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British Journal of Radiology (2007) 80, 657-667
© 2007 British Institute of Radiology
doi: 10.1259/bjr/97260522
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Full paper

Comparative evaluation of organ and effective doses for paediatric patients with those for adults in chest and abdominal CT examinations

K Fujii, MSc T Aoyama, PhD S Koyama, PhD and C Kawaura, PhD

Graduate School of Medicine, Nagoya University, Daikominami, Higashi-ku, Nagoya, Japan

Correspondence: K Fujii, Graduate School of Medicine, Nagoya University, Daikominami, Higashi-ku, Nagoya, Japan. E-mail: fujiikei{at}nirs.go.jp


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results and discussion
 Conclusions
 References
 
Patient doses in paediatric and adult CT examinations were investigated for modern multislice CT scanners by using specially constructed in-phantom dose measuring systems. The systems were composed of 32 photodiode dosemeters embedded in various tissue and organ sites within anthropomorphic phantoms representing the bodies of 6-year-old children and adults. Organ and the effective doses were evaluated from dose values measured at these sites. In chest CT examinations, organ doses for organs within the scanning area were 2–21 mGy for children and 7–26 mGy for adults. Thyroid doses for children were frequently the highest with a maximum of 21 mGy. In abdominal CT examinations, organ doses for organs within the scanning area were 3–16 mGy for children and 10–34 mGy for adults. Effective doses evaluated for children and adults were found to be proportional to the effective mAs of CT scanners, where linear coefficients were specific to the types of CT examinations and to the manufacturers of CT scanners. Effective doses in paediatric chest CT and abdominal CT examinations were lower than those in adult examinations by a factor of two or greater on average for the same CT scanners because of the lower effective mAs adopted in paediatric examinations.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results and discussion
 Conclusions
 References
 
Recent technological innovations in CT scanning, contributing to a large improvement in diagnostic image quality and the reduction of examination time, have resulted in an increase in the frequency of CT examinations not only for adults but also for children; the frequency of CT examinations in children aged from 0 to 15 years in developed countries is 6–11% [1, 2]. Increasing clinical use of paediatric CT has raised concerns about the possible detriment to the health of children caused by radiation exposure [3]. This is because CT examinations deliver relatively high exposure doses compared with other X-ray diagnostic techniques, and children are much more sensitive to radiation-induced cancer than adults [4] and have a longer time to accumulate the radiation effect throughout their lives. Another reason is that, until recently, paediatric CT examinations have been conducted using the same imaging protocols as those used in adult CT examinations at many medical facilities, resulting in higher doses than are received by adults [3].

Nowadays, the tube current of CT scanners is usually either adjusted according to the age or the weight of patients, or controlled by using automatic current modulation techniques to reduce exposure doses in paediatric CT examinations without affecting diagnostic image quality [5–11]. Because dose values in CT examinations using these scan techniques will be different from those using previous CT scanners and scan parameters, it is important to examine the dose values in paediatric CT examinations using modern CT scanners with dose reduction systems installed.

One of the methods used to estimate the doses for patients undergoing CT examinations is the calculation of organ and effective doses by using Monte Carlo simulations of photon interactions within a simplified mathematical model of the human body [12]. In this case, the organ and effective doses were calculated from the computed tomography dose index (CTDI) and dose length product (DLP) displayed on the console monitor of the CT scanner, and conversion factors derived from dose calculation software on the basis of Monte Carlo simulations [13]. Calculated dose values, however, should be verified through dose measurements using anthropomorphic or cylindrical phantoms and the same exposure conditions as the calculation [14]. Hence direct measurement of absorbed doses at various organ or tissue positions in anthropomorphic phantoms is important to evaluate organ and effective doses. Another benefit of using an anthropomorphic phantom and not a simple Perspex phantom for the measurement of exposure dose is the ability to assess organ doses in CT examinations using the automatic tube current modulation system of CT scanners. With this system, the tube current increases in such scan regions as the shoulder and pelvis or through the thicker, lateral projection where the attenuation of X-ray beams is larger, whereas it decreases in such regions as the thorax or in the anteroposterior projection where X-ray attenuation is smaller. The modulation of tube current in the CT scan for an anthropomorphic phantom is coincident to that for a human body.

In the present study we devised in-phantom dose-measuring systems by using newly developed photodiode dosemeters implanted in various tissue and organ sites of paediatric and adult anthropomorphic phantoms to evaluate organ and effective doses. Using these systems we investigated exposure doses for paediatric and adult patients undergoing routine chest CT and abdominal CT examinations at several hospitals in Japan, and compared the doses for paediatric patients with those for adults.


   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results and discussion
 Conclusions
 References
 
Paediatric phantom dosimetry system
Small-sized silicon spherical photodiodes with a diameter of 2.4 mm (Kyosemi KSPD1840C2; Kyosemi Corporation, Kyoto, Japan), were used as dosemeters, which were implanted in various tissue and organ sites in a paediatric anthropomorphic phantom. The photodiode, which is shown in Figure 1Go, has the advantage of isotropic sensitivity to incident photons except for the rear side, i.e. connection to lead wires. The sensitive surface of the photodiode was wrapped up with 15 µm thick aluminium foil for electromagnetic and light shielding, and a pair of twisted carbon fibre cables, which are tissue equivalent, were connected to derive the current signal from the photodiode.


Figure 1
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  		Figure 1. Silicon spherical photodiode, Kyosemi KSPD1840C2, used as the X-ray sensor of the in-phantom dosemeter, and the angular response of the dosemeter for longitudinal direction measured through a single rotation of the X-ray beam of the CT scanner. Incident angle of the X-ray beam is shown.

 
The angular response of the dosemeter, i.e. directional dependence of dosemeter sensitivity to incident X-rays, was examined by using a sequential CT scanner. A photodiode dosemeter was supported with thin polystyrene foam free-in-air at the centre of rotation of the CT scanner. The angular response of the dosemeter for lateral direction was entirely flat during a rotation of the X-ray tube. However, the response for the longitudinal direction, which is shown in Figure 1Go, indicated that the relative sensitivity for incident X-rays decreased to 0.4 at 180°. If X-rays with isotropic intensity were incident on the photodiode dosemeter from all directions, dosemeter sensitivity would be reduced by 5%. The sensitivity loss of the dosemeter at the rear side, however, would be negligible in our case, i.e. in-phantom use, where direct X-rays from the X-ray tube were incident from the lateral direction in CT examinations and scattered X-rays alone were incident from the longitudinal direction.

Output linearity and X-ray energy dependence of the dosemeter sensitivity were measured with an X-ray generator by placing the dosemeter at distances of 1–3 m from the X-ray tube. The dose calibration of each dosemeter was performed against a Radcal 1015 dosemeter with a 6 cm3 ion chamber attached, which was placed adjacent to a photodiode dosemeter, a few centimetres apart, at the same distance from the X-ray tube in an irradiation field. The ion chamber dosemeter is a tertiary standard, which was calibrated at a laboratory of the Japan Quality Assurance Organization in April 2001; dosemeter readings were calibrated to exposure dose values at nine points of effective or equivalent photon energies [15] from 20 keV to 72 keV. The values of exposure dose in roentgen obtained with the ion chamber dosemeter were converted to the values of absorbed dose for soft tissue by using the ratio of mass energy absorption coefficient of soft tissue [16] to that of air at the effective energy of X-rays used.

The output linearity of the dosemeter measured at a tube voltage of 120 kV and an intensity range of 2–80 mAs was found to be excellent over a dose range from 0.1 mGy to more than 10 mGy. The minimum detection limit of the dosemeter was estimated to be 0.1 mGy. The X-ray energy dependence of dosemeter sensitivity or the energy response of the dosemeter was measured with aluminium filters attached to the window of the X-ray tube at a constant tube voltage of 120 kV. Figure 2Go shows observed dosemeter sensitivity as a function of the effective energy of X-rays. This energy response could be utilized to derive a "conversion factor" to convert output voltage from the photodiode dosemeter to absorbed dose for soft tissue at the effective energy of X-rays used, where the conversion factor is the reciprocal of dosemeter sensitivity.


Figure 2
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  		Figure 2. X-ray energy dependence of dosemeter sensitivity.

 
The photodiode dosemeters were installed within a paediatric anthropomorphic phantom manufactured by Kyoto Kagaku (Kyoto, Japan), modelling the standard body of a 6-year-old Japanese child, 20 kg in weight and 115 cm tall. The phantom, shown in Figure 3Go, was composed of three types of tissue substitute corresponding to soft tissue, cortical bone and lung for which X-ray absorption was nearly equivalent to soft tissue, bone and lung, respectively. In total, 32 photodiode dosemeters were installed in various tissue and organ sites of the phantom, where the tissues and the organs were those with tissue weighting factors listed in the International Commission on Radiological Protection (ICRP) Publication 60 [4]. These sites, shown in Table 1Go, were decided from an anatomical chart and the three-dimensional CT images of the phantom, as shown in Figure 4Go.


Figure 3
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  		Figure 3. An adult phantom(left) and a paediatric phantom (right) manufactured by Kyoto Kagaku, Japan.

 

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  		Table 1. Number of photodiode dosemeters implanted in each tissue or organ required for the effective dose evaluation in ICRP Publication 60[4]

 

Figure 4
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  		Figure 4. Three-dimensional CT image of the paediatric phantom indicating the location of the 32 photodiode dosemeters (short bars) distributed in the phantom.

 
Adult phantom dosimetry system
Planar silicon pin photodiodes, Hamamatsu S2506-04 (Hamamatsu Photonics K K, Hamamatsu, Japan), were used as dosemeters, which were implanted in various organ sites in an adult anthropomorphic phantom. The photodiode, possessing a relatively large sensitive area of 2.8x2.8 mm2, was moulded out of black resin in which a silicon wafer with metal-plate backing was placed. The construction detail and the characteristics of the photodiode dosemeter were fully described in our previous paper [17]. This type of photodiode was adopted because of the perfect output linearity and the small energy dependence of the sensitivity compared with the spherical photodiode used in the paediatric phantom dosimetry system, although there is some larger size and directional or angular dependence of the sensitivity. To improve the angular response of the dosemeter, two photodiodes were glued together back to back and used as a single dosemeter with parallel connection [17]. This resulted in an approximately flat angular response of the sensitivity except for the rear side, i.e. connection to lead wires, of the dosemeter. The photodiodes were wrapped in 15 µm thick aluminium foil and connected to a pair of twisted carbon fibre cables.

The adult anthropomorphic phantom used, shown in Figure 3Go, is a human torso phantom (Kyoto Kagaku THRA1; Kyoto Kagaku), which represents a standard Japanese adult male, 60 kg in weight and 170 cm tall. The phantom, composed of three types of tissue substitute corresponding to soft tissue, cortical bone and lung, was used as both male and female with the left breast attached externally. Photodiode dosemeters, 32 in number, were installed at the positions of various tissues and organs of the phantom [18] as indicated in Table 1Go. Systematic uncertainties arising from the use of different types of dosemeters in paediatric and adult phantoms are negligible as both types of dosemeter were calibrated using the same ion chamber dosemeter.

Evaluation of organ and effective doses
Output voltage signals generated from 32 photodiode dosemeters were converted to absorbed doses for soft tissue, in which the conversion factor, which is the reciprocal of dosemeter sensitivity as shown in Figure 2Go, was estimated for each dosemeter separately at the effective energy of X-rays used. The absorbed dose to each tissue or organ dorgan was evaluated using the absorbed dose for soft tissue dsoft tissue from the equation:


Formula 001

where [(µen/{rho})organ /(µen/{rho})soft tissue] is the ratio of the mass energy absorption coefficient of the tissue or organ to that of the soft tissue.

Organ dose is used to refer to the mean absorbed dose for a specific organ [19]. For small organs such as the thyroid and gonads, the absorbed dose value obtained from a dosemeter implanted in the centroid of an organ was adopted as the organ dose. For organs with a large volume such as the lung, liver and colon, two to five dosemeters were set at the centroid of each organ subdivided equally, and the mean dose value was regarded as the organ dose. The organ dose for the colon was calculated according to ICRP Publication 67 [20].

The organ dose for red bone marrow Dbone marrow was evaluated from the equation:


Formula 002

where dabs,i is the absorbed dose for soft tissue at each measuring point in various bone tissues and Ai is the weight fraction of each red bone marrow. The weight fraction, i.e. contribution of individual red bone marrow to total weight, can be obtained from ICRP Publication 70 [21] and the values for children and adults are shown in Table 2Go.


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  		Table 2. Weight fractions of red bone marrow and bone minerals listed in ICRP Publication 70[21]

 
With regard to organ dose for bone surface, ICRP Publication 26 recommends that bone surface dose should be evaluated as the average dose within a 10 µm layer of endosteal cells and epithelial cells on bone surfaces [22]. Because estimation of the average dose within 10 µm of the endosteal cell layer on bone surfaces is difficult, the absorbed dose at the interface between cortical bone and soft tissue was evaluated as the bone surface dose. Absorbed doses at bone surfaces are known to vary according to the incident direction of X-rays to bone surfaces [15]. Because X-rays were expected to be incident on the surfaces of cancellous bones from all directions evenly, the organ dose for bone surface Dbone surface could be evaluated from the average of doses for cortical bone and for soft tissue:


Formula 003

where dabs,i is the same value as in Equation (2), Mi is the weight fraction of mineralized bone as shown in Table 2Go [21], and (µen/{rho})cortical bone and (µen/{rho})soft tissue are the mass energy absorption coefficients for cortical bone and soft tissue, respectively.

The absorbed dose for skin was measured by using two extra dosemeters attached to the surface of the front and the side of the adult or the paediatric phantom in the scan region of the CT examination where the direct X-ray beam was irradiated. The organ dose for the skin was evaluated by multiplying the average dose value of these two dosemeters by the ratio of the irradiated area to the gross surface area of the adult or paediatric phantom. The gross surface area of the adult phantom with imaginary head, arms and legs was estimated to be 1.60 m2, and that of the paediatric phantom with imaginary arms and legs was 0.76 m2.

Nine organs or tissues, i.e. adrenals, brain, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus, were categorized into "the remainder" designated in ICRP Publication 67 [20] for the evaluation of the effective dose. The average dose for these organs was adopted as the organ dose for the remainder. Muscle dose, however, was excluded because of the difficulty of dose measurement for muscle distributed in the whole body. Brain dose for the adult was assumed to be zero except for head examinations because of the headless phantom used.

The effective dose E was evaluated according to ICRP Publication 60 [4] and was given as an average value between males and females. The effective dose values obtained in our study, however, might be changed in the near future because tissue weighting factors to determine the effective dose were modified in new draft recommendations of the ICRP.

CT scanners
CT scanners used to measure exposure doses for adult and paediatric patients were modern multislice or multidetector CT scanners with 8 and 16 detectors from Toshiba (Toshiba Medical Systems Corporation, Otawara, Japan) and 16 and 64 detectors from Siemens (Siemens AG, Erlangen, Germany), which are listed in Table 3Go. Some of them have automatic current modulation systems to reduce exposure dose. The system installed in scanners A, B, C and D in Table 3Go is named "Real EC" (Toshiba, Japan) and is based on current modulation along the patient's z-axis, adjusting the tube current slice-by-slice automatically to achieve user-selected standard deviation (noise level) in the image data [23]. The system installed in scanner E is named "Care Dose" (Siemens, Germany) and is based on the angular modulation technique, continuously varying the tube current by calculating the X-ray attenuation in the patient for each X-ray projection angle [24, 25]. The X-ray attenuation profile is determined in the first 180° during a rotation of the X-ray tube by using the fixed tube current, and the tube current is modulated in the next 180° by the feedback of the attenuation profile date acquired in the previous 180° [25]. The system installed in scanners F and G is named "Care Dose 4D" (Siemens, Germany) and is an advancement of the previous Care Dose system; it is based on both angular and z-axis modulations [25, 26]. Although many of the paediatric and adult CT examinations were performed with these automatic current modulation techniques, paediatric chest CT examinations with scanners B and D, and paediatric abdominal CT examinations with scanners C and D, were implemented with manual selection of the fixed tube current technique based on the weight or the age of paediatric patients [27]. Because the CT scanners and examination conditions differed among hospitals, dose measurements were carried out with the scan parameters routinely used at each hospital, as indicated in GoGoGoTables 4–7Go.


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  		Table 3. Multislice CT scanners used for the measurement of exposure dose for paediatric and adult patients

 

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  		Table 4. Organ doses obtained in routine paediatric chest CT examination

 

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  		Table 5. Organ doses obtained in routine adult chest CT examination

 

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  		Table 6. Organ doses obtained in routine paediatric abdominal CT examination.

 

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  		Table 7. Organ doses obtained in routine adult abdominal CT examination

 
With regard to the scan parameters shown in GoGoGoTables 4–7Go, Siemens uses the term "effective mAs" to describe actual examination mAs, which is defined in the form [25]:


Formula 004

where beam pitch is defined as the ratio of table feed per gantry rotation to collimated beam width. With regard to scan parameters on Toshiba CT scanners, "effective mAs" was estimated by substituting values of average tube current, rotation time and beam pitch to Equation (4).

Measurement uncertainties
Exposure doses for chest CT and abdominal CT examinations were measured using paediatric and adult dosimetry systems and scan parameters in the examinations performed routinely in each medical facility, and the organ and effective doses were evaluated. Scan ranges of the chest CT and abdominal CT examinations were from the upper end of the lung apex to the lower region of the diaphragm and from the upper region of the diaphragm to the pubic symphysis, respectively, and scan length in each CT examination was approximately the same among medical facilities. The start and end positions of the CT scan, however, are slightly different between the phantom and the human body, and also among patients undergoing CT examination. Uncertainties of doses for organs on the periphery of the scan volume, e.g. the thyroid in adult chest scans and the testis in adult abdominal scans, were estimated to be a maximum of 50% and 60%, respectively, although Chapple et al [28] estimated the uncertainty to be 20%.


   Results and discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results and discussion
Conclusions
 References
 
Organ doses obtained in paediatric chest CT examinations are shown in Table 4Go. Doses for thyroid, lung, breast, oesophagus and liver irradiated by primary X-ray beam were 2.0–20.8 mGy, largely varying among CT scanners because of the differences in scan conditions and the types of CT scanners used. The thyroid dose in the case of scanners B, D and G was higher than any organ doses within the scan range. The paediatric chest CT examinations with scanners B and D were performed with fixed current and with scanner G were conducted with the highest effective mAs. The tube current at the position of the thyroid in the cases of B and D was higher than that in the cases of A and C, which used the current modulation technique. On the other hand, for CT scanners A, C, E and F, using the current modulation technique, the dose difference among organs within the scan range was small at less than 2 mGy.

Organ doses obtained in adult chest CT examinations are shown in Table 5Go. Doses for the organs positioned in the chest region were 7.1–25.8 mGy. Organ doses for lung and oesophagus were higher for all CT scanners except for E, followed by organ doses for thyroid, breast, liver, stomach and kidneys. Dose values observed for CT scanner E were the lowest in both paediatric and adult chest CT examinations because of the lowest effective mAs used in this scanner.

Organ dose values in paediatric chest CT examinations were compared with those in adult examinations for the same CT scanners. It is seen from GoTables 4 and 5Go that, although paediatric chest CT examinations delivered lower organ doses for lung, breast, oesophagus and liver than adult examinations, the thyroid dose for children in the case of scanners B, D and G was higher than that for adults. The tube current at the site of the paediatric thyroid using scanners B, D and G would be higher than that of the same site in adults. Because the cancer risk to the thyroid from radiation exposure in childhood has been estimated to be greater than that in adulthood or of any other organ in childhood because of the higher radiation sensitivity of the thyroid in childhood [4], the thyroid dose in paediatric chest CT scans should be lowered as much as possible without interfering with the quality of the chest CT image. The use of the tube current modulation technique and a commercially available thyroid shield is advisable to reduce the thyroid dose in paediatric chest CT examinations.

Organ doses in paediatric abdominal CT examinations are shown in Table 6Go, indicating that doses for organs from the liver to the bladder positioned in the upper abdominal and pelvic regions were 3.4–16.1 mGy. These values also varied among CT scanners because of the difference in scan conditions and the types of CT scanners used. Doses for organs within the scan area were the highest for scanner C using a fixed tube current. The difference in doses among organs within the scan range was small at less than 3 mGy for all CT scanners.

Organ doses in adult abdominal CT examinations are shown in Table 7Go. Doses for organs positioned in the upper abdominal and pelvic regions were 10.3–33.5 mGy, whereas doses for organs irradiated by indirect X-ray beam were below 1.0 mGy.

Organ dose values in paediatric abdominal CT examinations were compared with those in adult examinations for the same CT scanners. It can be seen from GoTables 6 and 7Go that, although paediatric abdominal CT examinations delivered lower doses for organs within the scan range than adult examinations in most cases, the paediatric scans in the case of scanner C gave organ doses that were as high as those for the liver, stomach, kidneys and colon in adult scans. This is because the same order of effective mAs is used in paediatric abdominal CT examinations as in adult examinations. This high effective mAs was required in the CT examinations at some facilities to maintain a lower diagnostic noise level as density differences between pathological lesions and normal parenchyma in a child's body are less than those in an adult's body because of the thinner layers of visceral fat in children.

Doses for red bone marrow with a high induced cancer risk were compared between paediatric and adult CT examinations. Doses for red bone marrow in children in chest CT examinations were lower than those in adults by factors of 1.6–4.9 (average 3.5), and in abdominal CT examinations by factors of 1.4–3.9 (average 2.7). Lower dose values for red bone marrow observed in paediatric CT examinations are due to reduced effective mAs and to the differences in the distribution of red bone marrow in the bodies of children and adults.

Organ dose values in paediatric abdominal CT examinations obtained using modern CT scanners were compared with those evaluated in Europe and the USA in 1989 [3]. Doses for liver and stomach in the paediatric CT examinations using scanners A–G were 4.3–16.1 mGy, whereas doses for these organs from 1989 were evaluated to be more than 30 mGy. These results indicate that doses in paediatric CT examinations using modern CT scanners are 2–6 times lower than those evaluated in the past.

Effective doses evaluated in chest CT and abdominal CT examinations are shown in Table 8Go. Effective doses in paediatric chest CT examinations were 1.3–7.4 mSv, whereas those values in paediatric abdominal CT examinations were 2.8–10.5 mSv. For the same CT scanners, Table 8Go shows that paediatric chest scan and abdominal scan protocols gave effective doses that were lower by a factor of 2.6 and 2.1 on average, respectively, than those in adult scans. Effective doses in paediatric CT examinations were reduced by adapting the tube current modulation technique depending on the weight or the age of the patient [5–9], or by using the automatic current modulation systems [10, 11, 23–26].


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  		Table 8. Effective doses and the ratios of the dose for adults to that for children, A/C, evaluated for each CT scanner, where Ave is the average of the ratios

 
Paediatric CT examinations performed with the same mAs as that required in adult examinations led to high exposure doses for paediatric patients [29]. In this situation, the lifetime CT-attributable cancer mortality risks of children at 5 years old were estimated to be approximately 2.5 times higher than those of adults at 30 years old [3]. However, the reduced effective doses in paediatric CT examinations allowed us to reduce radiation-induced lifetime cancer mortality risks for children at 5 years old to the same risk level as that for adults at 30 years old [3].

The organ and effective doses obtained with our 32 photodiode dosemeter system in adult abdominal CT examinations were compared with those evaluated using more than 100 thermo-luminescence dosemeter (TLD) chips implanted in a Rando phantom [30]. Organ doses obtained with the same CT scanner and technical parameters varied by 8–33% between the Rando phantom system and ours. The effective doses, however, were in good agreement with a difference of 1–6% in these systems [31].

The effective doses evaluated with our dosimetry system in paediatric CT examinations were compared with those published in the literature, where dose values were evaluated by Monte Carlo calculation. Pages et al [32] found effective doses for patients from 5 years to 10 years to be 1.4–6.6 mSv in chest CT and 4.3–19.9 mSv in abdominal CT examinations using single-slice and multislice CT scanners. Chapple et al [28] assessed effective doses for children from 5 years to 10 years and found values of 5.9–6.1 mSv in chest CT and 11.1–11.3 mSv in abdominal and pelvic CT examinations. Huda et al [33] evaluated the effective dose for children from 5 years to 10 years and found dose values of 3.7 mSv in abdominal CT examination. Although some of these dose values are proximate to the present results, dose values by Huda et al are lower and the values by Pages et al are higher than those in this study because of the great differences in CT scan volume and mAs values in abdominal CT examination.

GoFigures 5 and 6Go show the effective dose as a function of effective mAs in chest CT and abdominal CT examinations, respectively. The values of the effective dose for both children and adults were plotted on each graph. It is seen from GoFigures 5 and 6Go that the effective dose was proportional to the effective mAs. Experimental points attributable to paediatric CT examinations on each graph were located at lower effective doses. Two linear regression lines through the origin were calculated by using the method of least squares for each set of values obtained with the Toshiba (dashed lines) and Siemens (solid lines) CT scanners. The slope of the dashed line in chest CT examinations was approximately 0.08 (mSv mAs–1), indicating an effective dose of 8 mSv at 100 mAs with the Toshiba CT scanner. This slope was greater than that of the solid line, 0.06 (mSv mAs–1), obtained with the Siemens CT scanner. Slopes of the dashed and solid lines in abdominal CT examinations, indicated in Figure 6Go, were approximately 0.11 and 0.08 (mSv mAs–1), greater than those in chest CT examinations. This might be because of the irradiation by primary X-ray beam to organs of higher tissue weighting factor such as the stomach, colon and gonads in abdominal CT examinations.


Figure 5
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  		Figure 5. Effective dose as a function of effective mAs in chest CT examinations, where effective doses were obtained with both paediatric and adult CT scans. Linear regression lines through the origin were calculated from dose values obtained with Toshiba CT scanners(dashed line) and with Siemens CT scanners (solid line) separately.

 

Figure 6
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  		Figure 6. Effective dose as a function of effective mAs in abdominal CT examinations, where effective doses were obtained with both paediatric and adult CT scans. Linear regression lines through the origin were calculated from dose values obtained with Toshiba CT scanners(dashed line) and with Siemens CT scanners (solid line) separately.

 

   Conclusions
Top
 Abstract
 Introduction
 Methods and materials
 Results and discussion
 Conclusions
References
 
Patient exposure doses were measured in chest CT and abdominal CT examinations with modern multislice CT scanners from Toshiba and Siemens using paediatric and adult phantom dosimetry systems. Organ and effective doses for children and adults were evaluated from measured dose values. The results show that:

Lower effective mAs used in modern CT scanners delivered lower exposure doses in paediatric CT examinations than in the past, and therefore the incidence rate of radiation-induced cancer for children will be significantly lowered. The exposure dose values in paediatric and adult CT examinations evaluated in the present study will be a useful source of information for medical workers when explaining the health effects of radiation exposure to patients on a scientific basis, and will also give assistance to them when obtaining informed consent from patients and/or parents of paediatric patients.


   Acknowledgments
 
This study was supported in part by a grant-in-aid for Scientific Research (No. 16510034) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, which the authors greatly appreciate.

Received for publication July 21, 2006. Revision received November 14, 2006. Accepted for publication January 22, 2007.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results and discussion
 Conclusions
 References
 

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