British Journal of Radiology (2004) 77, 116-122
© 2004 British Institute of Radiology
doi: 10.1259/bjr/23591655
Barium enema: use of increased copper filtration to optimize dose and image quality
R E Morrell, MSc, Dip IPEM
1
A T Rogers, MSc, MIPEM
1
J C Jobling, FRCS, FRCR
2 and
K E Shakespeare, DCR(R)
2
Departments of 1 Medical Physics and 2 Radiology, Nottingham City Hospital, Hucknall Road, Nottingham NG5 1PB, UK
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Abstract
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The purpose of this study was to determine and validate the optimum copper filtration for adult double contrast barium enema examinations. Entrance surface dose rates to polymethyl methacrylate slabs and corresponding image intensifier input kermas, were measured for various added copper filters. Image contrast was assessed using a Leeds TO.10 test object. Copper filter thickness of 0.3 mm was chosen, as this reduced entrance surface dose rate by 56%, without substantially degrading image contrast due to kV and mA saturation. 20 sets of clinical films taken with each of 0.3 mm copper, 0.1 mm copper and no copper were reviewed following randomization, by a specialist gastrointestinal radiologist. Each set of digital spot and conventional films was allocated a score for each of three regions of the bowel, on a scale of 0–3 for perceived barium coating. The Kruskal–Wallis test showed no significant difference in perceived coating between the three groups (Digital spot: sigmoid colon p=0.207, splenic flexure p=0.103, hepatic flexure p=0.894. Screen–film: left colon p=0.803, right colon p=0.487, transverse colon p=0.905). All examinations but one were classified as diagnostic. The remaining one was classified indeterminate, due to poor distension of the colon. On adding 0.3 mm copper filtration, the mean dose–area product per examination was reduced by 57%, from 17.7 Gy cm2 to 7.6 Gy cm2. The estimated reduction in effective dose was 11%, from 3.0 mSv to 2.7 mSv. X-ray tube loading increased by 30%, but this caused no overheating with our local examination protocol and schedule. Additional filtration of 0.3 mm copper for adult double contrast barium enemas has now been implemented in routine clinical use at our hospital.
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Introduction
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The last two decades have seen a growing emphasis on radiation protection, across all areas of radiology. Reductions in radiation dose to patients and staff have been achieved through advances in technology and changes in clinical practice, supported by training guidelines which place a strong accent on radiation safety. Recent UK legislation has made the optimization of patient dose and image quality a legal requirement [1]. Here, the aim is not just to reduce dose, but to achieve the lowest dose that produces images of adequate quality to fulfil the intended purpose of the exposure.
Procedures involving fluoroscopy are a priority area for optimization, due to the relatively high doses that are involved. The National Radiological Protection Board (NRPB) estimate that barium enemas account for 13% of the UK collective dose from medical X-rays [2]. Their national dose survey in 1985 [3] and its subsequent reviews in 1995 [4] and 2000 [5] found dose–area products (DAPs) for enemas to be substantially greater than for other barium examinations. In addition, the potential introduction of colon cancer screening in the UK could increase the number of patients being referred for enema.
A number of operator-based dose reduction techniques have been put into practice and reported in the literature. These include reduction in fluoroscopy time [6], tube current [7] or number of radiographs [8], and selective use of the anti-scatter grid [9]. Each of these methods is associated with a reduction in the number or quality of the images available to operator and/or reporting clinician.
An additional approach to dose optimization is to alter the beam quality by means of filtration. Copper is a particularly suitable filter material, since it has a higher atomic number than aluminium, and hence a greater photoelectric absorption efficiency, but requires lower tube loading than heavy metal filters such as erbium, holmium, samarium and ytterbium [10, 11]. In addition, it is physically and chemically stable, widely available and inexpensive.
The authors have previously demonstrated that a 0.1 mm copper filter may be used throughout the double contrast barium enema examination, without any appreciable loss in technical or clinical image quality [12]. This filter reduced the mean DAP per examination by 37%. However, the requirement for optimization in IR(ME)R 2000 [1] prompts the question, "How much copper should be added?"
A small number of reported studies have used copper filtration for barium enemas. Geleijns et al [13] recommended that "additional filtration of at least 0.1 mm copper be applied" for double contrast colon examinations, whilst Kohn et al [14] considered 0.2 mm copper to give acceptable image quality for barium examinations with peak tube voltages between about 105 kV and 120 kV. Although the maximum published filtration for adult enemas is 0.2 mm copper, up to 0.3 mm has been successfully used by a group in Sweden for paediatric patients [15].
The current study sought to determine the optimum thickness of copper filter for adult enemas, i.e. the maximum amount of copper that could be used without losing diagnostic information from the images, or overloading the X-ray tube. Phantom measurements were used to predict the effect on radiation dose, image quality and tube loading of a range of copper filter thickness. The "optimal" amount of copper was chosen, based on these results, and its appropriateness was validated clinically. The reduction in DAP on adding this filtration was measured, and an estimate of the reduction in effective dose was made.
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Choice of filter
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Method
All measurements were carried out in a dedicated room, the main components of radiographic equipment being a Super 80 CP generator, with a Scopomatic 66 image intensifier, an SRO 25 50 undercouch tube and an SRO 33 100 overcouch tube (Philips Medical Systems, Best, The Netherlands). The total filtration provided by each X-ray tube and its housing was 2.5 mm aluminium equivalent at 80 kV. Each tube was fitted with a networked VacuDAP 2003 DAP meter (VACUTEC, Dresden, Germany), which provided a further 0.2 mm aluminium equivalent filtration. All equipment was subject to monthly and annual quality control checks, and the DAP meter calibration was adjusted to within 5% at 80 kV, throughout the study.
Radiation dose measurements were performed for a range of copper filters, and phantoms consisting of 34 cm by 34 cm polymethyl methacrylate (PMMA) slabs. Suitable phantom thickness was determined from previous height and weight records for 533 patients attending for barium enemas. An "equivalent diameter" was calculated for each patient, based on a cylinder of density 1 g cm-3, so that 
The distribution of patient equivalent diameters is shown in Figure 1. A phantom thickness of 24 cm was chosen to represent patients of median diameter, and 30 cm to represent the largest patients.
Phantom entrance surface dose rate and image intensifier input kerma were measured using two 60 cm3 ionization chambers (Radcal Corporation, Monrovia, CA). For the undercouch exposures, the first ionization chamber was placed directly on the couch, i.e. 53 cm from the focal spot, and the second was taped to the underside of the cassette carriage. The PMMA was supported 2 cm above the couch, and the explorator was positioned 5 cm above the PMMA. For the overcouch exposures, a single ionization chamber was positioned on top of the PMMA. The focus to PMMA exit surface distance was 100 cm.
Fluoroscopic image contrast was assessed using a Leeds TO.10 test object (Leeds Test Objects Limited, Boroughbridge, UK), with a range of copper filter and PMMA thicknesses. The test object was positioned in the cassette carriage, and the explorator kept at its maximum height. Live fluoroscopic images were scored by two independent observers, who were blinded to the amount of beam attenuation. In order to provide a single index of overall image quality, the number of visible contrast objects was summed over all the rows in the phantom.
The maximum tube heating rate was calculated assuming a full schedule of 12 patients per day, booked according to our standard system. Each day is divided into three sessions, starting at 09:00, 11:00 and 14:00. A maximum of four patients are booked in each session, at 15 min intervals. Table 1 shows the typical examination protocol. It is common for additional fluoroscopy and/or digital spot images to be required, but six screen–film radiographs are considered sufficient in the majority of cases.
In order to make a conservative estimate of tube heating, it was assumed that each examination consisted of 2 min of fluoroscopy, followed by 5 digital spot images taken with the undercouch tube, and 6 screen–film radiographs taken with the overcouch tube. The kV and mA for each component of the examination were taken from automatically controlled exposures of a 24 cm PMMA phantom, made with and without 0.3 mm added copper filtration. Tube and anode heat capacities and cooling rates were taken from the manufacturer's specifications.
Results
Figures 2 and 3 show the automatic kV selection for fluoroscopic exposures made with a 24 cm PMMA phantom and varying amounts of copper filtration, at the two field sizes used in clinical practice. The kV steadily increased with increasing filtration, but did not reach its maximum value of 110 kV.
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Figure 2. Automatic kV selection for fluoroscopic exposure of a 24 cm polymethyl methacrylate phantom (14 inch field).
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Figure 3. Automatic kV selection for fluoroscopic exposure of a 24 cm polymethyl methacrylate phantom (10 inch field).
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Figures 4 and 5 show the corresponding dose rates at the entrance surface of the phantom. The first 0.1 mm copper reduced the entrance dose rate by 34% for both field sizes, and each additional 0.1 mm gave a sequentially smaller dose reduction. Similar dose reductions were also obtained for digital spot and conventional radiographic exposures.
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Figure 4. Entrance surface dose rate for fluoroscopic exposure of a 24 cm polymethyl methacrylate phantom (14 inch field).
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Figure 5. Entrance surface dose rate for fluoroscopic exposure of a 24 cm polymethyl methacrylate phantom (10 inch field).
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When thicker phantoms were imaged, the kV and mA saturated as copper was added, resulting in a reduction in input kerma at the image intensifier input face. For a 30 cm phantom, the input kerma fell by 16% when 0.2 mm copper filtration was used, and 23% when 0.3 mm was used.
Table 2 shows the thickness of PMMA at which saturation occurred, for each copper filter, at the 10 inch field size. Since the equivalent diameters of the patient sample were normally distributed, the percentage of patients affected by this "factor saturation" effect was estimated from the area under the normal distribution. Factor saturation occurred at greater PMMA thickness on the 14 inch field size, since a lower image intensifier input dose was required. Patient images taken on the 14 inch field size were therefore less likely to be affected.
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Table 2. Polymethyl methacrylate (PMMA) thickness at which saturation occurred on a 10 inch field, and percentage of patients likely to be affected
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Table 3 shows the image quality scores for the two observers for the 10 inch field. Measured contrast was reduced on increasing the thickness of the PMMA. There was however no apparent trend in image scores with changing copper thickness.
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Table 3. Observer scores for TO.10 with varying amounts of copper and polymethyl methacrylate (PMMA) at a 10 inch field size
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On comparing Table 2
with Table 3, the image quality scores appeared to fall off rapidly at about 2 cm PMMA beyond factor saturation. Assuming this to be the case up to 0.4 mm copper, Table 4 demonstrates the PMMA thickness at which image contrast was expected to deteriorate, for each filter. The percentage of patients likely to be affected by this contrast degradation was calculated from the normal distribution, and is also shown. Again, the 14 inch views would not be so readily affected.
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Table 4. Phantom thickness at which image contrast was expected to deteriorate for various copper thicknesses (10 inch field), and percentage of patients likely to be affected by this
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The addition of 0.3 mm copper filtration increased the tube loading by about 30%, for a 24 cm PMMA phantom. The predicted heat generation in the anode during each examination and during each session did not exceed the anode heat capacity. The anode cooling rate was sufficient to prevent overheating, and the predicted heat generation for the whole day did not exceed the tube heat capacity.
Discussion
As might be expected, there was a steady decrease in entrance dose to the phantom, with increasing copper thickness. There was a smaller reduction at each increment due to the beam hardening effect of the copper. Although the fluoroscopic kVs did not reach their maximum value for a 24 cm PMMA phantom, saturation of both kV and mA did occur for thicker phantoms. This resulted in a reduction in input kerma at the image intensifier, since the unit could no longer compensate for the increased beam attenuation. This in turn led to a decrease in the signal to noise ratio, and hence contrast. The larger the patient, the less copper can be used before saturation occurs. This proved to be the limiting factor in determining the optimum amount of copper filtration. In practice, since patients are not cylindrical, more of the lateral and oblique views will suffer contrast loss than predicted by this model, but fewer of the anteroposterior and posteroanterior views.
The use of 0.4 mm copper was thought to be unacceptable since approximately 15% of patients' examinations may be affected by degradation in contrast, for an additional reduction in entrance surface dose of only 10%. The 0.3 mm copper filter seemed to provide an appropriate balance between the benefit of dose reduction and potential loss of contrast. The expected DAP reduction on moving from 0.1 mm copper to 0.3 mm copper was 33%, giving an expected DAP reduction of 56% relative to the standard 2.5 mm aluminium filtration.
Clinical diagnosis is based mainly on the radiographs, whilst the primary purpose of the fluoroscopic exposures is to ensure complete barium coating, and as a guide to positioning for the radiographs. Image quality is therefore more crucial for the radiographic exposures. These should not be affected significantly by factor saturation, since the automatic exposure control devices increase the exposure time to compensate for any reduction in dose rate at the detector. However, this may lead to an increase in the incidence of movement artefacts.
The addition of 0.3 mm copper increased the X-ray tube load by around 30%. Used with the examination schedule at this hospital, this did not put either the undercouch or overcouch tubes in danger of overheating. However, the effect on tube lifetime is as yet unknown.
Whilst phantom measurements had shown little degradation in contrast on changing to 0.3 mm copper filtration, the circular contrast details in the TO.10 test object offered a poor approximation to the clinical situation, where the subject of interest is a thin coat of contrast agent, lining the intestinal wall. In addition, the effects of factor saturation may be important for lateral and oblique views of more patients than predicted by the cylinder model. It was therefore important to verify that there was no significant effect on the diagnostic quality of clinical examinations.
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Clinical validation
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Method
The first 0.1 mm copper was mounted on the filter wheel inside the housing of each tube, along with an additional 1 mm aluminium. A further 0.2 mm copper was mounted between the X-ray tube and the DAP meter. For the undercouch tube, the copper was taped directly to the underside of the DAP meter. For the overcouch tube, the copper was taped to a Perspex slider, in order maintain access to the light beam.
DAP data had previously been collected for 60 examinations with no added filtration, and 449 examinations with additional filtration of 0.1 mm copper. On clinical implementation of the 0.3 mm copper filter, DAP was recorded for a further 24 examinations.
Effective doses for each of the three filter settings were estimated using PCXMC version 1.4, a PC-based Monte-Carlo dosimetry package [16]. The typical examination described in Table 1 was used, and the exposure factors and entrance doses were determined from 24 cm PMMA phantom measurements. It was assumed that the fluoroscopic projections and field sizes matched those used for digital spot images, and were used in the same proportion. The effect of the wedge filter used for decubitus views was not taken into account.
Since PCXMC requires a dose input without backscatter, the entrance surface dose measurements were divided by 1.3, in line with IPSM backscatter factors [17]. The standard adult phantom, with a height of 174 cm and a weight of 71.1 kg, was selected. The undercouch, circular radiation fields were approximated by square fields of equal area. 20 000 photons were used in each simulation.
Typical DAPs were also calculated, using the same input parameters. These were compared with the clinical DAP data.
The adequacy of clinical image quality was verified by comparing the hard copy images for 20 examinations carried out with no additional filtration, 21 with 0.1 mm copper, and 21 with the 0.3 mm copper filter. Examinations with fewer than six screen–film radiographs were excluded from the study. The cases were retrieved sequentially from the film archive and randomized, before being presented to a specialist gastrointestinal radiologist for blinded scoring. The scoring system used was a scale of 0 to 3 for perceived barium coating, where "0" corresponds to no coating, "1" to poor, "2" to fair and "3" to good. Each set of digital spot films was allocated a score for each of three regions of the large bowel: sigmoid colon, splenic flexure and hepatic flexure. Each set of screen–film radiographs was similarly allocated a score for each of left colon, right colon and transverse colon. The scorer was also asked to suggest reasons for scores below 3, and to classify each examination as "diagnostic", "non-diagnostic" or "indeterminate".
Results
Table 5 shows the mean patient equivalent diameter, fluoroscopy time and number of digital spot and conventional radiographs for each filter. The equivalent diameters, fluoroscopy times and number of digital spot images were log-normally distributed, so 95% confidence intervals were calculated using Cox's method [18]. The number of screen–film radiographs was approximately normally distributed. In each case, the 95% confidence intervals overlapped for all three branches of the study.
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Table 5. Mean patient equivalent diameter, fluoroscopy time, number of digital spot and conventional radiographs for each filter
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Table 6 shows the mean patient DAP for each of the filter combinations and the percentage reduction in mean DAP produced by each. The DAP distributions were log-normal, so the 95% confidence intervals were calculated using Cox's method [18].
The calculated DAPs for a typical examination are shown in Table 7. Values were 10% to 20% higher than the mean patient DAPs. However, predicted and measured DAP reductions agreed to within 5%. The estimated effective dose for each filter is shown in Table 8, along with the corresponding percentage dose reduction.
The majority of examinations in each filtration group were allocated some scores of less than three. Reasons suggested by the radiologist included patient obesity, retained fluid, poor distension of the bowel, overlying structures, and inappropriate printer settings. It was impractical to eliminate all affected examinations from the study, so it was instead assumed that such factors would contribute approximately equally to each arm of the study.
All examinations but one were classified as diagnostic. The remaining one was classified as indeterminate, due to poor distension of the colon. This examination was performed using 0.1 mm copper filtration.
The distribution of image quality scores for each filter combination was compared for each region, using Kruskal–Wallis one-way analysis of variance by ranks. The null hypothesis was that of no statistically significant difference between the populations from which the samples were drawn. The Kruskal–Wallis statistic for each region, and the associated probability of the samples coming from identical distributions, are shown in Table 9. Since the probability was in excess of 5% for each region, the null hypothesis could not be rejected. There was no significant difference between image scores for the three filters.
Discussion
The change from 0.1 mm to 0.3 mm added copper filtration reduced the mean patient DAP by 32%. The total DAP reduction on moving from no additional filtration to 0.3 mm copper was 57%. These results matched very closely the predictions made from the phantom measurements. The differences in DAP were statistically significant, since there was no overlap in the 95% confidence intervals for any two filter combinations. There was no significant difference in screening time, number of digital spot or screen–film images, or patient equivalent diameter, so that the reduction in dose may be attributed entirely to the increase in filtration.
At the standard filtration of 2.5 mm aluminium, the ratio of calculated effective dose to calculated DAP was 0.16 mSv (Gy cm2)-1. This is in good agreement with reported values of 0.14 mSv (Gy cm2)-1 to 2.0 mSv (Gy cm2)-1 [19]. The ratio increased to 0.29 mSv (Gy cm2)-1, when 0.3 mm copper filtration was added.
On changing from no additional filtration to 0.3 mm copper, the effective dose was reduced from 3.0 mSv to 2.7 mSv. This is a reduction of around 11%, which is a valuable achievement in terms of radiation protection of the patient, since effective dose is directly related to the risk of cancer induction [20].
Calculated DAPs were 10% to 20% higher than the clinical means. The main sources of error in the DAP calculation were the use of a typical protocol and exposure factors instead of individual patient parameters, and the modelling of patients as cylinders for determination of exposure factors. There was good agreement between the calculated and measured DAP reduction, on adding the copper filters.
The difference between calculated and measured DAP implied that effective doses were probably also overestimated by 10% to 20%. Additional sources of error in the effective dose calculation were the modelling of patients using a simple mathematical phantom, the use of square instead of circular fields, and statistical uncertainty in the Monte Carlo process.
The use of additional filtration did not appear to compromise the diagnostic adequacy of the examinations, since the radiologist was confident in making a diagnosis in all cases but one, and this exception was due to poor gaseous distension rather than poor image quality. No significant difference in perceived barium coating was found with the Kruskal–Wallis test. This confirmed the results of phantom measurements, that the reduction in contrast was small, and should not in general affect image quality in patient examinations.
The study highlighted the difficulties in defining and quantifying clinical image quality. Due to the number and complexity of contributing factors, it was not possible to separate out image contrast from technical aspects of the examination, such as true barium coating efficiency. The scoring clinician commented that image scoring was very subjective, despite efforts to be as consistent as possible. However, the key issue in terms of image quality is whether the images enable an appropriate diagnosis to be made.
Persliden et al [21] carried out a similar evaluation to compare image quality for analogue and digital barium enema equipment. Scores were allocated on a scale of 1 to 5 for noise, sharpness, contrast and overall impression, in each of four anatomical structures. Images were also graded as diagnostic or non-diagnostic, for inflammatory bowel disease. Despite finding a significant difference in noise and unsharpness between the two groups, they found no significant difference in diagnostic quality, or overall impression, suggesting that contrast is the key factor in determining the adequacy of image quality for barium enemas.
The approach taken by Hansson et al [15] was to carry out each examination with no added filtration, then repeat one of the views with the copper filter in place. This had the great advantage that the two filter options could be compared directly for identical views of the same patient, eliminating other factors that might affect image quality. The authors found that there was no visible difference in fine mucosal detail, with 2.5 mm aluminium filtration, and with an additional 0.3 mm copper. However, only one view was considered instead of the examination as a whole, and there was an increased radiation burden to the patients involved in the trial.
The gold standard measure of diagnostic adequacy is the retrospective outcomes audit. Several such studies have been done to determine the number of colonoscopically or histologically proven cancers missed by barium enema [6, 22–27]. However there is necessarily a delay of about a year before meaningful outcome data become available. In addition, only a few percent of barium enemas demonstrate cancer. An internal audit at this hospital showed that during the year March 2000 to 2001, cancers were histologically proven in 65 patients who had had an enema examination in the preceding 2 years. Of these, barium enema was the primary investigation in only 35 cases. Outcome data would therefore need to be collected over several months at least, in order to obtain adequate sample sizes.
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Conclusion
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Following phantom measurements of radiation dose and image quality, the use of 0.3 mm copper additional filtration was implemented in clinical practice. There was no significant difference in perceived contrast for hard-copy clinical images, and our gastrointestinal radiologist considered them to be of adequate quality for diagnosis. Tube loading increased by 30%, but with our local protocol and schedule, this did not put either the undercouch or overcouch tube at risk of overheating. Mean patient DAP was reduced by 57% and effective dose by 11%, compared with the standard 2.5 mm aluminium filtration.
Received for publication April 1, 2003.
Revision received July 18, 2003.
Accepted for publication August 29, 2003.
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Radiat Prot Dosimetry,
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ncm495v1.
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Abstract
Introduction
