Pediatric radiation dose management in digital radiography

Pediatric radiation dose management in digital radiography Reproduced by permission from: Neitzel U. Management of Pediatric Radiation Dose using Philips Digital Radiography. Pediatr Radiol 2004; 34 (Suppl 3):S227-S233.

U. Neitzel

Philips Medical Systems, Hamburg, Germany.

In the past, studies with computed radiography (CR) systems were not always conclusive with respect to the dose reduction potential for pediatric radiology [3,4,5]. Due to the large dynamic range of these systems there is also the risk that higher dose levels may go unnoticed, since the resulting image is not overexposed, but is actually of higher quality due to the reduced noise. The recent introduction of direct digital radiography (DR) systems based on flat-panel detectors opens up new possibilities for dose management and dose reduction in pediatric radiography. Compared with CR or screen-film imaging, the flat-panel detector offers a higher detective quantum efficiency (DQE), which may translate into a corresponding dose reduction potential, while retaining the same level of image quality. Moreover, in digital radiography the X-ray generation and image detection are usually integrated into a single computercontrolled system, making it posible to achieve much better control and monitoring of all parameters influencing the patient dose. This article describes how these principles of dose management are implemented and used in the Philips DigitalDiagnost DR system for pediatric applications.

The Philips DR system with flat-panel detector Figure 1. The Digital Diagnost. The modular design allows a wide choice of configurations.

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Radiation protection is of particular importance in pediatric radiology because of the higher sensitivity of tissue in childhood and adolescence, and the relatively longer life expectancy of young patients that may increase the chance of the development of radiogenic cancers. Measures are therefore needed to control and reduce the radiation dose from X-ray examinations, while retaining adequate diagnostic image quality. These have been described in official guidelines [1, 2]. The procedures and values given in these documents are mainly derived from experience with conventional screen-film radiography, and may require adjustments for digital imaging. In any case, the reference dose levels given for conventional imaging should also serve as the upper limit when digital systems are used.

Over the last few years, DR systems based on large-area flat-panel detectors have been introduced in routine clinical use [6, 7]. One family of DR systems is the Philips DigitalDiagnost, which is available in various configurations for table and upright examinations (Figure 1). All configurations employ a largearea (43 x 43 cm) cesium-iodide/amorphous silicon (CsI/a-Si) flat-panel detector with 143 µm pixel size (Trixell Pixium 4600, Moirans, France) (Figure 2). This type of detector is characterized by a DQE two to three times higher than that of screen-film combinations or CR detectors, and by a comparatively high resolution of 3.5 lp/mm, which makes it particularly suited for applications that require high image quality at low dose.

Figure 2.The large area (43 x 43 cm) flat-panel detector (Trixell Pixium 4600, Moirans, France) provides high image quality at low dose.

Figure 3. Example of an examination data set. Selecting a specific examination automatically sets the corresponding examination parameters.

In the DigitalDiagnost, the detector is fully integrated into the radiographic examination system, which also comprises a computercontrolled X-ray generator/tube combination and an acquisition workstation. The system allows to acquire both photo-timed and manual exposures with and without antiscatter grid. Radiation dose optimization for pediatric applications

Reduction of the patient dose according to the ALARA (as low as reasonably achievable) principle is not simply a question of selecting the right detector: it requires optimization of the whole imaging chain and selection of the appropriate imaging parameters. As the patient variation in pediatric radiology is comparatively large – ranging from premature babies with a weight below 1000 g to full-sized teenagers – widely different parameter settings may be necessary for optimal results, even for the same anatomical region, depending on the age group. The integrated technique guide In the DigitalDiagnost the key feature for managing the dose optimization process in clinical routine is an examination parameters data base, which is integrated into the system. The data base contains the full set of parameter settings for all examination types and patient classes (Figure 3). There are settings for manual and – where appropriate – photo-timed exposures for each examination and patient group. The most generally used settings are selected as the default mode. In this way the examination data base serves as an integrated technique guide, which

not only lists the technique factors but actually sets them on the generator and on the system. The user simply has to select the required examination and the age or weight group of the pediatric patient. The default settings supplied with the system have been derived from guidelines published by professional organizations, such as the European Guidelines on Quality Criteria for Diagnostic Radiographic Images in Pediatrics [2] and tested in cooperation with leading clinical sites to deliver satisfactory image quality at the lowest reasonably achievable dose level. However, if clinically necessary, the default parameters can be modified for any single exposure, or be reprogrammed according to the local requirements.

Reduction of patient dose requires optimization of the whole imaging chain.

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examination is selected, the appropriate prefilter is automatically positioned in the beam. Automated precollimation Appropriate collimation of the X-ray beam is important for both for radiation protection and image quality. The optimal collimation area depends on the individual patient and is the responsibility of the radiographer, taking into account the requirements of the case and the diagnostic question. However, since the full imaging area of the flat-panel detector is large and many field sizes in pediatric imaging are rather small, a presetting of the collimator related to the required examination/projection is helpful for a smooth workflow. In the DigitalDiagnost, collimator presets are programmed for each type of examination. When a specific examination is selected, the collimation is automatically set to the preprogrammed size. This feature corresponds to the “positive beam limitation” in conventional radiography when using different cassette sizes. The field size can then be adjusted by the radiographer according to the requirements of the individual case.

Figure 4. Different degrees of processing applied to a pediatric spine image. Figure 4a. Only postprocessing. Figure 4b. Standard (unsharp mask) postprocessing Figure 4c. UNIQUE postprocessing.


Automated prefilters Standard tube filtration in diagnostic radiology, as required by regulations, is 2.5 mm aluminum (Al) equivalent. Additional filtration in the X-ray beam can be used to remove the lowenergy component of the tube spectrum, which is completely absorbed in the patient and therefore increases the effective dose without contributing to image quality. For pediatric radiography, additional filters of 1 mm Al plus 0.1 or 0.2 mm copper are suggested, as recommended in the European Guidelines for Diagnostic Radiographic Images in Pediatrics [2], and in accordance with the usual practice in many countries. A practical problem sometimes arises when an X-ray room is used for both pediatric and adult examinations, because of the different filter settings required for these applications. The DigitalDiagnost has a preprogrammable, motorized filter wheel, which is coupled to the selection of the examination program at the operator’s console. As soon as a pediatric

Grid use in digital pediatric imaging Antiscatter grids are generally used when the level of scattered radiation is high enough to deteriorate the image quality in terms of contrast and signal-to-noise ratio. The level of scatter depends mainly on the volume being irradiated during the exposure. Pediatric patients may vary considerably in size depending on age and individual build. The use of antiscatter grids is not recommended for younger children and for the distal extremities in general, but may be necessary for adolescent and obese pediatric patients. This means that in a DR system, as in a conventional system, insertion and removal of grids (including exchange of different types) should be quick and easy. The DigitalDiagnost meets this requirement. The default configuration for each examination at each age group is part of the examination program. A mismatch between the program and the actual situation is indicated at the operator’s console and on the display on the tube housing. The need for anti-scatter grids in digital pediatric radiology needs further clinical study. Because reduced contrast due to scatter can be restored in digital images by appropriate image processing, the role of grids may be even more limited in digital imaging than in conventional screen-film radiography. On the other hand, an increase in exposure when using a grid is not absolutely

necessary, due to the large dynamic range of digital detectors. Digital imaging with and without a grid should be compared at equal patient doses and with individually adapted image processing. Automated exposure control Traditionally, phototimers (automatic exposure control, AEC) have found only limited application in pediatric radiography. One reason might be that bucky systems with AEC were deemed inappropriate for many types of examination, as such systems usually incorporate a grid that cannot be removed. As a consequence, pediatric exposures are often done on the tabletop using free cassettes. With the DigitalDiagnost system, the grid can be easily removed while the AEC measuring chamber remains in place. This allows phototimed exposures to be used for examinations both with and without grid, and can help to improve the consistency of image quality and the radiation dose level. The AEC allows operation at three different levels of detector exposure that can be linked to the examination data set, so that each examination of a certain type is always exposed with the appropriate dose level. AEC switching levels are set up at system installation in accordance with the requirements of the hospital; for pediatric installations the typical dose levels are comparable to film speeds of 400, 630, and 800. Image processing and display Image processing is an integral part of digital radiography systems, and serves to optimize the display of the digital image on a laser film hardcopy or a reading station monitor. Two stages of image processing can be distinguished: a preprocessing step and a postprocessing or display processing step. Preprocessing serves to correct for the detector pixel response (calibration) and, in particular, to adjust the dynamic range to the image data, a process also known as “exposure recognition” or “ranging”. Ranging is important, as the full dynamic range of the flat-panel detector used in the DigitalDiagnost is much larger than the latitude of conventional X-ray images. A preprocessing step combining image segmentation and histogram analysis is used to detect the relevant data range, which is then displayed with optimal contrast. Postprocessing further improves the visualization of subtle details. Philips DR and CR systems can be supplied with UNIQUE software [8]: a multifrequency processing procedure that that allows structural details to be enhanced while limiting noise (Figure 4).

Radiation dose monitoring and control

Optimization of the radiation protection according to the ALARA principle requires feedback on the actual dose levels and the quality of the acquired images in clinical routine, both for each individual examination and as averages. The latter are especially important for comparison with the diagnostic reference levels propagated in ICRP Publication 73 [9] and translated into legal requirements, such as those in the European Union Directive 97/43/Euratom [10]. While the image quality can be judged directly from the resulting image, determination of the dose needs special attention in digital radiography as the conventional indications of dose level in the form of film speed and film density are not available. Several dose quantities are in use to describe the radiation level used for an examination (Figure 5). The quantity best describing the radiation risk to the patient is the (absorbed) effective dose, but this quantity cannot be measured in clinical routine. The surrogate quantities usually adopted for radiation protection purposes are the entrance skin exposure (ESE) or the kerma-area product (KAP). The first is often used in the USA, while the second is more common in Europe. The KAP has the advantage that it also includes the effect of collimation on patient dose. For a given situation (patient size, examination type,

Figure 5.Typical examination beam geometry and related radiation dose quantities.

Optimization of radiation protection requires feedback on dose levels and image quality.


Exposure indicators (EI) are equivalent to the combination of film speed and film density.

Table 1. Exposure index (EI) scale for the Philips DigitalDiagnost system. Each EI value corresponds to a band of detector exposure values of approximately ±13% around the value given in the second column.

projection) the effective dose can be estimated from the KAP value using conversion factors [11] or Monte-Carlo simulation programs [12]. Kerma-area product The kerma-area product is the product of the dose (kerma) value of the incident radiation and the irradiated field size. Due to the inversesquare-law dependence of the dose value, the focal spot distance cancels out when calculating the KAP, i.e., it can be determined or measured at any distance from the focal spot, provided that the full beam is covered. Translucent ionization chambers directly attached to the tube collimator are often used to measure the KAP during clinical exposures. Given proper calibration of the tube output, the KAP value can also be derived from the generator settings (kV, mAs), taking into account a prefilter, if fitted. This method is applied in the DigitalDiagnost. It has the advantages that it needs no additional device in the beam, and that the KAP data can displayed and stored with the digital image. Exposure indicator For quality control purposes in digital radiography, the systems usually provide exposure indicators (EI), derived from the (mean) image signal and, therefore, related to the detector

Figure 6. Acquisition of long image formats by sequential, overlapping acquisitions. Figure 6a. Parallel movement of tube and detector leads to parallax mismatch at the image seams.

Figure 6b. Fixed-focal-spot principle: The tube remains stationary and only the detector moves, making seamless stitching possible.


exposure. They can be seen as the equivalent of the combination of film speed and film density that serves as an exposure indicator in conventional radiography. Exposure indicators in the various CR and DR systems differ in scale and definition, making it difficult to compare EI values from different vendors or installations. However, the main purpose of the exposure indicator is to allow longitudinal (time) comparisons of system operation in one installation. Also, since the EI is derived from the actual patient image, variations may occur due to the varying image content, even when a fixed or precisely comparable exposure setting is used. Exposure Indicator EI 160 200 250 320 400 500 630 800 1000 1250 1600 2000

Detector Exposure [µGy] 6.3 5.0 4.0 3.1 2.5 2.0 1.6 1.3 1.0 0.8 0.63 0.5

In the DigitalDiagnost the definition of the EI follows the speed definition of conventional screen-film systems quite closely (Table 1). The EI values are directly related to the (mean) detector entrance exposure, which in turn is derived from an appropriately defined mean pixel value. As the pixel values in a clinical image may vary greatly due to differences in the the anatomical structure under examination, the procedure used to determine the mean pixel value has a great influence on the resulting EI. Most systems use some type of histogram analysis for this purpose. In the DigitalDiagnost additional system information is available and used for the EI determination. For phototimed exposures, only the area of the activated measuring fields is used as the input for the calculation; for manually exposed images, only the central part of the collimated area is evaluated, excluding areas of direct radiation by means of a segmentation algorithm. This principle leads to highly consistent EI values with good correlation to the exposure level. Reporting exposure parameters and dose information The Digital Diagnost provides direct feedback on the exposure factors and dose values after each

image exposure by displaying the values at the operating console. All relevant exposure parameters (kV, mAs, ms, filter, grid use) and dose values (KAP and EI) are stored together with the acquired image, and are documented on the film hardcopy and included in the DICOM header for display at the radiologist’s reading station. Internally, the system collects all data for each X-ray exposure in a logfile that can be accessed by service and system specialists for further analysis. For instance, this feature can be used to compare the dose values applied in clinical routine use with the diagnostic reference levels.

Figure 7. Pediatric whole spine image obained with the method shown in Figure 6b at reduced dose (speed 800).

Special imaging procedures In pediatric radiology, long cassette formats are relatively frequently used for imaging the whole spine or the whole legs, e.g. for diagnosis of scoliosis. Because flat-panel detectors are generally limited in size to a maximum of about 43 cm x 43 cm, the imaging of long fields requires sequential acquisition of two or three partly overlapping images, which are then composed into one large image [13]. Two different approaches are possible: either tube and detector are moved in parallel along the patient (Figure 6a) or only the detector is moved while the tube is slightly tilted or the collimation is adjusted for the different detector positions (Figure 6b), i.e., the focal spot remains stationary. In the first method, parallax errors occur at the seams of the images, their magnitude being dependent on focus-detector distance, object-detector distance and the height of the X-ray field. These errors are completely avoided in the second method, because the position of the focal spot remains the same for all images. Whole spine or leg images are usually required for measuring geometric parameters such as skeleton angles and lengths. For this purpose, high detail resolution is not of importance, and a certain noise level can be accepted. Therefore, considerable dose reduction is possible, at least for follow-up examinations. Figure 7 shows a pediatric full spine image acquired at low dose with the DigitalDiagnost using the method illustrated in Figure 6b.

Conclusion Digital radiography systems based on flat-panel detectors provide improved physical performance in terms of DQE, which can be used to acquire images at reduced dose while maintaining image quality. This aspect is of particular importance in pediatric imaging. However, a more efficient detector alone is not sufficient to ensure consistent low-dose operation in clinical routine.

Moving the detector while keeping the tube stationary avoids parallax errors between images

Images can be acquired at reduced dose while maintaining image quality.


Optimization of clinical image quality at reduced radiation dose requires a systems approach.

Optimization of clinical image quality and reduction of radiation dose according to the ALARA principle requires a system approach which includes all components influencing patient dose.

compared with CR. In another study using an animal model, Ludwig et al. [15] found a dose reduction of as much as 75% when compared to 400 speed screen-film and CR images of the lumbar spine, while maintaining equal image quality.

Only a few clinical studies of pediatric applications of flat-panel based DR systems have been published to date. Ludwig et al. [14] showed that, for pelvic images in congenital hip dysplasia, the dose can be reduced by 50 % when

Further clinical research is necessary to fully exploit the potential of digital radiography systems for dose reduction in pediatric examinations. This requires the continued cooperation between manufacturers and clinical users of the systems

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