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1 All authors: Department of Diagnostic Radiology, Ruprecht-Karls-University of Heidelberg, Radiological University Hospital, Im Neuenheimer Feld 110, 69120 Heidelberg, Germany.
Received April 19, 2002;
accepted after revision December 17, 2002.
Address correspondence to M. Ganten.
Abstract
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MATERIALS AND METHODS. Routine chest radiographs (posteroanterior) of 30 patients that were obtained using flat-panel, storage phosphor, and film screen systems were compared. The visibility of 10 anatomic regions and the overall image quality criteria were rated independently by three radiologists using a 5-point scale. The significance of the differences in diagnostic performance was tested with a Wilcoxon's signed rank test. Dose measurements for the three modalities were performed.
RESULTS. The flat-panel radiography system showed an improved visibility in most anatomic structures when compared with a state-of-the-art conventional film-screen system and an equal visibility when compared with a storage phosphor system. The flat-panel system showed the greatest enhancement in the depiction of small detailed structures (p < 0.05) and achieved this with a reduction in overall radiation dose of more than 50%.
CONCLUSION. The visibility of anatomic structures provided by this flat-panel detector system is as good as if not better than that provided by conventional or storage phosphor systems while emitting a reduced radiation dose.
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Our study reports the results of a clinical study that compared the image quality and depiction of various anatomic regions using the same methodology as Fink et al. [7] and applied this methodology to an amorphous silicon digital radiography detector system, a thoracic film-screen system, and a storage phosphor system.
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Imaging Systems
The conventional film-screen system used as a standard reference was a
dedicated automatic chest system (Thoramat, Siemens, Erlangen, Germany) with a
standard X-ray tube and generator (Siemens) using wide-latitude 240-speed film
(Scopix, Agfa, Antwerpen, Belgium). The system worked with an automatic
exposure control, and the resultant exposure values were 125 kV, 395 mA, and
220-cm focus—film distance.
The self-scanning flat-panel detector based on a-Si technology (Revolution XQ/i, General Electric Medical Systems, Milwaukee, WI) stores images electronically. The technical details are described in the literature [8, 9]. The detector consists of a scintillator (cesium iodide layer) and an amorphous silicon photodiode combination on a glass substrate. X-ray photons are converted into visible light by the scintillation material. The photodiodes convert the visible light into electrons. The electronic signal is transmitted by thin-film transistors to an analog-to-digital converter to produce the digital image. Matrix size for this detector was 2048 x 2048 pixels with a pitch of 200 mm, leading to an active imaging area of 41 x 41 cm. The spatial resolution was 2.5 lp/mm. The automatic exposure control of the digital chest radiography system was adjusted to yield radiation doses comparable to the dose level of a conventional unit equipped with a 400-speed film-screen system, 125 kV, and a 180-cm focus—film distance. All flat-panel images were processed with a standardized postprocessing algorithm supplied by the manufacturer (XQ/i M3, General Electric Medical Systems) that had been adjusted to achieve a similar image appearance as the film-screen combination used in our department. The procedure included a multiresolution algorithm to perform edge enhancement and dynamic range reduction and an applied asymmetric contrast curve to match the film-screen combination. Hard copies were printed via a laser imager (LR 3300, Agfa, Mortsel, Belgium) on laser film (Scopix Laser 2B DL, Agfa).
The computed radiography system represents a storage phosphor system (ADC Compact, Agfa) and consists of a generator (Polyhydros H [radiator Opti 150/30/50c], Siemens) with a 1-mm aluminum filter. The automatic exposure control of the computed radiography system was adjusted to result in radiation doses comparable to the dose level of a conventional unit equipped with a 200-speed film-screen system, 125 kV, and a 180-cm focus—film distance. The applied postprocessing algorithms permitted the fabrication of images that closely resembled film-screen images and were provided by the manufacturer. The imaging system uses a laser printer (LR 3300, Agfa) and hard-copy films (Scopix, Agfa).
Image Evaluation
Three radiologists (residents in their third, fourth, and fifth years with
training in thoracic radiology from the beginning of their residencies)
evaluated the hard copies independently. The digital imaging methods were
somewhat compromised by standardized postprocessing and subsequent hardcopy
printing and by the fact that available optimization procedures (e.g., digital
magnification and edge enhancement) were not used.
To avoid potential bias as a result of a direct inter-modality comparison, we conducted separate evaluations for each imaging system. Digital images were postprocessed to match the conventional images in appearance to avoid any bias caused by differences in brightness or contrast between imaging modalities. Nevertheless, images from the three systems remained easily distinguishable because of their different appearances (film material and format). Patient names were either removed or obscured, and images were evaluated in randomized order without knowledge of patient history. The observers were allowed to handle the films and to use a spotlight if desired. All images were viewed under subdued ambient light using a light box with adjustable shutters. No time constraints or viewing distances were prescribed for the evaluation.
The visibility of 10 predefined anatomic regions (i.e., the depiction of lung parenchyma, soft tissue, and bone structures [Figs. 1A, 1B]), the quality of exposure criteria (i.e., the presence of under- or overexposed areas), and the subjective overall perception of image quality (which was influenced by contrast resolution and the presence of artifacts) were rated using a 5-point scale (1 = unsatisfactory, 2 = poor, 3 = fair, 4 = good, 5 = excellent).
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The 10 anatomic structures allow the evaluation of a range of different X-ray absorptions (including small and low-contrast structures) and include the right and left main bronchi, peripheral bronchi, apical and basal peripheral vessels, retrocardiac vessels, right pulmonary artery, descending aorta, left paravertebral diaphragm, and pedicles of the thoracic spine.
Dose Measurements
Patient examinations were performed under identical exposure conditions
(automatic exposure control using the same peak kilovoltage, tube current, and
grid) with the exception of a reduction of 50% for the patient radiation dose
in the flat-panel images.
Before the study, the exposure settings of the flat-panel detector radiography system were adjusted to provide a radiation dose equivalent to that of a 400-speed film-screen system. A 20-mm aluminum filter was used for comparative dose measurements on the receptor surface (dosimeter from RadCal, Monrovia, CA). There was a dose reduction of more than 50% with the flat-panel system (120 kV, 100 mA, 1.52 mAs, and 7 mGy in front of the grid) compared with the storage phosphor system (121 kV, 2.97 mAs, 14.8 mGy) and a 45% dose reduction compared with the conventional film-screen system (121 kV, 3.24 mAs, 12.8 mGy). Additional measurements of the skin dose using an anthropomorphic chest phantom (Alderson, Long Beach, CA) confirmed the results (55% dose reduction of the flat-panel system vs the storage phosphor system and 49% vs the film-screen system).
Statistical Analysis
Mean values were calculated for each criterion and image modality. The
resulting flat-panel detector ratings were compared with the respective
film-screen and storage phosphor scores. Score differences among the images
obtained with the three modalities were tested for significance using the
Wilcoxon's signed rank test. The Wilcoxon's signed rank test was adopted
because of the non-normal distribution of data and its potential to test for
differences between nonparametric variables.
A mean score difference with a statistical significance level of p less than or equal to 0.05 was defined as relevant if the difference was greater than or equal to 0.3 (we regard a score difference of < 0.3 as not apparent to the eye). Because image quality and visibility of the anatomic structures were rated subjectively by the observers, an assessment of interobserver agreement was evaluated. Spearman's rank correlation coefficient was performed; kappa statistics were not applicable in this study because of the large number of multivariate responses [10].
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There were no statistically significant inter-modality differences for the overall image criteria. The range was between scores of 3.66 and 4.77 using a 5-point scale (1 = unsatisfactory, 2 = poor, 3 = fair, 4 = good, 5 = excellent). The subjective image quality rating was satisfactory. No artifacts were observed.
The ratings and their statistical significances for the matched modalities are shown in Tables 1, 2, 3. Mean scores for the 10 anatomic regions are shown in Figures 2 and 3. The statistical description of integer values in Tables 1, 2, 3 as a normal distribution leads to an apparent increase in SDs.
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Although the radiation dose was reduced by approximately 50% compared with the other two modalities, we found that visibility for the 10 anatomic criteria on flat-panel images was best in the total calculated mean of ratings (mean scores: flat-panel-system, 3.6; storage phosphor system, 3.5; film-screen system, 3.3) (Figs. 4A, 4B, 4C).
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Analyzed for each anatomic region and checked for statistical significance using Wilcoxon's signed rank test, the flat-panel detector images were better than the storage phosphor system for the depiction of the fine lung structures, left main bronchus, peripheral bronchi, apical peripheral lung vessels, and basal peripheral lung vessels. The storage phosphor system better depicted the left paravertebral diaphragm and the pedicles (Table 1).
Compared with the film-screen images (Table 2), a statistically significantly better depiction of most (6/10) anatomic structures was found with the flat-panel detector images. Structures for which the flat panel was found superior included the right main bronchus, left main bronchus, peripheral bronchi, apical peripheral lung vessels, basal peripheral lung vessels, and the pedicles. No significant differences were found for retrocardiac structures.
Significant differences were observed in four anatomic criteria when the storage phosphor system and the film-screen system were compared. The storage phosphor system was better for the depiction of retrocardiac structures: the descending aorta, the left paravertebral diaphragm, and the pedicles. The film-screen system was better for the depiction of the basal peripheral vessels (Table 3).
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Previous experimental and clinical studies of limited size, most of which used experimental systems and detector prototypes, have been reported [11]. One prominent feature of digital detectors is the wide dynamic range. To visualize the extreme latitude, the computer system compensates automatically for the large exposure variations between the lung and mediastinum [12]. The dynamic range of the flat-panel detector is greater than 1:10,000, whereas that of the film-screen system is approximately 1:30 and that of the storage phosphor detector is 1:4000, according to the manufacturers' specifications. This wide dynamic range improves the contrast of features in poorly penetrated regions and enables a simultaneous adequate representation of tissues with substantially different absorption characteristics (e.g., bone, lung parenchyma). These differences in dynamic range may explain why the digital systems are superior to the film-screen systems in depicting mediastinal structures (such as the paravertebral diaphragm and pedicles) and in delineating the descending aorta.
Another important factor in image quality is the signal-to-noise ratio. Detective quantum efficiency is generally accepted as the most important parameter for characterizing the signal-to-noise performance of a detector system. Improvement of the detective quantum efficiency has been shown for the new detector system (65–70%) over the film-screen system (20%) and storage phosphor system (25%) [13]. The high detective quantum efficiencies of flat-panel digital systems are the basis of potential dose reductions. The high detective quantum efficiencies also explain the high image quality scores of the flat-panel system, even though the flat-panel images were obtained at a much lower dose than images from the other two systems.
The flat-panel system provides the same advantages as the storage phosphor system (e.g., postprocessing and archiving features) while also offering a better image quality (because of higher detective quantum efficiency and contrast detail resolution) and a reduction of radiation dose, which is applicable to the current standard of storage phosphor systems. However, technical developments in storage phosphor systems are in progress, and these points should be reevaluated when these techniques come into clinical use.
Although the sample size of the study was limited, the statistical comparison of the three detector systems was aided by the fact that images were obtained using each detector system on the same patients. In addition, patients who had significant changes between image acquisitions were excluded. As a result, the study revealed a number of significant results as analyzed for statistical significance by the Wilcoxon's signed rank test.
The depiction of anatomic structures on flat-panel images was significantly statistically better for 50% of the structures and was at least equivalent to the storage phosphor and film-screen systems for the remaining structures, with the exception of the retrocardiac area (paravertebral diaphragm and pedicles), which was rated better on storage phosphor images.
Several sources of bias could influence our results. Because of the relatively easy identification of conventional film-screen images, it was not possible to completely blind the observers to the detector system. To minimize this bias, we grouped each modality, and all cases were presented in randomized order. Flat-panel and storage phosphor radiographs were printed on hardcopy laser film after standard postprocessing procedures to obtain similar image evaluation conditions. Printing of the digital images reduces some of the advantages of digital systems—namely, the ability to magnify window and level settings and to use the image-enhancement tools available with soft-copy display.
Compared with the standard conventional film-screen system, the flat-panel system has definitive practical advantages. Retakes are rarely necessary because of the wide dynamic range that allows the use of over- or underexposed images. A preview image is shown on the monitor within 15 sec of the readout procedure, providing a quick and easy image quality check. The radiologist is able to concentrate on specific aspects of the abnormality shown on the image because of a number of postprocessing features such as interactive window level setting and contrast enhancement. The digital image can easily be integrated into an RIS (radiology information system) and PACS (picture archiving and communication system) environment.
Typically, as in similar studies dealing with flat-panel technology [14], observer preference studies are performed first and then followed up by studies of diagnostic accuracy. The advantage of observer studies is that they permit a wide range of anatomic features to be compared and may be used to determine how diagnostic studies are to be configured. The first-generation flat-panel amorphous silicon detector system showed an equal image quality, an equal or improved visibility of anatomic structures, and a 50% reduction in radiation dose compared with the storage phosphor system and the conventional film-screen system.
In conclusion, the new flat-panel detector system provides the advantages of a digital system while also offering the potential of dose reduction compared with the conventional and computed systems.
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