Pediatr Radiol (2008) 38:415–423 DOI 10.1007/s00247-007-0732-6

ORIGINAL ARTICLE

Computing effective doses to pediatric patients undergoing body CT examinations Walter Huda & Kent M. Ogden

Received: 14 May 2007 / Revised: 7 November 2007 / Accepted: 7 December 2007 / Published online: 15 January 2008 # Springer-Verlag 2008

Abstract Background The computation of patient effective doses to children is of particular interest given the relatively high doses received from this imaging modality, as well as the increased utilization of CT in all areas of medicine. Current methods for computing effective doses to children are relatively complex, and it would be useful to develop a simple method of computing pediatric effective doses for clinical purposes that could be used by radiologists and technologists. Objective To obtain pediatric effective doses for body CT examinations by the use of adult effective doses obtained from effective dose (E) per unit dose length product (DLP) coefficients, and energy imparted to a child relative to an adult. Materials and methods Adult E/DLP coefficients were obtained at 120 kV using the ImPACT CT dosimetry spreadsheet. Patients were modeled as cylinders of water, and values of energy imparted to cylinders of varying radii were generated using Monte Carlo modeling. The amounts of energy imparted to the chest and abdomen of children relative to adults (Ren) were obtained. Pediatric effective doses were obtained using scaling factors that accounted for scan length, mAs, patient weight, and relative energy imparted (Ren).

W. Huda Department of Radiology, Medical University of South Carolina, Charleston, SC USA K. M. Ogden (*) Department of Radiology, SUNY Upstate Medical University, 750 E. Adams St., Syracuse, NY 13210, USA e-mail: [email protected]

Results E/DLP values were about 16 μSv/mGy cm for males and about 19 μSv/mGy cm for females. Ren at 120 kV for newborns was 0.35 for the chest and 0.49 for the abdomen. At constant mAs, the effective dose to 6month-old patients undergoing chest CT examinations was found to be about 50% higher than that to adults, and for abdominal examinations about 100% higher. Conclusion Adult effective doses can be obtained using DLP data and can be scaled to provide corresponding pediatric effective doses from body examinations on the same CT scanner. Keywords Effective dose . CT . Pediatric dose

Introduction CT examinations are associated with a complex 3-D pattern of energy deposition in the body, and multiple organs and tissues are normally irradiated. The major radiation risk to patients from CT examinations is the induction of the stochastic effects of carcinogenesis and genetic effects [1]. The total patient risk can be obtained by summing the total organ doses multiplied by their respective risk factors. The International Commission on Radiologic Protection (ICRP) introduced the effective dose (E) that permits nonuniform exposures to be converted into the uniform radiation dose that has the same stochastic radiation risk [2]. The effective dose thus permits different types of radiological examinations to be directly compared [3]. The effective dose is generally regarded as the best available dose descriptor for quantifying these stochastic risks in diagnostic radiology [4]; it can be converted into a corresponding estimate of detriment if proper account is taken of the demographics (age/sex) of an exposed individual [5, 6].

416

Pediatr Radiol (2008) 38:415–423 140

Water cylinder radius (mm)

a 120

100

80

Abdomen Chest

60

40 0

5

10

15

20

Age (Years) 140

b Water cylinder radius (mm)

Quantifying the amount of radiation a patient receives during a CT examination is of interest for several reasons. Quantifying patient CT doses permits comparison with the corresponding doses for different types of X-ray and nuclear medicine examinations, as well as patient doses for similar CT scans performed at different institutions [7]. Furthermore, it is possible to compare doses from CT examinations with doses from other sources of man-made exposures, as well as with those received from natural background [8]. Image quality and patient doses for a given diagnostic task are always affected by technique factors such as X-ray tube voltage (kV) and CT output (mAs) [9]. Optimizing CT requires an attempt to balance the conflicting requirements of dose and image quality; i.e. one might attempt to keep the image quality constant at different X-ray tube voltage values and identify the kV that minimizes the patient dose [10]. Energy imparted to patients is a robust dosimetry quantity that depends on the selected technique factors, irradiation geometry, and the physical characteristics of the irradiated patient [11]. The energy imparted to patients undergoing conventional radiographic and fluoroscopy examinations can be readily determined by measurement or calculation [12, 13]. For a given projection in radiography, energy imparted can be converted into corresponding values of patient effective dose [14]. Energy imparted can also be determined for patients undergoing CT scans [15– 17] and converted into a corresponding patient effective dose [17, 18]. Methods have also been successfully developed to scale adult effective doses to pediatric effective doses with a reported accuracy of 5% for chest radiographs and 11% for abdominal radiographs [14]. We describe here a method that permits adult (body) effective doses to be determined by use of the dose length product (DLP) that is available for patients scanned on most commercial CT scanners. Values of adult effective dose

120

100

80

Abdomen Chest

60

40 0

20

40

60

80

Weight (kg)

Fig. 2 Water cylinder radii used to model the size (mass) of pediatric patients as a function of (a) age and (b) weight

can be adjusted to account for differences in techniques (mAs and scan length) and energy absorption factors to generate pediatric effective doses in body CT imaging.

Materials and methods 100

Weight (kg)

Modeling patients

10

0

5

10

15

20

Age (Years)

Fig. 1 Patient weight as a function of patient age taken from Huda and Vance [19]

Figure 1 shows how patient weight varies with patient age [19]. For patient dosimetry applications, the chest or abdomen of a patient can be modeled as an equivalent cylinder of water. The radius of the cylinder of water is chosen so that the mass of the cylinder is equal to the mass of the patient, assuming that the lengths of the body region and the cylinder are equal. Figure 2 shows the radii of the water cylinders that can be taken to model patient chests and abdomens as a function of age (Fig. 2) and weight (Fig. 2) [19]. As expected, the dimensions of the water cylinder increase with increasing patient age, and the radii of the water cylinder are smaller for the chest region than for the abdomen.

Pediatr Radiol (2008) 38:415–423

417

is defined as the energy deposited in the water phantom divided by the mass of the phantom that is directly irradiated. Mean section doses as a function of water cylinder radius have been generated using Monte Carlo modeling techniques on a GE Advantage HiSpeed CT scanner operated at four X-ray tube voltages ranging from 80 to 140 kV, and these results were used in this study [20]. At a given X-ray tube voltage, energy imparted is proportional to the product of the mean section dose and the square of the radius of the water cylinder. Our proposed method for pediatric CT dosimetry is based on the energy imparted to a child relative to that of a standard 70-kg adult. The calculations generate the ratio Ren, the energy absorbed by a given child undergoing a specified CT examination relative to that of an adult undergoing the same examination on the same scanner. Ren is computed with all scan parameters kept constant (kV, mAs, beam width, pitch, etc.) for a single rotation of the X-ray tube and does not take into account the differences in scan length between adults and children (see below). Effective doses Fig. 3 Mathematical anthropomorphic phantom that is used in the ImPACT CT dosimetry and is based on Monte Carlo dose computations performed by the NRPB. The vertical scale shows how the value of z varies with patient location

Energy imparted When a stationary water cylinder is irradiated by a single 360° rotation of the CT X-ray tube, the mean section dose

We used the ImPACT spreadsheet [21], which makes use of the Monte Carlo dose computations performed by the National Radiological Protection Board (NRPB) in the UK [22, 23]. This ImPACT spreadsheet was used to compute values of effective dose (E) in a mathematical anthropomorphic phantom (Fig. 3) as well as the corresponding values of DLP. Table 1 shows values of E/DLP conversion coefficients for normal-size adult males and females undergoing CT scans on four 16-slice scanners from the

Table 1 Values of E and DLP generated for adult males and females undergoing three scans on four commercial CT scanners operated at 120 kV and 100 mAs with a pitch of 1 Body region

z values (cm)a

CT scanner model

E (mSv/100 mAs) Male

Chest

35–70

Abdomen

20–44

Pelvis

0–20

a

LightSpeed 16 Brilliance 16 Sensation 16 Aquilion 16 LightSpeed 16 Brilliance 16 Sensation 16 Aquilion 16 LightSpeed 16 Brilliance 16 Sensation 16 Aquilion 16

Values of z (long patient axis) as shown in Fig. 3.

6.0 4.5 4.4 7.1 3.6 2.7 2.7 4.3 3.0 2.2 2.2 3.5

DLP (mGy cm)

Female

6.0 4.5 4.4 7.1 4.0 3.0 3.0 4.7 4.6 3.4 3.3 5.3

E/DLP (μSv/mGy cm) Male

347 248 266 423 238 170 183 290 198 142 152 322

17.3 18.1 16.5 16.8 15.1 15.9 14.8 14.8 15.2 15.5 14.5 14.5

Female

17.3 18.1 16.5 16.8 16.8 17.6 16.4 16.2 23.2 23.9 21.7 21.9

Average±SD Male

Female

17.2±0.7

17.2±0.7

15.2±0.5

16.8±0.6

14.9±0.5

22.7±1.1

418

Pediatr Radiol (2008) 38:415–423 1.4

Relative energy imparted

1.2

1.0

0.8

0.6

0.4

0.2 40

60

80

100

120

140

Water cylinder radius (mm)

Fig. 4 Values of relative energy imparted as a function of water cylinder radius; the data were arbitrarily normalized to unity at 100 mm

major vendors for scans performed at 120 kV, which are similar to values published in the European guidelines on quality and dose criteria for CT [24]. E/DLP conversion factors for a given scanner show little variation among vendors. Because variations in scanner design (X-ray tube, beam shaping filter, etc.) are expected to affect E and DLP in a similar manner, it is reasonable to use E/DLP presented in Table 1 for any type of commercial CT scanner. In body CT imaging, the pediatric effective dose (Ep) can be obtained from a corresponding adult effective dose (Ea) using the following expression: 


Ep ¼ Ea
Lp La
mAsp mAsa
Ma Mp
ðRen Þ ð1Þ where L is the scan length, mAs refers to the average product of the X-ray tube current exposure time product, M

1.0

Ratio εr /ε140

0.8

0.6

0.4

80 kV 100 kV 120 kV 140 kV

0.2

is the patient weight, and the subscripts of each parameter differentiate between pediatric (p) and adult (a) patients. Equation 1 assumes that the adult and pediatric scans are performed on the same body region using the identical CT scanner, which is operated using the same beam-shaping filter, X-ray tube voltage, detector configuration and X-ray beam width, and pitch ratio. The rationale for Eq. 1 can be understood from consideration of the energy imparted to a child relative to that imparted to an adult provided that both examinations are performed on the same scanner operating under the same conditions (i.e. kV, pitch, detector configuration, etc.). The first two scaling factors (i.e. scan length and mAs used) are directly proportional to the energy imparted to patients; doubling either mAs or scan length clearly doubles the energy imparted to the patient. The third term (Ma/Mp) accounts for the fact that a given amount of energy absorbed by small children results in higher doses than in adults, because their mass is smaller1. The final term (Ren) accounts for the differences in size between adult and pediatric patients, and quantifies how much less energy a given child absorbs compared to an adult irradiated by the same CT scanner in each 360° rotation of the X-ray tube.

Results Energy imparted Figure 4 shows values of energy imparted to cylinders of water of varying radii at a fixed 120 kV and assuming that all other technique factors (mA, scan time, X-ray beam width, pitch ratio, etc.) are kept constant. The data shown in Fig. 4 have been arbitrarily normalized to unity at a cylinder radius of 100 mm, and the solid line is a spline fit to the computed data points for ease of viewing. The data in Fig. 4 illustrate that as the water cylinder (i.e. patient size) increases, so does the total energy absorbed by the object in the CT scanner. It is notable that the rate of increase in energy absorbed falls with increasing object size, and would eventually reach a plateau value. Figure 5 shows the relative amount of energy absorbed by various-size cylinders of water at the four X-ray tube voltages available on most modern CT scanners. For each X-ray tube voltage, we computed the ratio of ɛr, which is

0.0 40

60

80

100

120

140

Water cylinder radius (mm)

Fig. 5 Values of energy imparted to a water cylinder relative to the energy imparted to a water cylinder that has a radius of 140 mm. (The choice of a value of 140 mm for normalization was guided by use of the largest patient sizes shown in Fig. 2.)

1

A 70-kg adult patient uniformly irradiated to 1 Gy absorbs 70 J of energy and receives an effective dose of 1 Sv (i.e. 14 mSv/J); a 35-kg patient uniformly irradiated to 1 Gy also receives 1 Sv but only absorbs 35 J (i.e., 28 mSv/J). In general, if the energy deposited is the same, and the patient mass is halved, organ doses and/or effective dose will double.

Pediatr Radiol (2008) 38:415–423 Table 2 Values of Ren for children undergoing body CT examinations at 80 and 120 kV

a

The 70-kg adult absorbs 1.0 (by definition). b Values at 120 kV also apply for scans performed between 100 and 140 kV.

419 Age (years)

0 0.5 1 2 5 15

Weight (kg)a

3.5 7.5 9.9 12.3 18.5 54

the energy absorbed by a water cylinder of radius r, to ɛ140, which is the corresponding energy absorbed by a water cylinder with a radius of 140 mm, chosen to reflect the size of cylinder that approximates an adult abdomen (Fig. 2). The computed ratio ɛr falls with decreasing cylinder radius, reflecting the fact that smaller cylinders absorb less energy, as depicted in Fig. 4. It is notable that the three curves obtained at 100, 120, and 140 kV gave very similar values, whereas those for 80 kV were markedly higher. Accordingly, we computed values of Ren for chest and abdomen examinations at 80 kV and 120 kV. Table 2 shows the values of Ren for chest and abdomen CT examinations performed at 80 and 120 kV. Relative to normal-size adults, one 360° rotation of the X-ray tube deposits one-third of the energy in a newborn chest and one-half of the energy in a newborn abdomen, when scanned at 120 kV and a constant mAs. The data in Table 2 permit the use of Eq. 1) to be used to scale any adult effective dose to the corresponding value that a child would receive on the same CT scanner. The Appendix illustrates the use of Eq. 1 to obtain effective doses to children undergoing chest and abdominal CT examinations. Effective doses Table 3 shows how pediatric effective doses compare with adult effective doses from the same mAs but taking into account differences in patient scan length. The results in Table 3 were obtained by setting mAsp to be equal to mAsa (Eq. 1) and using the specified scan lengths listed in this table. At the same mAs value (i.e. normalized) pediatric effective doses generally increase as the patient size reduces, and children scanned with the same technique as adults always receive higher effective doses. The data in Table 3 show that for a chest CT examination, the dose to a 6-month-old would be about 50% higher than to an adult at the same mAs; the corresponding increase for an abdominal scan would be about 100%. Figure 6 shows how the relative effective dose varies with patient age when both adults and children are scanned using the same mAs value, and for the same scan length. The data in Fig. 6 were obtained from Eq. 1 by setting the

Chest

Abdomen

80 kV

120 kVb

80 kV

120 kVb

0.42 0.48 0.52 0.55 0.63 0.92

0.35 0.41 0.44 0.47 0.55 0.89

0.58 0.61 0.64 0.66 0.71 0.94

0.49 0.53 0.56 0.58 0.64 0.92

mAs values and the scan lengths the same for adults and children. The data in Fig. 6 therefore reflect how E/DLP varies with patient age, with both adult and pediatric DLP data obtained in a dosimetry size that is the same (either 16 cm or 32 cm diameter2). Also depicted in Fig. 6 are relative E/DLP conversion coefficients recently reported by Shrimpton et al. [25], where the effective dose was computed using Monte Carlo computational methods applied to mathematical anthropomorphic phantoms. The two sets of data were set equal for a 5-year-old, and the agreement is very good as to how E/DLP changes with patient size between the two sets of data.

Discussion The computation of patient effective doses to children is of particular interest given the relatively high doses received from this imaging modality, as well as the increased utilization of CT in all areas of medicine [26, 27]. Concern about the amount of radiation dose delivered from CT has recently become a topic of major concern [28, 29]. Protocols for performing CT examinations in children are now being critically reviewed, and techniques are being adjusted to ensure that patient doses are kept as low as reasonably achievable (ALARA) without sacrificing the valuable diagnostic information that this imaging modality can offer to patients and their physicians [30–32]. For all of these reasons, it is important to be able to quantify the effective doses to patients undergoing CT examinations. Current methods for computing pediatric effective doses normally require significant medical physics expertise [17, 20], and might therefore be difficult to implement for CT

2 The E/DLP for an adult chest is 17.2 μSv/mGy cm when measured in a body phantom; DLPs obtained from dose measurements in head phantoms would be approximately double, but the E/DLP conversion factor would be approximately one-half. DLP is a measure of the total radiation incident on the patient, and the choice of phantom size is immaterial. It is only important to be consistent—one cannot quantify DLP using doses obtained with head phantoms and use E/DLP conversion coefficients using doses obtained with body phantoms.

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Table 3 Pediatric effective doses (E) relative to adult effective doses obtained with various scan lengths, and using the same mAs for adult and child CT examinations

Age (years)

Weight (kg)

0 0.5 1 2 5 10 15 18

a

Taken from data published by Huda and Vance [19].

3.5 7.5 9.9 12.3 18.5 32.5 54 70

Chest CT examination

Abdomen CT examination

Scan length (cm)a

Relative E at constant mAs

Scan length (cm)a

Relative E at constant mAs

10 14 15 18 22 28 34 35

2.00 1.53 1.33 1.38 1.31 1.22 1.12 1

6 8 9 10 13 16 20 20

2.94 1.98 1.78 1.65 1.57 1.31 1.19 1

practitioners such as radiologists and technologists. Our motivation was to develop a method for determining pediatric effective doses that: (a) is accurate for most patient dosimetry applications; (b) is simple to apply in a 10

Relative E/DLP

Chest CT examinations

Huda & Ogden (this work) Shripmpton et al (2006)

1

0

2

4

6

8

10

12

14

16

18

20

Age (years) 10

Relative E/DLP

Abdomen/Pelvis CT examinations

Huda & Ogden (this work) Shripmpton et al (2006)

1

0

2

4

6

8

10

12

14

16

18

20

Age (years)

Fig. 6 Relative values of E/DLP obtained in this study (solid circles/ solid line) obtained from Eq. 1 where the mAs and scan length (L) values for adults and children are taken to be equal. Note the DLP values for adults and children are obtained in the same size dosimetry phantom to permit these factors to be compared. The open circles are taken from the recent study of Shrimpton et al. [25], and both sets of data were normalized to unity for a 5-year-old child

clinical setting; (c) only uses generally understood parameters such as mAs and scan length; and (d) is applicable to any current or future scanner. In CT imaging, the use of high X-ray tube voltages (e.g., 120 kV) as well as high X-ray tube filtration results in relatively high photon energies between 50 and 70 keV [33]. In soft tissue, photoelectric absorption and Compton scatter are equal at 25 keV; given that the photoelectric effect is proportional to E−3, it is clear that in most of the interactions in patients for CT spectra will be Compton scatter. The number of Compton interactions for a soft-tissue target (i.e. patients) will be approximately proportional to the total mass of the irradiated object, because Compton processes are proportional to the electron density, and the latter is directly proportional to the physical density in soft tissues. As a result, the absolute energy absorbed by an equivalent cylinder of water will be expected to approximate the energy absorbed by a patient. Our computations of mean section doses in water cylinders (Fig. 4) were performed for a CT scanner in common use in the early 1990s; those scanners differ from current CT scanners in terms of irradiation geometry, X-ray tube, and beam filtration. CT design will clearly affect the absolute amount of energy absorbed by a water cylinder, but the relative amounts that two water cylinders absorb on a given scanner is unlikely to be affected by these differences in CT scanner design. In this work, we computed relative changes in energy imparted as the patient size changes with all other factors remaining constant. For example, changing the X-ray tube voltage by ±20 kV from the standard 120 kV had minimal impact on the relative values of energy imparted shown in Fig. 4. It is also notable that different scanners result in markedly different values of DLP as well as effective dose, which reflects differences in CT scanner design (e.g., filtration). Nonetheless, the ratio of these two parameters shows relatively little variation with the specific scanner design (Table 1), and variations in E/DLP values are only about 10% with changes in X-ray beam voltage (kV) [34]. The values of Ren presented in

Pediatr Radiol (2008) 38:415–423

Table 2 are thus likely to be adequate for generating typical effective doses to children undergoing CT examinations. It is important to note that the method proposed in this study is only applicable for body examinations and must not be used in head CT for two reasons: 1. The head of a newborn is proportionally larger than that of an adult and grows rapidly in the first two years. 2. The head does not contain any significant radiosensitive organs, and adjacent sensitive organs (e.g., thyroid) receive doses that can make major contributions to the head CT effective doses. In newborns, the small distance to adjacent tissues and organs will make the use of the simplistic scaling adopted here problematic. Accordingly, we do not recommend the use of Eq. 1 for estimating effective doses in head CT examinations. The issue of how effective dose varies with patient age is nontrivial and will require explicit consideration of the two complications described [17]. In this study, we have shown that E/DLP conversion factors can be used to obtain adult effective doses for use in Eq. 1. It is important to note that effective doses obtained with the conversion factors listed in Table 1 are approximate and only pertain to specific mathematical anthropomorphic phantoms used in the dose computations performed by the NRPB. In addition, these E/DLP conversion factors will be affected by operating parameters including the actual scan length and the choice of X-ray tube kV [34]. For the highest accuracy, it is important that the best available value of adult effective dose (Ea) is used when evaluating Eq. 1. There currently exist at least two commercially available software packages that permit the computation of adult CT effective doses: CT-Expo [35, 36] and ImpactDose [37]. There have also been recent advances in both computational methods that employ voxelized patient models for CT dosimetry [38] as well as direct measurements in anthropomorphic phantoms [39]. It therefore seems likely that more accurate adult effective doses will continue to become available for use in Eq. 1, which in turn will improve our estimates of pediatric effective dose. Our method of scaling pediatric doses is in very good agreement with the independent Monte Carlo-based effective dose computations reported by Shrimpton et al. [25], as shown by the data in Fig. 6. The results presented in Table 3 are also in very good agreement with the corresponding data reported by Khursheed et al. [40] for CT scanners that make use of beam-shaping filters (e.g., GE 9800 and Philips LX). These comparisons show that our approach to pediatric CT dosimetry appears to be robust and could be used to assess the validity of alternative methods of CT dosimetry that use either computational [17] or experimental approaches to CT dosimetry [41]. It is also important to note that our approach would permit the

421

generalization of any measured or computed effective dose for a patient of a particular size, or a specified irradiation condition such as the measurements made in anthropomorphic phantoms undergoing cardiac CT [42] or the Monte Carlo computations to voxel-based phantoms in MSCT [43]. The use of Eq. 1 for pediatric CT dosimetry also offers advantages of: (a) eight patient sizes that permits more accurate interpolation; (b) application to any scan length; (c) application to any selected body region; (d) flexibility in the choice of the adult effective dose. The principal limitation to the work presented in this paper is that our method does not permit the direct estimation of individual organ doses, which might be of interest when estimating individual organ risks. No account was taken of reported differences between the mathematical phantoms normally used to obtain adult effective doses and the size/locations of organs in a given patient [43]. Our method also implicitly assumes that the pediatric patients are of normal size, and it is unclear whether Eq. 1 could be used for estimating effective doses in obese children. The issues of how well mathematical anthropomorphic phantoms represent actual patients, as well as the obvious differences among patients, are likely to be addressed by the recent advent of voxel-based phantoms combined with Monte Carlo dosimetry techniques [44]. One important benefit of the method proposed in this work is the use of very simple physics concepts (energy imparted to the patient), as well as the absence of any advanced mathematics. Specifically, Eq. 1 contains no integrals and only refers to the dose quantity of specific interest, namely the patient effective dose. The remaining terms used in Eq. 1 are intuitive and understood by most operators involved in CT (i.e. mAs, scan length, patient weight) or are provided in this article (i.e. Ren data in Table 2). Accordingly, the use of the method proposed here should not require medical physics training and should permit any CT practitioner to quantify the dose to a child undergoing a given body CT examination. Acknowledgements The research was supported, in part, by the NIH (R01 EB000460). The authors acknowledge permission to use Fig. 3 by Dr. P.C. Shrimpton and Ms. S. Edyvean.

Appendix We show two sample calculations below that serve three purposes: (1) illustrate how our approach to pediatric CT (body) dosimetry can be implemented; (2) demonstrate the relative importance of key parameters that affect patient dose; and (3) provide values of pediatric effective dose when scanning with state-of-the-art protocols that modify techniques as a function of patient size.

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Pediatr Radiol (2008) 38:415–423

Chest CT scan

50 mAs/80 kV at a pitch of 1. The newborn effective dose can be computed using Eq. 1:

A 10-year-old girl is to have a chest CT examination on a 64-slice GE VCT using 80 mAs/120 kV at a pitch of 1.5. The 10-year-old effective dose can be computed using Eq. 1:




Ep ¼ Ea
Lp La
mAsp mAsa
Ma Mp
ðRen Þ




Ep ¼ Ea
Lp La
mAsp mAsa
Ma Mp
ðRen Þ

where each term in this equation can be readily determined as follows: –

where each term in this equation can be readily determined as follows: –

– – – –

Ea can be determined for an adult chest CT examination from consideration of the technique factors (120 kV/ 170 mAs), as well as the pitch (1.5) and the adult scan length (35 cm from Table 3). Selection of these factors on the console of the CT scanner would show that this examination corresponds to a DLP of 377 mGy cm and assumes that the widest X-ray beam width was used (40 mm). Applying the female E/DLP conversion factor of 17.2 μSv/mGy cm (Table 2) produces an adult effective dose of 6.5 mSv, typical of the value expected for adult chest CT examinations. Lp/La is 28/35 since the scan length is 28 cm for the child and 35 cm for the adult (see Table 3). mAsp/mAsa is 80/170, which reflects the expected twofold reduction when performing a body scan on a 10-year-old patient in comparison to an adult. Ma/Mp is 70/32.5 as given by the data in Table 3. Ren is 0.71, which can be obtained directly from Table 2 for the specified examination type (chest), X-ray tube voltage (120 kV) and patient age (10 years).

Ep ¼ 6:5 mSv
ð28=35Þ
ð80=170Þ
ð70=32:5Þ
ð0:71Þ Ep ¼ 3:7 mSv

Abdominal CT scan A newborn boy is to have an abdominal CT examination performed on a Siemens Sensation 16 scanner operating at

3

Note that adults would normally be scanned at 120 kV; the DLP at 120 kV would be 347 mGy cm, which corresponds to an adult effective dose of 5.2 mSv. 4 We have assumed that the E/DLP is independent of the X-ray tube voltage. E/DLP factors have a modest kV dependence, and in cardiac imaging E/DLP conversion factors have been shown to be 10% lower at 80kV compared to 120 kV [34].

– – – –

Ea can be determined for an adult abdominal CT examination by assuming that the adult is scanned at 80 kV (not the normal 120 kV)3, and using the normal adult current exposure time product of 190 mAs at a pitch of 1.0, and an adult scan length of 24 cm (Table 3). Selection of these technique factors on the console of the Siemens Sensation 16 CT scanner results in a DLP of 107 mGy cm, assuming selection of the widest beam width (24 mm). Use of the male adult conversion factor in Table 2 of 14.9 μSv/mGy cm shows that the corresponding adult effective dose is 1.6 mSv4; Lp/La is 6/24 because the scan length for a newborn is 6 cm, and 24 cm for an adult. mAsp/mAsa is 50/190, which reflects the expected fourfold reduction when performing a body scan on a newborn. Mat/Mp is 70/3.5 as given by the data in Table 1. Ren is 0.58 and is obtained directly from Table 1 for the specified examination type (abdomen), X-ray tube voltage (80 kV) and patient age/size (newborn male).

Ep ¼ 1:6mSv
ð6=24Þ
ð50=190Þ
ð70=3:5Þ
ð0:58Þ Ep ¼ 1:2mSv

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