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Eur. Radiol. (2001) 11: 1688±1696
DOI 10.1007/s003300000795
Svenja P. Hennigs
Marietta Garmer
Horst J. Jaeger
Reinhard Classen
Andreas Jacobs
Hans M. Gissler
Andreas Christmann
Klaus Mathias
Received: 20 July 2000
Revised: 16 November 2000
Accepted: 24 November 2000
Published online: 21 February 2001
Springer-Verlag 2001
)
S. P. Hennigs ( ) ´ M. Garmer ´
H. J. Jaeger ´ R. Classen ´ A. Jacobs ´
H. M. Gissler ´ K. Mathias
Department of Clinical Radiology,
Staedtische Kliniken Dortmund,
Beurhausstrasse 40, 44137 Dortmund,
Germany
E-mail: [email protected]
Phone: +49-2 03-5 08 59 56
Fax: +49-2 03-5 08 13 23
A. Christmann
Computing Center and Department
of Statistics, University of Dortmund,
August-Schmidt Strasse 12,
44221 Dortmund, Germany
Present address: S. P. Hennigs, Department
of Diagnostic and Interventional Radiology, Evangelisches und Johanniter Klinikum, Fahrnerstrasse 133, 47169 Duisburg,
Germany
CHEST
Digital chest radiography with a large-area
flat-panel silicon X-ray detector: clinical
comparison with conventional radiography
Abstract This was a radiologists'
preference study to compare a digital chest radiography system that
utilizes a large-area silicon flat-panel detector with conventional radiography for visualizing anatomic regions of the chest. Conventional and
digital posteroanterior (PA) and lateral chest radiographs were obtained in 115 patients. The PA and
lateral image pairs were compared
independently by three radiologists
rating the overall appearance, 11
anatomic regions in the PA, and 9 in
the lateral views. Statistical analysis
was performed with the Wilcoxon
signed-rank test with BonferroniHolm adjustment (p = 0.05). For the
PA view, the digital system performed significantly better for the overall
appearance and for all anatomic regions except for the peripheral pulmonary vasculature and hilum,
where no significant difference was
found. For the lateral digital images,
Introduction
Complete digitalization of X-ray departments and introduction of picture archiving and communicating systems (PACS) in hospitals are developments which have
made rapid progress in recent years. The advantages of
digital chest imaging compared with conventional
screen film system are the wide exposure latitude and
high dynamic range, as well as the possibility of postprocessing to avoid repeated examinations.
Thus far, two methods of digital chest radiography
are established, the storage phosphor and amorphous
selenium drum detector technique [1, 2, 3, 4, 5, 6, 7]. The
most recent technique for digital radiography are flat-
the regions trachea, costodiaphragmatic recess, and hilum were rated
significantly worse. The regions retrosternal and retrocardiac lung were
rated significantly better. The other
regions and the overall appearance
showed no significant differences.
The described digital chest radiography system showed statistically
superior visualization of anatomic
regions for PA and an ambiguous
performance for lateral images as
compared with conventional radiography. After changing some image processing parameters for the
lateral view, this system may be
suitable for digitalization of chest
radiography.
Keywords Observer performance ´
Digital radiography ´ Thorax ´
Radiography
panel detectors based on amorphous silicon thin-film
transistor technologies (TFT) using either scintillator/
photoelectric converter systems [8, 9, 10, 11, 12, 13, 14,
15] or amorphous selenium layers [16, 17, 18] for the
conversion of X-ray into electronic charge.
A new digital chest radiography system (DRS) that
uses a flat-panel X-ray detector based on a gadolinium
scintillator layer and an amorphous silicon photoelectric
converter/readout array (CXDI-11, Canon, Tokyo, Japan) has recently been introduced [19, 20]. The dynamic
range of the sensor is 104 and covers most of the exposure range for radiography. Detective quantum efficiency is high due to noise reduction in the electronic
system. Spatial resolution of the system is 3.1 line pairs
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per millimeter (lps/mm). These physical properties
should give this digital system some advantages as compared with the conventional system with respect to
lower noise level and a better image fidelity in combination with a sufficient spatial resolution.
Thus far, only clinical studies with bone radiography
or chest phantom studies with digital flat-panel systems
consisting of amorphous silicon have been reported [6,
8, 9, 10, 11, 12, 16, 21].
This paper reports a clinical test of DRS for chest
image quality, assessing overall appearance and visibility of anatomic regions compared with the conventional
screen-film combination as the diagnostic routine technique we use in our department.
Methods
Patients
From April to June 1998, 115 patients underwent PA and lateral
chest radiographs with both a conventional and the new digital
system. The patients were from different clinical departments and
required a chest radiograph for diagnostic purposes, mostly preoperatively. Because anatomic regions of the chest were analyzed,
only patients with an unremarkable conventional chest radiograph
of good technical quality were selected for investigation with the
digital system as well (selection by S. H. and M. G.). Well-exposed
radiographs with a good and interpretable visibility of all anatomic
structures were stated to be of good technical quality.
Posteroanterior and lateral radiographs using each imaging
technique were obtained within 24 h. The subjects included 76
male and 39 female patients with a mean age of 62 years (age range
22±86 years). Informed consent was obtained from each patient
after the nature of the procedure had been fully explained [22].
The study was ethically approved by an institutional review board.
Conventional screen-film radiography
Conventional standard PA and lateral examinations were performed with a dedicated automatic chest system (Thoramat, Siemens, Erlangen, Germany) with a stationary grid (ratio r = 12,
n = 40 lines per centimeter). Source-to-image distance was 200 cm,
and automatic exposure control was set at a phototiming of
125 kVp. Wide-latitude 400-speed films ( Ultravision UV 400,
Dupont, Bad Homburg, Germany) were used. Filtration was with
2.77-mm aluminum. The spatial resolution of the conventional
system was 5.6 lps/mm.
Digital radiography system (DRS)
Technical details and physical evaluation
The digital flat-panel detector (CXDI-11, Canon, Tokyo, Japan)
was used in place of the film in conventional radiography equipment. With the digital system, we used a standard X-ray tube (SRO
33/100, Philips, Hamburg, Germany) and a standard high-voltage
generator (HFG 650 R, Communications and Power Industries
CPI, Ontario, Canada). Patients were examined erect at a sourceimage distance of 200 cm. Studies were phototimed at 125 kVp.
The automatic exposure control of the digital chest system was
adjusted to result in radiation doses comparable to the dose level
of the conventional unit. A stationary grid was integrated into the
flat panel detector (ratio r = 12, n = 40 lines per centimeter).
The flat-panel sensor utilizes a thin-film transistor (TFT) metal
insulator semiconductor (MIS)-type photoelectric converter array
made from hydrogenated amorphous silicon (a-Si:H). The sensitivity peak wavelength of this material is between 540 and 620 nm. The
sensor consists of 2688 ” 2688 pixels with a pixel pitch of 160 mm. The
active area is 43 ” 43 cm (17 ” 17 in.). Each pixel comprises an a-Si
TFTand an MIS-type photoelectric converter fabricated together on
a glass substrate. The top of each element is covered with a transparent passivation layer. The sensor utilizes a scintillator coupled to the
array. The thin-film scintillator (200 mm) consists of terbium-coated
gadolinium dioxide sulfide (rare earth Gd 202S:Tb) as a squeezed
crystal shape with a peak wavelength of 545 nm. The incident X-ray
is absorbed by the scintillator and converted to visible light. Photo
carriers are generated in the semiconductor layer of the MIS-type
photoelectric converter by absorbed visible light, and accumulated
in the MIS capacitor. The accumulated carriers are read out by
switching the TFT [18, 19]. The conversion from analog to digital information takes place with a 14-bit resolution. The digital output is 12
bits per pixel, corresponding to 4096 gray levels. The maximum possible spatial resolution of the system is 3.1 lp/mm given by the pixel
size of 160 mm. Modulation transfer function (MTF) of the DRS is
independent of the amount of exposure. The MTF as a ratio of the
output modulation to the input modulation, expressed as a function
of spatial frequency was determined to be 28 % at a spatial resolution
of 2.5 lps/mm. Nyquist frequency of the DRS is 3.1 lp/mm. The MTF
of the DRS exceeds that of the screen-film system below the Nyquist
frequency. Contrast transfer function (CTF) of the DRS is superior
to that of the screen-film system at the spatial frequency below 2.5 lp/
mm. Dynamic range of the DRS is approximately 104. The substantial dynamic range of the screen-film system is approximately 101.5 so
that the dynamic range of the DRS significantly surpasses that of the
screen-film system. Since signal response of the DRS is linear, noise
response is proportional to square root of exposure. Noise equivalent quanta (NEQ) curve is approximately linear which means that
the effect of system noise can be neglected at a designated range of
exposure. Signal-to-noise ratio (SNR) is almost only quantum noise
limited. It is higher than that of conventional film-screen systems.
Detective quantum efficiency (DQE) and NEQ can therefore be
considered to completely describe the image quality (NEQ) or the
imaging characteristics of the sensor (DQE). The definitions of MTF
and CTF do not include the concept of noise. Since noise is always
present in an image, DQE (or NEQ) therefore give a better indication of the overall image sharpness and image quality [18, 19, 20].
Digital image processing
The digital data are transferred to special hardware for processing.
Digital image processing was performed according to the technique
available with the CXDI-II system. It can be broadly classified into
three stages: preprocessing, image processing, and postprocessing.
In the preprocessing the acquired digital data are subjected to
several steps compensating for the sensor characteristics which are
fluctuations caused by dark current, output values of the pixels,
and defective pixels. All the preprocessing steps are fully automatic and are identical for all acquired data.
Image processing are filtering and scaling operations that adjust the overall density and contrast of the chest image. It includes
segmentation, gray-scale conversion, dynamic range compression,
and adaptive unsharp mask processing (edge enhancement).
Segmentation is a process in which the image is analyzed and
the image feature values are derived. The derived image feature
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values are used as parameters for the various processes which are
performed later. Gray-scale conversion is a process for adjusting
the density of the overall image. The density is converted by a
look-up table (LUT). The LUT is produced by a sigmoidal function which shows a characteristic curve of the film similar to that of
an X-ray. Dynamic range compression is an image enhancement
technique which compresses the dynamic range of the pixel values
while preserving or even enhancing contrast detail. It allows the
simultaneous visualization of the lungs and the mediastinum.
Adaptive unsharp masking is a process for sharpening the image. It
selectively enhances the structured edges. Image-specific information is used to determine the degree of enhancement.
The image-processing parameters were initially adjusted during the installation phase of the system defined as suitable for
routine image processing for chest radiography by the manufacturer using the advantages given by the digital processing with
maximum local contrast distribution. All images were processed
with the same parameter set. No systematic study of parameter
optimization was conducted.
The gray-scale conversion could be adjusted for curve type,
brightness, and contrast. A sigmoidal chest curve type, brightness
of 5 (range 1±11), and contrast of 10 (range 1±11) was chosen for
PA views. For the lateral views a high-contrast curve type, brightness of 9, and contrast of 6 was chosen. The dynamic range could
be adjusted for low/high-density part compression, compression
level, and mask size. For the PA views the compressed direction
was low density, compression level was 6 (range 1±11), and mask
size was 8 (range 1±11). For the lateral views the compressed direction was high density, compression level was 5, and mask size
was 8. The adaptive unsharp masking process could be adjusted for
enhancement level and for contrast (frequency of sharpness). For
both the PA and lateral views enhancement level was chosen 4
(range 1±11) and contrast 6 (range 1±11).
A preview image with reduced resolution is displayed on the
operation panel 3 s after X-ray exposure to allow the technician to
check patient positioning and overall image quality. No radiographs were repeated in this study. No individually adjusted postprocessing of the images was used in our study.
A hard copy (35 ” 43 cm; Kodak Ektascan EHN7, Rochester,
N. Y.) is produced by using a laser imager (Kodak Ektascan laser
printer 190). Digital data can be transferred to a workstation monitor for postprocessing. The interface is DICOM 3.0 compatible.
Image evaluation
The 115 patient data sets, each of which included four images (a pair
of PA and a pair of lateral images), were compared and evaluated
independently by three board-certified radiologists (R. C., A. J.,
H-M. G.) with a high level of experience with chest radiography
(more than 5 years) but no particular experience with digital radiography. This resulted in a total number of 345 patient data sets. The
digital and conventional radiographs were not identified, but they
could be differentiated by the specific properties of the laser film for
the digital image. The images were anonymized so that the observers did not know patient names and details. All observers were given study protocols and guidelines and a sample radiograph was rated with the questionnaire before the official rating sessions. All image evaluations were done under the same standardized conditions.
As in the clinical situation, the observers had to evaluate complete cases. They were given the digital and conventional PA images and then the corresponding lateral images to compare them in
side-by-side viewing sessions to achieve direct comparison of digital vs conventional images.
The observers were first asked to rate the overall appearance of
the digital image on a five-point scale (1 = overall appearance very
good, 2 = overall appearance good, 3 = overall appearance adequate, 4 = overall appearance poor, 5 = overall appearance unacceptable).
The observers were then asked to compare the overall appearance of the digital and the corresponding conventional image. Then
they compared the visibility of 11 normal anatomic regions in the PA
images (trachea, carina, main bronchi, costopleural border, azygoesophageal recess, paraspinal stripe, peripheral pulmonary vasculature, hilum, mediastinum, soft tissue, and bones) and 9 normal anatomic regions in the lateral images (trachea, costodiaphragmatic recess, posterior cardiac border, retrosternal pulmonary structure,
retrocardiac pulmonary structure, fissures, hilum, thoracic spine,
and sternum). Costopleural border in the PAview means the total of
the borderline zone between the ribs and the parietal pleura. Costodiaphragmatic recess in the lateral view means the deepest part of the
lung dorsally formed by parietal pleura and diaphragm. Retrosternal
lung is the part of the lung in the lateral view which lies between
sternum and anterior cardiac border. Visible lung structure means
the presence of linear and round patterns representing bronchial and
vascular structures. Comparing the visibility of anatomic structures,
the observers were specifically asked to judge the sharpness of the
structures, their borders, and the contrast and visibility of details
within structures in terms of spatial resolution.
For the comparisons, the observers recorded their opinion by
means of a five point scale: 1 = digital much better than conventional; 2 = digital better than conventional; 3 = digital equal to
conventional; 4 = digital worse than conventional; and 5 = digital
much worse than conventional.
The results of the three observers were analyzed together and
separately to determine the interobserver variability. The radiological quality of the visibility of the anatomic regions was determined by guidelines of quality requirements [23] for chest radiography which were given to the observers with the study protocol.
These guidelines include the visualization of pulmonary vessels in
the peripheral parts, retrocardiac lung, mediastinum, hilum, and
costopleural border. Important details of the radiograph need to be
seen with a minimum of 0.7±1.0 mm for round and 0.3 mm in diameter for longitudinal image details. Critical structures, such as
small round peripheral and central details, need to be seen.
To evaluate the intraobserver variability, the ratings for the
overall appearance of the digital images and the comparison of the
overall appearance between the digital and the conventional images were repeated after 3 weeks in a randomized new order.
All images were interpreted under the same conditions including the same viewing box and the same light conditions.
Statistical analysis
All criteria considered were measured on an ordinal scale with five
possible values. The classical nonparametric two-sided Wilcoxon
signed-rank test is used to test for differences between the digital
and the conventional methods [24]. There were 11 anatomical criteria for posteroanterior images and 9 anatomical criteria for lateral images plus an overall appearance for each view. The Bonferroni-Holm procedure [25] is used to adjust for multiple comparisons. The multiple level of significance was set at 5 %. The twosided Wilcoxon signed-rank test for paired data was used at the
5 % level to test whether there are significant differences between
the first and second evaluation.
The Wilcoxon tests were done with StatXact 4 for Windows
[26]. The procedure PROC MULTTEST of SAS, version 6.12 [27],
was used for the Bonferroni-Holm procedure.
Spearman's rank correlation coefficient [28] was used to measure the interobserver variability, because all considered criteria
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Table 1 First evaluation of posteroanterior (PA) views, all observers: overall appearance of DRS, and comparison of digital and conventional chest radiographs for overall appearance and anatomic regions. N no. of observations
Parameter
Very good
Good
Adequate
Poor
Unacceptable
N
Overall appearance of DRS
49
161
103
32
0
345
DRS is ... than conventional
Comparison: digital and conventional Much
better
Better
Equal
Worse
Much
worse
N
P
Overall appearance
15
146
145
39
0
345
< 0.001
*
0
2
2
0
1
3
1
0
2
0
1
144
201
178
130
116
133
60
66
121
54
176
178
124
150
202
221
199
222
232
211
283
151
23
18
15
13
7
10
62
47
11
8
17
0
0
0
0
0
0
0
0
0
0
0
345
345
345
345
345
345
345
345
345
345
345
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
1.000
0.090
< 0.001
< 0.001
< 0.001
*
*
*
*
*
*
Anatomic regions
Trachea
Carina
Main bronchi
Costopleural border
Azygoesophageal recess
Paraspinal stripe
Peripheral pulmonary vasculature
Hilum
Mediastinum
Soft tissue
Bones
*
*
*
Significant differences at the 5 % level (Bonferroni-Holm test) are marked with asterisk
were measured on an ordinal scale. High positive correlation coefficients indicate a strong connection between the ratings of different observers. The computations were done with the SAS procedure PROC CORR [29].
Results
Overall appearance of digital chest radiographs:
first evaluation
The results of the first evaluation for the posteroanterior
view (Table 1) show that the overall appearance of DRS
was rated as very good in 14 % (49 of 345), as good in
47 % (161 of 345), as adequate in 30 % (103 of 345), and
as poor in 9 % (32 of 345) of patients. The value ªunacceptableº was not used by the observers for the posteroanterior view. For the lateral view (Table 2), the overall
appearance of DRS was rated as very good in 12 % (42 of
345), as good in 35 % (119 of 345), as adequate in 36 %
(124 of 345), and as poor in 17 % (58 of 345) of patients.
Two observers rated the overall appearance of one lateral radiograph as ªunacceptableº (0.6 %).
Comparison of digital and conventional chest
radiographs: first evaluation
Posteroanterior view
The comparison of the overall appearance of the digital
and conventional images showed a significant difference
in favor of the digital system at the 5 % level. For the
comparison of the anatomic regions DRS performed
significantly better than the conventional method for
nine regions. The exceptions are the parameters peripheral pulmonary vasculature and hilum, for which no
significant differences between the performance of the
digital and the conventional method were observed
(Fig. 1; Table 1).
Lateral view
The comparison of the overall appearance of the digital
and the conventional images showed no significant differences at the 5 % level. For the comparison of the anatomic regions between DRS and the conventional
method no significant differences at the 5 % level could
be found for the following criteria: posterior cardiac
border; fissures; thoracic spine; and sternum. At the 5 %
level of significance DRS was rated worse than the
conventional method for the criteria trachea, costodiaphragmatic recess, and hilum. It was rated significantly
better than the conventional method for the criteria
retrosternal and retrocardiac pulmonary structures
(Fig. 2; Table 2).
Interobserver variability
Spearman's rank correlation coefficients at first evaluation ranged for all considered criteria from ±0.18 to 0.58
with a mean of 0.23 for PA views, and from ±0.27 to 0.57
with a mean of 0.20 for lateral views. This shows a
moderate interobserver variability.
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a
b
Fig. 1 a, b Posteroanterior chest views of a 49-year-old female patient performed on the same day. Normal result. a Conventional
technique with reduced visibility of the bones: The lateral parts of
the ribs are not well seen and the thoracic spine is not visible
through the heart shadow. The pleuromediastinal borders are not
well seen. b Digital image with the digital chest radiography system
(DRS) technique shows a better overall appearance with good
visibility of the bony structures and more visibility of the mediastinal structures such as trachea, carina, aortic notch, and descending aorta. Hilum and peripheral pulmonary vasculature are
equally visible in comparison with the conventional technique in a
Overall appearance of digital chest radiographs: second
evaluation
The comparison of the first and the second evaluations
for the overall appearance of the digital images showed
that the observers rated DRS significantly better at the
5 % level for the posteroanterior view at the second
evaluation (Table 3). No significant differences between
the two evaluations for the overall appearance of the
digital images were found for the lateral view (Table 4).
In the second evaluation for the lateral view more
observers gave the rating ªadequateº than in the first
Table 2 First evaluation of lateral views, all observers: overall appearance of DRS, comparison of digital and conventional chest radiographs for overall appearance, and anatomic regions. N no. of observations
Parameters
Very good
Good
Adequate
Poor
Unacceptable
N
Overall appearance of DRS
42
119
124
58
2
345
Worse
Much
worse
N
DRS is ... than conventional
Comparison: digital and conventional Much
better
Better
Equal
P
Overall appearance
0
73
194
78
0
345
0.745
Anatomic regions
Trachea
Costodiaphragmatic recess
Posterior cardiac border
Retrosternal
Retrocardiac
Fissures
Hilum
Thoracic spine
Sternum
0
0
0
0
0
0
0
0
0
28
39
58
111
141
38
21
76
45
204
239
215
191
145
276
221
201
267
113
66
71
42
59
31
103
68
33
0
1
1
1
0
0
0
0
0
345
345
345
345
345
345
345
345
345
< 0.001
0.007
0.225
< 0.001
< 0.001
0.470
< 0.001
0.560
0.213
Significant differences at the 5 % level (Bonferroni-Holm test) are marked with asterisk
*
*
*
*
*
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a
Fig. 2 a, b Lateral chest views of the same patient as in Fig. 1 performed on the same day. Normal result. a Conventional technique
with good visibility of all anatomic structures. b Digital image with
the DRS technique shows a reduced visibility of the trachea and
hilum in terms of reduced sharpness and less demarcation to surrounding structures. The retrosternal and retrocardiac pulmonary
structures are better visible than in a
evaluation, in which the ratings ªgoodº or ªpoorº had
higher percentages.
Comparison of digital and conventional chest
radiographs: second evaluation
b
measured at both evaluation times. With a total number
of 345 ratings the PA images were rated identically in
221 cases (64 %) on the basis of the overall appearance.
The digital±conventional comparison showed the same
results in 234 cases (68 %) for the PA images. For the
lateral images with a total of 345 ratings, the corresponding numbers were 209 (61 %, overall appearance
of DRS) and 220 (64 %, comparison digital±conventional). The percentage of images for which the ratings
of both evaluations did not differ or differ by at most
one group (e.g. ªequalº in first evaluation and ªbetterº
in second evaluation for the overall comparison of DRS
and conventional) was between 94 and 98 %, and hence
was very high.
Comparison of overall appearance
The comparison of the overall appearance between
digital and conventional images showed no significant
differences (p = 0.298 and p = 0.935, respectively) between the first and the second evaluations for the posteroanterior as well as for the lateral views (Tables 5, 6).
Intraobserver variability
To measure the variability of the observer ratings the
criteria overall appearance of the DRS and comparison
of the overall appearance DRS vs conventional were
Discussion
Our results showed some advantages of this new digital
system for chest imaging as compared with conventional radiography: We found a significant increase in
subjective visibility of most normal anatomic regions
with the digital system for PA views; however, the results of the lateral images did not show these evident
differences.
Compared with conventional images, image fidelity
was improved in the digital images and a better visualization of low contrast regions, e.g. retrocardiac region,
carina, spine, and azygoesopageal recess, was achieved
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Table 3 First (1) and second (2) evaluation of overall appearance
of DRS, PA view (all observers; n = 345)
Evaluation 2
Very good
Good
Adequate
Poor
Unacceptable
Evaluation 1
Very
good
Good
Adequate
Poor
Unacceptable
46
3
0
0
0
10
113
37
1
0
0
39
61
3
0
0
6
25
1
0
0
0
0
0
0
Difference with respect to evaluations was significant at the 5 %
level (p < 0.001)
Table 4 First (1) and second (2) evaluation of overall appearance
of DRS, lateral view (all observers; n = 345)
Evaluation 2
Very good
Good
Adequate
Poor
Unacceptable
Evaluation 1
Very
good
Good
Adequate
Poor
Unacceptable
39
2
1
0
0
2
69
40
8
0
0
22
84
18
0
0
12
29
17
0
0
0
1
1
0
Difference with respect to evaluations was not significant at the
5 % level (p = 0.858)
Table 5 First (1) and second (2) evaluation of the comparison of
digital and conventional chest radiography for overall appearance,
PA view (all observers; n = 345)
Evaluation 2
Much better
Better
Equal
Worse
Much worse
Evaluation 1, DRS is ... with respect to conventional
Much
better
Better
Equal
Worse
Much
worse
15
0
0
0
0
12
91
34
9
0
0
23
118
4
0
0
6
23
10
0
0
0
0
0
0
Difference with respect to evaluations was not significant at the
5 % level (p = 0.298)
as shown by the results. The lower spatial resolution of
the digital systems compared with the conventional systems did not affect the subjective visibility of most of the
anatomic regions. It is compensated for by the higher
dynamic range and better contrast resolution. These are
the reasons for the better performance of the DRS. In
the PA images only ªperipheral pulmonary vasculatureº
and ªhilumº were not rated significantly better for the
digital images. This was most likely due to the lower
spatial resolution of the DRS which in these areas especially could not be compensated for by the high dy-
Table 6 First (1) and second (2) evaluation of the comparison of
digital and conventional chest radiography for overall appearance,
lateral view (all observers; n = 345)
Evaluation 2
Much better
Better
Equal
Worse
Much worse
Evaluation 1, DRS is ... with respect to conventional
Much
better
Better
Equal
Worse
Much
worse
0
0
0
0
0
5
34
27
7
0
1
18
150
25
0
0
1
41
36
0
0
0
0
0
0
Difference with respect to evaluations was not significant at the
5 % level (p = 0.935)
namic range and the better contrast resolution with the
given processing parameter definitions.
The results for the lateral images were statistically
equivalent but for three regions (trachea, costodiaphragmatic recess, and hilum) they were rated worse in
terms of impaired visualization and one lateral image
was evaluated to be of ªunacceptableº image quality by
two observers. This may be due more to failure in image
processing than to technical problems of the exposure
conditions or the physical properties of the digital flatpanel detector. It seems to be more difficult to obtain
advantages in the lateral images especially in regions of
high attenuation, such as trachea and hilum, with the
image processing techniques of the digital systems compared with the PA images. Preprocessing parameters
which were fixed before the study with the aim of maximum local contrast distribution will possibly need some
alterations for the lateral views in terms of changing parameters for unsharp masking and dynamic range compression especially for mediastinal and hilar structures
and borderline zones between soft tissue and pulmonary
tissue. Use of additional processing techniques may further enhance visibility especially in lateral examinations.
It is important to note, however, that use of processing to
enhance visibility potentially may lead to an increase in
false-positive diagnoses, especially if the appearance of
the radiograph is radically altered. Every attempt should
be made to use a consistent processing technique.
Comparing the results of our study with similar clinical studies evaluating digital chest radiography with the
rotating drum selenium detector [1, 4, 5, 30], the PA
images were also rated significantly better in most of the
anatomic regions as compared with the conventional
chest images. The rotating drum selenium detector
dedicated for chest imaging consisted of an aluminum
drum receptor coated with a layer of charged selenium
with an alteration of the electrical potential during Xray exposure. It had a spatial resolution of 2.5 lps/mm
and a high dynamic range comparable to the DRS.
These technical similarities might explain the good re-
1695
sults for both systems in comparison with the conventional technique. There are not many studies that evaluate both PA and lateral images. The authors of these
studies who did evaluate both PA and lateral images
also described minor differences between the digital and
conventional lateral chest images [1, 4]. Woodard et al.
[4] had similar results with impaired visualization of
some anatomical structures in the lateral digital images.
They concluded to change or use additional processing
techniques and to use an antiscatter grid. In our study an
antiscatter grid was used.
A comparison of the DRS with other flat-panel techniques, such as selenium flat-panel detectors or silicon
flat panel detectors with different scintillator materials,
is not yet possible because, to our knowledge, there have
not been other observer studies with use of clinical images yet. The technical data referring to spatial resolution, dynamic range, and detective quantum efficiency
are very similar between all of the flat-panel detectors.
Interobserver variability was moderate. This was
probably due in part to the relatively small range of responses given by the observers using mostly the range
between 2 and 4 out of the possibilities 1±5. This means
that their opinions differed but only within a small
range. Similar results were shown by Woodard et al. [4].
We additionally performed a second evaluation for
testing the intraobserver variability. For reasons of
practicability, only overall appearance of the DRS was
reevaluated and the comparison of the overall appearance between DRS and the conventional system was
performed at the second evaluation. The comparison of
the anatomic regions was not repeated. We found a
good to moderate reliability between the first and second evaluations with an average of 61±68 % identical
rating which consolidates the results for the digital
technique of the first evaluation. These results provide
the basis to conclude that the new digital system provides an at least equivalent detectability of normal anatomic structures.
For the PA images, the second evaluation of the overall appearance of the digital images showed significantly
better ratings. These different results could mean that it
was necessary for the observers ± not being experienced
with digital images ± to familiarize themselves with the
digital image impression. Consequently, the observers
were more familiar with the digital image on the second
occasion. The observers were familiar with the ªtraditionalº conventional image and might have a prejudiced
opinion on the digital image. This is a potential source of
bias because the observers knew which the digital images
were. The better rating of PA images was probably
caused by adaptation to the digital image impression, but
inconsistency of the readers cannot be excluded. There
was no general trend of all three readers toward a more
positive evaluation. The result improved for the overall
appearance for PA of DRS because the median of one
observer improved by one from 3.0 to 2.0. All the other
medians stayed the same; however, for the lateral images
the second evaluation of the overall appearance showed
no significant differences, although the rating ªadequateº was more often used in the second evaluation
which also could be a hint that the observers were more
familiar with the digital images.
However, the comparison of overall appearance of
conventional and digital images did not change significantly in the two readings which again appears to hint at
an inconsistency of the observers more than to the fact
that there were true differences. The general trend of all
three readers was stable for the comparison of overall
appearance. The results are subjective as well for the
interobserver variability than for the second evaluation,
but ultimately the assessment of radiographic images
involves objective measurements and subjective evaluations.
Other possible constraints of our study are that we
only investigated normal chest radiographs without pathology. But as a first clinical test of the DRS system, it
seemed important to determine image quality of normal
anatomic structures first, and in a second step to evaluate detectability of pathological lesions. This second
study is performed in our department at present. Obviously, the detection or delineation of normal pulmonary
structures is no indicator of the detection of pulmonary
abnormalities. The preprocessing parameters were
fixed at the beginning of the study and could not be
changed in between.
Our study judged the clinical usefulness of the
CXDI-11 silicon flat-panel system. The clinical comparison of an amorphous silicon flat-panel detector with
conventional technique for chest radiography demonstrated good performance for PA views and moderate
results for lateral views of the digital detector. Results
are encouraging and should motivate further clinical
studies that investigate patient dose reduction with an
adequate image quality [20, 31, 32], advantages of postprocessing, and the use of silicon-based flat-panel detectors for other radiological investigations; however,
we do recommend as first step to improve the image
quality of the lateral digital images by changing image
processing parameters.
Acknowledgements We thank R. Kühn, E. A. Hühn, R. Hennigs,
and the technicians of our department for their valuable help.
1696
References
1. Zähringer M, Krug B, Dölken W,
Gossmann A, Lackner K (1997) Kann
die digitale Selenradiographie in der
Thoraxdiagnostik die analoge
Röntgenaufnahmetechnik ersetzen?
Fortschr Röntgenstr 167: 4±10
2. Schaefer CM, Prokop M (1993) Storage
phosphor radiography of the chest. Radiology 186: 314±315
3. Heesewijk HPM, van der Graaf Y, de
Valois JC, Vos JA, Feldberg MA (1996)
Chest imaging with a selenium detector
versus conventional film radiography: a
CT-controlled study. Radiology 200:
687±690
4. Woodard PK, Slone RM, Gierada DS,
Reiker GG, Pilgram TK, Jost RG
(1997) Chest radiography: depiction of
normal anatomy and pathologic structures with selenium-based digital radiography versus conventional screenfilm radiography. Radiology 203:
197±201
5. Floyd CE, Baker JA, Chotas HG, Delong DM, Ravin CE (1995) Seleniumbased digital radiography of the chest:
radiologists' preference compared with
film-screen radiographs. AJR 165:
1353±1358
6. Rowlands JA, Zhao W, Blevis I,
Waechter D, Huang Z (1996) Digital
radiography and fluoroscopy with a
self-scanned, flat-panel, amorphous selenium detector (Abstr). Radiology
201(P):525
7. Mansson LG, Kheddache S, Lanhede
B, Tylen U (1999) Image quality for five
modern chest radiography techniques: a
modified FROC study with an anthropomorphic chest phantom. Eur Radiol
9: 1826±1824
8. Chaussat C, Chabbal J, Ducourant T,
Neyret R, Spinnler V, Vieux G (1998)
New cesium iodide/amorphous silicon
17º”17º digital flat-panel detector for
general radiography brings improved
performance and new operating modes
to support radiography, tomography
and digital tomosynthesis (Abstr). Radiology 209: 315
9. Neitzel U, Maack I, Rindt K, Spiess K
(1998) Digital radiography system with
a full-size, flat-panel X-ray detector:
evaluation of imaging performance
(abstr). Radiology 209(P): 161
10. Spahn M, Alexander J, Gmeinwieser J
(1997) Festkörperdetektoren aus amorphem Silizium und ihr künftiger Einsatz
in der medizinischen Röntgenbildgebung. Electromedica 65: 37±41
11. Hamers S, Freyschmidt J (1999) Digitale Radiographie mit einem elektronischen Flachdetektor: Erste klinische
Erfahrungen in der Skelettdiagnostik.
Kontraste 13: 2±6
12. Strotzer M, Gmeinwieser J, Völk M,
Fründ R, Feuerbach S (1999) Digitale
Flachbilddetektortechnik basierend auf
Cäsiumjodid und amorphem Silizium:
Experimentelle Untersuchungen und
erste klinische Ergebnisse. Fortschr
Röntgenstr 170: 66±72
13. Antonuk LE, El-Mohri Y, Siewerdsen
JH, Yorkston J, Huang W, Scarpine VE,
Street RA (1997) Empirical investigations of the signal performance of a
high-resolution, indirect detection, active matrix flat-panel imager (AMFPI)
for fluoroscopic and radiographic operation. Med Phys 24: 51±70
14. Antonuk LE, El-Mohri Y, Huang W
et al. (1997) Development of a high
resolution, active matrix, flat-panel imager with enhanced fill factor. SPIE
Med Imaging 3032: 2±14
15. Antonuk LE, Boudry JM, El-Mohri Y,
Huang W, Siewerdsen JH, Yorkston J,
Street RA (1995) Large area, flat-panel,
amorphous silicon imagers. SPIE Med
Imaging 2432: 216±227
16. Kengyilics SM, Davies AG, Launders
JH, Cowen AR (1998) Imaging performance of a direct digital flat-panel radiographic detector (Abstr). Radiology
209: 358
17. Lee DL, Cheung LK, Jeromin LS,
Palecki EF, Rodricks B (1997) Radiographic imaging characteristics of a direct conversion detector using selenium
and thin film transistor array. SPIE Med
Imaging 3032: 88±96
18. Lee DL, Cheung LK, Jeromin LS
(1997) A new digital detector for projection radiography. SPIE Med Imaging
2432: 237±249
19. Kameshima T, Kaifu N, Takami E, Morishita M, Yamazaki T (1998) Novel
large area MIS-type X-ray image sensor
for digital radiography. SPIE Med Imaging 3336: 453±462
20. Yamazaki T, Morishita M, Kaifu N,
Endo Y (1998) Development of digital
radiography system. In: Lemke JU,
Vannier MW, Inamura K, Farman A
(eds) CAR'98. Elsevier, Tokyo, pp
536±541
21. Strotzer M, Gmeinwieser J, Völk M,
Fründ R, Seitz J, Feuerbach S (1998)
Detection of simulated chest lesions
with normal and reduced radiation
dose: comparison of conventional
screen-film radiography and a flat-panel X-ray detector based on amorphous
silicon. Invest Radiol 33: 98±103
22. 41st World Medical Assembly (1990)
Declaration of Helsinki: recommendations guiding physicians in biomedical
research involving human subjects. Bull
Pan Am Health Organ 24: 606±609
23. Stender HS (1995) Leitlinien der Bundesärztekammer zur Qualitätssicherung in der Röntgendiagnostik. Dtsch
¾rztebl 92: 1691±1696
24. Campbell MJ, Machin D (1990) Medical statistics. A common sense approach. Wiley, New York, pp 142±143
25. Holm S (1979) A simple sequentially
rejective Bonferroni test procedure.
Scand J Statist:65±70
26. StatXact 4 for Windows (1998) Cytel
Software Corporation, Cambridge
27. SAS Institute Inc (1996) SAS/STAT
Software: changes and enhancements
for release 6.12. SAS Institute, Cary,
North Carolina, pp 124±125
28. Gardner MJ, Altman DG (1989) Statistics with confidence. Br Med J:48±49
29. SAS Institute Inc (1990) SAS Procedures Guide, version 6, 3rd edn. SAS
Institute, Cary, North Carolina, pp
209±236
30. Heesewijk HPM, Neitzel U, van der
Graaf Y, de Valois JC, Feldberg MA
(1995) Digital chest imaging with a selenium detector: comparison with conventional radiography for visualization
of specific anatomic regions of the
chest. AJR 165: 535±540
31. Völk M, Strotzer M, Gmeinwieser J,
Alexander J et al. (1997) Flat-panel Xray detector using amorphous silicon
technology: reduced radiation dose for
the detection of foreign bodies. Invest
Radiol 32: 373±377
32. Slone RM, Woodard PK, Jost RG, Sagel SS (1996) Comparison of 150- and
117-kVp techniques for clinical chest
imaging with a Thoravision seleniumbased digital system (Abstr). Radiology
201: 400