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First published online August 17, 2006
British Journal of Radiology (2007) 80, 177-185
© 2007 British Institute of Radiology
doi: 10.1259/bjr/19929794
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Full paper

Diagnostic reference levels and effective dose in paediatric cardiac catheterization

D G W Onnasch, PhD, F K Schröder, MD, G Fischer, MD and H-H Kramer, MD

Department of Paediatric Cardiology, University of Kiel, Schwanenweg 20, 24105 Kiel, Germany

Correspondence: Dr Dietrich Onnasch, Biomedical Engineering, Paediatric Cardiology, University of Kiel, Schwanenweg 20, Kiel 24105, Germany. E-mail: onnasch{at}pedcard.uni-kiel.de


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 
European states within the EEC are required to establish and use diagnostic reference levels (DRLs) in X-ray examinations. However, up to now there have been no DRLs for cardiac catheterization in children, nor as a rule is the effective dose estimated. We have evaluated the dose–area products (DAPs) for three different types of angiocardiography systems over a time span of 8 years. For each system DAP increased in proportion to the body weight (BW) over two orders of magnitude. The proportionality constant decreased over the years. To reduce the broad distribution of DAP the doses for cine acquisition (DAPA) and fluoroscopy (DAPF) were indexed with respect to the total numbers of acquired images (AN) and the total times of fluoroscopy (FT). DAPA/AN is directly proportional to BW with a high correlation (r = 0.896, n = 1346). Likewise, DAPF/FT is proportional to BW from 0.1 kg to 100 kg (r = 0.84, n = 2138). Therefore, by normalizing DAP to BW the growth dependent variation of DAP can be eliminated. There are numerous short examinations with very small total DAPs, which were separated from the group of diagnostic examinations. The mean DAP/BW of this group is 0.41 Gycm2 kg–1 (90th percentile: 0.81 Gycm2 kg–1, n = 1106). For interventional procedures in congenital heart diseases DAP/BW is significantly higher (p<0.001) (mean: 0.56 Gycm2 kg–1, 90th percentile: 1.16 Gycm2 kg–1, n = 883). There are significant differences between different types of interventional procedures, the mean values being between 0.35 Gycm2 kg–1 (occlusion of patent ductus botalli, n = 165) and 1.30 Gycm2 kg–1 (occlusion of ventricular septal defect, n = 32). For patients who are catheterized several times over the years, the cumulative effective dose (E) may reach high values, being especially high for patients with hypoplastic left heart syndrome (typically 11 mSv). E is derived from DAP/BW by use of a constant DAP/BW to E conversion factor, independent of the age of the patient. DAP/BW is appropriate to describe paediatric DRLs and is recommended instead of using mean DAP values for age groups.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results
 Discussion
 Conclusions
 References
 
According to the European Council Directive 97/43/Euratom of 30 June 1997 [1] all member states shall promote the establishment and use of diagnostic reference levels (DRLs) for radiographic examinations. These levels are expected not to be exceeded for standard procedures when good and normal practice regarding diagnostic and technical performance is applied. Article 9 of that directive expressly requires special attention for the radiological exposure of children and in interventional radiology. However, up to now there is no publication on such DRLs for cardiac interventional or diagnostic examinations of children. Therefore we analysed all paediatric cardiac examinations from 1995 to 2003 that had been performed in our hospital. During that time period, we used three different cardiography units.

Both the equipment and the types of examinations have changed over the last 10 years. Today we can distinguish three types of X-ray examinations in the paediatric catheterization laboratory: comprehensive diagnostic examinations; interventional procedures; and short examinations. First, while simple diagnostic questions may be examined by echocardiography only, prolonged diagnostic examinations are indispensable for the diagnosis of complex congenital heart diseases before and after cardiac operations, including several biplane angiographic recordings and measurements of local pressure curves and oxygen saturation. Second, an increasing number of powerful interventional procedures help to avoid cardiac surgery [2]. Among these is the hybrid approach that involves joint collaboration between the surgeon and the cardiologist. And third, after interventions or surgery, short examinations with a single short intravenous contrast injection or a fluoroscopic control may be necessary. This group also includes pacemaker insertion procedures using short times of fluoroscopy.

There are only a few studies on the radiation exposure of children during cardiac catheterization and its dependency on growth. For neonatal radiographic examinations Wraith et al [3] find a close relationship between the dose–area product (DAP) and the body weight (BW) of the neonate. Kuon and colleagues [4] report that the DAP during invasive cardiac procedures of adults was related to their body surface area. The close relationship between DAP and BW is also documented for other organs [5, 6]. In contrast to these findings, studies on dose measurements in the paediatric catheterization laboratory do not relate DAP to BW, but use age group statistics (0–1 year 1–5 years etc.) [7–12]. The authors found a broad distribution of DAP, entrance surface dose (ESD) and total times of fluoroscopy (TF). Because of its close relationship with the effective dose (E) and the associated excess relative risk of cancer (ERR), DAP is more useful than ESD as a DRL [4, 13]. ESD should be observed for cardiac examinations and interventions in adults, due to the risk of skin erythema or even ulceration [14, 15], which is uncommonly seen in children [16].

The aims of the present study are:

  1. to examine whether DAP has decreased following successive replacement of equipment over time,
  2. to investigate which are the main factors affecting the magnitude of DAP in the diagnostic examination and therapeutic interventional treatment of congenital heart diseases,
  3. to estimate a DRL for internal use in our hospital that can also be considered as a first proposal for establishing a generalized DRL,
  4. to develop a method for calculating E for each single radiological examination in the catheterization laboratory, and
  5. to assess the total E and ERR for patients with multiple radiological examinations of complex congenital heart diseases.


   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results
 Discussion
 Conclusions
 References
 
For 8 years, we gathered data from 2859 patients. BW was between 1.5 kg and 115 kg, with a median of 12.7 kg. The age range was between 1 day and 72 years. The mean and median ages were 918 days and 2091 days, respectively. Adult coronary angiography and coronary interventional procedures are not performed in our department. However, patients older than 18 years (n = 217) with complex congenital heart diseases were included, especially when interventional procedures such as atrial or ventricular septal occlusions might be performed. All data delivered from angiography systems and the subsystems were logged online by the physiological recording system [17].

During the evaluation period we used three different cardiography units in our hospital. The biplane Siemens Bicor/Digitron system (Siemens Medical Solutions, Erlangen, Germany) was used until June 1997. Cine acquisition was performed on analogue cine film and digitally. Frame rates were 50 frames s–1. DAP measurements were only performed in plane A (normal position of the X-ray tube under the table). 531 consecutive examinations were evaluated from mid-1995 to mid-1997.

When we modernized the catheterization laboratory in 1997, the biplane Philips DCI/LARC system (Philips Medical Systems, The Netherlands) was used for 6 months, in which digital acquisition and analogue film exposure ran simultaneously. The total DAP (from fluoroscopy and cinematography) was recorded in both planes (n = 188 examinations).

Since December 1997 the biplane Philips Integris 5000BH system (Philips Medical Systems, The Netherlands) has been used. This delivers DAP for fluoroscopy (DAPF) and cine acquisition (DAPA) separately, as a sum of both planes. Therefore it was not possible to analyse the data in terms of the direction of the projection. All cine runs are only recorded and archived digitally with 515x512x8 bit resolution. Frame rates of 12.5 frames s–1, 25 frames s–1 or, occasionally, 50 frames s–1 are selected dependent on the heart frequency and on the site of contrast injection. For the present paper we analysed all 2140 examinations performed between 1998 and 2003. Three groups are compared:

In a preceding study, we optimized the automatic exposure control of the Philips Integris system to minimize the X-ray exposure at constant image quality for cine acquisition in infants, children and adolescents [18]. As a result, in general the 0.4 mm copper plus 1.5 mm aluminium filter is selected with a set of exposure control programs being adapted to the patient size, the desired field of view and the frame rate. From the three programmable fluoroscopy modes (low, normal and high) of the system only the low and normal modes were applied, using 0.2 mm Cu and 0.4 mm Cu and 12.5 pulses s–1 and 25 pulses s–1, respectively. DAPF and DAPA as well as the TF and the total number of acquired images (NA) are recorded for each examination along with the patient's age, their BW and the type of examination.

Based on anthropomorphic phantom measurements and Monte Carlo calculations the DAP-to-E conversion factor (CF) can be estimated as being dependent on beam filtration, tube voltage and body size. For chest radiography and adult cardiology, values between 0.18 mSv Gy–1 cm–2 and 0.26 mSv Gy–1 cm–2 have been published [19–23]. For paediatric cardiology, phantoms representing children aged 0, 1 year, 5 years, 10 years and 15 years old are used to estimate growth dependent conversion factors [7, 9, 12, 24]. As a result CF decreases non-linearly with increasing age. However, when the BW of the phantoms is considered instead of the age, we have found that the product CF x BW is constant. Using the data from Schmidt et al [9] we calculated the CF x BW products (Table 1Go). They do not depend on the age or size of the patient but only on the projection. Similar results can be derived from the other above-mentioned papers on paediatric conversion factors. The mean values are 7.45kg mSv Gy–1 cm–2 and 11.07 kg mSv Gy–1 cm–2 for the posterioranterior (PA) and lateral (LAT) projection, respectively. As the viewing direction changes during most procedures and DAP is logged by our biplane angiographic system only as a sum from both planes, in this study we are using an average conversion factor to estimate E [mSv] from DAP/BW [Gy cm2 kg–1]:


Formula 001


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  		Table 1. Conversion factors according to Schmidt et al[9] for 65 kV and 3 mm aluminium filtration

 
Statistics
Before evaluation, all data were checked for erroneous measurements or transfer errors. In particular, we tried to eliminate those DAP values delivered by the Philips Integris system, which were supposed to be below the threshold level of 0.1 Gy cm2 (see first paragraph of the following section). Thus the numbers of fluoroscopic and angiographic DAP values differ. The plots of DAP vs BW showed that the relationship was best described in a log–log scale using a relative, not an absolute deviation. The proportionality constant was calculated as:


Formula 002

where x and y are the values of abscissa and ordinate and n is the number of (x,y) pairs. The mean percentage deviation was calculated as:


Formula 003

Distributions were described by mean values and the 50th, 75th, 90th and 95th percentile values. For the statistical comparison of the two distributions, the Wilcoxon–Mann–Whitney U-test was used. For the comparison of more than two groups the Kruskal–Wallis H-test was used. Values of p are reported to describe the results; those with p<0.05 were considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Methods and materials
 Results
Discussion
 Conclusions
 References
 
Measurements from the Philips Integris system with phantoms of different thickness, simulating the absorption range of infants, children and adolescents have been published elsewhere [25]. They showed that:

  1. the DAP values indicated by the system were correct and reproducible for thick absorbers and long radiation times,
  2. DAP measurements for small patients were often error-prone. The system ignored any single exposure below 0.1 Gy cm2 and any single fluoroscopic time below 6 s. In general the recorded DAPF, DAPA and TF are therefore too small. For premature infants and babies DAPF/TF is systematically too low, as the threshold of 0.1 Gy cm2 is reached only after a fluoroscopy time of at least 12 s. For cine runs of infants, the threshold of 0.1 Gy cm2 is reached only after 90 frames. Especially during complex interventional procedures there are many shorter runs taken. For adults, DAPF/TF may be too large, as in TF all single times of fluoroscopy smaller than 6 s are ignored, while DAPF is measured correctly. For the presented analysis we tried to eliminate all erroneous data.

In Figure 1Go the total DAP is plotted vs BW using log–log scales for the three angiography systems used in our hospital during the last 10 years. In each case there is a linear relationship between DAP and BW over two orders of magnitude with a rather broad distribution. The fitted lines have been calculated in such a way that the mean relative deviations (RD) are minimal. The proportionality with year of measurement decreased from 0.62 Gy cm2 kg–1 to 0.44 Gy cm2 kg–1 and then to 0.28 Gy cm2 kg–1. The 75th percentile of DAP/BW dropped from 1.14 Gy cm2 kg–1 to 0.90 Gy cm2 kg–1 then to 0.54 Gy cm2 kg–1. With this comparison one must take into account that the dose of the Bicor system was measured in only one plane.


Figure 1
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  		Figure 1. Plot of dose–area product (DAP) vs patient weight (BW) for three angiographic systems used from 1995 to 2003. n, number of examinations; r, correlation coefficient; RD, relative deviation, also indicated by the dashed lines.

 
The broad distribution can be explained by the huge variety of radiological examinations and procedures performed in paediatric cardiology, and the differences in the difficulty level among individual patients. Since for the newest equipment the DAP values for fluoroscopy (DAPF) and cine acquisition (DAPA) are logged separately together with TF and NA, we were able to analyse the data in more depth. By normalizing DAPF to TF and DAPA to NA we expected to remove dependencies related to the type of examination and the kind of disease. The results are plotted in Figure 2Go. Compared with Figure 1Go the correlations are higher and the relative deviations are smaller. Thus the increase in DAP proportional to BW for patients from birth to the adult age is clearly demonstrated.


Figure 2
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  		Figure 2. (a) Plot of the fluoroscopic dose–area product (DAPF) normalized to the time of fluoroscopy (TF) vs the patient weight (BW). (b) Plot of the dose–area product (DAPA) accumulating during cine acquisition, normalized to the total number of recorded frames (NA) vs BW. The data are from the Philips Integris system. For the DAPA plot all examinations with erroneous dose measurements were eliminated as far as possible. n, number of examinations; r, correlation coefficient; RD, relative deviation, also indicated by the dashed lines.

 
In the lower graph of Figure 2Go, all short cine runs were eliminated in order to remove erroneous DAPA values (see above). However, for DAPF the removal of erroneous values was hardly feasible (Figure 2Go, upper graph). With short times of fluoroscopy DAP may be lower than 0.1 Gy cm2 and the recorded DAPF value for infants is too small. On the other hand, as TF may be underestimated, because any fluoroscopic release shorter than 6 s is not considered by the system, yet contributes to the DAPF of large patients, the ratio DAPF/TF is often overestimated for adult patients.

Based on the proportionality relationship between DAP and BW we compared DAP/BW for various types of paediatric catheterization procedures. Results are presented in Table 2Go and Figure 3Go. In each subgroup there is still a broad asymmetric variation. The mean values between comprehensive diagnostic examinations and interventional procedures differ significantly (p<0.001). On average, the dose is 37% higher for interventions. In Table 3Go results are presented for selected plain interventional procedures such as occlusion of patent ductus arteriosus and atrial or ventricular septal defects, coil embolisation of collateral vessels and balloon dilatation of arterial and valvular stenosis. Again there are broad asymmetric distributions of DAP/BW, and the differences between all these groups are statistically significant (p<0.001). Wilcoxon rank-sum tests between each two groups deliver significantly (p<0.001) high dose levels for the ventricular septal occlusion and significant low levels for the occlusions of patent ductus arteriosus and patent foramen ovale. There are further therapeutic radiological examinations, where different kinds of interventional techniques are combined, not included in Table 3Go, yet included in the interventional group of Table 2Go. The mean effective dose given in Table 3Go is calculated using Equation (1).


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  		Table 2. Statistical data on patient weight and the total dose–area products during paediatric catheterization subdivided into comprehensive diagnostic examinations, interventional procedures and short control examinations

 

Figure 3
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  		Figure 3. Histogram of dose–area products per body weight (DAP/BW) for the groups of diagnostic examinations (light grey) and interventional procedures (dark grey). Axis labels represent the midpoint of 0.1 Gy cm2 kg–1 bands. The last band includes all DAP/BW values larger 4 Gy cm2 kg–1. The mean value, median (p50), 3rd quartile (p75), 90th and 95th percentile values (p90, p95) are marked for each group.

 

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  		Table 3. Statistical data on the dose–area product per body weight (DAP/BW) and the effective dose (E) for seven selected paediatric cardiac interventions

 
In paediatric cardiology many patients with complex congenital cardiac disease are catheterized several times during growth. For selected diseases, we gathered those patients who were examined three or more times over 12 years. For this evaluation dose data of patients who were already catheterized between 1991 to 1994 with the Siemens Digitron/Bicor system were also included. In Table 4Go the cumulative effective doses are presented, which were calculated from the respective DAP and BW values of each examination using Equation (1). For those cardiac diseases the typical total lifetime effective dose is 8 mSv (median), yet it may reach values over 100 mSv for some patients.


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  		Table 4. Statistical data on the total effective dose for patients with different congenital heart diseases who underwent multiple examinations over 12 years

 

   Discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
Conclusions
 References
 
The dose delivered to paediatric patients during imaging is a significant issue. A child receiving a high dose of ionizing radiation has many years in which to potentially develop malignancies associated with those X-rays. As in other radiological systems, in paediatric cardiology there is rapid development in both the medical use as well as the technical features of the laboratory. During the last two decades important technical modifications include (a) the introduction of isocentric biplane double C-arm gantries, (b) the transition to digital-only systems featuring last-image hold, pulsed fluoroscopy, various frame rates for acquisition and online DICOM archiving, (c) the use of high power X-ray tubes allowing strong copper filtration (0.2–0.4 mm Cu), and last but not least (d) the gradual increase in quantum detector efficiency of the image detector. The impact of these developments can be assessed from the comparison of three types of angiographic systems used in our hospital (Figure 1Go). From 1995 to 2003 the mean ratio DAP/BW was reduced by more than 50%. We should note that at the same time the proportion of therapeutic procedures increased considerably. This degree of dose reduction does not relieve us from the obligation to keep the radiation exposure as low as reasonably achievable. In a preceding study [18] we demonstrated that the dose can be reduced without impairing image quality by adapting automatic exposure control, generally optimized for adult patients, to the needs of paediatric cardiology.

The use of DRLs is helpful for continuously observing the radiation exposure both internally as well as between different facilities. Therefore, and to comply with legal requirements [1], the determination of DRLs for paediatric cardiac examinations and interventional procedures in the catheterization laboratory is overdue. For children, DRLs are published for some standard radiology examinations such as PA and LAT chest imaging [26]. Also, there are DRLs for cardiac diagnostic and interventional examinations of adults [23, 26]. As children with complex congenital heart disease are often catheterized several times and the risk of somatic radiation effects is higher than in adults, it is necessary to establish DRLs for paediatric cardiology as soon as possible.

However, while establishing the DRL concept is straightforward for simple radiographic examinations, it is less clear whether it is applicable to radiology procedures with broad distributions and long high-dose tails. The sources of DAP variation in paediatric cardiology are very different. They include the X-ray system specifications and performance, the kind of examination protocol and the quality of the preceding echocardiographic examination, patient pathology, in particular the complexity of the cardiac disease, operator skill and, last but not least, the size of the patient and the angle of projection.

In our study we were able to examine three sources of DAP variation: its dependence on the X-ray system, on the size of the patient and on the type of radiology procedure. Since DAP values from the two older angiographic units are only known as sums from fluoroscopy and cine acquisition, detailed analyses were impossible. The data from the Philips Integris unit allowed a more detailed analysis of DAP as a function of growth and type of procedure.

Generally in paediatric radiology and cardiology DAP is analysed as dependent on age [7, 9–12, 26]. We solved the problem of obtaining DRLs for children during growth by indexing DAP to BW. The size of the patient is the cause of the increase in DAP, not the age. The high correlation (r = 0.840 and 0.896) between DAPF/TF and BW, respectively, DAPA/NA and BW, from 1.5 kg to 100 kg (Figure 2Go) demonstrates that there is no need for using age groups. It is expected that the relationship becomes even closer when the DAP data logging includes small doses below the current threshold. The manufacturer has announced that they will modify their system in summer 2006 to correct the DAP recording.

Within an age group there is always a spectrum of different body sizes, which leads to a broad distribution of related DAPs. The normal BW range within an age group may vary by a factor of two. For a 10-year-old boy, the 3rd and 97th percentiles are 22.3 kg and 45.1 kg resulting in different X-ray absorptions. The strict proportionality between DAP and BW can be used to remove the growth dependent source of DAP variation. Therefore, we recommend using the quantity DAP/BW as DRL in paediatric radiology instead of different DAP values for age groups. The differentiation of boys and girls is not required either. The rationale for relating DAP to BW is that the mass of the heart and the volumes of its chambers are growing in proportion to the patient's body weight (not to the body surface area).

In the clinical routine the use of BW as a reference value is most straightforward. Alternatively, the diameter of the thorax (DT) was proposed, which can be estimated from the body weight and height, differentiating also between PA and LAT projections [27, 28]. However, neither Golder et al [5] nor Van de Putte et al [29] have found a closer relationship between DAP and DT than between DAP and BW. In addition, during an extended cardiac examination with a multiaxial gantry the direction of radiation is changed several times. Furthermore, the approach implies that DAP is recorded separately for the two planes of a biplane system, which is not the case for our system.

In this context the DRLs for adult cardiology should also be considered. According to Cusma et al [14] in coronary examinations and interventions the entrance dose differs significantly for patients below and above 83 kg. Kuon et al [4] presented the continuous increase in DAP with the body mass index and the body surface area. Golder and Weiner [5] have found a high correlation between DAP and BW for chest X-ray images. Therefore, the possibility that DAP should be related to BW also for other modes of application should be considered. It is possible that by doing this the DRLs would also be more specific for adult patients.

Because our objective was to obtain a useful DRL for paediatric cardiology, all short radiology examinations were separated from the group of prolonged diagnostic examinations. The differences in DAP/BW between these groups are highly significant (p<0.001) (Table 2Go). More interesting is that the mean values of DAP/BW for interventional procedures are also significantly larger (p<0.001) than for the diagnostic group.

In specifying DRLs for local use and as a first proposal for establishing reference levels for paediatric cardiology in general, the 75th or the 90th percentile value of the DAP/BW distributions may be considered. The higher value might be used to identify examinations or radiographic systems giving unusually high doses, yet without giving alarm too often in cases where well adjusted equipment and good practice is applied. Considering 0.81 Gy cm2 kg–1 for diagnostic examinations and 1.16 Gy cm2 kg–1 for interventional procedures, these levels may be compared with adult cardiology. For a 70 kg patient we get 57 Gy cm2 and 81 Gy cm2, respectively. These values are somewhat lower than the DRL published for coronary angiography (60 Gy cm2) and for percutaneous transluminal coronary angioplasty (120 Gy cm2) [26], although paediatric catheterization takes more time as a rule. This may be ascribed to the strong copper filtration of 0.4 mm that we usually use [18] as well as to different needs in diagnostic and therapeutic catheterization of patients with congenital heart disease and to institutional experience.

Admittedly, omission of several small contributions to the total DAP during extended examinations leads to an underestimation of the total X-ray exposure. Therefore the estimated DRL for paediatric heart catheterization must potentially be increased, especially when the X-ray equipment being used is not equipped with an automatic exposure control, optimized for cardiac examinations of infants and children. It would be desirable for other centres to acquire and publish DAP data sets on the radiology examination or treatment in complex congenital cardiac diseases in order to establish international DRLs for paediatric cardiac catheterization.

DAP/BW for different types of intervention is analysed in Table 3Go. For each of the groups of plain interventional cardiac interventions there are skewed distributions with significant differences (p<0.001). The rather high value for the occlusion of a ventricular septal defect is conspicuous, a complex procedure that requires long times of fluoroscopy and several angiocardiograms. Overall, in spite of large differences in the difficulty level among individual patients, the differences among various therapeutic catheter techniques are reflected in the mean DAP/BW.

In clinical use, however, we would not recommend using specific DRLs for different procedures, but using a single DRL for all the therapeutic interventions in congenital heart disease, taking into account that in practice different techniques are often used in a single examination. In this way the aim of keeping the radiation exposure as low as practicable could be promoted, especially in training new interventional cardiologists. The simultaneous use of echocardiographic guidance is paramount for the success of many percutaneous interventions. Moreover, the close cooperation between the interventional cardiologist and the cardiac surgeon is realized in the treatment of many congenital cardiac diseases. In addition, our system [17, 30] allows easy measurement of absolute cardiac distances and diameters online from the digital angiogram, a feature that greatly facilitates the selection of the correct device.

In agreement with our findings, Bacher and colleagues [16] also found higher doses for therapeutic interventions as compared with diagnostic catheterization for a limited number of examinations (n = 60). That difference was, however, not statistically significant, probably because they did not remove the weight-related variation of DAP of the patients being between 3 kg and 43 kg. They came to similar mean values for DAP and E, determining patient-specific effective doses by Monte Carlo calculations.

When the relationship between E and DAP/BW (Equation (1)) is used, E can be calculated from routine DAP measurements routinely, and is simpler to use than the concept of using age groups [7–12]. It would be desirable to determine the variation of the DAP/BW to E conversion factor with viewing direction and different X-ray spectra. The factor used in this paper was derived for a filtration of 3 mm Al (Table 1Go). According to Schmidt et al [9] it increases by 30% by adding 0.1 mm Cu. It is expected that the conversion factor is even higher with 4 mm Cu filtration, as used by us with the Philips Integris system [18]. Therefore, the values given for E in GoTables 3 and 4Go may be underestimated. Another source of uncertainty is that the conversion factor depends on the beam direction. We averaged the values for the two PA and LAT views, which are mostly used in paediatric cardiology. Factors for other projections are between these values.

For the assessment of the total effective dose of a patient who is examined several times during growth, E has to be determined for each catheterization and summed. Some results of such an approach are gathered in Table 4Go. Here the data from the three different systems used between 1991 and 2003 are combined. Based on those results the ERR can be estimated for a child with a complex cardiac disease such as a hypoplastic left heart syndrome (HLHS). Now the age and gender of the patient must be considered. According to Japanese data from the A-bomb survivors [31], the E-to-ERR conversion factor decreases from 15% Sv–1 for female infants to 8% Sv–1 for adolescents and for boys from 12% Sv–1 to 7% Sv–1. Correspondingly, for a HLHS patient ERR is typically 0.1%, yet can be 15 times as high in individual cases. That risk has to be weighed against the benefit of an optimal therapy.

Also such estimation presumes that the linear no-threshold theory [31] is correct and that each single short radiation exposure contributes to the risk of cancer. The concept ignores any self-healing process by the cell and the affected organism. These assumptions are increasingly called into question in the literature [32].

In addition to the uncertainty on the size of the DAP/BW to E conversion factor, other limitations of this study should be noted. Due to the erroneous thresholding of the DAP indication of the equipment used, the stated DRL for DAP/BW may be too low. Furthermore, one must be aware that there is rapid progress in the treatment of congenital cardiac diseases and the interventional paediatric procedures in particular. The data presented in Table 4Go give only a snapshot of the state-of-the-art techniques 3–10 years ago.


   Conclusions
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
References
 
For each of the three angiocardiography units used in the period of data analysis we found a linear increase of DAP with the BW over two orders of magnitude, whereas the constant of proportionality was decreasing over the years. The broad asymmetric distribution of DAP/BW can be abolished by indexing DAP to the time of fluoroscopy and the number of acquired frames, demonstrating that by normalizing DAP to BW the growth dependent variation in DAP can be eliminated. Therefore, we recommend using DAP/BW in paediatric cardiology as DRL instead of using different DAPs for age groups. Using the 90th percentiles, 0.81 Gy cm2 kg–1 is the reported DRL for the group of comprehensive diagnostic examinations of congenital heart diseases and 1.16 Gy cm2 kg–1 for therapeutic radiology treatments in paediatric cardiac catheterization. These levels are considered appropriate values to identify systems giving unusually high doses. In the absence of national or international DRLs for paediatric cardiac catheterization our data provide a way forward in establishing provisional DRLs. Further important sources of DAP variation are the complexity of congenital heart disease and the kind of interventional treatment. Continuous monitoring of DAP/BW will allow different facilities to be compared and will be useful for keeping radiation exposure as low as reasonably achievable, in particular in training new interventional cardiologists.

As the DAP/BW to E conversion factor is independent of age and gender, it is straightforward to assess E from DAP/BW. This allows comparison of the radiation exposure with CT examinations, for instance. On this basis the cumulative effective dose and the excessive relative risk of cancer of patients with complex heart diseases who are catheterized multiple times can also be estimated. For patients who had to be catheterized four times during their life because of their complex cardiac disease, the total mean effective dose was 19 mSv, 10 times higher than in single cases. Therefore, it is important to use effective beam filtration, image detectors with a high quantum detective efficiency and automatic exposure control programs adapted to the needs of paediatric cardiology.

Received for publication December 12, 2006. Revision received April 12, 2006. Accepted for publication June 30, 2006.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 

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