Dr. Willis is an Associate Professor, Department of Imaging Physics, The University of Texas M.D.Anderson Cancer Center, Houston, TX.
There is no doubt that all digital radiographic images share many
characteristics regardless of the technology with which they are
acquired simply because they are all digital approximations of an
analog projection. This is true even of older forms of digital
radiographs, eg, digitized ﬁlms and charge-coupled devices (CCD) mated
to X-ray–to–light converters. All digital radiography (DR) devices have
limitations with respect to spatial resolution (sharpness) and contrast
resolution because of the ﬁnite dimensions of their detector elements
(dels) or picture elements (pixels). All digital imaging devices are
subject to certain artifacts such as aliasing, moiré patterns, and
contouring because their images are an array of elements with ﬁnite
dimensions and quantization. All digital imaging devices separate the
process of acquisition from display and thus allow modiﬁed rendering of
the image for viewing. Digital radiography devices typically have wide
latitude, which results in low contrast if the digital image is mapped
to a display device of limited latitude without appropriate rescaling.
Recent variants of digital radiography cross traditional boundaries
between receptor technologies. A review of the basics of large
ﬁeld-of-view digital detectors predicted that:
is also likely that there will be a tendency to think of these devices
as equivalent, interchangeable commodities because they are similar in
physical size, appearance, and targeted applications. It must be
emphasized, however, that important differences exist among these
detectors, and differences in digital image quality among the various
systems are inevitable and may be quite large. To minimize confusion,
therefore, it is important for radiologists to have a working
understanding of this emerging technology.”1
radiography (CR) was the product name for the ﬁrst commercial device for
making radiographs using the process of photostimulable luminescence
(PSL), whereby X-ray energy projected onto a doped crystalline
material—the photostimulable phosphor (PSP)—produces a latent image of
excited electrons in local potential energy traps.2 The
number and locations of these excited electrons is a faithful
representation of the original projected X-ray energy. The electrons are
released from their traps by stimulation with light of a speciﬁc
wavelength (photostimulation). The electrons de-excite by the release of
photostimulable light (PSL), which can be measured and digitized to
create a digital image.
Rather than being the product, the
digital image was an intermediate in the process of making a
laser-printed ﬁlm image for interpretation. The incentive for a ﬁlm
company to introduce such a device was that by using a single receptor
type, a single receptor processor, a single laser ﬁlm, and a single
laser ﬁlm processor, the characteristic speed, contrast, and latitude of
any screen-ﬁlm combination could be simulated by manipulating the
digital image and could be duplicated as desired. The receptor could be
erased with white light and reused to acquire another projection.
Advances in automation and information technology soon made the digital
image more desirable than the laser ﬁlms. Recent improvements in CR
acquisition speed and integration are the basis for the argument that CR
should no longer be distinguished from DR.
The CR cassette form
factor contributed to the rapid acceptance of CR as a digital imaging
modality. CR systems built into upright exposure stations and
examination tables existed as competing products with cassette-based CR
systems almost from the beginning of the technology. Only recently have
the terms “integrated” or “cassetteless” been introduced to distinguish
this type of system from cassette-based CR. Perhaps the most important
aspect of integration is the possibility that the receptor system may
share information about how the radiographic projection was generated.
traditional method for developing the CR latent image was to use a
laser beam “ﬂying-spot” that was deﬂected by an alternating mirror
across the advancing PSP imaging plate in a raster fashion.3 Stimulated
light was collected and directed onto ≥1 photomultiplier tubes, whose
analog currents were converted into digital signals. The timing of the
analog-to-digital conversion in concert with the relative motion of the
laser spot and plate determined the spatial relationship of each digital
The line scan is a new method of developing the CR
image, which relies on an array of laser diodes and CCDs that move
across the PSP imaging plate stimulating and reading the image in a
rapid fashion. These arrays can be made small enough to ﬁt inside a
cassette or in an integrated CR system. Ironically, an early (circa
1987) integrated CR system for chest imaging, the Konica Direct
Digitizer (KDD), incorporated a linear scanning mechanism (rather than
advancing the PSP imaging plate) and used a special PSP material that
had an extremely short luminescence rate and concomitant rapid
spontaneous fading of the latent image. The KDD mechanism used a solid
state laser and galvanometer to deﬂect the beam.
improvements in speed and integration, CR remains distinct from other
forms of digital radiography. It is difﬁcult to imagine how PSP imaging
could ever be rapid enough to acquire images at 30 frames per second, as
needed for ﬂuoroscopy, which is within the capabilities of other
digital radiography devices. There is no latent image in non-CR digital
radiography, unless perhaps one considers the charges prior to readout
by the semiconductor arrays. There was a latent image in selenium drum
DR systems, in the form of unsatisﬁed charges remaining after exposure
to ionizing radiation. There is no possibility of double exposures in
DR, but they can occur in CR if the PSP is exposed a second time before
it is developed. Because PSL occurs over a ﬁnite time after stimulation,
there is always an additional source of blur (unsharpness) in the
direction of laser or laser array motion. This sort of anisotropic blur
is not present in DR systems, which have discrete detector elements ﬁxed
Because CR involves mechanical motion, there is always
the possibility of interference with the mechanical advance of the PSP
imaging plate, laser deﬂection mechanism, or scanner array; such
interference manifests in artifacts (Figure 1). In general, DR devices
are based on semiconductor arrays that are ﬁxed in space and are read
without mechanical motion. Slot scanning or fan-beam DR devices are an
exception to this general rule.
There is a fundamental difference
in the purpose and manner of gain and offset correction in DR and the
non-uniformity correction in CR (Figure 2). In CR, differences in
stimulation and light collection efﬁciency are generally corrected in 1
dimension. In DR, not only must the gain and offset of each del be
equalized, but the presence of defective dels must also be compensated
for by averaging neighboring dels.4 Newly defective dels in DR and interference with light collection in CR manifest differently in the digital image (Figure 3).5
are differences in the durability and replacement costs for CR
receptors versus DR receptors. Except for the newest structured phosphor
CR receptors, physical impact is not as much a concern for CR
receptors, and their cassettes can be replaced or repaired.
Photostimulable phosphor imaging plates are subject to processes of
mechanical damage and chemical oxidation but can be replaced at a
nominal cost. DR receptors do not last forever; they are subject to slow
aging processes, and their replacement cost is a substantial portion of
the capital investment for the total system.
“ghosting” processes in DR do not exist in CR. These false images arise
from residual signal in the X-ray–to–light conversion layer or from
residual charge in the thin-ﬁlm transistor itself.6-8 These are very different from inadequate erasure and double exposure in CR (Figure 4).
imaging plates are integrating dosimeters. Until they are read or
erased, they continue to build up charge from background radiation or
scattered radiation. This fogging process has not been reported for DR
detectors. Likewise, since DR images are immediately read-out, fading
processes that can occur for CR have not been reported for DR.
was introduced into clinical practice 2 decades ago. As a result,
processing of the digital image in CR is more mature, more clearly
deﬁned in the literature, and more clinically proven than it is in any
commercial DR system. Examples of image processing being exported
wholesale from CR onto DR detectors without any modiﬁcation or
customization can be found among some commercial systems. It would be
serendipitous if the processing that was developed for a speciﬁc CR
receptor were appropriate for any detector with different fundamental
properties, such as the mechanism of image formation, the sources and
characteristics of noise, and details such as matrix size, gray-scale
bit depth, and characteristic function.
Owing to the early
introduction of CR into clinical practice, CR exposure indicators for CR
are better documented and more useful than those for DR.
Notwithstanding standardization efforts for a receptor exposure
indicator suitable for both CR and DR by the American Association of
Physicists in Medicine (AAPM) TG 116 and the International
Electrotechnical Commission (IEC), current literature describes CR
exposure indicators, provides target ranges, explains how they can be
used for patient dose management purposes, and describes potential
interferences.9 Such information about DR exposure indicators is scant.
Again, because of the maturity of CR as a clinical modality, performance and quality measurements have been established for CR10 but not yet for DR. AAPM TG 150 represents the beginning of an effort to establish such metrics, but this only began in 2007.
are some historical arguments for retaining the CR nomenclature for PSP
imaging systems. There are 20 years of scientiﬁc literature describing
performance characteristics and clinical uses of CR that would not be
found by a search engine when using “DR” as the key ﬁeld.
naming of DICOM Modality Objects also obscures the differentiation
between CR and DR. The “CR Object” was an early and primitive object for
digital radiographic images. The “DX Object” was developed later to
correct some of the shortcomings of the CR Object. As a result of its
late development, and because some PACS vendors were slow to accommodate
the DX Object, some DR vendors adopted and still use the CR object.
There are many beneﬁts to using the DX Object, for both CR and DR
images; therefore, both CR and DR vendors should migrate to the DX
All digital radiographic images
share many characteristics that are inherent to their discrete
approximation of a plane projection of X-rays through anatomy that is
continuously variable with respect to intensity and spatial
distribution. A variety of receptors have been applied to the task of
acquiring a digital radiography. The convergent evolution of form and
function of these devices has led many to conclude that distinctions
between CR and DR serve no useful purpose. Although categorization of CR
as part of the umbrella of DR is tempting and contributes to a broader
view of digital radiography, there are speciﬁc reasons why CR should be
considered a separate imaging modaity. Similar reasons may exist for
maintaining distinctions for other receptor technologies. Perhaps an
alternative nomenclature that would include CR and all forms of DR might
be electronic radiography.
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- Willis CE, Thompson SK, Shepard SJ. Artifacts and misadventures in digital radiography.Appl Radiol.2004;33(1):11-20.
- Yorkston J. Flat-panel DR detectors for radiography and ﬂuoroscopy.
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conversion x-ray detectors on x-ray exposure and exposure history. In:
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- Siewerdsen JH, Jaffray DA. A ghost
story: Spatio-temporal response characteristics of an indirect-detection
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- Seibert JA, Shelton DK, Moore EH. Computed radiography X-ray exposure trends. Acad Radiol.1996;3:313-318.
JA, Bogucki TM, Ciona T, et al.Acceptance Testing and Quality Control
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