IMAGE QUALITY AND DOSE MANAGEMENT IN DIGITAL RADIOGRAPHY: A NEW PARADIGM FOR OPTIMISATION H. P. Busch 1 and K. Faulkner 2,

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1 Radiation Protection Dosimetry (2005), Vol. 117, No. 1 3, pp doi: /rpd/nci728 Advance Access published on February 3, 2006 IMAGE QUALITY AND DOSE MANAGEMENT IN DIGITAL RADIOGRAPHY: A NEW PARADIGM FOR OPTIMISATION H. P. Busch 1 and K. Faulkner 2, 1 Department of Radiology, Krankenhaus der Barmherzigen Brudder, Nordalle, Trier, Germany 2 Quality Assurance Reference Centre, Unit 9, Kingfisher Way, Silverlink Business Park, Wallsend, Tyne and Wear NE28 9ND, UK The advent of digital imaging in radiology, combined with the explosive growth of technology, has dramatically 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 owing to the higher detective quantum efficiency (DQE). These fundamental differences in comparison 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. INTRODUCTION The advent of digital imaging in radiology combined with the explosive growth of computer technology has dramatically 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, 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 stateof-the-art in diagnostic projection imaging using X rays. 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 characterised 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. Postprocessing of images is a double edged sword. On the one hand, it may improve image quality; on the other hand, 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. 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. Postprocessing by individual and automatic procedures is necessary. Corresponding author: keith.faulkner@nhs.net 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 Ó The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org

2 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 capable of. 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 in the number 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 of 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 a complete metal inplant 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 and 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 H. P. BUSCH and K. FAULKNER 144 reach the necessary quality with lowest dose. This can be achieved only by clinical studies and 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. Userdefined default post-processing and individual postprocessing 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 CDRAD 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, standardisation 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 (11,15). Figure 1 shows the evaluation of CDRAD images for three storage phosphor systems (FCRXG1, ADC-Solo and ADC-70), a flat detector system (Digital Diagnost) and a film/screen combination at 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 such as the FCR XG1 and the ADC-Solo and older systems such as the 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 (4 ma s) and a speed class of 200 (16 ma s) for storage phosphor (ADC-70). For the same quality

3 DIGITAL RADIOGRAPHY: A NEW PARADIGM FOR OPTIMISATION 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) and ADCV70 (Agfa). 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) and ADC-70 (Agfa). this means a 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 ADC70). 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 panel detector system and a film/screen combination. After blinding the imaging method and demonstrating only three areas of the image, four 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 with film screen (speed class 400), the flat panel detector system (Digital Diagnost speed 1600) had been graded into the same quality class medium than storage phosphor 145

4 H. P. BUSCH and K. FAULKNER Figure 3. Relation between image quality classes and dose, depending on imaging methods. Parentheses indicate speed class. system speed classes 400 and 200. Higher dose values for the flat panel detector system (speed 800, 400 and 200) lead to a significant improvement in image quality. Lower dose values for storage phosphor systems (speed classes 800 and 1600) was graded into the low image quality class. For a clinical diagnostic question first the image quality class has to be determined. Then depending on 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. Individual post-processing not only 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 postprocessing to this pattern similar to the whole image. Evaluation of this pattern on image display monitors or hard copy films will provide a impression of the influence of post-processing to the image quality. 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 (e.g. spatial resolution or contrast detectability) or by direct evaluation of the digital images using computer QA programs (e.g. signalto-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, such as signal-to-noise ratio, dynamic range or homogeneity. These parameters can be assessed by direct analysis of the digital image. Testing of image documentation can be performed by digital test patterns, such as the SMPTE-Test ( org). 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. ACKNOWLEDGEMENTS This research was partially supported by the EC radiation protection programme DIMOND III research Contract (FIGM-CT ). REFERENCES 1. Busch, H. P., Faulkner, K. and Malone, J. F. Image quality criteria applied to digital radiography. Radiat. Prot. Dosim. 57(1 4), (1995). 2. Busch, H. P. and Jaschke, W. Adaption of the quality criteria concept to digital radiography. Radiat. Prot. Dosim. 80(1 3), (1995). 3. Busch, H. P. Need for new optimisation strategies in CR and direct digital radiography. Radiat. Prot. Dosim. 90(1 2), (2000). 4. Busch, H. P., Bosmans, H., Faulkner, K., Peer, R., Vano, E. and Busch, S. Dose management with new digital imaging techniques. Presentation ECR (B-0561/ SS 1013) (2002). 5. Busch, S. Bildqualität und dosis in der digitalen radiographie. MD Thesis, Fakultät für Klinische Medizin, Klinikum Mannheim der Universität Heidelberg, Germany (2002) (in German). 6. Brenner, D. J., Elliston, C. D., Hall, E. J. and Berdon, W. E. Estimated risks of radiation induced fatal cancer from pediatric CT. Am. J. Roentgenol. 176, (2001). 7. Chotas, H. G. and Ravin, C. E. Digital chest radiography with a solid-state flat-panel x ray detector: contrast-detail evaluation with processed images printed on film hard copy. Radiology 218, (2001). 8. Floyd, C. E., Jr, Warp, R. J., Dobbins, J. T., III, Chotas, H. G., Baydush, A. H., Vargas-Voracek, R. and Ravin, C. E. Imaging characteristics of an amorphous silicon flat-panel detector for digital chest radiography. Radiology 218, (2001). 9. Geijer, H., Beckman, K. W., Anderson, T. and Persliden, J. Image quality vs radiation dose for a flatpanel amorphous silicon detector: a phantom study. Eur. Radiol. 11, (2001). 10. Hamers, S., Freyschmidt, J. and Neitzel, U. Digital radiography with a large-scale electronic flat-panel 146

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