Practical guidelines for radiographers to improve computed radiography image quality
N
Pongnapang, PhD
Faculty of Medical Technology, Mahidol University, Bangkok,
Thailand
ABSTRACT
Computed Radiography
(CR) has become a major digital imaging modality in a modern
radiological department. CR system changes workflow from the
conventional way of using film/screen by employing photostimulable
phosphor plate technology. This results in the changing perspectives
of technical, artefacts and quality control issues in radiology
departments. Guidelines for better image quality in digital
medical enterprise include professional guidelines for users
and the quality control programme specifically designed to
serve the best quality of clinical images. Radiographers who
understand technological shift of the CR from conventional
method can employ optimization of CR images. Proper anatomic
collimation and exposure techniques for each radiographic
projection are crucial steps in producing quality digital
images. Matching image processing with specific anatomy is
also important factor that radiographers should realise. Successful
shift from conventional to fully digitised radiology department
requires skilful radiographers who utilise the technology
and a successful quality control program from teamwork in
the department. © 2005 Biomedical Imaging and Intervention
Journal. All rights reserved.
Keywords: Computed radiography, image quality, quality
control
INTRODUCTION
The evolution of medical imaging towards totally digital
imaging has accelerated over the past decade [1].
Since its introduction two decades ago, computed radiography
(CR) has now become the main player in acquiring, processing
and displaying digital images. CR is a process of delivering
images that is similar to conventional screen/film system.
The main difference between the two systems is that CR processes
the optical signals based on a phenomenon called “photostimulated
luminescence”, rather than from a prompt emission of
light, as in the case with screen-film radiography. In CR,
the imaging plate containing storage phosphor is inserted
in a cassette similar to a screen-film system, exposed to
x-rays, and the signal trapped by the plate read by the scanning
of a laser light beam. A photomultiplier tube then enhances
the signal coming from the light guide [2,3].
The advantages of CR are its large dynamic range, digital
format, portability, and post-processing capability. The technology
of CR continues to improve in concomitant with the development
of digital technology.
HOW CR AFFECTS WORKFLOW IN A DIGITAL
IMAGING DEPARTMENT
Radiologists and radiographers are the two main professionals
involved in the provision of radiology services. An efficient
workflow in any radiology department is therefore dependent
on how these two professionals plan the department. The present
article however, will focus only on the role of the radiographers,
which mainly concerns the acquisition of general radiographic
projections. This process can be employed using either screen-film
or digital radiographic modalities, such as computed radiography.
In order to achieve the best of productivity, it is imperative
that one has a basic understanding of an efficient workflow.
In a digital imaging enterprise, a unique number of tasks
make up the process of performing radiographic examinations
that could be significantly different from a conventional
screen-film system. A common digital imaging workflow includes
examination scheduling, patient transportation, patient preparation,
data access, examination acquisition, image processing, retrieval
of historical comparison studies, and image duplication. The
process may also incorporate repeat examinations due to technical
factors or loss. Figure 1 shows an image processing work flow
in a digital imaging department using CR technology.
[View this figure] |
Figure 1 Diagram showing image
processing flow in a digital imaging department
employing CR technology. |
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OPTIMISATION OF CR IMAGES
The impression that CR images can always be adjusted after
exposing the CR with x-rays is not necessarily true. There
are several factors affecting the quality of CR images, and
radiographers or technologists are the key persons who are
responsible in delivering good quality radiographs, with reasonable
radiation dose given to the patients. Quality control of the
technical parameters and radiographic positioning are therefore
critical to a CR image. Optimisation of a CR image quality
may be achieved by optimizing the following factors:
Positioning and collimation
The routine practice of radiographers includes correct positioning
of the organ of interest at the centre and collimating the
x-ray field just to cover the organ; this will deliver a good
quality image with an acceptable contrast. Proper collimation
reduces scattered radiation in the region of interest and
reduces the noise that degrades the radiographic contrast.
This good practice is still valid with CR, and most image
processing software employed in CR relies on the fact that
the image collimator edge is detected, so that the contrast
may be optimised. Failure of the software to define the image
boundary may be caused by a number of factors.
For example, a radiographer may be used to take two projections
of a hand radiograph in one 18 cm x 24 cm film. This however,
is not a good practice with CR technique, since double or
multiple exposures on a single photostimulable phosphor (PSP)
can lead to a failure of the image processing software to
detect the image boundary. Matching the positioning and collimation
with the image processing parameters is also crucial. Some
radiographers may take a radiograph of a lumbosacral spine
without collimation, thus making the radiograph looks more
like an image of the Kidney-Ureter-Bladder (KUB) technique.
Image processing will eventually fail to process since the
input information is totally different.
Exposure techniques
In order to introduce CR as a replacement for conventional
film-screen technique, the common thinking is that it would
be reasonable to adhere to the same exposure techniques to
help the radiographers to adapt to the newer technology. But
this is not necessarily the case. CR may be operated at a
different film speed, and then optimizing the exposure technique
accordingly. Existing CR has a speed similar to medium speed
film-screen system while spatial resolution is still generally
inferior [4].
The idea of reducing radiation dose to patients when switching
from screen-film system to CR may not always be valid. To
keep the same signal to noise ratio, CR needs 20% more radiation
exposure as we treat CR as medium speed film. Reduction of
radiation dose to the patient will then results from reduction
of reject rate due to poor exposure technique. As a result
of poorer intrinsic spatial resolution of the PSP, radiographers
need to make sure that when they set up exposure factors,
i.e., the mA station from small focal spot should be selected
when imaging bones or other high resolution required body
parts.
Consideration should be made to the detection efficiency
of kVp and K-absorption edge of the PSP, which is totally
different from that of the conventional screen-film system.
Matching kVp with the pre-set range offered by the image processing
is also important. Some radiographers may still use too low
kVp for chest radiographs. Employing a standard high kVp technique
when the pre-set kVp range for image processing may be higher
prevents optimisation of the image quality.
A proper adjustment of exposure technique is therefore still
crucial in any radiography practice. Although an increased
radiation exposure would yield a higher signal-to-noise ratio
and better low contrast detectability in PSP, this would clearly
violate the “As low as Reasonably Achievable”
(ALARA) principle.
[View this figure] |
Figure 2 Some examples of artefacts
in CR (a) an image with loss of contrast as a result
of improper selection of image processing; (b) the
same image as 2a shows acceptable image quality
as a result of proper selection of image processing. |
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Image processing selection
CR vendors will normally provide various software packages
for image processing. Proper selection of an image processing
algorithm specific to each type of x-ray examination is thus
important. The technical skills of radiographers definitely
play a crucial role in determining the quality of the radiographic
image. Even though a CR image may be adjusted to improve the
image visibility in the cases of over- or under-exposures,
it would still be impossible for an image processing to improve
the visibility of clinical features that were not available
in the raw image. This effect of image processing is illustrated
in Figure 2. Image processing may not be substituted for poor
positioning techniques and inadequate intrinsic contrast from
improper setting of radiographic exposures, or any information
outside the edge of the imaging plate for that matter.
Lifetime of the PSP
One of the major advantages of CR is that the phosphor plate
is reusable. However, there are a number of factors that may
affect the lifetime of an imaging plate. The plates are subjected
to normal wear and tear from scratches, scuffs, cracks, and
contamination with dust and dirt, which may interfere with
the production of a good image. The establishment of a well-organised
quality control program will play an important role in assessing
the clinical quality of the imaging plate. This may easily
be carried out by artefact assessment and uniformity evaluation
across the plate.
CR ARTEFACTS
The artefacts in radiographic images are seen as any fault
impressions visible on the produced radiographic images. These
artefacts are distracting and may lead to poor diagnostic
accuracy. Although many radiographers may be already accustomed
with artefacts appearing in conventional x-ray images, artefacts
in CR, require special attention. This is due to the fact
that CR artefacts may be produced from various components
of the CR system itself [5]. Artefacts may
also be generated by the users who are not aware of the proper
imaging techniques or selection of appropriate image processing
protocols [6,7]. Since
CR is also very sensitive to scattered radiation, it is vital
that anti-scattered grids be used as in conventional radiography.
Radiographers should be concerned of the effects of the aforementioned
factors, since these may generate unwanted artefacts that
could not be corrected by any image processing algorithm.
[View this figure] |
Figure 3 (a) Image artefact resulting
from double exposure of the imaging plate. This
is a composite image showing a femur superimposed
on a chest radiograph; (b) artefact caused by a
towel that was used to help in positioning a paediatric
patient. Due to the wider dynamic range of CR comparing
to conventional film-screen system, radiographic
contrast from the towel is readily seen; (c) artefact
resulting from dirt collected inside the light-guide
in the CR reader leading to the formation of a bright
horizontal line (near the bottom of the image). |
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[View this figure] |
Figure 4 A QC image using a phantom
embedded with test patterns such as low and high
contrast objects, spatial resolution bar phantom,
and gray scale objects (Leeds TOR from the University
of Leeds, U.K.). (a) 70 kVp, 2 mAs; (b) 70 kVp,
0.5 mAs. Note the increase in noise level. |
|
Implementing a competent quality control program and the
proper training of new staff members who will operate the
system is therefore still crucial in a digital imaging enterprise.
Periodic maintenance from vendors will also contribute to
the quality management program by avoiding unwanted circumstances
that would degrade the overall quality of the clinical images.
Figure 3 demonstrates some common artefacts generated from
CR.
SYSTEM CALIBRATION AND QUALITY CONTROL
To ensure the production of high-quality radiographic images
from a CR, a well-organised acceptance testing following the
system installation must be carried out. Although the system
may have already been calibrated by the manufacturer prior
to the installation, the current working environment and conditions
in a hospital may be different. Medical physicists will play
a role during the acceptance testing by determining that the
calibration of the system was made in accordance with the
current environment and conditions of the newly-acquired x-ray
system. Task Group 18 of the Diagnostic Committee of the American
Association of Physicists in Medicine has undertaken the task
of establishing a standard of performance for Quality Control
(QC) of CR equipment [8].
A periodic quality control program is still necessary even
after a successful completion of an acceptance testing. The
medical physicist is responsible for performing acceptance
testing and setting up the quality control program for the
CR system. QC processes for CR are no less important than
they are for conventional screen-film radiography. The design
of the program needs to be modified to fit the differences
that are unique to the characteristics of the CR and good
quality control program needs cooperation between radiographers
and medical physicists. Radiographers perform daily and periodic
check of quality control items that do not require complicated
dose measurement procedures or reject analysis and image quality
evaluation. Medical physicists should be responsible for performing
the review of QC activities, patient dose assessment and annual
quality assessment of the CR system. Figure 4 shows an example
of images obtained from an image quality phantom (Leeds TOR,
University of Leeds, U.K.). Table 1 summarises a QC program
for CR listing the various tasks, frequency and individuals
to be responsible.
CONCLUSION
Practical guidelines for better image quality in computed
radiography is mainly concerned with the professional skills
of the users and the establishment of an efficient quality
control program specifically designed to produce the best
quality of clinical images. Another important factor is the
level of teamwork among the users. Radiologists should support
and encourage staff in the radiology department to appreciate
the importance of an effective quality control program. In
addition, radiographers who utilise the technology should
also receive proper training on developing professional skills
concerning CR technology and must also play an important role
in the quality control program. A successful digital radiology
enterprise will undoubtedly earn immeasurable benefits from
an effective quality control program and skilful radiographers
who correctly utilise the technology.
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Received 29 July 2005; received
in revised form 2 October 2005; accepted 17 October 2005
Correspondence: Faculty of Medical Technology, Mahidol University, Siriraj Hospital, Bangkoknoi, Bangkok 10700 Thailand. Tel.: +66 2 419 7173; Fax: +66 2 412 4110; E-mail: mtnpp@mahidol.ac.th (Napapong Pongnapang).
Please cite as: N Pongnapang,
Practical guidelines for radiographers to improve computed radiography image quality, Biomed Imaging Interv J 2005; 1(2):e12
<URL: http://www.biij.org/2005/2/e12/>
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