artifacts and misadventures in digital radiography

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www.appliedradiology.com APPLIED RADIOLOGY s 11January 2004

C

onsidering the sales hyperbole

associated with digital radiogra-

phy (DR), one may wonder if itis even possible to produce a non-

diagnostic digital image. Certainly DR

is more tolerant of inappropriate expo-

sure factor selection than is conventional

film-screen radiology. However, classic

technical errors (such as malpositioning,

patient motion, incorrect patient identifi-

cation, incorrect examination, and dou-

ble exposure) still occur in the usual

frequency.1 The unfortunate truth is that

DR is subject to many of the same inac-

curacies as conventional radiography, inaddition 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 inaccu-

racies and recover unacceptable images.

Artifacts and digital systemsAn artifact is a feature in an image that

masks or mimics a clinical feature. The liter-

ature classifies artifacts according to

causative agent, such as hardware, software,

or operator,2-5 although artifacts can also be

categorized by the mechanism of interfer-

ence with image acquisition, processing, or

display.6 Misadventures encompasses the

entire gamut of technical errors that can pro-

duce unsatisfactory images.

The reports cited above concentrated

on computed radiography (CR), that is,

any imaging system that relies on photo-

stimulable luminescence to make a radi-

ographic 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 radi-

ographs from the photoelectric interac-

tion of X-rays with the detector itself,

indirect digital radiography (IDR) sys-

tems that are sensitive to the light pro-

duced by an intensification screen, and

optically coupled direct radiography

(OCDR) systems that use optical compo-

nents to focus fluorescence onto 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.

DR systemsDigital radiographs in this study were

produced with a variety of devices in

clinical service at our institutions or avail-

Artifacts and misadventures

in digital radiography

Cha rles E. Willis, PhD, DABR; Ste phen K. Tho mpson, MS, DABR; S. Jeff She pard, MS, DABR

Dr. Willis is an Associate Professor inthe Department of Radiology, BaylorCollege of Medicine, Houston, TX. Mr.Thompson is a Medical Physicist withMemorial Medical Center, Modesto,CA.Mr. Shepardis a Senior MedicalPhysicist in the Department of Diagnos-tic Physics, University of Texas M.D.Anderson Cancer Center, Houston, TX.

This article is based on material origi-nally presented in: Willis CE. Artifactsand misadventures in digital radiogra-phy. Presented at the Society of Com-puter Applications in Radiology Meeting.

SCAR University Course 305, 20th Sym-posium. Boston, MA, June 710, 2003.


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able 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 simi-

lar 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 imagesRegardless of the acquisition technology,

good radiographic images can be produced

only when certain fundamental require-

ments 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.7 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.8,9Observers tend

to complain about noise in DR imagesexposed at one-fourth to one-half of the

appropriate level. On the other hand, arti-

facts 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 exposurefactor in DR must rely on the value of a

derived exposure indicator, which itself is

subject to interference. However, pro-

grams that monitor the exposure indicator

have been shown to be effective in con-

trolling exposure factor in DR.10

System installation and maintenanceIn order to make good images, the DR

device must be properly calibrated, con-

figured, maintained, and operated. Every

individual DR device must be calibrated

for overall gain and uniformity.11 This is

usually performed during the initial

installation and should be repeated peri-

odically. Acquisition devices should be

calibrated for gain or sensitivity, and to

compensate for nonuniformity (Figures

4 and 5). Display devices should be cali-

brated to provide output according to the

DICOM Part 14 Grayscale Display

Function, including hard-copy devices,

12 s APPLIED RADIOLOGY www.appliedradiology.com January 2004

FIGURE 1. (A and B) Underexposed computed radiography (CR) images demonstrate increase in quan-

tum mottle and loss of contrast in dense features. These constitute approximately 9% of all rejected images.

These images were acquired on an Agfa CR System (Agfa Medical Systems, Ridgefield Park, NJ).

FIGURE 2. (A) Underexposed direct digital radiography (DR) at 125 kVp and 4.4 mAs caused

an exposure data recognition failure. (B) Repeated exposure at 7 mAs. These images were

acquired on a GE DR system (GE Medical Systems, Milwaukee, WI).

A B

A B

DR ARTIFACTS AND MISADVENTURES


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A B



FIGURE 3. (A) Overexposed computed radiography (CR) images demonstrate loss of contrast in skin and dense features. These constitute approxi-

mately 5% of all rejects. (B) Repeated exposure. These images were acquired on an Agfa CR System (Agfa Medical Systems, Ridgefield Park, NJ).

FIGURE 4. (A) Improper calibration of a computed radiography (CR) system causes loss of contrast in dense features. (B) Exposure repeated

after calibration for nonuniformity and sensitivity. These images were acquired on a Fuji CR System (Fujifilm Medical Systems, Stamford, CT).

A B


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soft-copy devices, and displays used for quality control (QC).12

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 images.

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. Multi-

ple 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, includ-

ing 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 radiolo-

gist. When CR is introduced into an imaging operation, photo-

timers in all X-ray rooms need to be recalibrated to deliver the

appropriate exposure.13 The methodology for phototimer cali-

bration is different because DR density is adjustable.

Scheduled and unscheduled service should be done in a thor-

ough and timely manner, including reporting and documenting,

cleaning, and repairs. Operator functions include cleaning, report-

ing 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


FIGURE 5. Improper calibration of digital radiography (DR). Note

interfaces of four detectors visible at arrows. This image was acquired

on a Canon DR system (Canon USA, Inc., Lake Success, NY).

FIGURE 6. (A) Computed radiography (CR) corduroy artifact (arrows) is an

interference pattern generated from the interaction of the line rate of a fixed

scatter reduction grid, the sampling rate of the detector, and display zoom fac-

tor and pixel dimensions. This image was taken on an Agfa CR system (Agfa

Medical Systems, Ridgefield Park, NJ). (B) These CR cassettes are marked

to indicate orientation when inserted into the CR scanner. Each orientation

was appropriate only for a specific scanner model. Inserting in the incorrect ori-

entation caused jams. (C) CR image displayed according to exam-specific

processing parameters in the main department. (D) Same image displayed

according to settings in orthopedic department. Images B, C and D were

acquired on a Fuji CR System (Fujifilm Medical Systems, Stamford, CT) .

A

B

C


D


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performing scheduled maintenance that

includes preventive maintenance and soft-

ware 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, as well.

Training and system operationDigital radiography systems must be

operated properly to produce good

images. Technologists are often unfamil-

iar with DR features and functions, and

may require additional training beyond

vendor applications training. Technolo-gists need to select the proper examina-

tion; 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, tech-

nologists need to know how to recover

from errors without repeating examina-

tions 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 analy-sis.14 Electronic images can disappear

without a trace: counting the films

remaining in the film bin is no longer a

useful method for determining the num-

ber 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 inani-

mate object is involved (Figure 11), or

FIGURE 7. Artifacts on clinical images constitute 3% of all rejects. (A) Artifact observed (arrow) after technologist frees jam from computed radiography

(CR). (B) CR plate discoloration (arrow). (C) CR plate defect in two different orientations (arrows). Images A, B, and C were acquired on an Agfa CR sys-

tem (Agfa Medical Systems, Ridgefield Park, NJ). (D) Dust on Fuji CR collection optics (arrows) (Fujifilm Medical Systems, Stamford, CT).

A B

C D


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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 erasuremechanism failure.

Image processingAppropriate 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 pri-

mary purpose of image processing is to maximize the contrast of the

part of the image that contains relevant clinical details.15,16 To do this,

the DR system locates either the boundary of collimation or the bor-

der of the projected anatomy, and disregards details outside this


FIGURE 8. (A) Artifact on computed radiography (CR) image

(arrows) released by technologist and reported by the radiologist.

(B) Same artifact (arrows) reported later on another image from the

same machine by the same radiologist. Images were acquired on an

Agfa CR system (Agfa Medical Systems, Ridgefield Park, NJ).

FIGURE 9. It is difficult to distinguish debris on this computed radiogra-

phy (CR) imaging plate from foreign bodies (arrows). The technologist

must open the cassette and inspect the plate or re-expose the cas-

sette. These errors constitute


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FIGURE 10. (A and B) Missing lines or pixels in computed radiography (CR) can indicate memory problems, digitization problems, or communi-

cation errors. Images were acquired on an Agfa CR system (Agfa Medical Systems, Ridgefield Park, NJ).

FIGURE 11. Double exposures with computed radiography (CR) requiring a single repeat study. (A) Backboard or gurney side rails. (B) Cas-

sette left in buckey tray during fluoroscopy. Images were acquired on an Agfa CR system (Agfa Medical Systems, Ridgefield Park, NJ).

AB

A B


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FIGURE 12. (A and B) Double exposures with computed radiography (CR) requiring two repeats.

Images were acquired on an Agfa CR system (Agfa Medical Systems, Ridgefield Park, NJ).

FIGURE 13. Exposure Data Recognizer (EDR)

failure, an inability of the software to determine

collimation boundaries. This type of failure can

be caused when the operator fails to follow colli-

mation rules or when interferences prevent the

software from detecting the collimation bound-

aries. It results in incorrect histogram analysis

and inappropriate rescaling. Image acquired on

the Fuji computed radiography system (Fujifilm

Medical Systems, Stamford, CT).

FIGURE 14. (A) Error in computed radiography (CR) automatic detection of the collimation field. (B) Manually collimated images. Images were

acquired on an Agfa CR system (Agfa Medical Systems, Ridgefield Park, NJ).

FIGURE 15. (A) Inappropriate processing

of pediatric digital radiography image by

vendor-supplied parameters suitable for

adults. (B) Image processed by parameters

modified by the customer. Images acquired

on GE DR system (GE Medical Systems,

Milwaukee, WI).

A B

A B

A B



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boundary. Errors in collimation can causemistakes in detection of the boundary, with

a dramatic loss of image contrast (Figures

13 and 14).

The secondary function of image pro-

cessing 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 sharp-

ness of some features while compromis-

ing the contrast and sharpness of others, as

well as modifying the image to make it

appear more like a conventional transillu-minated film. This secondary image pro-

cessing is applied in a manner that is

usually specific to the anatomic projec-

tion. Errors in the selection of the

anatomic projection can cause inappropri-

ate processing (Figure 15).

An auxiliary purpose of image process-

ing is to improve the usability of the digital

image.17 This includes imprinting demo-

graphic overlays, adding annotations,

applying borders and shadow masks, flip-

ping and rotating, increasing magnifica-tion, 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 radi-

ographic technique, compensating for

poor calibration of acquisition and dis-

play devices, and surreptitious deletion of

nondiagnostic images. Image processing

to recover nondiagnostic images to pre-vent re-exposure should be a last resort,

not a routine activity. Routine reprocess-

ing indicates a problem with automatic

image processing or technical practice.

Access to image-processing software is

essential to develop and maintain appro-

priate processing parameters.

Automatic image processing involves

assumptions about the radiographic tech-

nique, the composition of anatomic

region imaged, and the use of collima-

tion. A number of factors can interferewith the automatic detection of the

boundaries of the radiation field, includ-

ing nonparallel collimation, use of multi-

ple 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.

ConclusionsA multitude of factors affect DR

image quality, and no device or operator

is immune to unacceptable images. A

strategy for addressing images that just

didnt turn out right must be imple-

mented. Responsibilities for document-

ing, reporting, and taking corrective

action must be clearly established.

Wider dynamic range means that tech-

nologists have to pay attention to expo-

sure indicator values, instead of bright-ness and contrast. Without this attention,

patient dose will escalate. If exposure

indicator logs are available, they need to

be evaluated. If they arent, this will

need to be done manually. Vendors need

to make such logs available in conve-

nient digital form. New technologies

should be developed for dealing with

pediatric examinations and patients with

prosthetic devices. New image process-

ing 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 diagnostic-quality images.

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Conference on Computer Applications in Radiology.

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Table 1. Secondary image processing methods in CR

Process names by vendor*

Processing method Fuji Agfa Kodak

Remapping grayscales Gradation processing Sensitometry Look-up-table (LUT)Edge enhancement Frequency processing Edge contrast Unsharp mask

Noise reduction

Decomposition and modification Multi-objective frequency Musicaprocessing

Adjustment of tone scale for Dynamic range control Latitude reduction Enhanced visualizationlimited display processing (EVP)

Correction of streaks in Tomographic artifactlinear tomography suppression (TAS)

Dual energy imaging Energy subtraction

*Fuji CR (Fujifilm Medical Systems, Stamford, CT); Agfa CR (Agfa Medical Systems, Ridgefield Park, NJ); Kodak CR (Eastman Kodak, Rochester, NY).These items are different methods, but have equivalent purposes.


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