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1 Department of Medical Physics and Radiation Protection, Ghent University,
Proeftuinstraat 86, Gent B-9000, Belgium.
2 Department of Radiology, Ghent University Hospital, De Pintelaan 185, Gent
B-9000, Belgium.
Received February 11, 2003;
accepted after revision April 17, 2003.
Address correspondence to K. Bacher
(klaus.bacher{at}ugent.be).
Abstract
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SUBJECTS AND METHODS. Three groups of 100 patients each were examined using the amorphous silicon flat-panel detector, film-screen, or computed radiography systems. All patient groups were matched for body mass index, sex, and age. To measure the entrance skin dose, we attached 24 calibrated thermoluminescent dosimeters to every patient. The calculation of the effective dose, which represents the risk of late radiation-induced effects, was based on measurements on an anthropomorphic phantom. Image quality of all three systems was evaluated by five experienced radiologists, using the European Quality Criteria for Chest Radiology. In addition, a contrast-detail phantom study was set up to assess the low-contrast detection of all three systems.
RESULTS. The amorphous silicon flat-panel detector radiography system allowed an important and significant reduction in both entrance skin dose and effective dose compared with the film-screen radiography (x 2.7 decrease) or computed radiography (x 1.7 decrease) system. In addition, image quality produced by the amorphous silicon flat-panel detector radiography system was significantly better than the image quality produced by the film-screen or computed radiography systems, confirming that the dose reduction was not detrimental to image quality.
CONCLUSION. The introduction of digital flat-panel radiography systems based on amorphous silicon and cesium iodide is an important step forward in chest imaging that offers improved image quality combined with a significant reduction in the patient radiation dose.
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Because computer technology and storage capacity have developed rapidly during recent years, PACS (picture archiving and communication systems) have become more important, and the accurate implementation of a PACS depends on digital radiography techniques. Digital radiography systems offer an instant image display, a wide dynamic range, and a linear signal response [2]. Moreover, digital images allow flexibility in processing and archiving, thereby providing a solution to the major disadvantages of the film-screen radiography systems.
The first step in digital chest radiography was the use of storage phosphor plates. Introduced some 20 years ago [4], these computed radiography systems are widely used because of their compatibility with existing radiography equipment. However, conflicting results have been reported concerning comparisons of the image quality and radiation dose delivered by computed radiography systems with those of conventional film-screen radiography combinations [2].
Recently, full-field digital amorphous silicon flat-panel X-ray detector radiography systems based on cesium iodide and amorphous silicon have become commercially available. These systems combine all advantages of digital radiography with a higher quantum detection efficiency than is attainable with film-screen or computed radiography systems [5]. Several studies have confirmed the excellent image quality provided by the amorphous silicon flat-panel detector radiography system [2, 5–7], but none has measured and calculated the patient radiation dose in correlation with the diagnostic performance of the detector. Hence, our aim was to compare the patient radiation dose delivered by a full-field digital amorphous silicon flat-panel detector radiography system with those of state-of-the-art conventional film-screen and storage phosphor-based computed radiography systems in clinical chest imaging. Image quality performance was evaluated to ensure that the potential dose reduction was not detrimental to quality.
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Image Acquisition
Standard posteroanterior and lateral chest radiographs were obtained with
the patients standing at a focus-to-detector distance of 180 cm. All imaging
was performed at 125 kVp using automatic exposure control. Exposure values
(milliampere-seconds) for each acquisition were noted.
Digital radiography.—The amorphous silicon flat-panel detector radiography system used in this study (Siemens, Erlangen, Germany; Trixell, Moirans, France) consisted of an X-ray tube (Optilix 150/30/50 HC-100, Siemens; focal spot size, 0.6 mm), a high-voltage generator (Polydoros LX 30 or 50 Lite, Siemens), and a motorized receptor wall stand with the flat-panel detector mounted behind a stationary anti-scatter grid (80 lines per centimeter; ratio, 15:1). This amorphous silicon image detector is equipped with a 43 x 43 cm X-ray sensing surface with a 3000 x 3000 matrix and a 143-µm pixel size. The detector consists of a needle-structured thallium-doped cesium iodine scintillator layer and an amorphous silicon thin-film transistor array. The cesium iodine scintillator converts the X rays into the visible light that is deposited directly onto the amorphous silicon matrix and is subsequently converted by an amorphous silicon photodiode into an electrical charge, resulting in a 14-bit digital signal. The automatic exposure control of the flat-panel detector radiography system was adjusted to a 400-speed class. After acquisition, the digital data were sent to a PACS workstation (MagicView, Siemens) for assessment of image quality.
Computed radiography.—The computed radiography system consisted of an X-ray tube (Opti 150/40/73 C, Siemens; focal spot size, 0.6 mm), a high-voltage generator (Polydoros 80S; Siemens), and a wall stand with an in-height adjustable cassette holder. A moving grid (40 lines per centimeter; ratio 12:1) was used to reduce scatter. Each of the storage phosphor cassettes used (GP DirectView cassettes, Eastman Kodak, Rochester, NY) has a 35 x 43 cm detection surface, with a 2048 x 2500 pixel matrix and a pixel size of 168 µm. After the patients were irradiated, the storage screen data were reviewed on a dedicated unit (DirectView CR-900, Eastman Kodak) by scanning the screen line-by-line with a laser beam. The resulting digital images were sent to a PACS workstation (MagicView, Siemens).
Conventional film-screen radiography.—Conventional film-screen radiography was performed using a dedicated automatic chest film changer (Thoramat, Siemens) combined with a 250-speed wide-latitude asymmetric film-screen system optimized for thoracic imaging (InSight Thoracic imaging film and screen, Eastman Kodak). The X rays were generated using an X-ray tube (Optitop 150/40/80 HC-100, Siemens) with a focal spot size of 0.6 mm and a generator (Polydoros 80S, Siemens). A moving antiscatter grid was used (40 lines per centimeter; ratio, 12:1).
Dose Measurements
Entrance skin dose.—For the measurement of the entrance skin
dose, 24 calibrated thermoluminescent dosimeters (Harshaw TLD-100, Thermo
Electron, Solon, OH) were attached to each patient at six well-defined and
reproducible locations. To ensure the reproducibility of the location of all
thermoluminescent dosimeters, we used the square pattern in the light beam of
the X-ray tube. For the posteroanterior acquisition, we spread 20
thermoluminescent dosimeters equally over five locations on the back of the
patient: the center and all four corners of the light field. For the lateral
radiographs, we attached four thermoluminescent dosimeters to the right side
of the patient (toward the X-ray tube) in the center of the light pattern.
Each set of 24 thermoluminescent dosimeters was used to measure radiation dose
in 20 successive patients to obtain a high signal-to-noise ratio. We analyzed
the thermoluminescent dosimeters using a Harshaw 3500 reader (Thermo
Electron).
Effective dose.—The concept of the effective dose,
introduced by the International Commission on Radiological Protection, is used
to measure nonuniform radiation exposure
[8]. The effective dose
(E), representing the risk of late radiation-induced effects such as
malignancies, is defined by the expression:
 | (1) |
In this equation, HT is the equivalent radiation dose to tissue T and wT is the weighting factor representing the relative radiation sensitivity of the tissue T. An overview of the tissue-weighting factors is given in Table 2.
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Equivalent radiation doses to the tissues of the organs listed in Table 2 were determined by placing 166 calibrated thermoluminescent dosimeters on an anthropomorphic (representing an average-sized man) Rando phantom (The Phantom Laboratory, Salem, NY) on regions that represented these organs and tissues. We adopted the distribution of bone marrow over the body described by Christy [9]. The choice of the thermoluminescent dosimeter locations was based on a complete CT scan of the phantom. The distribution of the 166 thermoluminescent dosimeters over the phantom is also given in Table 2. Posteroanterior and lateral chest radiographs of the Rando phantom were obtained (125 kVp; focus-to-detector distance, 180 cm) on the digital flat-panel detector, computed, and film-screen radiography systems. To obtain dose measurements well above the detection limit of the thermoluminescent dosimeters, we set the exposure to 300 mAs.
We used the data from the thermoluminescent dosimeters to calculate the mean equivalent organ doses per milliampere-second. Multiplying this result by the mean registered exposures of the three patient groups, we derived the mean equivalent organ doses (measured in microsieverts [µSv]) for the flat-panel detector, computed, and film-screen radiography systems (Table 2). We used equation 1 to calculate the effective dose (Table 3).
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Image Quality
Scoring of patient images.—Five experienced chest
radiologists assessed all chest radiographs and recorded the score of the
image quality on a questionnaire. All images were interpreted independently.
Each radiologist rated the visibility and radiographic quality of 12 anatomic
regions as either clearly visible (scored as 1) or not (scored as 0). The
choice of the anatomic structures used for image evaluation was based on the
European Guidelines on Quality Criteria for Diagnostic Radiographic Images
[10]. The evaluated regions
are indicated in Table 4. The
mean overall score was calculated using the individual scores of all evaluated
regions.
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For the digital images (flat-panel detector radiography and computed radiography systems), the radiologists were allowed to adjust the image brightness and contrast as well as to magnify the images. All digital images were scored on a 21-inch (53 cm), high-contrast gray-scale monitor (SMM 21140P, Siemens) with a resolution of 1280 x 1600.
Contrast-detail phantom study.—Because no reference images were available, the analysis of image quality based on patient data was subjective. Therefore, a contrast-detail phantom study (using Contrast-Detail Phantom for Digital and Conventional Radiography, version 2.0, University Hospital Nijmegen, St. Radboud, The Netherlands) was set up for a more objective analysis of the image quality produced by the three radiography systems. A detailed description of the CDRAD 2.0 phantom can be found elsewhere [2, 7, 11]. This phantom was used to assess the minimum contrast required to visualize objects of different sizes above the signal-to-noise threshold.
The phantom was placed between two layers of 5-cm polymethylmethacrylate to simulate patient scatter. Three images of the phantom were acquired with all three radiography systems under the same conditions as were used to image the patients. All images were scored independently by the five radiologists using the methodology described by the manufacturers of the phantom [11]. The reviewers were again allowed to adjust the image brightness and contrast of the digital images as well as to magnify the images. The results were presented in contrast-detail curves.
Statistical Analysis
The Kruskal-Wallis test was used to compare the age and body mass index of
the three patient groups. Differences between two mean values were tested for
significance using the two-tailed Mann-Whitney test (95% confidence level).
For the comparison of the image quality (expressed as a percentage) between
two imaging systems, chi-square statistics were used. To check the reliability
of the image quality evaluation by the five independent radiologists, we
assessed the interobserver correlation by calculating Spearman's rank
correlation for all image quality criteria for all observer combinations. A
significant correlation indicated that the scores of the observers could be
combined into a mean score for the image quality parameters
[6]. All statistical
calculations were performed using MedCalc software (MedCalc, Gent,
Belgium).
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Mean equivalent radiation doses to all tissues for the posteroanterior and lateral radiographs are compared in Table 2. Table 3 summarizes the results of the exposure and dose measurements. For both the posteroanterior and lateral acquisitions, a highly significant reduction in exposure, entrance skin dose, and effective dose was found with the amorphous silicon digital flat-panel detector radiography system. For the posteroanterior image, the flat-panel detector radiography system showed a reduction (p < 0.0001) in effective dose from 18.8 µSv with the computed radiography system and with 23.2 µSv for the film-screen radiography system to 9.6 mSv. For the lateral image, the effective dose decreased (p < 0.0001) from 43.5 µSv with the computed radiography system and 77.0 µSv with the film-screen radiography system to a value of 27.1 µSv with the flat-panel detector radiography system.
Comparing the entrance skin doses allowed similar conclusions to be made. In the posteroanterior images, the mean entrance skin dose decreased from 164.9 µGy with the computed radiography system (p = 0.0001) and 199.0 µGy with the film-screen radiography system (p < 0.0001) to 66.8 µGy with the flat-panel detector radiography system. For the lateral images, a decrease in the mean entrance skin dose was from 733.6 µGy with the computed radiography system (p = 0.009) and 1286.2 µGy with the film-screen radiography system (p = 0.008) to 346.7 µGy with the flat-panel detector radiography system.
For the image quality study, a significant interobserver agreement was found (p = 0.034), indicating that the individual scores of all observers could be combined into an averaged value. Analysis of image quality showed a mean overall score (95%) for the amorphous silicon digital flat-panel detector radiography system that was significantly better than the mean overall score of the computed radiography system (85%, p = 0.0339) or the film-screen radiography system (82%, p = 0.0129). Table 4 shows the scores for all regions with all three systems, together with the level of significance for the difference of the computed radiography and film-screen radiography systems compared with the flat-panel detector radiography system. When comparing the amorphous silicon flat-panel detector radiography system with the computed radiography system, four regions were judged to be significantly better visualized with the flat-panel system: the medial border of the scapulae, the peripheral vessels, the trachea and proximal bronchi, and the spine. The flat-panel detector radiography system scored significantly better than the film-screen radiography system in the same four regions. In addition, the visualization of the retrocardiac lung and mediastinum was significantly better with the flat-panel system than with the film-screen radiography system.
No significant difference in mean overall quality was found (p = 0.7032) between the computed radiography system and the film-screen radiography system. In two regions (the retrocardiac lung and mediastinum and the spine), however, the computed radiography system performed statistically better than did the film-screen radiography system (p < 0.0001).
The contrast-detail phantom study showed the flat-panel detector radiography system had a significantly better low-contrast performance than the computed radiography (p = 0.002) or film-screen radiography (p = 0.001) systems. No significant differences in low-contrast detectability were found between the computed radiography and film-screen radiography systems (p = 0.3). The contrast-detail curves are presented in Figure 2.
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Digital radiography will play an important role in this evolution because conventional radiographs are the most frequently obtained images in medical imaging. Furthermore, chest radiographs represent about 25% of all diagnostic radiography examinations [1] and are often obtained repeatedly for the followup of patients.
Recently, amorphous silicon radiography systems with direct readout capabilities became commercially available. The diagnostic performance of this new amorphous silicon flat-panel detector radiography technology still requires evaluation, but it is expected to be at least as good as that of conventional radiography. Previous experimental and clinical studies have shown that excellent image quality is achieved with the silicon flat-panel detector radiography system compared with the image quality produced by the conventional film-screen radiography and the computed radiography systems [2, 3, 6, 7]. In a phantom study of Strotzer et al. [12], the depictions of linear-structured and micronodular-simulated lesions were significantly better on the amorphous silicon flat-panel detector radiography system than on conventional film-screen radiography. The two systems were equally capable of revealing pulmonary nodules and reticular patterns. In a CT-controlled clinical study, Garmer et al. [3] concluded that the diagnostic performance of the amorphous silicon flat-panel detector radiography system was equivalent or superior to that of the film-screen radiography system. Fink et al. [6] illustrated an improvement in the visibility of various anatomic structures on flat-panel detector chest radiographs compared with conventional film-screen radiographs. In our study, the quality of flat-panel detector chest radiographs was significantly superior in four anatomic regions (the medial border of the scapulae, the peripheral vessels, the trachea and proximal bronchi, and the spine) compared with that of computed radiographs and film-screen radiographs, findings consistent with those of the previously described studies. In addition, the visualization of the retrocardiac lung and mediastinum was significantly better with the amorphous silicon flat-panel detector radiography system compared with that of the film-screen radiography system.
An important characteristic of the amorphous silicon flat-panel detector radiography system is that its quantum detection efficiency is higher than either computed radiography or film-screen radiography systems [3, 5]. Quantum detection efficiency combines spatial resolution and image noise to provide a measure of the signal-to-noise ratio of all frequency components of the image [13]. Hence, a higher quantum detection efficiency provides improved capability to reveal an object in a noisy background [5], in addition to the possibility of reducing the patient radiation dose with no loss of diagnostic information. Previous studies have postulated that a dose reduction might be possible with the amorphous silicon flat-panel detector radiography system [3, 5, 6]. In our study, the flat-panel detector radiography system showed a strong and significant dose reduction compared with that possible with computed radiography or film-screen radiography systems.
We measured and calculated the radiation dose for three comparable patient groups using the entrance skin dose and the effective dose. On the basis of the speed classes of the amorphous silicon flat-panel detector radiography (400) and film-screen (250) radiography systems, a dose reduction of a factor of 1.6 could be expected. However, the high quantum detector efficiency of the flat-panel detector radiography system resulted in the dose being significantly lower than that delivered by the film-screen radiography system. For the combination of a posteroanterior and a lateral acquisition—the most common imaging combination in clinical chest radiography—the flat-panel detector radiography system showed a reduction in entrance skin dose of a factor of 3.6 compared with the film-screen radiography system and 2.2 compared with the computed radiography system. For the effective dose, the dose reduction factors were 2.7 and 1.7 compared with the film-screen radiography and the computed radiography systems, respectively. This finding corresponds with the assertion of Aufrichtig [7] that an amorphous silicon flat-panel detector radiography system only needs 30% of the exposure required by a film-screen radiography system to achieve the same contrast-detail detectability. The image quality assessment proved that despite the significant dose reduction, image quality had been not affected. The overall performance of the amorphous silicon flat-panel detector radiography system was significantly superior to the performances of the computed radiography and the film-screen radiography systems.
Another important advantage of digital radiography systems is the wide dynamic range and histographic equalization [3]. Hence, low-contrast regions such as the mediastinum are better visualized, as was illustrated by the statistically better score of both digital systems compared with the score of the film-screen radiography system in imaging the mediastinal region (Table 4). The contrast-detail phantom study confirmed the superior low-contrast detection of the amorphous silicon flat-panel detector radiography system as compared with the low-contrast detection of the film-screen and computed radiography systems (Fig. 2). This result corresponds with the results reported by Garmer et al. [3], who found that mediastinal abnormalities were more clearly seen with the amorphous silicon flat-panel detector radiography system than with a conventional film-screen radiography technique, and by Schaefer et al. [14], who found that the computed radiography system was superior to the film-screen radiography system for revealing mediastinal lesions.
Other studies have proven that limiting the spatial resolution of digital systems, in contrast to film-screen radiography systems, does not influence the diagnostic performance. MacMahon et al. [15] showed that a pixel size of 200 µm is sufficient for the detection of necessary details using computed radiography systems. Aufrichtig [7] proved that an amorphous silicon flat-panel detector radiography system with a pixel size of 200 µm is superior to a dedicated film-screen radiography system in revealing small objects.
Although the radiation dose is already low in acquisitions with amorphous silicon flat-panel detector radiography systems, additional dose reductions may be possible to achieve. In a phantom experiment, Strotzer et al. [12] did not find any significant difference in image quality of a flat-panel detector image obtained with a standard radiation dose and a flat-panel detector image obtained with a dose reduction of 50%. Therefore, the amorphous silicon flat-panel detector radiography systems would be appropriate for pediatric use, where it is crucial to keep the patient radiation dose as low as possible. Amorphous silicon flat-panel detector radiography systems are not exclusively designed for thoracic imaging; they have already proven their value in skeletal radiography [16], particularly in revealing cortical bone defects and fractures [17]. Flat-panel detector radiography has the additional advantage over computed radiography in that it offers an instant image display and the elimination of the need for cassettes. Therefore, amorphous silicon flat-panel detector radiography systems could be a cost-effective replacement for conventional radiography systems in the future.
In conclusion, digital flat-panel detector radiography systems based on amorphous silicon and cesium iodide are an important step forward in chest imaging, offering improved image quality and a significant reduction in the radiation dose delivered to patients.
Acknowledgments
We thank the technical staff of the radiology department of the Ghent
University Hospital for their cooperation in measuring patient radiation
doses.
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
), computed radiography (
), and
digital flat-panel detector radiography (•) systems. Data points were
obtained by averaging responses of five radiologists who reviewed images
independently. Bars on either side of symbol indicate range.