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1071
Review
Digital Skeletal Kenneth
A. Buckwalter1
Radiography
and Ethan
M. Braunstein
Skeletal radiography accounts for a large proportion of the plain film images generated in most radiology departments, yet it has been underemphasized in the investigation of digital im-
aging technologies. Unique features of skeletal radiography provide a challenge in the evaluation and investigation of digital skeletal imaging. This review summarizes the results of ongoing research in digital skeletal radiography. Elementary concepts of digital sion
imaging
are
of nominal
reviewed
contrast
to provide
and
Article
resolution
a foundation
for
requirements
discus-
for clinically
useful digital skeletal radiography. Methods of acquiring digital skeletal images are reviewed, and digital image display, image compression, and basic image processing techniques are dis-
it is important to understand of digital skeletal radiography general practice.
In order to understand
the advantages and limitations before it is incorporated into
these problems,
it is necessary
to
review some basic concepts of digital imaging. These will be reviewed with emphasis on the spatial and contrast resolution requirements of skeletal radiography. The technologies used
to acquire digital radiographs with
one
images
another.
will be discussed
Additionally,
will be discussed
in the context
features
and compared of skeletal
of image
display,
image
archive,
Currently, nearly 30% of the images generated in most radiology departments are in digital format [1]. This includes
Chest
vs Skeletal
sonography, scintigraphy, CT, MR imaging, some angiography, and computed radiography. Most likely, the bulk of conventional projectional radiographs also will be acquired
Technical requirements for routine and mobile chest radiography are better defined than those for skeletal imaging. The 1 4 x 1 7 in. (36 x 43 cm) cassette is the standard size of adult chest radiographs. Film sizes for skeletal radiography vary widely. The high-kilovoltage technique and wide-latitude screen-film combination generally used in chest imaging are different from the low-kilovoltage technique and high-contrast screen-film combination used in skeletal radiography. Additionally, spatial resolution requirements for skeletal imaging vary depending on the purpose of the examination. Unlike chest radiographs, skeletal radiographs may have uses other than diagnostic ones. Measurements obtained from the images are important for preoperative planning of joint replacement surgery. This may impede the acceptance of digital skeletal radiographs by referring orthopedists, es-
cussed
digitally
agnostic
with
emphasis
on
in the future.
accuracy,
specific
Issues
skeletal
such
and clinical
gation before computed Although much research
as cost-effectiveness, dineed further investiis universally accepted.
aging of the chest [2, 3], few articles
conducted on digital imhave been published on
digital
reviews
skeletal
evant research
imaging.
published
This
article
on digital skeletal
the clinically
rel-
radiography.
Although chest radiography accounts for as much as 40% of all medical imaging performed in the United States [3], skeletal radiography accounts for a large fraction of the total number of plain film examinations. Thus, a large number of
skeletal radiographs Received September 1
Both authors:
potentially
may be digital. For this reason,
3, 1 991 : accepted
Department
of Radiology,
after revision Indiana
December
University
May
1992 036i-803x/92/i585-1
Radiography
4, 1991.
Hospital,
Room x-64, 926 W. Michigan
to K. A. Buckwalter. AJR 158:1071-1080,
processing.
applications.
efficacy
radiography has been
and image
unique
071 © American
Roentgen
Ray Society
St., Indianapolis,
IN 46202-5253.
Address
reprint
requests
BUCKWALTER
1072
pecially
with
regard
to soft-copy
display
and minified
AND
hard-
copy images.
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Digital
vs Conventional
When
assessing
development, on a light
to compare
radiogra-
screen-
[4]. Film has multiple functions. Combined screen, it serves as the X-ray detector. After
it functions
box,
of computed
it with conventional
it displays
as the image archive. the image.
When placed
Obviously,
film cannot
perform all three tasks equally well. As a detector, excellent spatial resolution. However, the exposure
film has latitude
of screen-film combinations is limited and nonlinear. As an archival medium, film is excellent, although a single radiograph cannot be displayed in multiple locations simultaneously. As
a display device, film is also excellent, showing a wide range of film densities on a properly exposed radiograph. The widespread use of conventional radiography is an indication of the success of film-screen technology. Combining the functions and image archive requires
of image capture, some compromise
image display, in the optimi-
zation of each task. For example, the limited and nonlinear exposure latitude of a screen-film detector places a critical dependency on proper radiographic technique. This is particularly important for skeletal radiography, which is usually performed with a narrow-latitude gin for error. Additionally, the
technique, leaving less marwide variation in body and
extremity thicknesses may result in underexposure in one part of a radiograph and overexposure in another part of the same radiograph. As film serves both as an archival medium and a display medium, lost, misplaced, or stolen radiographs can result in a permanent loss of information. The inability to view the original radiographic image simultaneously in two locations may compromise the delivery of clinically relevant interpretations. This is particularly important in an outpatient
setting,
and most
outpatients.
of skeletal
Another
limitation
the inability to reproduce film does
not faithfully
radiography of screen-film
is performed
on
radiography
is
an exact copy of the image.
duplicate
all of the findings,
Copy
particularly
the more subtle ones. Unlike conventional radiography, these film functions are uncoupled in digital radiography. Uncoupling allows optimization in the design and implementation of devices to perform each task. Each of these functions-image capture, archival, and display-will be discussed with specific applications to skeletal imaging. A review of the basic concepts of digital imaging is useful in understanding some limitations of computed radiography.
Basic
Concepts
Conventional radiographic
of Digital
and
are continuous;
medium.
as an m x n matrix size. The spacing of the pixels
One may express system
by recording
tom. Although
the spatial the resolving
the modulation
To the naked eye, number
of elements appear to be making up the image, and each element has an infinitely variable gray value. A digital radiograph is a quantized representation of the spatial modulation of the X-ray beam after it passes through the patient. The
resolution power
of an imaging
of a line-pair
transfer
function
phan-
provides
a
more complete assessment of spatial resolution, line-pair resolution is easier to measure. A large number of visible line-
pairs per millimeter for conventional [5], to 8 lp/mm
means a higher resolution.
Measurements
skeletal radiographs vary from 3-7.8 lp/mm [6], and 1 0-1 2 lp/mm [7]. In comparison,
chest and gastrointestinal
radiographs
have a resolution
of 4
lp/mm [6]. These measurements suggest that, aside from mammography, conventional skeletal radiography requires more spatial detail than any other plain film technique [8]. At least two pixels are required to represent the line and
gap of a line-pair obtain
a specific
phantom spatial
on a digital
resolution,
number of pixels spatial resolution
image.
it is necessary
the pixel size. The size of the image
will dictate
In order
to
to adjust
the total
required to represent the image at a specific (see Appendix). Given equivalent-sized pix-
els, a larger image will require more total pixels to maintain the same spatial resolution as a smaller image. Additionally, a fixed matrix size may result in adequate detail for a smaller image but be inadequate for a larger image. This is particularly important in skeletal image sizes.
radiography,
which
requires
multiple
The dynamic range of digital images depends on the number of bits assigned to each pixel. This is typically 8, 10, or 1 2, representing 256, 1 024, and 4096 possible gray values per pixel. Bits are grouped in 8-bit units called bytes for storage and computer processing. An image with 8 bits per
pixel requires requires
1 byte, whereas
2 bytes
for storage.
an image with 10 bits per pixel Thus,
a change
from
an 8-bit to
a 10-bit per pixel image requires twice the memory or storage space. Image transmission speed and archival space depend on the size of a digital image. Image size is the product of the number of pixels in the image matrix times the number of
bytes
per pixel.
Doubling
the spatial
resolution
quadruples
the image size; however, increasing the gray scale range may not increase the image size in bytes. The data required for digital representation of conventional radiographic examinarequirements
that is, an infinite
May 1992
with respect to the original size of the image determines the spatial resolution of the image. Each pixel has an assigned gray level, and the range of available gray levels defines the image’s dynamic range, which is determined by the number of bits per pixel. The distribution of the gray level values within the dynamic range is the contrast resolution.
tions may be considerable.
Radiography
film is an analog images
of the problems
AJA:158,
individual sample points (pixels) are usually obtained in a rectangular pattern, with evenly spaced samples arranged in rows and columns. The number of pixels in the rows (m) and columns (n) determines the size of the image matrix, usually
expressed
the development
phy, it is important film technology with the imaging
Radiography
BRAUNSTEIN
for a single
A conservative
estimate
1 4 x 1 7 in. image
of storage
is 40 megabytes
[9] based on a 4096 x 4976 x 1 6 bit image matrix (approximately requires
5.7 lp/mm resolution). four 1 4 x 1 7 in. image
Because an average study equivalents, 1 60 megabytes
of storage would be needed for each examination. By comparison, a 30-image set of 51 2 x 51 2 x 12 bit CT images is only 16 megabytes.
AJR:158,
Contrast Skeletal
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DIGITAL
May 1992
and Spatial Radiography
Resolution
Requirements
SKELETAL
for Digital
Several experiments have been devised to determine the effects of altered spatial and contrast resolution on diagnostic accuracy and lesion detectability in digital skeletal imaging [5-7, 1 0-1 5]. Both spatial and contrast resolution are important in determining the costs of digital radiography equipment, picture archiving and communication systems (PACS), and image workstations. These experiments suggest that resolution requirements depend on the imaging task.
Higher resolution images are needed to detect subtle changes. One particularly demanding task is the detection of subperiosteal resorption on hand radiographs. This was investigated by Murphey [1 1], who found that a spatial resolution of 5.7 lp/mm or less resulted in a loss of diagnostic
accuracy
in detecting
conventional
subperiosteal
radiographs.
cation is the detection
Perhaps
resorption a more
of nondisplaced
compared
widespread
fractures.
with appli-
Murphey
et
al. [6] tested a series of nondisplaced fracture images varying in resolution from 0.72 to 5.75 Ip/mm and found that all fractures in a series of 56 radiographs could be detected at a resolution of 2.88 lp/mm (0.1 6-mm pixel size). Wilson et al. [1 4] compared high-resolution (S lp/mm) photostimulable phosphor digital extremity radiographs with conventional radiographs and found conventional radiographs slightly supenor in the detection of fractures. They believed that it is possible to compensate for this discrepancy during clinical practice, but concluded that conventional radiography should be available when digital images are equivocal if digital radiography is used as the primary imaging method in acute extremity trauma. Wegryn et al. [7] found that high variability
between observers digital radiographs
limited identifiable differences between obtained at 1 .25 lp/mm resolution and
conventional radiographs. Images were also reviewed at 2.5 lp/mm.
that 1 .25 lp/mm resolution some subtle abnormalities
with more subtle findings Wegryn et al. concluded
was adequate for most tasks, but required higher resolution.
Unfortunately, it is not always possible to determine how much detail will be required to make a confident diagnosis. A study comparing the effects of varying resolution on the detection of subtle gastrointestinal mucosal abnormalities
suggested malities
that more experienced at lower
levels
of resolution
same is true with digital skeletal
observers
detected
abnor-
[1 6]. It is not clear
images.
if the
Early experience
in
digital skeletal radiography suggests that a resolution of 1.5 lp/mm may be sufficient for teleradiology and may result in diagnostic images such as those required for gross fracture assessment, but 2.5 lp/mm resolution is preferable [1 2, 17]. Greater resolution may be necessary in some instances, such as diagnosing subperiosteal resorption, subtle extremity fractures, or screening for child abuse. Magnification techniques can increase the effective resolution and may be helpful for certain examinations. An advantage of digital imaging is the ability to alter the gray scale after the image has been acquired. To some extent, enhanced contrast resolution may partially compensate for the decreased spatial resolution [1 8]. One benefit is the
reduction
in the number
of examinations
repeated
because
RADIOGRAPHY
1073
of suboptimal exposure technique in conventional radiography, with a cost savings of $0.60/examination (1 985 dollars) as estimated by Merritt et al. [19]. A substantial radiation savings, particularly important in children, is another advantage [20]. Serial examinations, such as scoliosis radiographs and leg-length scanograms, may be obtained at reduced exposure while maintaining diagnostic information [21 22]. ,
Digital
Image
Capture
Digital images can be acquired in several ways. In the simplest method, a conventional radiograph is converted to a digital image. One may digitize a video camera image of a radiograph with an analog-to-digital converter. Although the images can be acquired rapidly, both spatial and contrast resolution are poor, typically a Si 2 x 51 2 x 8 bit image matrix. These images are not suitable for primary image interpretation. In one series describing a clinical trial of teleradiology [23], clinically significant discrepancies between conventional and digital images were found in only 1 .6% of comparisons, but a disproportionate number of these were skeletal radiographs. Laser film digitizers are capable of performing rapid highresolution film digitization. Radiographs are scanned line by line in “raster” fashion. The transmission of light from a small laser beam passing through the film is detected by a photo diode and converted to a digital number by an analog-todigital converter. Typically, the size of the beam is 100-200 m, representing a maximum spatial resolution of 2.5-S Ip/ mm. Contrast resolution varies from 8 to 1 2 bits depending on the device. More direct methods of image capture involve digitization of the X-ray image from an image intensifier, the standard technology used in digital angiography. Typically, 51 2 x 12 or 1 024 x 1 024 image matrices are available. The spatial resolution depends on the size of the image intensifier. Digital arthrography can be performed with these units [24-28], allowing subtraction of the bones and enhanced visibility of the contrast distribution. All three injections of a triple-cornpartment wrist arthrogram can be done [24] concurrently with digital subtraction. Preliminary results of K-edge digital subtraction arthrography of hip prostheses suggest that this may be a useful technique [26]. Unlike conventional temporal subtraction, which depends on rigid immobilization, K-edge subtraction can be performed at any time during an exarnination. Energy-selective filters are used to enhance the visibility of iodine-based contrast agents. Limited experimental trials of large image intensifier systems measuring 47 cm or more in diameter have been conducted. Overall system resolution of one of these systems is 1 .8 lp/mm [29]. A small series of patients with pulmonary nodules imaged on this system showed no statistically significant difference in detection compared with conventional
chest radiographs. cations
has
been
found the prototype
No large-scale published,
but
series with skeletal the
authors
of this
unit useful in the assessment
appliseries
of trauma
victims. Positioning is greatly facilitated by the fluoroscopy capability, but significant technical problems such as low
1074
BUCKWALTER
AND
spatial and contrast resolution, veiling glare, and poor scatter control may limit their application. Electronic X-ray detectors offer a more direct way to capture the X-ray image in a digital format. These include spot
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scanners,
slit and slot scanners,
and charge-coupled
devices
(CCDs). Spot scanners produce an image by measuring the transmission of a pinpoint X-ray beam as it passes through the patient. There is essentially no scatter radiation, and there is high utilization of the X-ray dose. However, the relatively low spatial resolution and slow scan times are not suitable for imaging applications. This geometry is used in dual-energy X-ray absorptiometry equipment for measuring bone densitometry. The high precision (less than 1 %) and low dose of this equipment suggest that it will displace dual-photon absorptiometry and quantitative CT in the screening and serial evaluation of patients with osteoporosis [30]. Slit scanners use a row of detectors (typically 1 024 or 2048) that move in conjunction with a fan-shaped X-ray beam to scan the desired body region. Although total scan time may be several seconds for the selected anatomy, the effective exposure time is in the milliseconds range. There is low scatter with this geometry, measuring less than 1 % [31]. However, conventional X-ray tubes may not produce sufficiently high X-ray output to enable penetration ofthick tissues. Thus, the pelvis, lateral lumbar spine, lateral cervicothoracic junction, and other thick body parts cannot be imaged in most patients. The slit geometry has been used by Picker (Picker International, Inc., Highland Heights, OH) in a dedicated digital chest unit. A further enhancement incorporates a dual-energy detector enabling energy subtraction radiography [32]. With this machine, it is possible to determine the mineral content of bone in nonoverlapping rib segments [33]. The accuracy of calcium quantification is high, and there is good correlation of posterior rib mineral measurements with dual-photon absorptiometry of the lumbar spine (r = .77), suggesting appliCation in the assessment of patients with osteoporosis.
Fig. 1.-A, cassette.
Detail view of conventional
posteroantenor
radiograph
BRAUNSTEIN
May 1992
A more familiar application of digital slit radiography is found on most commercial CT scanners and allows one to obtain a scout scan to plan the position of the individual CT slices. Movement of the couch past a stationary X-ray tube and detector array creates a projection radiograph. Leg length discrepancy scanograms can be acquired in this manner with good correlation between the CT method and conventional radiographs [34, 35]. Radiation dosage is three to six times less with CT [35], and special modifications of the equipment are unnecessary. However, patients cannot be imaged in the standing position, and the table may not accommodate longer
legs. A digital flying spot scanner was designed to overcome these limitations [36] and provides accurate distance and angular measurements. Slot scanners are similar to slit scanners except that the Xray beam and detector apertures are widened, enabling simultaneous detection of multiple lines of image data. This geometry reduces scatter compared with conventional radiography and allows more efficient use of the available X-ray beam. Slot scanners may be useful in imaging thicker body parts such as the pelvis with minimal compromise of image quality. However, the detector array is more complicated [37], and additional development is needed before a clinical device is available. CCDs contain an integrated solid-state two-dimensional detector array. Widespread use of CCDs is common in consumer video cameras. They are small and sensitive to low light levels. Their size limits the ability to directly image most body parts except in dental radiography, although they can replace conventional television vidicon tubes in fluoroscopy equipment. Multiple CCDs can be placed end to end to form a slot scanner detector. While most direct digital methods require special or modified equipment, it is possible to acquire digital images by using photostirnulable phosphors [38] without modifying the X-ray equipment (Figs. 1 and 2). Imaging plates containing europiurn-activated barium fluorohalide compounds capture the
of the hand obtained
B, Computed radiograph of same hand obtained with high-resolution (5 lp/mm) an image with slightly less contrast than that on conventional radiograph. C, Unsharp mask filtering of image in B. Trabecular detail is more conspicuous two images is identical.
AJR:i58,
storage
at 52 kVp and 15 mAs by using a single-emulsion, phosphor,
and image
appears
same
technique.
to contain
more
Default
image
detail,
although
single-screen
processing resolution
produced of these
AJR:i58,
DIGITAL
May 1992
Fig. 2.-Computed
radiographs
of forearm
SKELETAL
107S
RADIOGRAPHY
1
day after radius and ulna osteotomies. A and B, Both conventional (A) and processed (B) images are printed cies around hardware
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loosening,
simultaneously. (arrowheads)
an image processing
Lucen-
simulate
artifact.
X-ray image. These plates are similar to X-ray intensifying screens. However, conventional screens emit light when exposed to X-rays. In addition to this, photostimulable phos-
phors emit luminescence
image that can be recovered
academic centers. Film digitizers are an integral part of PACS, used to convert conventional radiographs into digital form for
from minutes
to hours after the
light and is available
for immediate
reuse. The
plates can be used indefinitely, although repetitive limits their life span in practice. The imaging plates are available in conventional cassette sizes and can be used in unmodified X-ray equipment. A commercially available computed radiography system developed by Fuji and marketed by Philips (Philips Medical Systems, Shelton, CT) has a nominal spatial resolution of 2.5 lp/ mm for all image plate sizes. A rn, caas.ette size would be represented by a 1 780 x 21 60 image matrix size at this resolution. A high-resolution 8 x 1 0 in. (20 x 25 cm) plate, which has a resolution of S lp/mm, is available. Higher resolution systems are in development [39]. The dynamic range of photostimulable phosphor plates is more than 1 0,000:1 [38], much greater than for conventional screen-film radiography. In practice, this means that radiographic technique is not as critical as that in conventional radiography when using the imaging plates. This is particularly important in skeletal radiography when high-contrast, narrowlatitude screen-film combinations and low-kilovoltage technique are used. Techniques that would result in severely underexposed or overexposed radiographs can produce diagnostic images with the photostimulable plates. There are no comprehensive published surveys of the use
ofdigital
in the United States
to the
(light) proportional
phosphor handling
and distribution
machines
excited by light, usually from a screen stores a latent X-ray
initial exposure. A special reader scans the phosphor with a laser, recording the light output emitted from the screen at each point. The phosphor plate is erased by exposure to
high-intensity
X-ray absorptiometry
at present (DePass J, LUNAR Radiation Corp., Madison, WI, personal communication). More unusual devices such as slit scanners are fewer in number and usually are located in
radiation
incident X-ray exposure when laser. Thus, a photostirnulable
energy
imaging equipment.
Excluding
cross-
sectional techniques and scintigraphy, it is likely that the most widely distributed digital technology is image intensifier based, used primarily for digital angiography. A relatively small number of digital arthrograrns are performed with this equipment. Spot scanners are used almost exclusively for bone densitornetry measurements, and there are approximately 430 dual-
archive and display or transmission and remote printing. These devices are located primarily in a few larger centers with a strong commitment to PACS development and implementation. Photostimulable phosphor technology is the most widely
used technology
in the capture
of projectional
radiographic
images. In 1 990, 30 photostimulable phosphor imaging devices were installed in the United States, 250 in Japan, 35 in Europe, and three in parts of Asia outside Japan [40]. The acceptance of this technology is partly related to the absence of a need for specialized X-ray equipment. Photostimulable phosphor imaging can coexist with conventional radiography, and a dedicated PACS system is not mandatory for implementation. The larger number of installed systems in Japan is partly related to government subsidies associated with the use of computed radiography. Our experience at the Indiana University Medical Center with digital radiography is concentrated at the James Whitcomb Riley Hospital for Children. A total of 71 ,000 imaging studies were performed at this 262-bed pediatric hospital in 1 990. Of these, 27,700 (39%) were computed radiographic examinations performed with a photostimulable phosphor device. Skeletal examinations are almost exclusively performed with computed radiography and account for 9700 examinations, approximately 35% of the computed radiographic studies. The balance of the computed radiographic examinations are portable chest radiographs. The remainder of the plain film studies account for 26,980 examinations (38%). These examinations would be computed radiographs if the equipment capacity were expanded. Digital fluoroscopy and digital angiography account for 2650 (4%) of the total examinations. Cross-sectional technique account for 8550 (1 2%)
and nuclear medicine
the examinations the examinations
performed. performed
studies
account
for 5200 (7%) of
Thus, a total of 44,1 20 (62%) of each year at this hospital gen-
1076
BUCKWALTER
erate digital images. At present,
all images are photographed
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on film prior to interpretation. However, a remote, low-resolution television display and temporary image archive are located in the intensive care unit for immediate review of the computed radiography images by the clinical staff.
Digital
Image
Archiving
Archiving and transmission of digital images remain in a developmental stage. The set of image format and hardware standards developed by the American College of Radiology and the National Electronic Manufacturers’ Association [41] represents an attempt to facilitate this process. Skeletal images have some unique properties that should be considered in the development of PACS. By conservative estimates, an average radiographic examination will generate approximately 40 megabytes of data [9]. Without some form of image compression, it will be impossible to store and transmit images efficiently with present technology. Images can be compressed by reversible or nonreversible algorithms. Reversible compression results in storage of an image representation that is an exact duplicate of the original. Irreversible compression means that the stored representation is not exact. Image compression is described in terms of compression ratios, which represent the amount of storage required by the original image divided by the amount of storage required by the compressed image. Typically, reversible compression results in compression ratios up to 3:1 [42]. Irreversible compression ratios can be extremely high, from 1 5:1 to 90:1 [43], depending on the technique. However, image fidelity begins to suffer at higher compression ratios. Although the time to archive or transmit an image is reduced proportionately, more time may be required to process the images, possibly reducing the overall time savings. Experimental hardware has been designed to allow rapid compression of 1024 x 1 024 and 2048 x 2048 matrix images with a
compression
ratio of 10:1 by using a full-frame
bit-allocation
algorithm [44]. The matrix size and image source determine the amount of compression that can be applied without loss of significant information [45]. In general, higher compression ratios are more acceptable with images of larger matrix size. For many
compression strategies, the termed the spatial correlation,
similarity of adjacent is an important factor
pixels, in pre-
dicting the level of compression that can be applied. In general, images such as digital chest radiographs contain pixels with a high degree of spatial correlation and can be cornpressed more than CT or MR images that contain less well correlated pixels [46]. Skeletal radiographs contain many small structures such as trabecular bone as well as sharply defined, high-contrast margins such as the cortical bone-soft tissue interface. These features may be obscured with higher compression ratios [42, 44, 47]. Bramble et al. [48] investigated the effects of an irreversible compression algorithm (Fourier quantization) on the detectability of subperiosteal resorption in the hands. Compression ratios of 1 6:1 and 28:1 were applied to Si 2 x
AND
BRAUNSTEIN
AJR:158,
May 1992
51 2 x 12 bit images without a statistically significant loss in diagnostic quality. Although there was no statistically significant difference between 7- and 8-bit compressed images, the
confidence
of the radiologists
in establishing
the presence
of
subperiosteal resorption was reduced when interpreting the 7-bit images. The experience of these investigators suggests that empirical testing may be required to determine the optimal compression ratio allowable for a given task. Further research efforts are needed to evaluate both the effects of
varying
amounts
on the diagnostic
Digital
Image
and different accuracy
image compression
of skeletal
algorithms
images.
Display
Film, either used in the conventional manner or as output from a laser film printer, will remain an important means of
displaying
radiographic
images
[4]. A benefit
of printing
film
images of digital radiographs is the ability to alter image size. Printing a 1 4 x 1 7 in. image on a 1 0 x 14 in. (25 x 36 cm) film results in a 41 % reduction in film area. By one estimate, an overall film cost reduction of 31 .5% can be achieved if chest radiographs account for 40% of the volume [20]. This number may be misleading, as it applies only to film savings and does not account for purchase of computed radiographic equipment, maintenance, and other costs. Minified film images may cause problems in the preoperative planning of joint reconstructive surgery. Joint prosthesis templates are reproduced at specific magnification factors and may not be used on minified images without resizing the template. This is not practical, as the amount of minification may not be standardized. Additionally, distance and size
measurements
made from minified
images
are more critical
because of the smaller performed electronically
pixel size. Template fitting could be or automatically with computer as-
sistance,
require specialized
but this would
equipment
in the
surgeon’s office. Images may be minified by decreasing the total number of pixels comprising the displayed image or by reducing the size of each of the individual pixels in the displayed image. If the total number of pixels is decreased, there is a potential loss of information. If the size of the individual pixels is decreased, small structures may be more difficult to see, especially if the
viewing distance remains image size on the detection
constant. The effect of altered of specific skeletal abnormalities
has not been investigated. Experiments performed with chest images suggest that this may be an important consideration. One study found a positive correlation between overall image size and the detection of small nodules from video displays [49]. A related study evaluating the detection of nodules on a chest phantom image showed no significant difference
between
minified
film images
and conventional
radiographs
[SO]. An evaluation of storage phosphor chest radiographs vs conventional chest radiographs [51 ] revealed that some of the physicians participating in the study would be less confident interpreting rninified film images, although the effect of image minification on diagnostic accuracy was not assessed.
AJR:158,
DIGITAL
May 1992
SKELETAL
Conventional television monitors cannot display high-contrast, high-resolution images. It is estimated that displays
1077
issue of higher resolution monitors may be more important in chest radiography than skeletal radiography owing to the larger image format. Most high-resolution skeletal work is done on smaller body parts. The dynamic range of television monitors limits the image display to fewer than 256 gray levels (8 bits). To some extent, windowing can overcome this limitation. Additionally, the luminance of a radiographic image displayed on a television monitor is roughly an order of magnitude less than the lumi-
and it suggests that an unprocessed version of the image be available at the time of interpretation. Unfortunately, this increases the time required for interpretation. The two basic types of image manipulation are gray-scale transformation and spatial filtering. Gray-scale transformations involve the mapping of a given pixel value to a specific gray level. One of the simplest forms of image processing is a linear gray-scale transformation. This is the process that occurs when adjusting the window and level of an image. The transformation allows a specific range of pixel values to be displayed over the full dynamic range of the display device, enhancing contrast. More complex gray-scale transformations involve analysis of the distribution of pixel values in the image. Histogram equalization is one way to reassign gray values in this manner. If the display parameters are fixed, it would be advantageous to ensure that all of the available gray values are equally used, regardless of the intensity distribution in the original image. Histogram equalization performs a gray-scale reassignment that attempts to use every available gray value equally. This tends to enhance low-contrast objects in the image, making them more visible. Adaptive histogram equalization is a modification involving
nance of a conventional
separation
capable
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RADIOGRAPHY
of 3300
x 4096
pixel
resolution
are needed
to
produce an image equivalent in quality to a radiograph [52]. At present, 2K x 2K displays are available, but 4K monitors with 72-Hz noninterlaced display rates present a technical challenge [53]. Enlargement or zooming of a portion of the image can display the image at full resolution, but requires additional time for interpretation. The lower resolution 2048 x 2048 matrix displays correspond roughly to a 0.2-mm pixel size (2.5 lp/mm) for a 1 4 x 1 7 in. radiograph. It has not been proved that this is sufficient for routine imaging. The same 2048 matrix display can be used to display a smaller format 8 x 1 0 in. radiograph
at roughly
a 4 lp/mm
radiograph
resolution.
displayed
The
on a view box,
resulting in lower contrast sensitivity [54, 55]. The efficacy and accuracy of diagnosis of skeletal images from workstations need to be investigated. A few investigations comparing the display of chest or CT images have not demonstrated the equivalence of video monitors to film. One study comparing film to 1 000-line television display of CT images showed sensitivity was better and reporting time shorter when interpreting film images [54]. A comparison
between tions
film and workstation
of the knee
image
interpretation
longer
[56].
displayed
greater
showed
A comparison
of digitized monitor
with film [57].
required
plain chest
showed
equal
although
2.7 times radiographs
sensitivity
but
A receiver-operating-char-
acteristic (ROC) study of chest radiographs line display showed no significant difference and video displays [58].
Image
of MR examina-
performance,
from a workstation
on a 1 000-line
specificity
interpretation equivalent
that used a 2048between the film
Processing
The objective of image processing is to improve diagnostic accuracy. The uncertainty about the effects of image processing on specific image features and the extra effort required have affected its acceptance in routine practice [59]. Although alteration of an image may improve the detectability of one type of abnormality, it may obscure another type, an effect observed in digital chest radiography. When unsharp masking is used, the visibility of pneumothorax is enhanced, but the detection of interstitial disease is diminished [60]. This and other examples suggest that a priori knowledge of the anticipated findings be used to select the imaging processing algorithm. However, important findings may be obscured with this approach. This has important medicolegal implications,
of an image into subregions
that are individually
histogram equalized. This technique has been applied to the analysis ofdigitized radiographs ofdestructive skeletal lesions [1 3]. Equalized images were superior in determining cortical breakthrough and presence of periosteal reaction as determined by ROC analysis. Other investigations of equalized digitized skeletal radiographs did not find the technique useful, although specific detection tasks were not investigated and only qualitative analysis was applied in the evaluation [10]. Although visually similar images can be obtained with scanning equalization radiography [61], an advantage of histogram equalization is that it is a postprocessing technique that can be applied after the image has been obtained. Additionally, no specialized radiographic equipment is required. More complex forms of image processing involve filtering of an image. A variety of techniques can be used. One of the most popular is unsharp masking (Fig. 1 C). Unsharp masking does not increase the resolution or information content of the image, although some structures in processed images appear to contain more details because of the enhancement of high spatial frequencies. In general, the contrast resolution of unsharp masked images is altered. This may obscure lowcontrast objects, depending on the selection of the filter
parameters
[62]. One study
analyzed
the effect
of unsharp
mask filtering on the detectability of artificial cortical bone defects in human femur specimens. Computed radiographs of the specimens were obtained at 2.5 lp/mm spatial resolution with stimulable phosphor plates. Analysis of the observations with ROC methodology demonstrated that unsharp mask filtering did not help, especially with decreased kernel sizes and pronounced enhancement [5]. Aside from the effects of unsharp mask filtering, the investigators found that phosphor plate imaging was at least equal to conventional screen-film radiography with respect to contrast resolution in the detection of cortical bone defects. This is significant as
1078
BUCKWALTER
spatial resolution images.
is lower
for the phosphor
AND
BRAUNSTEIN
ners,
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Conclusions principles
of digital
radiography
have
been
sum-
marized to provide a background for discussion of some of the properties of digital skeletal radiographs. Most of the research in computed radiography has been performed with chest images. However, skeletal radiography accounts for a significant proportion of the total number of plain film examinations performed in many radiology departments. Although
of the research
results
can be applied
to skeletal
unique properties of skeletal radiography consideration. These properties include
spatial resolution
and high contrast
of skeletal
require the high
images as well
as the wide variety of anatomic structures examined. Additionally, planning of joint replacement surgery from preoper-
ative radiographs is a process unique to skeletal radiography. present, precise matching of standard manufacturer-supplied templates to the radiographs requires near life-sized images.
At
An advantage
the tasks display.
of digital
imaging
of image acquisition, This
to perform
separation these
is the ability
image
facilitates
these separate
film combines
tasks,
archiving,
optimal
tasks.
to separate
and image
design
of devices
Conventional
but compromises
radiographic must
be made.
Perhaps the most severe compromise is the limited dynamic range of conventional screen-film technology. This is particularly important because conventional radiographic technique and screen-film combinations for skeletal radiography have narrow latitude with little margin for exposure error. Additionally, the use of film as both archival and display media limits the use of the radiographic image to a single site at any
one time. These limitations are largely eliminated in digital imaging. Even if the images are printed on film, multiple identical
copies
can be generated
concurrently
at physically
separate sites. This advantage is particularly desirable in a busy outpatient setting. Excluding screen-film mammography, skeletal radiography requires greater spatial resolution than any other plain film technique.
Nominal
line-pair
resolution
measurements
of con-
ventional screen-film combinations for skeletal radiography vary from 8 to 1 2 lp/mm, while those for chest radiography are approximately 4 lp/mm. The most recent studies with digital skeletal images report that the minimum spatial resolution requirement for computed radiographs is between 2.5 and 5 lp/mm. Resolution requirements vary depending on the imaging
these required
task;
unfortunately,
requirements in special
it is not always
in advance. circumstances,
Higher
or scoliosis
evaluation.
possible
to predict
resolution
may be
such as for detecting
subperiosteal resorption. Conversely, may be acceptable in the assessment ment
image-intensified
digital
and photostimulable
fluoroscopy,
spot
scan-
imaging
plates.
Laser
phosphor
scanners convert conventional radiographs into digital images. These devices will play an important role in enabling conventional radiographs to be incorporated into a PACS environment. Image-intensified digital fluoroscopy is widefilm
The basic
imaging, additional
May 1992
Digital skeletal images can be acquired in many different ways. Some of the most important devices are laser film
plate-generated
digitizers,
some
AJR:158,
The
subtle
lower spatial resolution of gross fracture alignenhanced
contrast
spread
because
application intensifiers
of
digital
angiography
and
has
to digital arthrography. Large-field-of-view have been built and can generate digital
images,
but technical
scanners
are widely
image
skeletal limit their usefulness. Spot
problems distributed
limited
and are mainly
used
for dual
X-ray absorptiometry of the skeleton. The most promising technology to date is one based
on
photostimulable phosphor imaging plates. These plates are compatible with conventional radiography equipment. Images can be filmed with a conventional laser camera. This eliminates the immediate need for a PACS environment and provides a bridge between conventional and future technologies. It may be extremely difficult to justify the purchase of such a
system as it is replacement additional direct revenues. benefits of the system are The most appropriate time
technology and does not generate Without a PACS network, some lost. Careful analysis is required. to consider such a commitment is
in the creation, replacement, or expansion of a radiology department. Digital images may be displayed on film or television monitors. Conventional television monitors cannot display high-
contrast, high-resolution images. High-resolution 2K x 2K television monitors are currently available, but 4K monitors may be necessary to display some images at full resolution. Availability of higher resolution monitors may not be a problem with skeletal images, as most high-resolution skeletal imaging is done on smaller body parts. These images may be dis-
played
at full resolution
monitors
are expensive,
out a hospital remain
on a 2K monitor. and many
if film is to be eliminated.
an important
means
High-resolution
may be required
of image
through-
Film will probably
display
because
of its
superior contrast and spatial resolution, although the film will originate as laser camera output. The objective of image processing is to improve diagnostic accuracy. Two basic forms of image processing are grayscale transformation and spatial filtering. Both have been applied to skeletal images. Adaptive histogram equalization of destructive
skeletal
lesions,
a gray-scale
transformation
technique, improved detectability of cortical breakthrough and periosteal reaction. Unsharp mask filtering of cortical bone defects in human femur specimens, a spatial filtering technique, showed no improvement in detectability of the defects. Additional work is needed to determine if image processing can improve
diagnostic
performance.
and
wider dynamic range of most digital radiography systems may partly compensate for reduced spatial resolution. Additionally, the ability to alter contrast can reduce exposure requirements and lower patient dose, particularly important in children.
Appendix The following required
formula expresses
for a given nominal
line-pair
the number of pixels (matrix size) resolution
and film-cassette
size.
AJR:158,
DIGITAL
May 1992
Matrix size
[(nominal
=
resolution
dimension
x [(nominal
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sette
As an example, resolution
would
in lp/mm)(2
in inches)
resolution dimension
(25.4
pixels/lp)(one
an image
cassette
mm/in.)]
in p/mm) (2 pixels/Ip) in inches)
a i 4 x 17 in. cassette require
matrix
size
(25.4
=
(other cas-
mm/in.)].
and nominal of i 778
2.5 lp/mm
x 21 59 pixels.
By modifying the formula, the pixel size can be obtained matrix size and the image field size. Pixel size in mm
SKELETAL
line-pair
of such a configuration,
multiply
size.
the pixel
size in millimeters by two and take the reciprocal. Thus, this system has a nominal resolution of 2 lp/mm. These calculations are useful in comparing the resolution of various systems as well as the storage requirements for images generated by different systems.
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given the
As an example, a 1024 x i 024 matrix with a i 0 x i 0 in. (25 x 25 cm) image size is composed of 0.25-mm pixels. To calculate the nominal
RADIOGRAPHY
Seeley GW, Fisher HD, Stempski MO, Bergstrom M, Bjelland J, Capp MP. Total digital radiology department: spatial resolution requirements. AJR 1987:148:421-426
47. 48.
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1080
BUCKWALTER
magnification hand radiographs. Radiology 1989:170: 133-1 36 49. Fisher PD, Brauer GW. Impact of image size on effectiveness imaging systems. J Digitallmaging 1989;2:39-41
interpretation
51.
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JA, Ritchie GW. Signal detection in digital with an image intensifier. J Digital Imaging
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1989:2:42-47
58.
55. 56.
LIST OF BOOK 1 006 1 01 0 1028 1 064 1 070 1 086 1 1 14 1 134
1 1 50 1 1 60
with multiscreen
digital
workstation
vs hardcopy
May 1992
format.
AJR
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1991:157:81-85 57.
Brauer GW, Fisher PD, Hanley chest-phantom images acquired
53.
BRAUNSTEIN
of digital
50.
52.
AND
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AND VIDEOTAPE
REVIEWS
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