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British Journal of Radiology (2006) 79, 899-904
© 2006 British Institute of Radiology
doi: 10.1259/bjr/57138583
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Full paper

Comparison of radiation doses to patients undergoing standard radiographic examinations with conventional screen–film radiography, computed radiography and direct digital radiography

G Compagnone, PhD1, M Casadio Baleni, MD2, L Pagan, MSc1, F L Calzolaio2, L Barozzi, MD2 and C Bergamini, PhD1

1 Medical Physics Department, 2 Accident and Emergency Department, S. Orsola-Malpighi Hospital, Via Massarenti 9, 40138 Bologna, Italy


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
New flat-panel direct digital radiography equipment has recently been installed in our Accident and Emergency Department; its characteristics and versatility are well suited to the work undertaken in this environment. The aim of this study was to compare radiation doses to patients undergoing standard radiographic examinations using conventional screen–film radiography, computed radiography and direct digital radiography; entrance surface dose and effective dose were calculated for six standard examinations (a total of 10 projections) using standard patient exposure parameters for the three imaging modalities. It was found that doses for computed radiography (all examinations) were higher than the doses for the other two modalities; effective doses for direct digital radiography were ~29% and ~43% lower than those for screen–film radiography and computed radiography, respectively. The image quality met the criteria in the European guidelines for all modalities.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusions
 References
 
In Accident and Emergency (A&E) departments, patients undergo radiological examinations to establish their clinical condition. The importance of having a radiologist to report the images is documented in the literature; discrepancies of up to 58% have been reported between primary care physicians and radiologists [1].

In our hospital, there are eight departments where X-ray equipment is used. One of these is the A&E department, comprising three X-ray rooms where approximately 50 000 standard radiographic examinations out of a total of 400 000 are performed annually.

In addition to outpatient casualties presenting at A&E, the department also performs radiological examinations for inpatients at night and at the weekend, when the other radiology departments in the hospital are closed. It is for these reasons that new technologies may be installed in A&E departments first.

With digital radiography, namely computed radiography (CR) and direct digital radiography (DDR), becoming a viable technology for acquiring X-ray images, departments are looking to replace conventional screen–film radiography (SFR).

Potential advantages of digital systems over conventional radiography are well known [2], i.e. that they have a greater dynamic range, wider exposure latitude, post-processing facilities available, and that there is improved access to images by clinicians and decreased film costs. New flat-panel DDR equipment has recently been installed in the A&E department in our hospital because its characteristics and versatility are well suited to the work undertaken in this environment, e.g. being able to manage taking X-rays of critically ill patients with relative ease.

The aim of this study was to compare radiation doses to patients undergoing standard radiographic examinations using SFR, CR and DDR; entrance skin dose (ESD) and effective dose (E) were calculated for six standard examinations (a total of 10 projections) using standard patient exposure parameters for the three imaging modalities. ESD and E are considered to be efficient and powerful parameters in the protection of patients [3, 4].


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusions
 References
 
In 2003, the SFR system in the A&E department was replaced with a CR system and in May 2004, the X-ray equipment in X-ray Room 2 was replaced with a Siemens Axiom Aristos FX radiography system with a flat panel detector (Siemens, Erlangen, Germany). The Axiom Aristos is a multifunctional system which enables virtually the entire range of radiographic applications to be performed in a single room. A brief description of the X-ray equipment used before and after May 2004 is given in Table 1Go.


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  		Table 1. Description of the X-ray equipment used in the Accident and Emergency Department (before and after May 2004)

 
The SFR system comprised Kodak TMAT G/RA film (Kodak, Rochester, NY) and Trimax Regular screen (nominal speed class 400). The CR system comprises two Kodak CR readers (CR-850 and CR-900) and three sizes of general purpose image plates, i.e. 18 cmx24 cm, 24 cmx30 cm and 35 cmx43 cm. The image plates contain a europium-activated barium fluoro-halide phosphor with 1792x2392, 2048x2500 and 2048x2500 matrix sizes, and 0.100 mm, 0.120 mm and 0.172 mm pixel pitches, respectively.

The DDR system comprises a 0.5 mm thick thallium doped caesium iodide phosphor (CsI:Tl). The readout array, to which the phosphor is coupled, consists of a 43 cmx43 cm amorphous silicon (a-Si) photodiode and a thin film transistor (TFT) array (3000x3000 pixels and 0.143 mm pixel pitch). The X-rays interact with the phosphor and release light photons. The light from the phosphor promotes charge in the photodiode array with the amount of charge being proportional to the intensity of the incoming X-ray photons. Each TFT element is then sequentially addressed so that the charge in the photodiodes is read out and digitized. The detector resolution is limited by pixel pitch; this may be reduced by the effect of light spread in the phosphor layer. The images were finally reported by a consultant radiologist on a Kodak Autorad workstation.

ESD were calculated for six standard radiographic examinations (a total of 10 projections): anteroposterior (AP) Abdomen, posteroanterior (PA) Chest, lateral (LAT) Chest, AP Lumbar Spine, LAT Lumbar Spine, LAT Lumbo-Sacral Joint, AP Pelvis, AP Skull, LAT Skull, AP Urinary Tract.

To calculate E, first we measured the X-ray tube output using a technique previously described [5]. For each X-ray tube, 30 air kerma measurements were made with an ionization chamber (model 90X6-6, connected to a Radiation Monitor Controller model 9010; Radcal Corporation, Monrovia, USA) held in a scatter-free support on the central axis of the X-ray beam (Table 1Go). Instruments are calibrated annually, with the calibration traceable to an SIT (National Calibration Service in Italy) centre.

The Harpen mathematical model [6] was then used to obtain the X-ray tube output for every tube voltage (kVp), milliampere-seconds product (mAs) and focus-to-skin distance (FSD) used in each of the clinical protocols (provided by the radiologist). A backscatter factor of 1.35 was used to calculate the ESD from air kerma, as suggested in the European Guidelines [7]. Finally, E has been calculated from ESD using the NRPB conversion coefficients [8].

In order to correctly apply these factors, it is necessary to know the tube potential used in clinical protocols and the total X-ray tube filtration. The total tube filtration is measured annually as part of a quality assurance programme; acceptance, status and constancy tests are performed by the Medical Physics Department (the department is certified to UNI EN ISO 9001-2000).

Assessment of image quality was undertaken by Consultant Radiologists within A&E; images from the three modalities, i.e. SFR, CR and DDR, were qualitatively evaluated to ensure that the quality criteria for diagnostic radiographic images of the European Guidelines [7] were met.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusions
 References
 
The parameters used for standard radiographic examinations on adult patients are reported in Table 2Go. It should be noted that for some of the examinations performed with the Siemens Axiom Aristos equipment, the FSDs are different to those used with SFR and CR as the system is self-positioning and self-centring; this is a feature of the equipment.


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  		Table 2. Parameters used for standard radiographic examinations using conventional screen–film radiography (SFR), computed radiography (CR) and direct digital radiography (DDR)

 
Manufacturers of digital systems often recommend the use of low tube potentials, but this will lead to less penetrating beams and hence possibly higher doses. It is important, therefore, to ensure that systems are optimized [9]. For Kodak CR systems, the exposure index (EI) is the average raw data pixel value within the anatomical area exposed [10]: an EI ~2000 is indicative of optimum exposure level. EI is directly related to patient dose by the equation EI = 1000xlog10(exposure in mR)+2000 [10]. Although the use of low tube potentials is often recommended, clinically diagnostic images can be achieved using high tube potentials. An example of this is the PA chest radiograph obtained at 125 kVp (Figure 1Go); an EI very close to the value recommended by the manufacturer was obtained, i.e. EI = 1970, and the quality criteria of the European Guidelines [7] were satisfied. With the digital systems we generally used a higher tube potential and lower mAs to reduce the patient dose (Table 2Go). Careful adjustment of these parameters will allow patient doses to be optimized without sacrificing image quality by taking full advantage of the post-processing tools, because higher tube potential can reduce contrast [11].


Figure 1
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  		Figure 1. Posteroanterior(PA) chest radiograph obtained with computed radiography: male, 45 years old, 125 kVp, focus to skin distance (FSD) = 151 cm, Exposure Index = 1970 (the Exposure Index recommended by the manufacturer is 2000). The image satisfies the quality criteria for PA chest radiographs given in the European Guidelines [7].

 
In this study, doses to patients were assessed by calculating ESD for six standard radiographic examinations (a total of 10 projections), and E for complete examinations (GoTables 3 and 4Go).


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  		Table 3. Entrance surface dose(mGy) for standard radiographic examinations using conventional screen–film radiography (SFR), computed radiography (CR) and direct digital radiography (DDR), and percentage differences between the systems

 

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  		Table 4. Effective dose(mSv) for standard radiographic examinations using conventional screen–film radiography (SFR), computed radiography (CR) and direct digital radiography (DDR), and percentage differences between the systems

 
5 months after the installation of the DDR equipment, a survey was undertaken on 175 A&E patients to establish the preferred imaging modality. Approximately 30% of examinations were necessarily performed using the CR imaging modality, e.g. patients being X-rayed using mobile radiographic equipment. Of the remaining patients, approximately 86% were X-rayed using the DDR equipment.


   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusions
 References
 
Dose comparisons between CR, SFR and DDR have been reported for chest radiography [12, 13]; the results of this study confirm the findings of the two reported studies, i.e. that CR generally results in higher ESDs than those in SFR and DDR. Comparison between DDR and SFR doses shows that, in general, DDR results in lower ESDs than those in SFR; the AP Skull projection is the only one where the ESD for DDR is higher than that for SFR. A possible reason for this is that AP Skull is not performed by high tube potential technique with DDR. Table 2Go shows that AP Skull and Abdomen are the only examinations where both tube potential and FSD for DDR are lower than tube potential and FSD for SFR, respectively: the effect of these two factors is an overall decrease in the mAs value for DDR equipment compared with SFR system. This reduction in mAs is greater for Abdomen than for AP Skull: as a consequence, the AP Skull is the only projection where ESD for DDR is higher than ESD for SFR. However, E for complete Skull examination for DDR is still lower than that for SFR.

Table 4Go shows that for complete examinations, the values of E for DDR are approximately 29% and 43% lower than those for SFR and CR, respectively. E has been calculated from ESD using the NRPB conversion coefficients [8]; the coefficients increase with applied tube potential and to a lesser extent with tube filtration.

The magnitude of any dose saving that can be achieved with different imaging modalities is controversial [13]: some authors found a dose reduction with CR systems of approximately 50% compared with a conventional 200-speed SFR system [14], whilst others have reported the same values of ESD and E for the LAT Lumbar Spine radiograph when a CR system replaced a 300-speed SFR system [15]. Other authors have reported a dose increase in CR systems of 33–58% compared with a 400-speed SFR system [16]. The findings of this study are therefore neither completely unexpected nor in contradiction with those of other trials. It would seem reasonable to state that doses with the use of CR systems are approximately the same as those for a 300-speed SFR system, but it has been also reported that the speed class quoted by manufacturers does not predict the actual relative speeds of the SFR systems [17] and there are wide differences in image quality for similar speed systems [18]: therefore the importance of optimization, whichever system is being used, must be stressed. The CR system used in this study has been optimized together with the manufacturer engineers: nevertheless, a full optimization may take a long time because it is a dynamic process where radiologists, physicists, radiographers and manufacturers are involved day after day in the achievement of continuous improvement.

On the whole, CR increases doses for a single exposure, but the larger dynamic range reduces the number of examinations that have to be repeated [19, 20], e.g. for stretcher patients or for patients in intensive care where the FSD often cannot be standardized.

However, the increase in CR patient doses found in this study should be put into perspective, e.g. chest examinations. In this example, the additional E per complete examination (1 PA projection + 1 Lateral projection) using CR was approximately 0.01 mSv and 0.02 mSv compared with SFR and DDR, respectively.

The risk of developing a fatal or other cancer, or other serious defect (including hereditary effects), during the course of life for the population undergoing chest examinations in the A&E Department (about 20 000 examinations/year) is the same as for the whole population, i.e. 7.3% per Sievert [3]. Thus, nearly 1 million patients would each have to undergo complete chest examinations with the use of a CR system in the A&E Department to produce 1 and 2 additional health defects in this population compared with SFR and DDR systems, respectively. Moreover, the probability of a fatal cancer being induced in an individual patient is dependent on the age of the patient: e.g. for patients exposed after the age of 70 years, the risk can be assumed to be reduced to one-third of the average risk as the patient is likely to die before the cancer develops [21]. Similar calculations could be made for all other examinations.

Potential advantages of digital systems over conventional radiography are well known [2], i.e. that they have a greater dynamic range, wider exposure latitude, post-processing facilities available, and that there is improved access to images by clinicians and decreased film costs. The post-processing tool is a very important and useful characteristic provided that the individual performing this task is adequately trained [22]. The DDR equipment has been shown to have the following advantages:

  1. the radiographer can stay close to the patient throughout the examination;
  2. the equipment is self-positioning and self-centring around the patient, and this characteristic is very useful for stretcher patients;
  3. the image acquisition times are much shorter than in SFR or CR;
  4. an increased patient throughput (a further survey showed that out of a total of 175 patients, 60% of examinations are now performed using DDR);
  5. the rapid availability of images for timely clinical decision-making, which may be vitally important for severely injured patients.

When the workload is heavy, the patient throughput can be maximized by using the DDR equipment for chest, abdomen, pelvis, spine and skull examinations, and by using CR for extremities.

The image quality characteristics of flat-panel technology are theoretically better than those of SFR and CR because at a comparable resolution the DDR has a higher detective quantum efficiency (DQE) [23]; this advantage can be utilized in reducing the patient dose whilst maintaining image quality and without changing the signal-to-noise ratio. The appropriateness of this approach would, however, require verification. All images met the criteria in the European Guidelines for all modalities and were used for reporting by the Consultant Radiologists. Figure 2Go shows chest images obtained using CR and DDR. It shows that both systems satisfied the quality criteria for diagnostic radiographic images [7]: full inspiration; symmetrical reproduction of the thorax; scapulae outside the lung field; reproduction of the whole rib cage above the diaphragm; visualization of the retrocardiac lung and the mediastinum; sharp reproduction of the trachea and proximal bronchi, the borders of the heart and aorta; visualization of the spine through the heart shadow. Finally, the radiologists prefer the appearance of the DDR images.


Figure 2
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  		Figure 2. Posteroanterior(PA) chest radiographs obtained with computed radiography and direct digital radiography. (a) Direct digital radiography: female, 47 years old, 125 kVp, focus to skin distance (FSD) = 155 cm. (b) Computed radiography: male, 28 years old, 125 kVp, FSD = 151 cm. Both images satisfy the quality criteria for PA chest radiographs given in the European Guidelines [7].

 

   Conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
References
 
This study compared radiation doses to patients undergoing standard radiographic examinations using SFR, CR and DDR; ESD and E were calculated using standard exposure parameters. Effective doses for DDR were found to be ~29% and ~43% lower than those for SFR and CR, respectively. The image quality met the criteria in the European Guidelines for all modalities. Even though in the A&E department most examinations are now performed with the DDR system, we are aware that the optimization effort has to also continue for the CR equipment, because digital systems need to be optimized very carefully by a dynamic process that may take a long time. Conventional technologies are soon to be replaced by digital ones and although DDR equipment has been shown to have many advantages, further work is required to specifically look at image quality and dose for real patients.


   Acknowledgments
 
We express our thanks to the Reviewers for their valuable advice regarding this study.

Received for publication May 6, 2005. Revision received March 13, 2006. Accepted for publication April 12, 2006.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 

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  9. Peters SE, Brennan PC. Digital radiography: are the manufacturers' settings too high? Optimisation of the Kodak digital radiography system with aid of the computed radiography dose index. Eur Radiol 2002;12:2381–7.[Medline]
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