Download Quality- and Dose- Management

Image Quality And Dose Management In Digital Radiography: A New Paradigm For
Optimisation
H P Busch *, K Faulkner #
* Department of Radiology, Krankenhaus der Barmherzigen Brudder, Nordalle, Trier, Germany, [email protected]
# Quality Assurance Reference Centre, Unit 9, Kingfisher Way Silverlink Business Park, Wallsend,
Tyne and Wear, NE28 9ND, [email protected] (corresponding author)
Abstract. The advent of digital imaging in radioalogy, combined with the expolosive growth of technology,
has dramtcally improved imaging techniques. This has led to the expansion of diagnostic capabilities, both in
terms of the number of procedures and their scope. Throughout the world, film / screen radiography systems are
being rapidly replaced with digital systems. Many progressive medical institutions have acquired, or are
considering the purchase of, computed radiography systems with strorage phosphor plates or direct digital
radiography systems with flat panel detectors. However, unknown to some users, these devices offer a new
paradigm of opportunity and challenges. Images can be obtained at a lower dose due to the higher detective
quantum efficiency. These fundamental differences in compasrison to conventional film / screens necessitate the
development of new strategies for dose and quality optimisations. A set of referral criteria based upon three dose
levels is proposed.
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1.
Introduction
The advent of digital imaging in radiology, combined with the explosive growth of computer
technology, has dramatically improved imaging techniques. This has lead to the expansion of
diagnostic capabilities both in terms of the number of procedures and their scope. Throughout the
world, conventional fluoroscopic and film/screen radiography systems are rapidly being replaced with
digital systems. Many progressive medical institutions have acquired or are considering the purchase
of computed or direct digital radiography systems with either storage phosphor plates (computed
radiography) or flat panel detectors. These systems represent the current state of the art in diagnostic
projection imaging using x-rays.
2.
Digital Imaging
Conventional film-screen radiography and digital radiography are very different approaches to x-ray
projection imaging. Both methods share common physical principles and involve exposure to ionising
radiation. In conventional film/screen radiography, the exposed film is the result of a well established
procedure. X-ray film is a permanent record, which cannot be manipulated thereafter. In digital
radiography the information is acquired by detectors and stored in digital form within a computer. Post
processing by individual and automatic procedures is necessary.
However, unknown to most users and potential users, these devices offer a new paradigm of
opportunities and challenges for projection radiology. The imaging capabilities of digital radiography
are characterized by a high detective quantum efficiency (DQE), a broad dynamic range and
possibilities of post processing, archiving and transfer of data. This means that images can be acquired
at a lower dose as a result of the high quantum efficiency. Post processing of images is a double edged
sword. On the one hand, it may improve image quality, on the other if used inappropriately it may
decrease the quality and create artefacts that can be confused with pathology [1-16].
These fundamental differences in comparison to conventional film/screen necessitate the development
of new strategies for quality optimisation.
3.
Clinical Strategies
Projection radiography of chest, skeleton, and gastrointestinal tract are currently the most frequently
performed radiographic examinations in hospitals and practices. It is now well documented that when
physicians frequently request paediatric computed tomography (CT) examinations, they must modify
the CT machine’s scanning parameters in order to lower the paediatric CT dose. There are similar
opportunities to reduce patient dose for a variety of examinations in digital projection-radiography.
The choice of the appropriate examination parameters must be a compromise between the clinical
situation and the radiation dose to the patient.
The broad range of imaging capabilities of digital radiography allows the clinician the ability to adapt
the dose and image quality parameters to the clinical situation. For successful patient management,
clear goals and a precise investigation strategy are necessary.
The primary goal of the radiologist is to give the referring clinician the information he requires. The
examination should be performed to achieve this aim with the lowest possible dose value.
The strategy for using digital imaging devices can be described by two points:
1. Applying the ALARA-principle (i.e. the dose to the patient should be as low as reasonably
achievable).
2. Image quality should be as good as necessary for the diagnosis, not necessarily as good as the
digital x-ray system is independent of the dose value capable of.
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4.
Optimisation Strategies
Strategies for optimisation start with a consideration of the diagnostic requirements for a given clinical
situation. Improved clinical practice recommendations should lead to a reduction of the numbers of
referrals for investigations in the first instance and thereby, to a reduction in radiation exposure [17].
The choice of imaging method has to be undertaken on the basis of evidence based medicine. When
developing clinical pathways for digital projection radiography as a diagnostic tool, the task is to meet
the requirements of the referring physician. Therefore optimisation means making the imaging method
and adjusting the technique parameters this clinical task with lowest risk for the patient and the staff.
Within clinical pathways referral guidelines are an excellent starting point. After the decision for an
imaging method has been made the necessary image quality has to be determined. The necessary
image quality depends on the clinical question, which has to be answered [18,19].
For instance the evaluation of a fracture without dislocation requires a high image quality, control of
the position of a fracture requires a medium image quality and control for complete metal
explanation requires a low image quality. Depending on the imaging method three dose levels (or
speed classes if conventional films/screens are used) can be assigned (i.e. speed class 400, 800, 1600).
This means that the referral guidelines for conventional radiography may be amended for use in digital
radiology. The existing referral guidelines have to be completed by an additional parameter, the image
quality class (low, medium or high) when using digital radiography. As a reference, a film/screen with
the speed class 400 can be regarded the quality class “medium”.
According to this definition, imaging parameters, including post processing, have to be optimised to
reach the necessary quality with lowest dose. This can be achieved only by clinical studies not by test
phantom exposures. In this way new optimisation strategies have to be defined for digital radiography.
It is important that optimisation includes post processing which depends on different detectors and
exposure/dose parameters.
For an imaging chain including post processing and documentation a tool is needed for description and
then for standardisation. Post processing methods used are dependent on the manufacturer. User
defined default post processing and individual post processing are only partly known. Consequently,
we have to handle the issue of post processing with a “black box” model.
This "black box” can be described by the relation of an input function to the output function. A useful
input function would be a digital test pattern placed like a “Trojan horse” within a digital image. The
output function can then be analysed in a numerical way. A more practical approach would be to
perform an exposure of a test phantom with typical organ parameters to study the image quality on
dedicated display monitors or laser films on a viewing box. A good example of this phantom is the
CRRAD 2.0 phantom (Artinis-Medical Systems B.V.). In several different squares are more
cylindrical objects with different diameters and depths. These have to be detected by an observer.
Exposures with different dose values and scattering material can characterise the low contrast
detectability of a system. Using this phantom the whole imaging chain can be studied. Phantom
measurements cannot optimise the imaging chain, but can assess the imaging chain for quality control,
standardization and comparison purposes. Optimisation for a defined quality level can only be
undertaken based upon clinical experience and objective clinical studies.
Studies in the literature compared imaging capabilities of different digital imaging methods by
CDRAD exposures. Figure 1 shows the evaluation of CDRAD images for three storage phosphor
systems (FCRXG1, ADC-Solo, ADC70), a flat detector system (Digital Diagnost) and a film/screen
combination and different dose values. Four radiologists evaluated the low contrast object to
determine the sensitivity. The results show that the sensitivity drops down with lower dose values. The
flat detector system had the highest sensitivity. There was a significant difference in the results of state
of the art storage phosphor systems like the FCR XG1 and the ADC-Solo and older systems like the
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ADC-70. As a consequence for dose management it could be demonstrated that the same sensitivity
can be reached with the flat detector speed class 800(4mAs) and a speed class of 200(16mAs) for
storage phosphor (ADC-70). For the same quality this means difference of dose by a factor of 4. On
the other hand this figure demonstrates significant differences of image quality for different
generations of computed radiography systems (FCR XG1 to ADC 70).
FIG. 1.
Sensitivity (%)
60
50
Digital Diagnost
FCR XG1
Film/Screen
ADC-Solo
ADC-70
40
30
20
10
0
16mAs
(200)
8mAs
(400)
4mAs
(800)
2mAs
(1600)
mAs
(Speed- Class)
Figure 1:
Comparison of the sensitivity by the CDRAD phantom for
Different digital imaging methods and different dose values
Flat detector:
Digital Diagnostic (Philips)
Computed radiography: FCR XG1 (Fuji)
ADC-Solo (Agfa)
ADCV70 (Agfa)
The results of the CDRAD studies could be confirmed by the evaluation of images of organ phantoms
like the abdomen phantom. Images with speed classes of 200, 400, 800 and 1600 had been taken with
the three storage phosphor systems, the flat detector system and a film/screen combination. After
blinding the imaging method and demonstrating only 3 areas of the image 4 radiologists graded the
quality into the classes high, medium and low. The class medium had been defined by an exposure of
film/screen speed class 400. Figure 2 shows the example of an organ phantom graded by radiologists
by three regions. The results show the improvement of image quality for the flat detector system
(Digital Diagnost) and the possibilities to reduce dose. Compared to film screen (speed class 400) the
flat detector system (Digital Diagnost – speed 1600) had been graded into the same quality class
“Medium than storage phosphor system speed class 400 and 200. Higher dose values for the flat
detector system (speed 800, 400, 200) leads to a significant improvement in image quality. Lower dose
values for storage phosphor systems (speed class 800, 1600) was graded into the “low” image quality
class.
FIG. 2. Image Quality and Dose
High
Digital Diagnost (200, 400, 800
Medium
Digital Diagnost (
)
1600)
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FCR XG1
Solo
ADC70
Film/Screen
Low
FCR XG1
Solo
ADC70
(200, 400
(200, 400
(200, 400
(
400
(
(
(
)
)
)
)
800, 1600)
800, 1600)
800, 1600)
(Speed Class)
Figure 2: Grading of the image quality of an exposure of an
abdomen phantom for different imaging methods and
dose values
Flat detector:
Digital Diagnostic (Philips)
Computed radiography: FCR XG1 (Fuji)
ADC-Solo (Agfa)
ADC-70 (Agfa)
For a clinical diagnostic question first the image quality class has to be determined. Then depending of
the imaging method the speed (dose) class has to be chosen corresponding to the necessary image
quality. A suggestion for further strategy of dose and quality management is drawn in figure 3.
FIG. 3. Relation between image quality classes and dose depending on imaging methods
( ) Speed class
High
Medium
Low
Flat detector(400)
Stor.Phos. (200/400)
Film/screen (200)
Flat detector(800)
Stor.Phos. (400)
Film/screen (400)
Flat detector(1600)
Stor.Phos. (800)
Film/screen (800)
Figure 3: Suggestion for quality and dose management in digital projection radiography
Individual post processing can increase but also decrease image quality. To demonstrate the influence
of individual post processing it would be helpful to integrate a fixed digital test pattern into the raw
image like a “Trojan horse”. For example, this digital test object could simulate a lead bar pattern and
contrast detail phantom and to apply post processing to this pattern similar to the whole image.
Evaluation of this pattern on image display monitors or hard copy films will give in impression of the
influence of post processing to the image quality.
5.
Quality Control
Quality Control may be performed by phantom exposures which assess the whole imaging chain.
Qualitative evaluation can be obtained at the level of digital stored images, monitors or film
documentation. This can be achieved by subjective assessment of image quality displayed on
dedicated monitors or films (for example, spatial resolution or contrast detectability) or by direct
evaluation of the digital images using computer QA programs (for example, signal/noise).
For digital radiography the imaging chain can be divided into imaging acquisition and documentation
on monitors and laser films. Image quality of a test phantom exposures can be evaluated by digital
parameters like signal/noise ratio, dynamic range or homogenity. These parameters can be assessed by
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direct analysis of the digital image. Testing of image documentation can be performed by digital test
patterns like the SMPTE-Test. These can be evaluated on a monitor or laser film. The data should be
stored and displayed as curves to demonstrate the results over a time period.
In conclusion digital projection radiography offers new possibilities for dose and quality management
and quality control. The aim is an optimal adaptation of image quality to the diagnostic task with a
reduction of risk for the patient and staff by lowering the necessary dose level.
6.
Acknowledgements
This research was partially supported by the EC radiation protection programme DIMOND III
research Contract (FIGM-CT-2000-00061).
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