detector for clinical exposure settings (entrance dose values up to 135 µGy). For entrance dose values of 218 µGy or higher, no significant difference in low-contrast detectability was found. Figure 3 illustrates that the entrance dose value for the amorphous selenium should be about 135 µGy (3.10 mAs) to obtain an equal contrast-detail performance of an amorphous silicon flat-panel radiograph obtained at an entrance dose of 54 µGy (1.25 mAs). The latter situation reflects the clinical practice in excellent agreement (Table 3).
In conclusion, chest radiographs can be acquired using a digital amorphous silicon flat-panel system with a significantly lower patient radiation dose than an amorphous selenium system, thereby producing image quality that is equal to or even superior to that of an amorphous selenium flat-panel detector system. Further research should reveal whether both systems provide the same diagnostic accuracy. Excellent agreement was obtained between the clinical image evaluation and the contrast-detail study, indicating the value of the CDRAD phantom as a tool for objective image quality analysis in digital radiography.
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
) and amorphous selenium (
) images taken with phantom
entrance dose of 54 µGy (A), with AEC (B), or with phantom
entrance dose of 135 µGy (C). Data points were obtained by averaging
responses of five radiologists who reviewed all images independently. Bars
indicate SD of 15 image scorings (5 observers x 3 images).
