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Clinical Comparative Study with a
Large-Area Amorphous Silicon
Flat-Panel Detector: Image Quality
and Visibility of Anatomic Structures on
Chest Radiography
Christian Fink 1, 2
Peter J. Hallscheidt 1
Gerd Noeldge 1
Annette Kampschulte 1
Boris Radeleff 1
Waldemar P. Hosch 1
Günter W. Kauffmann 1
Jochen Hansmann 1
OBJECTIVE. The objective of this study was to compare clinical chest radiographs of a
large-area, flat-panel digital radiography system and a conventional film-screen radiography
system. The comparison was based on an observer preference study of image quality and visibility of anatomic structures.
MATERIALS AND METHODS. Routine follow-up chest radiographs were obtained
from 100 consecutive oncology patients using a large-area, amorphous silicon flat-panel detector digital radiography system (dose equivalent to a 400-speed film system). Hard-copy
images were compared with previous examinations of the same individuals taken on a conventional film-screen system (200-speed). Patients were excluded if changes in the chest anatomy were detected or if the time interval between the examinations exceeded 1 year. Observer
preference was evaluated for the image quality and the visibility of 15 anatomic structures using a five-point scale.
RESULTS. Dose measurements with a chest phantom showed a dose reduction of approximately 50% with the digital radiography system compared with the film-screen radiography
system. The image quality and the visibility of all but one anatomic structure of the images
obtained with the digital flat-panel detector system were rated significantly superior ( p ≤
0.0003) to those obtained with the conventional film-screen radiography system.
CONCLUSION. The image quality and visibility of anatomic structures on the images
obtained by the flat-panel detector system were perceived as equal or superior to the images
from conventional film-screen chest radiography. This was true even though the radiation
dose was reduced approximately 50% with the digital flat-panel detector system.
A
Received May 3, 2001; accepted after revision
August 23, 2001.
1
Department of Diagnostic Radiology, University of
Heidelberg, INF 110, 69120 Heidelberg, Germany.
2
Present address: Department of Diagnostic Radiology
(E0101), German Cancer Research Center, Im
Neuenheimer Feld 280, 69120 Heidelberg, Germany.
Address correspondence to C. Fink.
AJR 2002;178:481–486
0361–803X/02/1782–481
© American Roentgen Ray Society
AJR:178, February 2002
ctive matrix flat-panel radiography
detectors based on cesium iodide
and amorphous silicon provide
digital radiographs with high spatial and contrast resolution and have the potential to allow
a reduction in radiation dose [1–3]. Previous
experimental and clinical studies, mostly with
experimental systems and detector prototypes
of limited size, have shown excellent results
compared with conventional film-screen systems [1–9]. We report the results of an observer preference study comparing the image
quality and visibility of anatomic structures on
reduced-dose digital chest radiographs with
those on conventional film-screen radiographs.
Materials and Methods
Patients
In a 4-month period, posteroanterior and lateral
chest radiographs were obtained from 112 consecutive asymptomatic patients using a digital flat-
panel detector radiography system. All patients
were from the outpatient oncology clinics of the local surgery department and were examined for a
routine follow-up.
All patients had previous posteroanterior and
lateral film-screen chest radiographs within a period of 1 year (mean, 192 days; minimum, 34 days;
maximum, 365 days) before the actual examination
with the digital system.
Before the image evaluation, all hard copies
were checked for changes in the chest anatomy
(e.g., changes resulting from therapy or pulmonary
disease) between the two examinations. Three patients were excluded from the study because of alterations of the chest anatomy caused by thoracic
surgery. All other patients showed normal findings.
Another nine patients were excluded from the
study because the film-screen images were on loan
to other hospitals at the time of image evaluation.
Imaging Systems
Digital radiography.—The digital radiography system (Revolution XQ/i; General Electric Medical Systems, Milwaukee, WI) consisted of an X-ray tube, a
481
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Fink et al.
generator, and a large-area amorphous silicon flat-panel
detector. The structure and properties of this singlepiece detector panel are described in detail elsewhere
[5]. Briefly, the panel consists of a cesium iodide scintillator and an amorphous silicon photodiode array
fabricated on a glass substrate. The thallium-doped cesium iodide scintillator converts X rays into visible
light. The scintillator material has a needlelike microstructure and is applied on the silicon photodiode array. The amorphous silicon photodiode array converts
the light into an electric charge, which is read out by
electronics and converted to a 14-bit signal. The detector has a matrix size of 2048 × 2048 pixels, with a
pixel size of 200 µm. The active area of the detector
measures 41 × 41 cm. The maximum spatial resolution achieved with this detector is 2.5 line pairs per
millimeter [5]. The digital radiography system also included a standard X-ray tube (Maxiray 100; General
Electric Medical Systems) and a standard high-voltage
80-kW generator (General Electric Medical Systems).
Images were obtained with an automatic exposure
control that was adjusted to provide a radiation dose
equivalent to that provided by a 400-speed film-screen
system. Exposure conditions were 125 kVp and 160
mA for the posteroanterior radiographs and 125 kVp
and 250 mA for the lateral radiographs. The film focus
distance was 180 cm. A grid (78 lines per centimeter;
ratio, 13:1) was used. All digital images were processed with a standardized postprocessing set that was
defined in collaboration with the manufacturer before
the patient examinations. On the basis of the postprocessing algorithms supplied by the manufacturer, postprocessing was adjusted to achieve a similar image
appearance to the film-screen combinations used in our
department (Cronex UV-G, DuPont, Bad Homburg,
Germany; and Ultra-Vision C, Sterling Diagnostic Imaging, Newmark, Denmark). Briefly, the processing included a multiresolution algorithm to perform edge
enhancement and dynamic range reduction and then
applied an asymmetric sigmoidal look-up table to
shape the contrast curve to match the screen-film combination. Finally, all digital images were transferred to
a laser imager (LR 3300; Agfa, Mortsel, Belgium) and
printed on laser film (Scopix Laser 2B DL, Agfa).
Film-screen radiography.—As a standard reference, a 200-speed (Cronex UV-G) conventional
film-screen system was used in this study. The conventional chest radiography system consisted of an
automatic film changer (Thoramat; Siemens, Erlangen, Germany), a standard X-ray tube (Opti 150,
Siemens), and a high-voltage 70-kW generator (Tridoros 712 MP, Siemens). The system worked with
an automatic exposure control using 125 kVp for the
posteroanterior and lateral radiographs. The film-focus distance was 220 cm. A moving grid (40 lines
per centimeter; ratio, 12:1) was used.
Dose Measurements
According to the manufacturer’s specifications, the
exposure settings of the flat-panel detector radiography system used in this study were adjusted to provide a radiation dose equivalent to that of a 400-speed
film-screen system. We assumed this would result in
a dose reduction of approximately 50% compared
with the conventional film-screen radiographs obtained with 200-speed film. Dose measurements
were performed with a dosimeter (RadCal, Monrovia, CA) using an anthropomorphic chest-phantom (Radiology Support Devices, Long Beach, CA)
before the patient examinations, which were performed under identical exposure conditions (automatic exposure control using the same peak
kilovoltage, tube current, grid, etc.) as for the patient
examinations. Dose measurements for the patient
examinations were not performed.
Image Evaluation
Hard-copy images were interpreted independently by three observers (a board-certified general radiologist and two residents in the sixth and
second years of training). All observers had training in thoracic radiology. As described previously,
the postprocessing of the digital radiographs was
optimized to achieve a similar image appearance
to the conventional radiographs. However, the images of both modalities were easy to distinguish
because of the different film material and format.
Images from the two modalities were analyzed
separately to exclude a potential bias from the direct intermodality comparison. Patient names were
either removed or obscured, and no information
about patient history was given. Furthermore, all
cases were presented in a randomized order.
The observers were allowed to handle the films
and to use a spotlight if desired. No time or viewing distance constraints were given. The image quality and the visibility of defined anatomic structures,
consisting of lung parenchyma, soft tissue, and bone
structures (Fig. 1) were evaluated using a 5-point scale
A
B
Fig. 1.—Anatomic structures used for image evaluation.
A and B, Posteroanterior (A) and lateral (B) radiographs show lung parenchyma (squares ), soft tissue (arrows ), and bone (rectangle ).
482
AJR:178, February 2002
Digital Versus Conventional Chest Radiography
TABLE 1
Dose Measurements with
Anthropomorphic Chest
Phantom Using Same
Exposure as for Patient
Examinations
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Imaging System
Parameter
Speed
Peak kilovoltage
Milliamperage
Milliampere-seconds
Surface dose (mGy)
Dose reduction (%)
Digital
Film-Screen
= 400
125
160
2.16
132
50.1
200
125
395
5.1
259
TABLE 2
Quality of Radiographs Obtained with Digital and Conventional
Radiography Systems
Imaging System
Image Quality of
Posteroanterior radiographs
Lateral radiographs
Digital
Film-Screen
4.11 (0.67)
3.93 (0.70)
3.74 (0.73)
3.51 (0.73)
∆
p
0.37
0.42
0.0001
0.0001
Note.—Data are mean ± SD of scores for image quality evaluated using a 5-point scale (1 = unsatisfactory, 2 = poor, 3 = fair,
4 = good, 5 = excellent). ∆ = difference of mean scores.
(1 = unsatisfactory, 2 = poor, 3 = fair, 4 = good, 5
= excellent).
Image quality was defined by the quality of exposure (i.e., the presence of under- or overexposed areas throughout the image) and the overall image
quality, which was influenced, for example, by contrast and the presence of artifacts. The anatomic
structures used for the image evaluation were similar
to those of previous studies dealing with chest radiography [2, 8, 10] and were chosen to allow the evaluation of a range of anatomic structures. These
structures included small structures such as the peripheral vessels and low-contrast structures such as
the retrocardiac vessels.
Specifically, the structures included in the posteroanterior chest radiographs were the main bronchi on
both sides, the peripheral bronchi, peripheral lung
vasculature of the left apex and the right base, retrocardiac and retrodiaphragmatic lung vessels, the right
pulmonary artery, the descending aorta, the paravertebral diaphragm on the left side, and the pedicles of the
upper six thoracic vertebrae (Fig. 1A). In the lateral
radiographs, the anatomic structures consisted of retrocardiac lung vessels, the hilus, lung vessels in the
heart shadow, the diaphragm at the heart shadow, and
the upper third of the thoracic spine (Fig. 1B).
correlation. A significant correlation indicates that
the scores of the observers may be combined for an
overall score for image quality and anatomic visibility. As a result of the high level of correlation measured in this study, we have reported averaged
results of all three observers.
Results
Dose Measurements
The dose measurements performed with
the anthropomorphic chest phantom before
the patient examinations showed a dose reduction of 50.1% with the digital system
compared with the conventional film-screen
system (Table 1).
Image Evaluation
The quality of the images obtained with the
digital flat-panel detector system was rated sigTABLE 3
nificantly superior to the quality of those obtained with the conventional film-screen
radiography system (Table 2). The mean
scores for the image quality of the digital images were 4.11 and 3.93 (posteroanterior and
lateral views, respectively) compared with
3.74 and 3.51 for the film-screen images. The
difference (∆) of the mean scores between the
two systems for the image quality was 0.37
and 0.42 (posteroanterior and lateral views, respectively). For the image quality, a significant
interobserver agreement was shown by the
Spearman’s rank correlation ( p ≤ 0.02).
The visibility of all but one anatomic structure on the images obtained with the digital
system was rated significantly superior to the
images obtained with the reference filmscreen system (Tables 3 and 4). One exception was the basal peripheral lung vessels in
the posteroanterior view. An insignificant ad-
Visibility of Anatomic Structures on Posteroanterior Radiographs Obtained
with Digital and Conventional Radiography Systems
Imaging System
Statistical Analysis
Mean values and standard deviations were calculated for the image quality and anatomic structures
for each modality. Differences between the images
obtained with the digital radiography system and the
film-screen radiography system were tested for significance using the Wilcoxon’s signed rank test.
Because the image quality and visibility of the
anatomic structures in this study were rated subjectively by the observers, an assessment of interobserver agreement was desirable to get an impression
of the objectivity and reliability of the image evaluation. Kappa statistics were not applicable because of
the large number of multivariate responses caused
by the number of observers and response categories
[11]. Instead, Spearman’s rank correlation was performed for all criteria for all observer combinations.
The level of significance was calculated for each
AJR:178, February 2002
∆
p
3.23 (0.96)
3.40 (0.87)
3.58 (0.65)
0.42
0.33
0.28
0.0001
0.0001
0.0001
3.29 (0.90)
3.10 (1.14)
3.10 (0.86)
3.05 (0.78)
2.98 (1.03)
2.77 (0.92)
0.24
0.12
0.33
0.0003
NS
0.0001
4.00 (0.70)
3.65 (0.81)
4.03 (1.16)
3.38 (0.91)
3.72 (0.72)
3.34 (0.98)
3.54 (0.91)
3.09 (0.97)
0.28
0.31
0.49
0.29
0.0001
0.0001
0.0001
0.0001
Structure
Right main bronchus
Left main bronchus
Peripheral bronchi
Lung vessels
Apical peripheral
Basal peripheral
Retrocardiac and
retrodiaphragmatic
Right pulmonary artery
Descending aorta
Paravertebral diaphragm
Pedicles of thoracic spine
Digital
Film-Screen
3.65 (0.81)
3.73 (0.76)
3.86 (0.63)
Note.—Data are mean ± SD of scores for image quality evaluated using a 5-point scale (1 = unsatisfactory, 2 = poor, 3 = fair,
4 = good, 5 = excellent). ∆ = difference of mean scores. NS = not significant.
483
Fink et al.
TABLE 4
Visibility of Anatomic Structures on Lateral Radiographs Obtained with
Digital and Conventional Radiography Systems
Imaging System
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Structure
Retrocardiac lung vessels
Hilus
Lung vessels in heart shadow
Diaphragm at heart shadow
Upper third of thoracic spine
Digital
Film-Screen
3.73 (0.85)
3.80 (0.65)
3.74 (0.73)
4.19 (0.78)
3.51 (0.90)
3.09 (0.83)
3.47 (0.74)
3.31 (0.78)
3.83 (0.90)
3.30 (0.87)
∆
p
0.64
0.33
0.43
0.36
0.21
0.0001
0.0001
0.0001
0.0001
0.0003
Note.—Data are mean ± SD of scores for image quality evaluated using a 5-point scale (1 = unsatisfactory, 2 = poor, 3 = fair,
4 = good, 5 = excellent). ∆ = difference of mean scores.
vantage was noted for the digital system for
these structures.
For the lung parenchymal structures, the
biggest difference noted between the two systems was the visibility of the small lung vessels in the retrocardiac space on the lateral
view. The mean score for the digital images of
this structure was 3.73, compared with 3.09 of
the conventional film-screen images (∆ = 0.64).
For the soft-tissue structures, the biggest differences between the two radiography systems
were observed for the delineation of the diaphragm in the posteroanterior view. The mean
score for the digital images of this structure
was 4.03, compared with 3.54 for the filmscreen system (∆ = 0.49) (Table 4).
For the bony structures, the biggest differences between the two radiography systems
were observed for the visibility of the pedicles of the upper thoracic spine on the
posteroanterior view. For these structures, the
mean score of the digital images was 3.38,
compared with 3.09 for the images of the
film-screen radiography system (∆ = 0.29)
(Table 4). For most anatomic structures, a
significant interobserver agreement was
shown by the Spearman’s rank correlation.
Discussion
Large-area flat-panel detectors based on cesium iodide and amorphous silicon recently
became commercially available for digital radiography [5]. Previous experimental and clinical studies, mostly with experimental systems
and detector prototypes of limited size,
showed excellent results of these radiography
detectors compared with state-of-the-art conventional film-screen radiography [1–9].
One feature of these new digital detectors is
the higher detective quantum efficiency com-
A
B
Fig. 2.—Posteroanterior chest radiographs of 62-year-old man with colorectal cancer.
A and B, Improved delineation of descending aorta (arrowheads ) is seen on digital radiograph (A) compared with film-screen radiograph (B).
484
AJR:178, February 2002
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Digital Versus Conventional Chest Radiography
A
B
Fig. 3.—Lateral chest radiographs of 62-year-old man with colorectal cancer.
A and B, Improved visibility of retrocardiac vessels (arrowheads ) is seen on digital radiograph (A) compared with film-screen radiograph (B).
pared with film-screen radiography. “Detective
quantum efficiency” describes the efficiency of
the transduction of the input signal into the
output signal. In particular, detective quantum
efficiency is defined as the ratio of the squared
signal-to-noise ratio at the output of the detector to the squared signal-to-noise ratio at the
input of the detector. Usually detective quantum efficiency is measured as a function of
spatial frequency [1, 2, 5, 6]. Detective quantum efficiency is generally accepted as the
most important parameter for characterizing
the performance of a detector system [5, 9].
The detector used in this study showed a significantly higher detective quantum efficiency
in theoretic analysis and experimental testing
than did film-screen systems [1, 2, 5]. The high
detective quantum efficiency of these flatpanel digital detectors is the basis for a potential dose reduction.
In this study, the exposure settings of the
digital system were adjusted to provide a radiation dose equivalent to that of a 400-speed
film system. This adjustment resulted in an
approximately 50% dose reduction with the
digital images, as shown in the dose measurements using an anthropomorphic chest
phantom. However, dose measurements were
not performed for the patient examinations.
The images obtained by the flat-panel digital system were considered better or at least
equivalent to the images obtained with the reference conventional film-screen system. The
AJR:178, February 2002
potential of a dose reduction without substantial loss of image quality was shown in various
experimental studies with this kind of digital
detector [3, 7, 9]. In a clinical study with 15
patients, Strotzer et al. [8] reported that at 50%
dose reduction, a flat-panel detector system
produced equivalent or superior image quality
compared with the reference film-screen radiography system.
One limitation of our study is that, like
similar studies dealing with this new technology [2, 8–10], our results are based on observer preference rather than on diagnostic
performance of the radiography systems.
Typically, observer preference studies are
performed first and are then followed by diagnostic accuracy studies. The advantage of
observer studies is that they allow a wide
range of anatomic features to be compared,
and they may be used to determine which diagnostic studies are required to further validate a new technology.
Another limitation of our study is that although the postprocessing of the digital images
was adjusted to produce a similar image appearance to conventional radiographs, the images of
both modalities could be easily distinguished
because of their different film material and format. This limitation is a potential source of bias
that could have influenced our results if the observers preferred one radiography system to the
other. To reduce this source of bias and to avoid
direct intermodality comparison, the images of
the different radiography systems were evaluated in separate sessions, patient names were either removed or obscured, and all cases were
presented in a random order.
Spearman’s rank correlation was performed
to get an impression of the interobserver agreement and therefore of the objectivity and reliability of the image evaluation. (Kappa statistics
were not applicable in this study because of the
large number of multivariate responses [11].) A
significant positive correlation was observed for
the image quality and for most anatomic structures for all observer combinations.
In this study, all digital images were printed
on hard-copy laser film after a standard postprocessing procedure in order to have similar
conditions for the image evaluation of the digital images and the film-screen images. This
procedure causes a potential bias against the
digital system because possible advantages of
the digital technology using various postprocessing features (e.g., digital magnification,
edge-enhancement) were not implemented.
In this study, the largest differences for the
visibility of anatomic structures between the
two modalities were noted for the mediastinal
structures (such as the delineation of the descending aorta in the posteroanterior view)
(Fig. 2) or for structures with high-contrast
differences (such as the retrocardiac vessels
in the lateral view) (Fig. 3). This finding may
be due to the wide dynamic range of these
digital detectors compared with conventional
485
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Fink et al.
film-screen radiography systems [1, 2, 6].
The wide dynamic range improves the contrast of features in poorly penetrated regions
(e.g., the mediastinum) and enables a simultaneous adequate representation of tissues
with substantially different absorption characteristics (e.g., bone, lung parenchyma) [1, 6].
Garmer et al. [4] reported similar findings
when the imaging of mediastinal abnormalities was significantly improved with the flatpanel digital radiography system compared
with film-screen radiography.
In conclusion, the image quality and the visibility of structures on the images obtained
with a flat-panel detector based on amorphous
silicon were perceived as equal or superior to
images obtained with a conventional filmscreen radiography system. This finding was
true even though the radiation dose of the digital radiographs was reduced approximately
50% compared with the film-screen radiographs. Further clinical studies are desirable to
compare the diagnostic accuracy of the flat-
486
panel detector system to that of conventional
film-screen systems.
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