Digital radiography (DR) is an innovative technology for capturing the radiographic projection in digital form. But there is a general lack of appreciation for potential artifacts in these systems. This article describes fundamental requirements for producing diagnostic quality radiographic images and provides examples of the consequences of violating these requirements when using DR.
is an Associate Professor in the Department of Radiology, Baylor
College of Medicine, Houston, TX.
is a Medical Physicist with Memorial Medical Center, Modesto, CA.
is a Senior Medical Physicist in the Department of Diagnostic
Physics, University of Texas M.D. Anderson Cancer Center,
This article is based on material originally presented in:
Willis CE. Artifacts and misadventures in digital radiography.
Presented at the Society of Computer Applications in Radiology
Meeting. SCAR University Course 305, 20th Symposium. Boston, MA,
June 710, 2003.
Considering the sales hyperbole associated with digital
radiography (DR), one may wonder if it is even possible to produce
a nondiagnostic digital image. Certainly DR is more tolerant of
inappropriate exposure factor selection than is conventional
film-screen radiology. However, classic technical errors (such as
malpositioning, patient motion, incorrect patient identification,
incorrect examination, and double exposure) still occur in the
The unfortunate truth is that DR is subject to many of the same
inaccuracies as conventional radiography, in addition to many new
ones that are a direct result of the way the DR image is generated.
An understanding of the causes of both new and old problems is
necessary in order to avoid these inaccuracies and recover
Artifacts and digital systems
An artifact is a feature in an image that masks or mimics a
clinical feature. The literature classifies artifacts according to
causative agent, such as hardware, software, or operator,
although artifacts can also be categorized by the mechanism of
interference with image acquisition, processing, or display.
"Misadventures" encompasses the entire gamut of technical errors
that can produce unsatisfactory images.
The reports cited above concentrated on computed radiography
(CR), that is, any imaging system that relies on photostimulable
luminescence to make a radiographic image. In contrast, this
article also provides examples of problems that may be encountered
with any DR system, including
direct digital radiography
(DDR) systems that make digital radiographs from the photoelectric
interaction of X-rays with the detector itself,
indirect digital radiography
(IDR) systems that are sensitive to the light produced by an
intensification screen, and
optically coupled direct radiography
(OCDR) systems that use optical components to focus fluorescence
charge coupled detectors
(CCD). In this context, DR includes any radiographic image acquired
without photographic film, thus excluding film digitizers whose
artifacts are described elsewhere.
Digital radiographs in this study were produced with a variety
of devices in clinical service at our institutions or available to
us, including Agfa CR (Agfa Medical Systems, Ridgefield Park, NJ),
Fuji CR (Fujifilm Medical Systems, Stamford, CT), GE DR (GE Medical
Systems, Milwaukee, WI), and Canon DR (Canon USA, Inc., Lake
Success, NY). Although not shown, we have similar experiences with
Kodak CR (Eastman Kodak, Rochester, NY), Lumisys CR (now owned by
Eastman Kodak), Konica CR (Konica Medical Imaging, Inc., Wayne,
NJ), and Hologic DR (Hologic, Inc., Bedford, MA).
Acquiring good quality images
Regardless of the acquisition technology, good radiographic
images can be produced only when certain fundamental requirements
are met. Appropriate radiographic technique must be used, which
includes the proper tube potential (kVp), beam current (mAs),
source-to-image distance (SID), collimation, alignment of the X-ray
central ray, and positioning of detector and subject for the
specific anatomic projection.
The detector must receive enough X-rays to make a good image.
Whether from underexposure or misalignment of a scatter reduction
grid, too few X-rays produce noisy images (Figures 1 and 2). Too
many X-rays are a disservice to the patient and may also produce
poor images (Figure 3). Although DR is more tolerant of incorrect
exposure factor selection, it cannot make up for extra noise, loss
in subject contrast, and signal out of its range of adjustment.
"Exposure factor creep" is a well-known phenomenon related to
the wide exposure latitude of DR.
Observers tend to complain about noise in DR images exposed at
one-fourth to one-half of the appropriate level. On the other hand,
artifacts are generally not apparent until the exposure exceeds 10
times the appropriate level. Technologists soon learn to avoid the
unpleasant circumstance of repeating an underexposed study by
routinely increasing the radiographic technique. Thus, the
potential for gross overexposure exists in DR. Image optical
density (OD), the usual indicator of proper exposure, is arbitrary
in DR. Managment of exposure factor in DR must rely on the value of
a derived exposure indicator, which itself is subject to
interference. However, programs that monitor the exposure indicator
have been shown to be effective in controlling exposure factor in
System installation and maintenance
In order to make good images, the DR device must be properly
calibrated, configured, maintained, and operated. Every individual
DR device must be calibrated for overall gain and uniformity.
This is usually performed during the initial installation and
should be repeated periodically. Acquisition devices should be
calibrated for gain or sensitivity, and to compensate for
nonuniformity (Figures 4 and 5). Display devices should be
calibrated to provide output according to the DICOM Part 14
Grayscale Display Function, including hard-copy devices, soft-copy
devices, and displays used for quality control (QC).
Processing algorithms used by the DR system must be designed to
anticipate the use of this display function in order to properly
render the image for display. Not all vendors adhere to this
standard. Calibrated, high-quality QC monitors are essential on
every acquisition system and QC workstation. Adjusting image
processing on an uncalibrated monitor leads to unsatisfactory
During the initial installation, it is important to make sure
that the DR system is properly configured with the most up-to-date
version of software, hardware, and durable goods. Multiple vintages
of imaging plates exist for CR, and some are not universally
compatible with CR hardware. The software and settings should be
consistent with the versions and settings in operation with other
individual DR systems at the site, including examination-specific
parameter settings (Figure 6). All DR systems have an internally
calculated estimate of exposure. The DR system may need to be
configured to report this value to the digital image management
system, and the image management system may need to be configured
to display it to the radiologist. When CR is introduced into an
imaging operation, phototimers in all X-ray rooms need to be
recalibrated to deliver the appropriate exposure.
The methodology for phototimer calibration is different because DR
density is adjustable.
Scheduled and unscheduled service should be done in a thorough
and timely manner, including reporting and documenting, cleaning,
and repairs. Operator functions include cleaning, reporting service
interruptions, removing the unit from clinical service,
re-introducing the unit into clinical service, and documenting
service events (Figure 7). Service engineer functions include
performing scheduled maintenance that includes preventive
maintenance and software and hardware upgrades, as well as
unscheduled maintenance or repairs. On-site support for DR requires
a team with expertise not only in image acquisition, but in picture
archiving and communications systems (PACS), radiology information
systems (RIS), technologist workflow, image quality, and networks,
Training and system operation
Digital radiography systems must be operated properly to produce
good images. Technologists are often unfamiliar with DR features
and functions, and may require additional training beyond vendor
applications training. Technologists need to select the proper
examination; in addition, they must properly associate demographic
and examination information to the image, properly manipulate the
detector, and review the image before releasing it to the image
management system. Beyond this, technologists need to know how to
recover from errors without repeating examinations and need to
follow exposure factor control limits. Quality control processes
must be in place to detect and correct unsatisfactory images
(Figures 8, 9, and 10). Digital radiography requires a new approach
to QC and study reject analysis.
Electronic images can disappear without a trace: counting the films
remaining in the film bin is no longer a useful method for
determining the number of repeated images.
Double exposure is a classic operator error that constitutes
approximately 2% of all rejected images. The consequence of double
exposure can be either a single repeated examination, when an
inanimate object is involved (Figure 11), or two repeated
examinations when two patients are involved (Figure 12). In DR,
double exposures can also be caused by power interruptions and
communications errors, as well as by inadequate erasure secondary
to overexposure or erasure mechanism failure.
Appropriate digital image processing is key to producing good DR
images. All DR systems have extremely wide latitude, which means
that connected to a display system with a relatively narrow dynamic
range, DR images have extremely low contrast. The primary purpose
of image processing is to maximize the contrast of the part of the
image that contains relevant clinical details.
To do this, the DR system locates either the boundary of
collimation or the border of the projected anatomy, and disregards
details outside this boundary. Errors in collimation can cause
mistakes in detection of the boundary, with a dramatic loss of
image contrast (Figures 13 and 14).
The secondary function of image processing is to customize
contrast in the region of interest (Table 1). This type of image
processing includes modifying the image to enhance the contrast and
sharpness of some features while compromising the contrast and
sharpness of others, as well as modifying the image to make it
appear more like a conventional transilluminated film. This
secondary image processing is applied in a manner that is usually
specific to the anatomic projection. Errors in the selection of the
anatomic projection can cause inappropriate processing (Figure
An auxiliary purpose of image processing is to improve the
usability of the digital image.
This includes imprinting demographic overlays, adding annotations,
applying borders and shadow masks, flipping and rotating,
increasing magnification, conjoining images for special
examinations like scoliosis, and modifying the sequence of views.
This processing may require a separate QC workstation.
Image processing is not a panacea. Misuses of image processing
include compensating for inappropriate radiographic technique,
compensating for poor calibration of acquisition and display
devices, and surreptitious deletion of nondiagnostic images. Image
processing to recover nondiagnostic images to prevent re-exposure
should be a last resort, not a routine activity. Routine
reprocessing indicates a problem with automatic image processing or
technical practice. Access to image-processing software is
essential to develop and maintain appropriate processing
Automatic image processing involves assumptions about the
radiographic technique, the composition of anatomic region imaged,
and the use of collimation. A number of factors can interfere with
the automatic detection of the boundaries of the radiation field,
including nonparallel collimation, use of multiple fields on a
single imaging plate, poor centering, implants (especially when
they overlie the boundary), and violation of collimation rules
provided by the vendor. For example, placement of gonadal shields
is no longer trivial, but may adversely affect image quality.
A multitude of factors affect DR image quality, and no device or
operator is immune to unacceptable images. A strategy for
addressing images that "just didn't turn out right" must be
implemented. Responsibilities for documenting, reporting, and
taking corrective action must be clearly established. Wider dynamic
range means that technologists have to pay attention to exposure
indicator values, instead of brightness and contrast. Without this
attention, patient dose will escalate. If exposure indicator logs
are available, they need to be evaluated. If they aren't, this will
need to be done manually. Vendors need to make such logs available
in convenient digital form. New technologies should be developed
for dealing with pediatric examinations and patients with
prosthetic devices. New image processing strategies may be needed
with these special patients, as well.
Thorough training and active onsite support of the technical
staff are crucial. For many, this is a completely different way of
thinking about the imaging process. Technologists are generally
eager to become involved and master this new technology, but they
need proper training and guidance to use it effectively to produce