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British Journal of Radiology (2003) 76, 398-405
© 2003 British Institute of Radiology
doi: 10.1259/bjr/76222078
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Full Paper

Survey of posteroanterior chest radiography in The Netherlands: patient dose and image quality

R T M van Soldt1, D Zweers2, L van den Berg1, J Geleijns2, J Th M Jansen1 and J Zoetelief1

1 Interfaculty Reactor Institute Delft University of Technology, Mekelweg 15, 2629 JB Delft and 2 Department of Radiology, Leiden University Medical Centre, Albinusdreef 2, 2333 ZA Leiden, The Netherlands

Correspondence: J Zoetelief


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion and conclusions
 References
 
Council Directive 97/43/Euratom (Medical Exposure Directive) states that member States of the European Union shall promote the establishment and use of diagnostic reference levels for radio-diagnostic examinations. Dose surveys can form the basis for the establishment of diagnostic reference levels. In view of the implementation of the Medical Exposure Directive in the Netherlands, a survey of dose and image quality has been performed for posteroanterior (PA) chest radiography in 2001. In this survey, 25 participants were selected from a list of 175 Dutch hospitals, whereas in a previous PA chest survey (about 10 years ago) participation was voluntary and participants came predominantly from the south-western part of the Netherlands. For conventional screen–film PA chest radiography, the present results for patient dose and image quality are quite similar to those results from the previous survey. The fraction of conventional X-ray systems utilizing lung compensation filters has remained approximately the same. For dedicated digital chest radiography systems, image quality is better than for conventional systems, but doses vary and can assume relatively high values. The results indicate that there are still possibilities for dose reduction, without loss of image quality. The 75 percentile value of the entrance surface dose distribution is approximately 0.13 mGy.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion and conclusions
 References
 
Council Directive 97/43/Euratom (Medical Exposure Directive) [1] states that member States of the European Union shall promote the establishment and use of diagnostic reference levels for radio-diagnostic examinations and the availability of guidance for this purpose, having regard to European diagnostic reference levels where available. Dose surveys can form the basis for the establishment of diagnostic reference levels. For diagnostic radiology, the International Commission on Radiological Protection (ICRP) [2] states that diagnostic reference levels are a form of investigation level, which apply to an easily measured quantity, e.g. the absorbed dose in air at the surface of a simple standard phantom. In practice, diagnostic reference levels can initially be selected as a percentile point on the observed distributions of doses to patients. In addition, the ICRP [2] recommends that the values should be selected by professional medical bodies, be reviewed at suitable intervals and be specific to a country or region.

In the past decade, a number of surveys have been performed in the Netherlands to determine patient doses for various types of examination. These surveys comprised examinations of chest [3, 4], breast [5], gastrointestinal tract [6, 7], CT [8] and non-cardiovascular radiology procedures [9]. In general, during these studies, measurements were made at X-ray units in different hospitals, which participated voluntarily. The majority of these studies aimed at the determination of effective dose, as this quantity is assumed to be most directly related to the risks to patients due to exposure to ionizing radiation. In some of the surveys, image quality was also assessed.

In view of the implementation of the Medical Exposure Directive [1] in the Netherlands, a survey of dose and image quality was conducted for posteroanterior (PA) chest radiography in 2001. In the present survey, 25 participants were selected from a list of 175 Dutch hospitals, ranked by increasing zip code, by taking each seventh hospital. All selected departments of radiology agreed to participate. In the previous survey [3, 4], participation was voluntary, which may have had some influence on the results. The survey was carried out approximately 10 years ago, when only screen–film systems were in use. It was expected that there would be more digital systems in use nowadays. A comparison of the present examination conditions and doses with the previous results might therefore indicate progress achieved since the early 1990s with regard to patient dose and image quality.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion and conclusions
 References
 
A LucAl chest phantom [10] was used for simulation of a standard patient at the various X-ray units, both for dose determination and for assessment of image quality. The phantom is relatively lightweight and made from polymethylmethacrylate (PMMA) and aluminium plates. It simulates the attenuation and scatter characteristics in the lung-field region of an average patient, for X-ray spectra typically used in chest radiography. The same type of phantom was used in the previous PA chest survey in the Netherlands [3, 4]. The exposure of the phantom at the various X-ray units was made according to the local procedures applied for actual patient examinations, including tube voltage setting, applied filtration (use of additional lung filters where applicable), focus-to-film distance (FFD) or source-to-image receptor distance (SID) employed, focal spot size selected and automatic exposure control (AEC) settings. AEC was used on all X-ray units. A radiograph of the phantom was made to enable determination of the average optical density using a Macbeth 931 densitometer (Kollmorgen Corp., Newburgh, New York, USA). For assessment of image quality, one of the PMMA plates was replaced by a contrast-detail phantom [11] made out of PMMA of the same thickness.

Measurements of the entrance and exit surface air kerma (without backscatter) were made with an R100 solid state detector connected to a PMX-III readout system (RTI Electronics AB, Mölndal, Sweden). The measurement results were stored on a laptop computer employing oRTIGo PMX software (RTI Electronics AB, Mölndal, Sweden). Due to its lead backing, the detector is insensitive to backscattered radiation. The energy dependence in the radiation quality range (tube voltage: 50 kV to 150 kV, total filtration: 2.2 mm Al to 8.0 mm Al) is specified by the manufacturer to be within ±5%. Air kerma measurements with the R100 detector connected to the PMX-III system were compared with those obtained by using a calibrated (combined standard uncertainty of 1% at 68% confidence level) Farmer ionization chamber (NE2571, Nuclear Enterprises, Reading, UK) connected to a Keithley 617 electrometer (Keithley Instruments Inc., Cleveland, Ohio, USA). For a total filtration of 3 mm Al and tube voltage in the range 50 kV to 150 kV (half value layer (HVL) range 2.0–5.6 mm Al), a maximum air kerma ratio of 1.06±0.02 was found between results from the two systems, with the values obtained from the R100/PMX-III combination being the lower. For HVLs ranging from 3.9 mm Al to 13.9 mm Al, obtained from different tube voltage and filter combinations, the air kerma ratio ranged from 0.95±0.02 (tube voltage of 50 kV with a total filtration of 13 mm Al and an HVL of 3.9 mm Al) to 1.11±0.02 (tube voltage of 150 kV and a total filtration of 50.5 mm Al and on HVL of 13.9 mm Al). The results of a European intercomparison of dosimeters used in diagnostic radiology [12] have suggested that measurements with good accuracy are equivalent to deviations from the correct value by less than 10%. The PMX-III combination fulfils this requirement.

The angular dependence of the response of the R100 detector has been investigated at tube voltages of 50 kV and 150 kV (and total filtration of 3.0 mm Al), where an angle of 0 degrees corresponds to normal incidence. Relative to normal incidence, values of 0.90, 0.81, 0.58 and 0.18 (tube voltage: 50 kV) and 0.95, 0.88, 0.65 and 0.28 (tube voltage: 150 kV) were found for angles of 15, 30, 45 and 60 degrees, respectively. For entrance dose measurements, the uncertainties due to the angular dependence of the response of the detector will be negligible at the tube voltages used in chest radiography, when the R100 detector is properly positioned. For exit dose measurements, the underestimation of the air kerma measurement using the PMX-III combination has been calculated to be a factor of approximately 1.5 at 150 kV, using a radiation transport code and taking into account the angular dependence of the response of the R100 detector.

For the Advanced Multiple Beam Equalization Radiography (AMBER) system (Delft Instruments N.V., Delft, The Netherlands), the R100/PMX-III system cannot be used for dose measurements since the lead backing of the detector would disturb the system too much. In this case an ionization chamber (Physikalisch Technische Werkstatte, Freiburg, type N 233612) connected to an electrometer was used for determination of entrance and exit air kerma measurements.

The PMX-III has also been used for measurement of peak tube voltage and HVL. In the tube voltage range from 45 kV to 155 kV (anode/filter combination: W/Al) the manufacturer states an accuracy of ±2%, a precision of 1% and a resolution of 0.1 kV, in low frequency mode. For measurement of the HVL, a comparison has been made between the results from the PMX-III and those from HVL measurements according to ICRU [13] for tube voltages in the range 50 kV to 125 kV. The maximum difference found was a few per cent at relatively low filtration. In principle, the PMX-III is not capable of obtaining correct HVL values when additional copper filters are applied. A comparison of the uncorrected PMX-III HVL value with the results obtained according to the ICRU method at a tube voltage of 125 kV with an additional filtration of 1 mm Cu showed only a difference of less than 1%. Values of the exposure time were read from the generator control.

Calculations of effective dose were made using the Monte Carlo code PCXMC [14] on the basis of the measured entrance air kerma (excluding backscatter) and the exposure conditions (radiation quality, field size, projection, FFD or SID). The calculations were made for an adult hermaphrodite phantom. In the case of the application of a lung filter, the lung dose was derived for the radiation quality and entrance air kerma in the lung region, whereas for the organs outside the lung region the entrance air kerma and the radiation quality outside the lung region were taken. The organ doses thus obtained were combined to obtain effective dose.

For the AMBER system, PCXMC cannot be used due to the complex distribution of entrance air kerma for this technique [15]. From the entrance air kerma area product of 83±23 mGy cm2 measured previously for a series of 126 patients during PA chest examinations [15], an entrance air kerma of 55±15 µGy can be estimated. The entrance air kerma found in the present survey is 45.8 µGy. Although the entrance air kerma values are similar, the effective dose of 50 µSv measured previously for the AMBER system in the patient study [15] has to be corrected for the current survey for two reasons: (1) the field size used in the patient study was too large for the LucAl phantom; and (2) the corrections to effective dose for size of the phantom compared with patient dimensions required modification. For AMBER, the correction for air kerma area product was previously 1.7 [15], which, combined with the correction for the relatively large field size for the phantom, results in a total correction factor of 2.6, thus yielding an effective dose of about 20 µSv for AMBER PA chest radiography for patients.

For the conventional system, however, an effective dose of 30 µSv [15] had already been estimated corresponding to an entrance air kerma of 30 µGy, whereas in the previous PA chest survey an effective dose of 8 µSv was estimated corresponding to an entrance air kerma of 27 µGy [4]. The difference by a factor of three can be explained as follows. To correct for the difference in dimensions between the LucAl phantom and the patients, the effective dose has been multiplied by a factor of two, which was obtained as the ratio of the air kerma area products for patients to that for the phantom. For paediatric PA chest radiography an approximately linear increase in air kerma area product was observed as a function of age, whereas the effective dose was approximately constant [16]. The remaining difference by a factor of approximately 1.5 can be quantitatively explained by the observation that doses to organs in the vicinity of the lung, e.g. stomach, spleen and oesophagus are relatively high [4] compared with organ dose conversion factors calculated for PA chest radiography [16]. This is probably because the same field size was used for the phantom and for the patients.

For assessment of image quality a contrast-detail (CD) phantom [11] has been used. Detailed information on the assessment of image quality from radiographs of the CD phantom scored by human observers has been published previously [18]. The CD phantom is a 1 cm thick PMMA plate, with a matrix structure, which replaces one of the central PMMA plates of the LucAl chest phantom. A matrix element of the CD phantom is a square with sides of 1.5 cm, containing two cylindrical holes of the same diameter and depth (but only one hole for the three largest diameters). One cylinder is located at the centre and the second one at any of the four corners of the element. Along one axis of the matrix the depth of the cylinders varies and along the other axis their diameter varies, both in 15 logarithmic steps from 0.3 mm to 8.0 mm. Radiographs of the CD phantom were examined by three observers. They indicated separately for each of 15 diameters (d) the depth (h) of the pair of test spots just visible. As a measure of image quality, the parameters IQF [11] and K [18] were used. The Image Quality Factor (IQF) is defined as: Go


where:

hi is the depth of the ith cylinder

di is the diameter of the ith cylinder spot just visible.

K is defined as: Go


IQF and K were calculated for n=15 in this study. For digital systems a hard copy on film was used for assessment of images of the CD phantom. It should be noted that the hard copy was made according to the local procedures of the department of radiology and the quality of the hard copy has not been further optimized.

The linear fits in the figures were obtained from the least squares method and uncertainties in the slopes of the lines refer to one standard deviation. For a comparison of the present and the previous [3, 4] PA chest survey, mean values and standard errors of the mean are given for a number of parameters. The significance of differences between the two surveys is determined at the 95% confidence level.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion and conclusions
 References
 
The participants in the survey and an indication of their type of chest X-ray unit are given in Table 1Go. The sequence in the table is different from the code used for presentation of the results. There is a good geographical spread of participants and types of hospitals over the Netherlands. In the present survey, 7 out of the 25 participants used digital systems for PA chest radiography, five of which were systems dedicated to chest radiography. No digital systems were included in the previous PA chest survey [4]. The fraction of conventional screen–film systems utilizing lung filters amounts to 25%. This is quite similar to the previous study, approximately 10 years ago [4]. The nominal screen–film speeds were of class 400.


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  		Table 1. Participants in the 2001 Dutch posteroanterior (PA) chest survey listed alphabetically by city, giving the hospital names and cities and an indication of the type of chest X-ray system

 
Measurement results and recorded data from the survey are given in Table 2Go. Effective dose and image quality parameters are given in Table 3Go. The measured tube voltage ranged from 118.3 kV to 154.4 kV, whereas the tube voltage settings varied from 117 kV to 150 kV. On average the measured tube voltage is slightly higher (1.5%) than the tube voltage selected, but remains within the accuracy stated by the manufacturer of the PMX-III, except for two situations. For participant I, the ratio of tube voltage measured to tube voltage selected is 1.045. For participant X, the ratio of measured tube voltage to tube voltage set is 1.06±0.02, which is somewhat larger than expected. In all cases the accuracy of the tube voltage fulfils the European requirement [19], i.e. a maximum deviation of 10% and the Dutch requirement [20], i.e. a maximum deviation of 5%, taking the measurement uncertainty into account.


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  		Table 2. Measurement results and data recorded in the 2001 Dutch posteroanterior (PA) chest survey. Ka,entrance is the phantom entrance air kerma (without backscatter) and Ka,exit the exit air kerma (without backscatter)

 

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  		Table 3. Effective dose and image quality in the 2001 Dutch posteroanterior (PA) chest survey

 
A comparison of the current and the previous surveys is given in Table 4Go. The average tube voltage presently measured is relatively high and slightly increased compared with the previous PA chest survey (Table 4Go). Additional copper filtration is commonly applied (Table 2Go) as was the case during the previous survey [4]. The first HVLs did not differ significantly. Except for two cases, the SID was 200 cm (Table 2Go). This is similar to the SIDs in the previous survey. The mean exposure time is reduced significantly. The present exposure times (Table 2Go) fulfil the European recommendations (less than 20 ms) given in the example of good radiographic technique for PA chest radiography [21]. It should be borne in mind, however, that the present values are recorded from the generator control unit, whereas the exposure times during the previous survey were measured. The results for entrance air kerma from the present and previous surveys are not significantly different. The average film density found in the present study is significantly larger than that measured in the previous survey (Table 4Go). The effective doses (Table 4Go) from the present and previous surveys are not significantly different when the result for the screen–film system of speed class 200 was omitted from the previous survey. Mean image quality, expressed as IQF, is not significantly different in the current survey compared with the previous one.


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  		Table 4. Comparison of parameters and results from the 2001 survey with those from the early 1990s survey [3, 4]

 

   Discussion and conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion and conclusions
References
 
The image quality in terms of K is shown in Figure 1Go as a function of the inverse of the effective dose, as a linear dependence is expected theoretically [18]. A distinction is made between conventional screen–film systems, conventional screen–film systems where a lung filter is applied, AMBER, digital systems for general use and digital systems dedicated to chest radiography (Thoravision, Philips Medical Systems, Best, The Netherlands). The dose values shown for the systems for which a lung filter is applied correspond to an incorrectly low HVL since the HVLs were determined in absence of the lung filter. Linear fits are shown for the conventional screen–film systems and for the five Thoravision systems.



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  		Figure 1. Image quality parameter K as a function of the inverse of effective dose. The fits shown refer only to the conventional systems or only the Thoravision systems. The slopes and standard deviations are 4.3±2.4 and 1.7±4.4, for conventional and Thoravision systems, respectively, indicating that the slope is not significantly different from zero at the 95% confidence level.

 
For the conventional screen–film systems, the image quality parameter K is independent of effective dose. Differences in effective dose are expected as the exit doses vary from 9.2 µGy to 24.5 µGy. It can be concluded from Table 2Go that there is no clear correlation between film density and exit dose. The two highest film densities are found for screen–film systems with relatively low exit doses. Antiscatter grids were used with X-ray units, but the data on grids are incomplete since they were not always accessible. The antiscatter grids used by the different systems are quite similar. It is, therefore, most likely that differences in exit doses are due to differences in film processing. It should be borne in mind, however, that the range of effective doses (a factor of about three) is relatively small compared with the results from previous surveys. The average effective dose found in the present survey for the conventional screen–film systems was 14.5 µSv which is somewhat lower than the value of 17.5 µSv found in the previous survey [4]. When, however, the largest dose value from the previous survey is omitted the average effective dose becomes 14.6 µSv, which is almost identical to the present value.

For the conventional screen–film systems, tube voltage increased on average compared with the previous survey. The resulting first HVL, however, remained the same, i.e. on average 7.4 mm Al. The increase in average film density between the present and previous surveys result in neither a significant increase in effective dose nor in an improved image quality as might have been expected.

For the screen–film systems where lung filters are applied and the AMBER system, relatively high doses are found, as a result of an increased exposure in chest regions with a larger attenuation, such as the mediastinum and the cardiac region. The image quality parameter derived for the AMBER system is correct for the lung region. For the systems applying lung filters the image quality was determined in absence of the lung filter, i.e. for too low an HVL. The image quality parameter as a function of HVL is shown for screen–film systems in Figure 2Go. The dependence of K on HVL is not significant. Consequently, the influence of the presence or absence of the lung filter on image quality will be marginal for the lung region. The improvement in image quality for the regions outside the lung cannot be judged from the present measurements. For the AMBER system it has been shown that the image quality in the regions with relatively low transmission, i.e. in front of the retrocardiac, subdiaphragmatic and mediastinal regions, is improved [15].



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  		Figure 2. Image quality parameter K as a function of the half value layer (HVL) for conventional systems. The slope of the fit is not significantly different from zero (0.006±0.044).

 
The five Thoravision systems exhibit a similar image quality but a difference in effective dose by a factor of approximately 3 (Figure 1Go, Table 3Go). The lower two effective doses are close to the average effective dose for the conventional screen–film systems. The three remaining Thoravision systems deliver the highest effective doses found in the present survey. The recorded filtration was slightly different (in three cases 0.2 mm Cu filtration and in two cases 0.1 mm Cu) resulting in slightly different first HVLs (about 9.0 mm Al in the case of 0.2 mm Cu and about 7.7 mm Al in the case of 0.1 mm Cu). This difference in HVL cannot explain the differences in effective dose (Figure 3Go). The ratios between the highest and lowest doses (approximately the same HVL) are the same for effective, entrance and exit doses. Apparently, the dose differences are mainly due to difference in threshold setting of the image receptor systems, which does not significantly influence image quality. Dose reduction is recommended for three of the five systems.



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  		Figure 3. Effective dose as a function of half value layer (HVL). The fit refers to the conventional systems, including those with lung filters. The slope is significantly different from zero at the 95% confidence level (slope -1.9±0.6).

 
The use of a contrast-detail phantom for the assessment of image quality has acknowledged limitations [22]. However, the present finding of lower K values (better image quality) with the Thoravision systems is in agreement with other published data [23–27]. These studies used a variety of methods to assess image quality ranging from judgement of images by radiologists to physical techniques.

Two digital general-purpose phosphor plate systems were employed for PA chest radiography in the present survey. The effective doses measured for these two phosphor plate systems are within the range measured for the conventional systems (Figure 1Go and Table 3Go), but between one and two standard deviations higher than the mean value. The image quality might be reduced due partly to the fact that the hard copies on film had quite low film densities, i.e. 0.90 and 0.97 optical density (OD), compared with an average of 1.66 OD found for conventional screen–film systems. It should be stressed that the hard copies were made according to the local procedures and were not optimized. For one of the systems, image quality was similar to that of conventional systems. For the second system image quality was rather poor. From a study of modern chest X-ray units [29], it was concluded that phosphor plate systems of generation IIIN show image quality similar to that of a screen–film system and phosphor plate systems of generation I show an image quality that is worse compared with a screen–film system. These findings are qualitatively in agreement with the present results.

A comparison can also be made with the European criterion for patient dose in PA chest radiography [21], i.e. an entrance surface dose (including backscatter) of 0.3 mGy. The entrance air kerma data in Table 2Go do not include backscatter. Backscatter factors of cuboid (surface: 30 cm x 30 cm and depth 15 cm) phantoms of various materials, including PMMA, positioned at a focus-to-surface distance (FSD) of 100 cm have been calculated for various X-ray spectra [28]. Calculations of backscatter factors have also been made for PA chest examinations of a male anthropomorphic phantom at FSDs of 150 cm and 177.5 cm for a number of X-ray spectra [17]. For first HVLs of approximately 8 mm Al, the backscatter factor for the cuboid PMMA phantom at a field size of 25 cm x 25 cm is 1.67, whereas for the anthropomorphic phantom at a field size of 35 cm x 40 cm the backscatter factor is 1.47. This difference can be expected due to the lower density of the lungs in the anthropomorphic phantom. On the assumption that backscatter factors are approximately 1.5, the highest entrance surface dose (including backscatter) is 0.21 mGy, which is well below the value of 0.3 mGy. The 75th percentile value of the entrance surface dose distribution, which has been proposed as method for the establishment of a reference level [2], is approximately 0.13 mGy for the present survey.


   Acknowledgments
 
The co-operation of the participants in the survey is gratefully appreciated.


   Footnotes
 
Current address for L van den Berg, University Hospital Rotterdam, Dr Molewaterplein 40, 3015 GD Rotterdam, The Netherlands. Back

The Dutch Ministry of Health, Welfare and Sports has financially supported the work described here (obligation number 26213897). Back

Received for publication October 31, 2002. Revision received February 20, 2003. Accepted for publication April 10, 2003.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion and conclusions
 References
 

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