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CT doses in children: a multicentre study

¹ Vrije Universiteit Brussel, Applied Sciences Faculty, Pleinlaan 2, 1200 Brussels and ² Academisch Ziekenhuis Vrije Universiteit Brussel, Radiology Department, Laarbeeklaan 101, 1090 Brussels, Belgium

	Abstract

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
We evaluated examination protocols used for common CT procedures of paediatric patients at different hospitals in Belgium in order to determine whether adjustments related to patient size are made in scanning parameters, and to compare patient doses with proposed reference levels. Three paediatric hospitals and one non-paediatric hospital participated in the study. Weighted CT dose-index (CTDI_w), dose–length product (DLP) and effective dose (E) were evaluated for three patient ages (1 year, 5 years and 10 years) and three common procedures (brain, thorax and abdomen). CTDI_w and DLP values higher than the reference levels were found for all types of evaluated examination. E ranged from 0.4 mSv to 2.3 mSv, 1.1 mSv to 6.6 mSv, and 2.3 mSv to 19.9 mSv for brain, thorax and abdomen examinations, respectively. All centres but one adapted their protocols as a function of patient size. However, no common trend in the selection of protocols was observed. Some centres divided the whole range of patient size into only two/three groups by age, while others classified the patients into six groups by weight. It was also observed that some centres used the same mAs for the total range of patient sizes and decreased the pitch factor for small children, which resulted in higher doses. This indicates the importance of careful selection of technical scan parameters. If CT parameters used for paediatric patients are not adjusted on the basis of examination type, age and/or size of the child, then some patients will be exposed to an unnecessarily high radiation dose during CT examinations.

	Introduction

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CT, a technique that produces non-superimposed, cross-sectional images of the body, was introduced into clinical practice in 1972. After its introduction, rapid technological development of this imaging modality (fast acquisition and reconstruction times, spiral acquisition mode, multislice CT) has resulted in a continuous expansion of CT examinations. In the last 10 years the use of CT has grown considerably. As a result, the numbers of examinations have increased to the extent that CT has made a substantial impact on not only patient care, but also patient and population exposure from medical X-rays. This relatively high dose modality, which represents about 3–7% of all X-ray examinations, contributes up to 41% of the collective dose from diagnostic radiology in some countries [1, 2]. Therefore special attention should be given to quality assurance programmes including quality control measurements and patient dosimetry.

The European Commission, in order to ensure optimization of performance and patient protection in CT procedures, has established a set of quality criteria for CT examinations on adult patients. These criteria have been published in the "European Guidelines on Quality Criteria for Computed Tomography" (EUR 16262) [3].

Children are increasingly being referred for CT examinations. This increased frequency is largely caused by the advent of fast helical scanning, which reduces the need for sedation and allows the evaluation of younger, sicker or less co-operative children. But often little attention is paid to adapting examination protocols developed for adult patients to suit children, which results in much higher doses than are necessary for an adequate level of image quality [4]. There is a need to improve the optimization of this high-dose imaging modality for this especially vulnerable section of the population. Children are more sensitive to the effects of ionizing radiation than adults. At ages up to 10 years they are in general more sensitive by a factor of three [5].

The "4th Framework European Programme" in paediatric radiology has concentrated on developing quality criteria guidelines for a selection of common paediatric CT examinations. A draft paediatric document has already been prepared based on the adult CT document [6, 7]. The draft document comprises general principles associated with good imaging technique, quality criteria and guidelines on radiation dose to the patient.

In the USA "The Food and Drug Administration" (FDA) centre has published a set of recommendations in order to keep radiation doses during CT as low as reasonably achievable, especially for paediatric and small adult patients. They stress the importance of adjusting CT scanner parameters appropriately for each individual's weight and size, and for the anatomic region being scanned [8].

The objective of this study was to carry out a survey to assess paediatric patient dose for common CT examinations at different hospitals in Belgium in order to:

1. compare the doses with the dose criteria proposed in the European draft document;

2. determine whether adjustment related to patient age and/or body size is made in the scanning parameters that determine the radiation dose.

	Materials and methods

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Invitation letters to participate in the study were sent to 30 hospitals distributed throughout the country. A positive response was obtained from only four centres (with a total of seven CT scanners). Three of the hospitals corresponded to specialist paediatric centres and one was a non-paediatric centre. From the CT units analysed, five were single-slice helical scanners, and two were multislice scanners (multirow detector array).

Dosimetric phantoms
In paediatric radiology, in order to cover the very wide range in size from a newborn baby to a 15-year-old adolescent it is appropriate to have more than one standard patient size for the purpose of establishing reference dose levels and comparing doses for specific examinations. It has been found that five standard sizes corresponding to neonates, 1-, 5-, 10-, and 15-year-old patients are sufficient to cover the whole paediatric age range [7, 9]. These head and trunk sizes can be represented by cylindrical phantoms. Table 1 summarizes estimated diameters of polymethyl methacrylate (PMMA) cylinders which are equivalent (in terms of mass and thickness) to transverse slices of mathematical phantoms which represent patients of different ages [7, 10]. Mathematical phantoms are computational hermaphrodite models where body contours and organs are defined by mathematical expressions. In order to estimate directly the dose to a patient from CT a dosimetric phantom representing the size of the patient is needed. Measurement of doses to a particular size of dosimetric phantom will provide a direct indication of the dose in an irradiated section of the patient for a particular anatomical section and patient size. However, when characterizing relative patient exposure in order to allow comparison of performance in CT, the absolute size of the dosimetric phantom is less important. Therefore, the use of one standard-sized dosimetric phantom is more appropriate. For CT dosimetry in relation to adults the use of standard cylindrical phantoms of 16 cm diameter for the evaluation of the head and neck region, and of 32 cm diameter for the evaluation of the trunk region are recommended [3]. For paediatric CT dosimetry a 16 cm diameter phantom can be used as an appropriate standard in relation to all types of CT examination on children of all ages [7, 11]. This single dosimetry phantom provides a reasonable representation of patient size over the paediatric age range. The ratio of the doses to a transverse section of the 16 cm phantom relative to the dose to the newborn or the 15-year-old phantom will be approximately two [7, 12].

where D(z) is the dose as a function of position z along the axis of rotation of the scanner, and T is the slice thickness. In practice the integration range is ±7 slice thicknesses (CTDI_FDA as defined by the US Food and Drug Administration) or ±50 mm (CTDI₁₀₀ as defined by the European Guidelines in CT [3]).

The CTDI₁₀₀ can be determined using a pencil ionization chamber positioned along the axis of rotation of the scanner, with its centre in the centre of the scan-plane. The CTDI₁₀₀ is derived from the expression:

where D is the dose measured with the chamber and L is the sensitive length of the chamber (100 mm for this case). In practice CTDI₁₀₀ is expressed in terms of absorbed dose to air. It can be measured in air (CTDI_100,air) or in phantom (CTDI_100,phantom).

In the European CT guidelines [3] two quantities are proposed for setting reference dose levels: weighted CTDI (CTDI_w) per slice (serial scanning) or per rotation (helical scanning), and dose–length product (DLP) per complete examination.

The CTDI_w is defined by the relation:

where CTDI_100,c represents the CTDI₁₀₀ measured at the centre of the dosimetry phantom, and CTDI_100,p represents an average of measurements of CTDI₁₀₀ at four different positions 10 mm below the surface of the phantom. CTDI values can vary with nominal slice thickness, especially for the narrowest thicknesses.

The DLP per slice is defined as the product of the normalized CTDI (i.e. _nCTDI_w (mGy mAs^-1)) and the slice thickness T:

where C is the X-ray tube current-time product (in mAs) per serial slice or helical rotation. For a series of many slices of the same thickness, the total DLP is obtained by multiplying the DLP for one slice by the total number of slices or rotations (N).

For multislice CT, where more than one slice can be acquired simultaneously, the CTDI₁₀₀ and the DLP are defined as:

where M is the number of simultaneously acquired slices each of thickness T. This M factor is included in the expression in order to consider the total beam width collimated by the tube collimators (X-ray collimation). The CTDI_w depends on the selection of exposure settings; the DLP depends on the volume of irradiation and overall exposure for an examination.

Free-in-air axial measurements of CTDI_100,air for different tube voltage-slice thickness combinations were made at each CT unit participating in the study. A pencil ionization chamber (model PC-4P, Capintec, Pittsburgh, PA) connected to a Capintec electrometer (model 192) and positioned on the axis of rotation was used for the measurements. The CTDI_w was calculated from Equation 4 using the values of CTDI₁₀₀ measured for the 16 cm standard phantom. The phantom was positioned, with the help of lasers, symmetrically in the centre of the gantry. The pencil chamber was inserted in the centre and at four positions in the periphery of the phantom. The position of the phantom and chamber within the gantry was verified from the image obtained.

Effective dose assessment
In order to compare CT procedures with other types of radiological examinations and to estimate the related radiological risk, the effective dose (E) was also assessed [5]. E can be estimated from measurements of free-in-air axial doses utilizing normalized organ specific dose factors (f(organ,z)). These conversion factors have been determined for paediatric mathematical anthropomorphic phantoms using Monte Carlo methods [13].

where w_i is the tissue weighting factor of the organ and

+z and -z are the upper and lower boundaries of the scan volume. If the table feed and slice thickness are not numerically equal, a correction has to be applied according to the pitch factor:

For multislice CT the pitch is defined as:

	Results

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Results from all participating hospitals are presented in relation to three patient ages (1 year, 5 years, and 10 years) and three types of common procedure (brain, chest and abdomen). We found that all units but one (unit C) have developed their protocols in relation to the size of the paediatric patient. In units A, B, E and F the protocols have been adapted to the age of the patient. In unit G the protocols have been adapted to the weight of the patient. In unit D the brain protocol was adapted to the age of the patient, and the chest and abdomen protocols were adapted to the weight of the patient. Unit C used for all procedures but one (brain examination) the same protocols for adults as for children (Table 2).

	Discussion

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Normalized organ specific dose factors for CT examinations are available from the literature for tomographic paediatric models representing children of 8 weeks and 7-years-old [13], and for tomographic adult models representing a male and a female of standard size [14]. For children of 1-, 5-, 10- and 15-years-old, organ doses were interpolated from the values available. Organ doses for the baby, child, and adult phantoms were evaluated separately by summing the conversion factors for those slices representing the scanned body region (brain, chest or abdomen). Afterwards, appropriate interpolation (exponential for ages 1- and 5-years, linear for ages 10- and 15-years) was done as a function of the age. For dose calculation purposes, weight-specific protocols have been converted into age-specific protocols using the characteristics of the mathematical anthropomorphic phantoms defined by Cristy [10]. The observed CTDI_w and DLP values have been compared with the initial reference levels proposed for Europe (Table 6) [7]. CTDI_w values for brain examinations were within reference values, with the exception of unit D for the 10-year-old group. For chest examinations, units E and F presented CTDI_w values above the reference for the 1-year-old group. For abdomen examinations, units C, E and F presented CTDI_w values above the reference for the 1-year-old group. Unit F was also above the reference for the 5-year-old group. Unit C was just within the reference for the 5-year-old group. All DLP values for brain examinations were within reference levels, with the exception of unit D for the 10-year-old group. For chest examinations unit E presented DLP values above the reference for the 1-year-old group. For abdomen examinations unit C presented DLP values above the reference for the 1- and 5-year-old groups. Unit F was just above the reference for the 5-year-old group.

For brain examinations, unit D presented the highest effective dose values, followed by unit C. The lowest values were observed in general for units B and E. For chest examinations, high effective doses were observed for units C, E, F and G. The lowest values were observed for units D and A. For abdomen examinations the highest dose values were observed for units C, E, F and G. The lowest doses corresponded to units D, B and A; an exception was that unit B presented a high dose value for the 10-year-old group.

The results obtained have been compared with values from other studies recently published in the literature (Table 7). It can be observed that the values of E for brain and chest procedures reported in this study were lower than the values reported by Huda for the same types of procedure [12, 17, 18]. For abdomen procedures, the range of E observed in our study was larger than the range reported by other authors [12, 16], with both lower and higher values. It has been observed that there is no common trend in the selection of protocols according to the patient size. The centres applied their selection criteria based on results obtained from different sources: their own experience, data available in literature and manufacturer recommendations. Some centres divided the whole range of patient size into only two or three groups by age (unit A: <1-year, >1-year –brain examination, <10-years, >10-years –chest and abdomen examinations; unit B: <6-years, >6-years –all examinations; unit C: <10-years, >10-years –brain examination; units E and F: <1-year, >1-year and <10-years, >10-years –brain examination, <10-years >10-years and <13-years, >13-years –chest and abdomen examinations), while some centres classified the patients into six groups by weight (units D and G). It was also observed that while some centres adapted both mAs and slice thickness/table feed parameters according to the patient size (units A, C and D –brain examination; unit B –all examinations; unit G –chest and abdomen examinations), other units changed the slice thickness/table feed settings (unit A –chest and abdomen examinations, units E and F –all examinations), whereas others changed only the mAs settings (unit D –chest and abdomen examinations). From the technical parameters that can be selected by the CT operator, those that have a high influence on dose and image quality are the mAs, tube voltage and pitch. These parameters are the main cause of the wide variation in doses between different CT units. This is particularly observed between units of the same model: units B and C – 1 and 5 year old abdomen examinations, DLP ratio of 3.4; units D and G – 1 and 5 years old abdomen examinations, DLP ratio of 2.6 to 3.4. Another cause of dose variation is the model of scanner and its physical characteristics: gas or semiconductor detectors, single or multirow detectors, and the tube voltages available. Zeman et al reported that solid state detectors allow doses approximately 30% lower than those of gas detectors [20].

In CT scanning the detector dose should be approximately constant for any size of patient in order to maintain a constant level of image mottle in the CT scans. The mAs selected for examinations on children can be reduced compared with the mAs selected for adults with no detrimental effect on the resultant quality of the image. Two arguments support this fact [21]. First, the percentage of energy fluence transmitted through a patient of small mass is much higher than the percentage transmitted through a patient of large mass. Second, the exit photon energy slowly decreases with decreasing patient mass. A decrease in photon energy increases the image contrast. Therefore, the increment in contrast due to the reduction in photon energy will allow image for small patients to have a better contrast-to-noise ratio than those obtained for adult patients using the same tube voltage settings. The potential for increased noise caused by lowering the mAs in younger patients will be counterbalanced by the smaller size of younger patients. The dose is directly proportional to the selected tube current–time product; therefore a reduction in mAs by 50% results in a reduction of dose by half.

Lucaya et al found that for high-resolution thorax CT a reduction in mAs from 180 to 50 did not show differences in image quality and a reduction in dose of 72% was obtained [22]. Robinson et al found that paediatric abdominal CT scans could maintain diagnostic quality with a least a 50% reduction in dose from the manufacturer's suggested protocol [23]. Kamel et al reduced the tube current–time product used for CT of the paediatric pelvis from 240 mAs to 80 mAs, achieving a substantial reduction in dose without a recognisable deterioration of diagnostic image quality [24]. Donnelly et al suggested the use of tube current ranging from 40 mA to 140 mA and 60 mA to 170 mA for single-detector helical CT of the chest and abdomen, respectively [4]. Grees et al studied the effects on dose and image quality of an attenuation-based on-line modulation of the tube current system. They found the mAs product to be reduced typically by 10% to 60% depending on patient geometry and anatomical region, without a decrease in image quality [25]. Wong et al developed an objective method to select mAs for CT brain examinations of paediatric patients based on the size of the head [26]. They observed that the use of this method allowed patients to receive a radiation dose reduced by as much as 246% while keeping the image noise to an acceptable and consistent level. In our study we observed for two multislice CT units (D and G) both high and low dose values in comparison with the other scanners. McCollough et al evaluated a multislice CT system and observed a great improvement in acquisition speed with a comparable image quality relative to single-slice scanner from the same manufacturer [27]. However, they noted a significant increase in radiation dose (approximately 50%) relative to single-slice CT. The great variability in body size among the paediatric population makes the adjustment of scanning parameters necessary for optimum performance since they are the main determinants of radiation dose to the child. It has been shown by Huda et al how CT technique factors result in relatively high patient doses when these are not adjusted on the basis of patient size [17]. They found effective doses of 9.6 mSv and 5.4 mSv for 10 kg and 70 kg patients, respectively, for a CT chest when the same settings were used.

It is not straightforward to draw a conclusion on whether size or age related parameter selection is more appropriate for a better patient dose distribution in paediatric CT. From the two units which used weight-related parameter selection, unit D presented the lowest doses for chest examination and low doses for abdomen examination, while unit G presented medium to high doses for abdomen and chest examinations (brain examination is age-based in unit D and is not performed in unit G). These results could have been influenced by the fact that the mAs values selected by unit D were those values recommended by the manufacturer for that specific scanner model, while the mAs values selected by unit G were those values recommended by the study of Donnelly et al [4], which are based on a scanner model (single-detector) other than the model at unit G (multidetector). It was observed that when the age or weight range is divided into groups of large size there is the potential risk of giving excessively high doses to the small ages/low weights, or excessively low doses to the large ages/high weights within the groups (unit C –abdomen examination, unit A –brain examination).

	Conclusion

Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 
Results obtained clearly show large differences between hospitals for the same type of examination and age group. The ratio of the highest E value to the lowest E value ranged from 1.1 to 4 for brain examinations, 1.2 to 5.4 for thorax examinations, and 1 to 6.6 for abdomen examinations. In the small sample analysed (four centres) all CT units but one have adjusted their protocols on the basis of patient's age or weight, which can be interpreted as a first step towards optimization of performance and patient protection. However, the highest values of E did not always correspond to the unit with non-adapted paediatric protocols as might be expected. None of the units analysed presented the highest or lowest dose values for all the examinations. Therefore, no conclusion can be drawn as to which technique is better for parameter selection.

In some CT units the pitch factor was decreased for small children, resulting in higher doses while the mAs settings were the same for the whole range of patient sizes. Pitch and dose are inversely related: a decrease in pitch by half increases the dose by two. This shows the importance of careful selection of technical parameters for each type of examination. Scanning techniques have become more sophisticated, and radiologists are faced with an expanding array of options, including the selection of scanning parameters. This is observed in multislice scanners where scanning parameters such as effective tube-current, slice collimation, slice-width and volume pitch are also available. If CT parameters used for paediatric patients are not adjusted on the basis of examination type, age and/or size of the child then some paediatric patients may be exposed to an unnecessarily high radiation dose during CT examinations. In the present study, unit C used the same settings for adults and children for abdomen CT. This resulted in E values of 17.1 mSv for a child aged 5 years compared with 10.5 mSv for an adult. Using the attributable lifetime risk multiplicative model [5] the radiation-related risks were estimated to be 2400 and 790 induced cancers per million exposed children and adults, respectively. It can be seen that paediatric CT can result in significantly increased lifetime radiation risk over adult CT.

Received for publication September 11, 2002. Revision received April 22, 2003. Accepted for publication June 2, 2003.

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Top Abstract Introduction Materials and methods Results Discussion Conclusion References

 

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