DOI:10.2214/AJR.05.0733
AJR 2007; 188:1138-1144
© American Roentgen Ray Society
Neonatal Chest Computed Radiography: Image Processing and Optimal Image Display
Steven Don1,2,
Bruce R. Whiting2,
Jacquelyn S. Ellinwood3,
David H. Foos3,
Keith A. Kronemer1 and
Richard A. Kraus1
1 Mallinckrodt Institute for Radiology, St. Louis Children's Hospital,
Washington University School of Medicine, 510 S Kingshighway, St. Louis, MO,
63110.
2 Electronic Research Laboratory, Mallinckrodt Institute for Radiology,
Washington University School of Medicine, St. Louis, MO.
3 Eastman Kodak Company, Rochester, NY.
Received April 29, 2005;
accepted after revision November 10, 2005.
Address correspondence to S. Don
(dons{at}mir.wustl.edu).
Presented at the 2004 annual meeting of the American Roentgen Ray Society,
Miami Beach, FL.
Supported in part by National Institutes of Health research grant 1 R41
HD40747-01.
The employment status of J. S. Ellinwood and D. H. Foos at Eastman Kodak
Company did not influence the data in this study.
Abstract
OBJECTIVE. The purpose of this study was to determine soft-copy
image display preferences of brightness, latitude, and detail contrast for
neonatal chest computed radiography to establish a baseline for future work on
low-dose imaging.
CONCLUSION. Observers preferred brighter images with higher detail
contrast and narrow to middle latitude for soft-copy display compared with the
typical screen-film hard-copy appearance. Future research on low-dose neonatal
chest imaging will be facilitated by an understanding of optimal soft-copy
image display.
Keywords: chest • digital images • neonatal imaging • PACS • pediatric radiology
Introduction
Traditional screen-film radiographs have a characteristic
density/log (exposure) response (Hurter and Driffield curve or tone scale)
that is sigmoidal in shape and determines the density and contrast of
screen-film images at a specific peak kilovoltage and effective tube current
[1]. An underexposed image
falls in the toe of the characteristic curve, and an overexposed image falls
in the shoulder of the characteristic curve.
The signal response in computed radiography (CR) is linear with exposure,
resulting in a wider dynamic range: 100- to 1,000-fold greater than that of
screen-film radiography. The result is better tolerance of underexposure and
overexposure with CR, but the unprocessed raw image data do not have the
proper image contrast and density necessary for image interpretation
[2,
3]. Image processing of raw CR
digital data frequently results in the screen-film radiographic appearance
that radiologists are accustomed to interpreting. This appearance typically is
achieved because of laser printing of the image on film. Hospitals, however,
have begun to convert to soft-copy image display.
Image processing, because of the unprocessed raw image data, must include
recognition of the amount of exposure the X-ray plate receives in the relevant
anatomic regions. Grayscale rendition is then applied to achieve appropriate
overall brightness in the displayed image. Gray-scale rendition is performed
to simulate the Hurter and Driffield curve and to produce an image acceptable
to radiologists. Specialized image processing, such as edge enhancement and
signal equalization, also may be used to emphasize features in the CR images
[2,
3].
Because of concerns about patient radiation dose, it is desirable to
perform CR examinations with a dose level as low as reasonably achievable
while maintaining an acceptable level of image noise
[4]. These requirements can be
met by changing radiographic parameters, such as peak kilovoltage and
effective tube current, or by using image processing to suppress noise while
preserving image features. Many neonatal dose-reduction studies have been
performed with laser-printed film and fixed image processing
[5-7].
In those studies, investigators tested the effects of dose reduction on the
diagnosis of hyaline membrane disease and pneumothorax
[5-7].
Soft-copy image display parameters must be controlled to prevent
interacting effects from confounding dose-reduction experiments. Soft-copy
image display preferences for adult chest CR were determined in a previous
study [8], but requirements for
soft-copy image display preferences for neonatal chest CR are unknown. Because
of concerns about radiation sensitivity in children, research is being
conducted into methods of reducing radiation dose in CR. The purposes of this
research are to determine user preferences for neonatal chest CR soft-copy
image display as a baseline for future soft-copy work on low-dose imaging and
to gain an understanding of which factors are important in the optimal
presentation of neonatal images. The effects of low-dose exposure on observer
performance in the detection of disease can then be studied.
Materials and Methods
Image Acquisition
This study received the approval of our institutional review board. All
image data were handled according to the Health Insurance Portability and
Accountability Act (HIPAA). Images were obtained with a Kodak 400 CR system
(Eastman Kodak Company). Five chest radiographs of healthy neonates and five
chest radiographs of neonates with clearly discernible pneumothorax were
selected. Raw image data were collected at a quality-control workstation with
all image processing turned off. These data were transferred to another
workstation for removal of all patient identification data and then were sent
to an analysis workstation.
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Fig. 1A —Normal neonatal chest radiographs illustrate response curve to
brightness adjustments for neonatal chest computed radiography. Lungs appear
progressively lighter from lowest brightness to highest brightness.
Low-brightness image.
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Fig. 1B —Normal neonatal chest radiographs illustrate response curve to
brightness adjustments for neonatal chest computed radiography. Lungs appear
progressively lighter from lowest brightness to highest brightness. Reference
T-MAT G (Eastman Kodak Company) image.
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Fig. 1C —Normal neonatal chest radiographs illustrate response curve to
brightness adjustments for neonatal chest computed radiography. Lungs appear
progressively lighter from lowest brightness to highest brightness.
High-brightness image.
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Fig. 1D —Normal neonatal chest radiographs illustrate response curve to
brightness adjustments for neonatal chest computed radiography. Lungs appear
progressively lighter from lowest brightness to highest brightness. Graph
shows adjustment of characteristic curve from lowest to highest brightness. At
fixed input code value, lowest brightness images have higher or more lung
density output and highest brightness images have lower or more bone density
output.
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Imaging Processing
The raw image data were processed with a prototype image-processing
algorithm [8]. Three variables
were altered: brightness, detail contrast, and latitude. A control image for
each patient was acquired to simulate the characteristic response curve of
T-MAT G film (Eastman Kodak Company). This screen-film combination is common
in pediatric chest radiography.
Brightness is increased on a CR image through a shift in the response curve
to the right (Fig. 1A,
1B,
1C,
1D). This control is analogous
to the level in CT image display in which the lungs are dark at a mediastinal
level and bright at a lung level.
Detail contrast is increased on a CR image through an increase in the slope
of the linear portion of the response curve. Low-detail contrast images appear
grayer, and high-detail contrast images are more black and white (Fig.
2A,
2B,
2C). This control is analogous
to the window in CT in which shallow lung window contrast minimizes the
contrast difference between water and soft tissue and steep mediastinum window
contrast accentuates the difference between water and soft tissue.
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Fig. 2A —Normal neonatal chest radiographs show detail contrast adjustment of
response curve for neonatal chest computed radiograph. See reference image in
Figure 1B for comparison.
Difference in contrast between air in stomach and ribs is evident. Low-detail
contrast image appears gray.
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Fig. 2B —Normal neonatal chest radiographs show detail contrast adjustment of
response curve for neonatal chest computed radiograph. See reference image in
Figure 1B for comparison.
Difference in contrast between air in stomach and ribs is evident. High-detail
contrast image is more black and white than A.
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Fig. 2C —Normal neonatal chest radiographs show detail contrast adjustment of
response curve for neonatal chest computed radiograph. See reference image in
Figure 1B for comparison.
Difference in contrast between air in stomach and ribs is evident. Graph shows
adjustment of response curve. As detail contrast increases, characteristic
curve becomes steeper (T-MAT G, Eastman Kodak Company).
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Latitude adjustment on a CR image changes the overall contrast of an image
without affecting local contrast or resolution. For a wide-latitude image, one
compresses the large-area, low-frequency data of the histogram information and
subtracts that from the histogram of the image. The overall appearance of the
image is grayer yet retains local contrast. For example, the contrast between
a rib and adjacent lung is maintained, but the contrast between the free-in-
air exposure and soft tissues around the humerus is decreased (Fig.
3A,
3B). This manipulation
highlights features such as lung markings and bone detail and minimizes
differences such as density difference between lungs. Examples of the effect
of altering the image-processing parameters of brightness, detail contrast,
and latitude for a neonate with pneumothorax are presented in Figure
4A,
4B,
4C,
4D,
4E,
4F.
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Fig. 3A —Normal neonatal chest radiograph with latitude adjustment of
response curve for neonatal chest computed radiograph. See reference image in
Figure 1B for comparison.
Difference in free-in-air exposure and soft tissue around humerus is evident.
Wide-latitude image appears gray.
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Fig. 3B —Normal neonatal chest radiograph with latitude adjustment of
response curve for neonatal chest computed radiograph. See reference image in
Figure 1B for comparison.
Difference in free-in-air exposure and soft tissue around humerus is evident.
Graph shows adjustment of characteristic curve. As latitude increases,
steepness of response curve decreases. This finding applies to
low-spatial-frequency data only.
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Each case was processed with five brightness levels, four detail contrast
levels, and seven latitude levels. One hundred ten combinations of 140
possible combinations of each patient's image were made. The processing
parameter matrix was not symmetric, and the narrower-latitude and lower-detail
contrast images were not acquired because observation
[8] had shown the range of
lowest latitude and contrast performed the worst. With this criterion, 30
image combinations per patient were eliminated without alteration of the
results, allowing reduction of computational time and enhancement of observer
participation.
Combinations were achieved by adjustment of the tone scale with a fixed
shape that approximately matched the tone scale curve of T-MAT G film. The
middle brightness level was chosen as the level that would yield an optical
density of approximately 1.65 on a T-MAT G film image of the lung region.
Brightness adjustments were achieved by shifting the tone scale along the
input axis in increments of 75 and 150 code values above and below the middle
brightness code value for each image to yield lowest, lower, middle, high, and
highest brightness settings. At the reference detail contrast setting, that
adjustment would yield optical densities in the lung region of, from low to
high, 2.25, 1.95, 1.65, 1.35, and 1.15. Detail contrast adjustments were
achieved by pivoting the tone scale at set increments around the input value
corresponding to a density of 1.0. Latitude adjustments were achieved by
reducing the low-frequency contrast at set increments while maintaining the
detail contrast, achieved through a signal-equalization filtering step.
The control image-processing parameters were chosen as the image at middle
brightness (one that would yield an optical density of approximately 1.65 on
T-MAT G film in the lung region), low-detail contrast (T-MAT G film contrast
of 3.1), and narrow latitude (one that would maintain the tonal
characteristics of low-frequency data for a detail contrast level of 3.1).
These parameters would simulate on soft-copy display the hard-copy screen-film
appearance of neonatal chest radiographs.
Image Viewing
One hundred ten images were acquired for each of 10 sets of neonatal CR
images for a total of 1,100 images. These images were randomized and presented
in 10 sessions of 110 images. The images were viewed with a stand-alone PC
workstation on a high-resolution (2,000 x 2,500) cathode ray tube
monitor (model DR 110, Data Ray). The monitor was calibrated, and lighting was
controlled to eliminate glare on the screen. The workstation was in a quiet
room for elimination of extraneous noise.
Each test image was presented along with the control processed image. The
observer toggled between the test image and the control image and controlled
the toggle rate and amount of observation time before rating. Three pediatric
radiologists with certificates of added qualification in pediatric radiology
participated in the image review. These radiologists were familiar with
soft-copy interpretation. Each radiologist rated each image on a nine-point
viewing scale relative to the control image as in previously published adult
work [8]: 4, image quality
markedly better, diagnosis likely altered; 3, image quality clearly better,
diagnosis might be altered; 2, image quality somewhat better, diagnosis should
be the same; 1, image quality slightly better, diagnosis will be the same; 0,
no difference; -1, image quality slightly worse, diagnosis will be the same;
-2, image quality somewhat worse, diagnosis should be the same; -3, image
quality clearly worse, diagnosis might be altered; -4, image quality markedly
worse, diagnosis likely altered.
Statistical Analysis
A regression model with a second-order polynomial was used to analyze the
data. The factors of brightness, latitude, and detail contrast were treated as
continuous. A logarithmic transform of the latitude was used to improve
conformance to the polynomial fit. Brightness, detail contrast, and
log-latitude levels were centered to reduce the correlation between predictor
values, thus clarifying significance testing. In addition, health condition
(presence or absence of pneumothorax) was included as a discrete and fixed
effect. Observer and patient effects were treated as discrete and random, and
the patient effect was considered nested within the health condition.
The peak soft-copy viewing preference range of detail contrast and latitude
cell at each brightness level was determined. The best-rated cell at each
brightness level was identified. The 95% CI was determined for the difference
between the predicted mean of the cell with the best rating and the predicted
mean of the cell being checked. All cells identified as not significantly
different from the best rating cell were included in the peak preference range
for each brightness level.
Results
The regression results showed that brightness (p < 0.01) was
the single most important factor in determining user imaging and diagnostic
preferences (Table 1). Also
significant in order of contribution to the variance were observer, detail
contrast, latitude, and patient (Table
1). The presence or absence of pneumothorax was not statistically
significant (p = 0.14).
There was a peak cell-rating difference of three points between the
user-preference predicted best cell rating at the lowest brightness (-1.7) and
the user-preference predicted best cell rating at the highest brightness
(1.3). At the lowest brightness, all scores were significantly less than 0,
indicating that the images at this brightness level, regardless of detail
contrast and latitude, were less desirable than the reference image in image
and diagnostic quality (Table
2). At the middle brightness level, which included the reference
image, the predicted ratings were near 0
(Table 3). The ratings at the
highest brightness and highest detail contrast and narrow latitude had the
highest predicted score (Table
4). At none of the image brightness settings was there a
statistically significant difference between the ratings for healthy patients
and those for patients with pneumothorax.
Discussion
Despite the presence of CR in pediatric imaging for nearly 20 years
[9], to our knowledge no
scientific methodology has been developed for addressing specific soft-copy
viewing preferences. Years ago, a hard-copy CR film included two images, one
imitating a screen-film radiograph and a second that enhanced edge structures,
such as catheters [8,
9]. Currently, one hard-copy CR
film melds the screen-film appearance and edge enhancement. Our findings
clearly indicate that soft-copy viewing preferences in neonatal chest CR are
different from the established hard-copy screen-film radiographic preferences
[8]. Our findings establish
baseline soft-copy settings for future use in image display and dose-reduction
research.
More radiology departments are shifting to soft-copy display and
eliminating film entirely. Much work has been done to meet the challenge of
PACS and soft-copy display
[10]. Research has focused on
comparing soft-copy display with hard-copy film
[11], resolution of the
monitor [11], CR with other
digital radiographic systems
[12,
13], and monitor luminance
[14-16].
In these studies, the images were initially displayed from fixed, preset
image-processing algorithms. In our study, the image-processing parameters
were the tested variables. In three of the previous studies
[12-14],
it was explicitly stated that radiologists were allowed to adjust window and
level but not spatial filtering. In one study
[14], the radiologists
manipulated window and level settings 90% of the time. In only one study
[8], with adult subjects, were
preferred soft-copy display settings evaluated.
Our study of neonatal chest CR images showed that pediatric radiologists
find improved image and diagnostic quality of softcopy display with the
brightest image setting and with high-detail contrast and narrow to middle
latitude processing settings. Adjusting the CR image display resulted in a
higher rating by the radiologists compared with the reference screen-film
radiographic appearance. The CR viewing preference was different from the
screen-film radiographic appearance of the characteristic response curve of
T-MAT G film, which is middle brightness, narrow latitude, and less detail
contrast. Wide-latitude images were not preferred for routine viewing.
The parameters that were varied in this experiment (brightness, detail
contrast, and latitude) are the building blocks of image quality. As such, the
results can be generalized for use with most CR systems, provided the
interface with image processing has the required flexibility. Although the
particular image processing (rendering) parameters are different among CR
systems, the functionality of image processing can be related to these three
fundamentals of image quality.
The results of this study can be applied to soft-copy and hard-copy
interpretation, provided the image-processing parameters are adapted to
compensate image appearance for the dynamic range, gray-scale resolution, and
spatial resolution differences among printers and soft displays. The concept
of the building blocks of image quality has been detailed
[17].
In a previous study [8] of
posteroanterior chest radiographs of adults, the radiologists preferred a
wide-latitude (highly equalized) image with less detail contrast, different
from the user preferences we found. One explanation may be that adult chest
images have a broader histogram range than chest images of neonates. This
difference is attributable to the size difference between neonates and adults.
The anatomic structures of neonates are less attenuating than are those of
adults. The typical peak kilovoltage used for neonatal chest radiographs is
60-70 kVp, whereas that for adult chest radiographs is much higher, in the
range of 120-140 kVp. The higher peak kilovoltage used for adult chest
radiographs results in lower contrast and a wider range of optical density
than on neonatal chest radiographs. A wider-latitude image compresses the
range of optical density for optimal display of soft-copy chest images of
adults.
The presence or absence of pneumothorax did not significantly affect viewer
preference. This finding implies that there is no need for routine alteration
of image display for routine evaluation for pneumothorax. This study, however,
did not test ability to diagnose subtle cases of pneumothorax. A previous
study of neonatal chest images showed that edge enhancement, which was not
investigated in this study, improved detection of subtle pneumothorax
[18].
There were limitations to this study. Only three radiologists from a single
site were used to select user preferences. The statistically significant
difference between radiologist preferences (p < 0.01)
(Table 1) indicates that
image-display preference is a highly personal choice. In addition, only one
vendor's CR equipment was used. The results cannot be extrapolated to other
types of monitors, uncontrolled viewing situations, and other vendors' CR and
digital radiography systems.
Direct extrapolation of the findings of this study to current clinical
practice as a department transitions to soft-copy interpretation is
problematic given the limitations of this study. Setting up and performing
experiments such as ours require access to raw image data, imaging physicists,
and time. To aid clinical departments in the transition, radiologists should
encourage the vendors of digital imaging equipment and PACS workstations to
develop a library of cases that include various ages, body parts, and disease
processes. When new equipment is installed, radiologists can review the cases
to find their personal preferences.
The results of this study establish baseline soft-copy display parameters
for neonatal chest CR. These parameters are clearly different from those for
the screen-film hard-copy laser-printed images to which radiologists have
become accustomed for interpreting CR images. Scientific research on image
processing and dose reduction requires establishing baseline image processing.
Future improvement in image processing and studies on the effects of dose
reduction on detection of disease processes can be conducted with controlled
imaging parameters for reducing and eliminating confounding factors in
statistical analysis.
In conclusion, this study showed that pediatric radiologists prefer high
brightness, high detail contrast, and a narrow to middle latitude image
processing for viewing neonatal chest CR images compared with a soft-copy
control image on which tonal characteristics and brightness are mapped to
yield an optical density of approximately 1.65 in the lung region. The
presence or absence of pneumothorax did not affect viewing preferences.
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Abstract
Introduction
