1. The British Journal of Radiology, 76 (2003), 51–56 E 2003 The British Institute of Radiology
DOI: 10.1259/bjr/53215511
A method for the systematic selection of technique factors
in paediatric CT
C J KOTRE, PhD and S P WILLIS, DCR
Regional Medical Physics Department, Newcastle General Hospital, Newcastle-upon-Tyne NE4 6BE, UK
Abstract. A method for the systematic selection of paediatric CT technique factors is described. The approach is
based on the assumption that the level of image noise acceptable for a given adult CT image is also acceptable
for the equivalent paediatric examination. A simple exponential attenuation model is proposed. Effective linear
attenuation coefficients were initially established from a series of phantom measurements simulating head, chest
and abdomen examinations at 120 kVp, then extended for a range of tube potentials and beam qualities using a
beam spectral model. Application of the method is demonstrated using phantoms representing head, chest and
abdomen sections for neonate and ages 1 year, 5 years, 10 years, 15 years and adult.
Paediatric CT examinations are performed relatively measure to determine the perimeter of the section required,
rarely in most centres, and the range of patient sizes then dividing by p to give the diameter of the equivalent
encountered is large. It is therefore difficult to make a circular section. All equivalent diameters used in this paper
selection of technique factors that produces a consistently are measured in this way and it should be noted that these
optimized balance between radiation dose and image are not directly interchangeable with those derived by
quality. A number of recent publications have suggested different means, e.g. from patient height and weight, in
that paediatric CT doses may be unnecessarily high in other work [5–7]. By use of the equivalent diameter, the
some cases [1, 2], information that has been reported in the mAs per slice required to give a similar signal-to-noise
UK national news [3]. Recent UK legislation requires that ratio relative to that for the adult case in CT can be
the dose of ionizing radiation is kept as low as reasonably approximated using a simple monochromatic attenuation
practicable consistent with the diagnostic purpose, and model. The relationship is
specifically requires that special attention is paid to
medical exposures of children [4]. It is also required that mAsP SP ~mAsA SA e{keff ðdA {dP Þ ð1Þ
detailed examination protocols are put in place for all where dA is the standard adult equivalent diameter, e.g.
diagnostic X-ray work. In this paper a method for that of ‘‘Reference Man’’ [8], dP is the paediatric
selecting technique factors is suggested such that the equivalent diameter, mAsA and SA are the tube current–
images produced for paediatric patients will be similar in time product per slice and slice width, respectively, for the
terms of signal-to-noise ratio to the images produced in adult examination, mAsP and SP are the same quantities
the equivalent adult examination, the technique factors for for the paediatric examination and meff is the effective
which will be much better established in most centres. linear attenuation coefficient for the body segment being
The approach is based on the following assumptions: scanned at the tube potential being used. Equation 1
(i) The technique factors for the equivalent adult includes the slice widths SA and SP specifically, as these
may change between adult and paediatric examinations
examination are known.
given the smaller size of paediatric patients. In addition,
(ii) The image signal-to-noise ratio in the equivalent manipulation of slice width is used to modify the total
adult examination is suitable for diagnosis in the number of incident X-ray photons in the experiments
paediatric case. reported below. It should be noted, however, that reducing
(iii) The attenuation and relative proportions of body slice width without reducing mAs per slice does not result
tissues are similar in adults and paediatrics. in a dose saving where contiguous scanning is employed,
(iv) The tube voltage is not altered between the adult indeed the patient dose may well increase on single-slice
and paediatric versions of the examination. scanners owing to the increased proportion of penumbra
(v) The reconstruction filter used in the paediatric present at small slice widths. In all cases the slice widths
examination is not radically different from that refer to the reconstructed image slice thickness rather than
used in the adult examination (this point is the collimated thickness of the fan beam.
Equation 1 is based upon the assumption that the
discussed further below).
problem may be adequately modelled in terms of simple
A useful approximation to patient cross-sectional size is exponential attenuation. Clearly the patient exit spectrum
the equivalent diameter, d. For the purposes of this paper, is not monochromatic, but the heavy beam filtration
this is measured directly on the subject using a tape normally used in CT plus the filtering effect of the patient
makes the monochromatic assumption more appro-
Received 4 December 2000 and in final form 6 August 2002, accepted priate for CT than for some other radiological imaging
12 August 2002. techniques.
The British Journal of Radiology, January 2003 51

2. C J Kotre and S P Willis
Initial check of model validity technique factors are not commonly altered for gender.
Axial scans of the phantoms were carried out on the
A simple initial experiment was carried out to confirm Siemens Somatom Plus 4 scanner at 120 kVp, 12 mm Al
the validity of the attenuation model proposed above.
total filtration, using nominal 10 mm slice widths. Effec-
Uniform Perspex CT dosimetry phantoms of 32 cm and
tive attenuation coefficients for other tube potentials and
16 cm were axially scanned on a Siemens Somatom Plus 4
scanner filtrations are derived below.
scanner (Siemens AG, Munich, Germany) at 120 kVp. The
To obtain values of meff for each body segment, the level
beam filtration of this scanner was estimated from measure-
of quantum noise within a region of uniform tissue-
ments of half value layer (HVL) using data generated with
equivalent material was measured for each of the seven
a beam spectral simulation programme [9]. The filtration
phantoms using fixed technique factors. For the chest,
was estimated to be 12 mm Aluminium (AL), comparable
noise measurements were confined to the near water-
with the 10 mm Al minimum quoted by the manufacturer.
Substituting the phantom diameters into Equation 1 and equivalent material of the phantom heart region. Five
using a value of 0.22 cm21 obtained from the spectral circular areas of interest were analysed on each image and
simulation programme for the linear attenuation coeffi- the geometric mean value of the pixel SD calculated. Gross
cient of Perspex at a depth of 16 cm, gave a ratio between non-uniformities in the phantoms were avoided and efforts
the paediatric (16 cm) mAs–slice width product and the were made to use the same location for areas of interest
adult (32 cm) mAs–slice width product of 0.03. This large when comparing noise measurements from the same
ratio was approximated in practice using 300 mAs, 10 mm section.
slice width for the 32 cm phantom and 50 mAs, 2 mm slice To measure the level of quantum noise within the
width for the 16 cm phantom. Multiple noise samples were phantoms accurately, any additional noise components
taken from each image to obtain a mean noise level in due to inhomogeneity of the phantom material at small
terms of pixel standard deviation (SD) and an estimate of scales and system noise had to be eliminated. Initial
the standard error on the mean (sem). For the 32 cm measurements showed that the material of the commercial
diameter phantom, the SD was 13.1 (sem50.25) and for paediatric phantoms was indeed inhomogeneous at
the 16 cm diameter phantom scanned at 0.03 of the adult small scales, giving rise to an additional noise component.
mAs–slice width product, the SD was 13.6 (sem50.16). Although the adult phantoms were more uniform, they
This level of agreement was felt to be encouraging, given also produced a small additional noise component. All
the very large reduction in patient dose per slice implied by noise measurements were therefore made at two widely
this choice of technique factors. separated values of mAs so that the quantum noise and
phantom inhomogeneity components could be separated
using the relationship
Experimental derivation of effective linear
attenuation coefficients 1
Ap2 {p2
1 2
2
ps ~ ð2Þ
Measurements of perimeter were made directly on a set A{1
of male and female adult anthropomorphic phantoms
(Alderson Research Labs, Stamford, CT) based on the
where sS is the pixel SD representing the non-quantum
dimensions of ‘‘Reference Man’’ [8] and paediatric
phantoms (ATOM Ltd, Riga, Latvia) [10] constructed to noise sources (including both phantom and system noise),
the same dimensions as the software phantoms of Christy s1 is the pixel SD measured at the high value of mAs, s2 is
[11]. These were then converted to the diameter of a circle the pixel SD measured at the low value of mAs and A is
with the same circumference. These dimensions are taken the ratio between the high and low mAs values used. The
as typical for neonate and ages 1 year, 5 years, 10 years, 15 value of this ratio in the experiments was 10.4. The pure
years and adult in the experimental work that follows quantum noise contribution for each measurement was
(Table 1). then calculated by quadrature subtraction of the phantom
Head, chest and abdomen examinations were performed and system noise, ss established for that phantom from
on the set of phantoms in order to obtain meff values for Equation 2, from the total noise SD. As the mean
head, chest and abdomen sections. Adult male and female Hounsfield unit (HU) value for the paediatric phantoms
results were combined to produce a result equivalent to was found to be significantly above the expected value for
an adult hermaphrodite on the assumption that adult CT water equivalent tissue (in the range 70–90 HU), an
additional correction for this anomalously high attenua-
tion was necessary. The relative numbers of photons
Table 1. Typical values of equivalent diameter for use in contributing to the image were corrected to what they
Equation 1 would have been for zero HU and the quantum noise
estimations were adjusted accordingly.
Age (years) Equivalent diameter (cm) The reciprocal of the fractional change in quantum
Head Abdomen Chest noise with equivalent diameter for each body segment was
then squared to obtain the fractional change in number of
Adult 17.6 23.4 27.6 X-ray photons forming the image. Finally, the values of
15 17.2 20.5 23.2
meff were obtained by plotting the natural logarithm of the
10 15.9 17.5 19.4
5 15.3 16.9 16.9 relative number of photons forming the image against the
1 13.7 13.4 13.2 equivalent diameter of the phantom section. The slopes of
Neonate 11.6 9.9 9.9 the fits to these experimental points gave the required
effective attenuation coefficients.
52 The British Journal of Radiology, January 2003

3. Selection of technique factors in paediatric CT
Extension of meff values using an X-ray spectral scanners were used, a Siemens Somatom Plus 4 (estimated
simulation programme total filtration 12 mm Al) and a Toshiba Asteion (Toshiba
Corporation, Tochigi, Japan) (estimated total filtration
A beam spectrum simulation programme [9] was 4 mm Al). Head, chest and abdomen sections for each of
employed to extend the experimentally derived meff the available ages were scanned at 80 kVp, 120 kVp and
values over a more useful range of tube potentials and 140 kVp (Somatom Plus 4) and 120 kVp (Asteion), using
beam filtrations. The programme was used to model the a 10 mm nominal slice width. Adult mA-slice width
incident X-ray beam and the attenuation of the various product values were based on those found in a regional
body sections in terms of thicknesses of water and bone CT dosimetry survey, and the values for the paediatric
for the head, water for the abdomen and water and air in
scans were obtained from Equation 1 using the equivalent
the case of the chest. The parameters of the model were
diameters in Table 1 and the effective linear attenuation
adjusted to match the measured beam HVL and
coefficients from Tables 2–4. A realistic field of view to
experimentally derived effective attenuation coefficient
suit the size of section being scanned was chosen in each
for the head, chest and abdomen sections at 120 kVp
case. As the paediatric phantoms are hermaphrodite,
(constant potential, 10 ˚ target angle) as measured above.
averaged male and female adult measurements were used
The kVp and total tube filtration were then varied to
to produce an equivalent hermaphrodite adult.
produce the required range of meff values, which are given
Where the mAs value required for the smaller phantom
in Tables 2–4 for the head, abdomen and chest examina-
sections fell below the minimum mAs available for a full
tions, respectively. In each table the experimentally derived
value is shown in bold type and the values extrapolated rotation of the scanner (37.5 mAs for the Somatom Plus 4,
from it in normal type. 30 mAs for the Asteion), the number of photons per slice
was further decreased by reducing the slice width. The
relationship between slice width and relative number of
Experimental validation photons detected was previously established for each
scanner in a separate series of water calibration phantom
A further series of scans of the anthropomorphic scans. Where slice width reduction was employed, a
phantoms was made to verify that application of the further correction was required to account for the increase
method of paediatric factor selection proposed above in measured phantom noise component as the effect of
results in images with a similar signal-to-noise ratio to noise averaging across the slice width is reduced. Noise
those produced by the equivalent adult examination over a measurements were again corrected to remove the effects
realistic range of tube potentials and beam filtrations. Two of phantom local inhomogeneity and the elevated
attenuation of the paediatric phantom material.
Table 2. Values of effective attenuation coefficient for use in
Equation 1 (head examination)
Tube potential Total filtration (mm Al)
(kVp) Results
3 6 9 12
Figure 1 shows an example of the trend in mean noise
80 0.222 0.219 0.216 0.214 levels measured in the experimental validation for the
100 0.208 0.205 0.203 0.201 abdominal examination. This example is for 120 kVp at
120 0.198 0.196 0.194 0.192
12 mm Al total tube filtration. Error bars of ¡2 standard
140 0.190 0.188 0.186 0.185
errors on the mean, calculated for each point, are shown
together with the best fit line of zero gradient. The noise
results are plotted against nominal phantom age, with the
Table 3. Values of effective attenuation coefficient for use in
Equation 1 (abdomen examination) hermaphrodite adult point being placed at age 30 years.
This is an arbitrary choice but is consistent with the age at
Tube potential Total filtration (mm Al) which the relationship between body weight and age
(kVp) reaches a plateau [8]. Complete results for the abdomen,
3 6 9 12
head and chest validations are given in Table 5.
80 0.210 0.208 0.207 0.206 Despite a careful analysis of propagation of errors
100 0.197 0.195 0.194 0.194 through the experimental process, including the applica-
120 0.187 0.186 0.184 0.184 tion of Equation 2 and the subtraction of the structure
140 0.178 0.178 0.177 0.176 noise from the total measured noise, it was found that the
zero gradient line did not pass through the error bars of
¡2 standard errors on the mean for a number of the
Table 4. Values of effective attenuation coefficient for use in experimental points. This would seem to indicate that
Equation 1 (chest examination) some sources of error have not been adequately accounted
for. Possible candidates for the ‘‘missing’’ error include
Tube potential Total filtration (mm Al)
(kVp)
variations in the choice of noise sampling region between
3 6 9 12 the scans taken to establish the phantom structure noise
80 0.168 0.165 0.163 0.162
and the subsequent validation scans and variations in
100 0.157 0.154 0.153 0.152 noise between individual scans. Despite this, no systematic
120 0.149 0.147 0.146 0.145 trend in noise level with phantom age is seen across the
140 0.143 0.141 0.140 0.138 results taken as a whole, indicating broad agreement with
the expected flat noise levels.
The British Journal of Radiology, January 2003 53

4. C J Kotre and S P Willis
Figure 1. Experimentally measured
mean noise level plotted against
phantom age for abdomen examina-
tions at 120 kVp, 12 mm Aluminium.
Error bars of ¡2 standard errors on
the mean calculated for each point
are shown together with the best fit
line of zero gradient.
Table 5. Summary of image noise results, (mean pixel standard deviation) for the experimental validation on phantoms for head,
abdomen and chest sections, (standard error on the mean in parentheses)
Phantom age (years)
0 1 5 10 15 Adult
Head
a a
S80 kV 5.0 (0.13) 5.8 (0.16) 6.2 (0.15) 6.6 (0.14) 6.5 (0.19) 6.0 (0.08)
a
S120 kV 1.6 (0.21) 2.0 (0.33) 2.9 (0.18) 2.4 (0.27) 2.7 (0.21) 2.9 (0.09)
a
S140 kV 1.4 (0.23) 0.7 (0.80) 1.6 (0.28) 1.7 (0.32) 2.1 (0.27) 2.4 (0.10)
a
T120 kV 1.7 (0.56) 1.7 (0.72) 1.3 (0.68) 1.6 (0.72) n/e 3.1 (0.20)
Abdomen
a
S80 kV 8.7 (0.41) 9.4 (0.13) 9.1 (0.44) 10.0 (0.28) 9.1 (0.31) 10.5 (0.29)
S120 kV 3.5 (0.50) 4.2 (0.23) 4.5 (0.45) 4.2 (0.37) 4.6 (0.32) 4.2 (0.28)
S140 kV 3.3 (0.30) 3.1 (0.19) 3.4 (0.34) 3.3 (0.38) 3.2 (0.41) 3.1 (0.29)
T120 kV 2.4 (0.70) 2.7 (0.33) 2.0 (0.67) 2.4 (0.72) 2.2 (0.72) 2.4 (0.41)
Chest
a a a
S80 kV 13.0 (0.53) 10.2 (0.31) 9.7 (0.98) 10.6 (0.13) 11.5 (0.26) 9.0 (0.29)
a
S120 kV 4.5 (0.37) 3.5 (0.36) 4.9 (0.51) 4.6 (0.56) 4.8 (0.22) 4.8 (0.24)
S140 kV 3.1 (0.63) 3.8 (0.25) 3.9 (0.60) 3.9 (0.60) 4.0 (0.24) 4.2 (0.25)
T120 kV 2.5 (0.51) 2.0 (0.47) 2.3 (1.06) 2.3 (1.06) 2.9 (0.34) 3.5 (0.30)
S, Siemens Somaton Plus 4 scanner; T, Toshiba Asteion scanner; n/e, quadrative sun could not be evaluated.
a
Result falling outside two standard errors on the mean of a best fit line of zero gradient.
Application of the method in practice ratio of paediatric to adult mAs–slice width product
for abdominal examinations as a function of phantom
It is envisaged that the relationship of Equation 1 would age, based on the average equivalent diameters given in
be applied as follows. Measure the perimeter of a Table 1. Curves are shown for 80 kVp, 100 kVp, 120 kVp
representative part of the section to be scanned using a
and 140 kVp, and are based on the values of meff from
tape measure on each individual patient and divide this
Table 2 corresponding to a total filtration of 9 mm Al.
by p to obtain the equivalent diameter. Substitute the
The curves shown are spline fits to calculated values at age
paediatric equivalent diameter into Equation 1 along with
0 years, 1 year, 5 years, 10 years and 15 years and 100%
the appropriate value of meff from Tables 2–4 and a value
for the nominal adult 30-year-old. At first sight the curves
for dA, taken from Table 1 for the appropriate body seg-
are surprising both for the very small proportions of adult
ment. The results of this calculation could be tabulated or
shown graphically as mAs against perimeter for standard mAs–slice width product required for younger children
examinations. and for the very rapid increase from the teens to adult-
hood. The curve shapes do, however, seem more reason-
able when the rates of change in the equivalent diameters
Discussion from Table 1 are considered together with the exaggera-
The magnitude of the dose saving expected from the use tion of those changes resulting from the application of the
of Equation 1 is illustrated in Figure 2, which shows the exponential relationship given in Equation 1. The noise
54 The British Journal of Radiology, January 2003

5. Selection of technique factors in paediatric CT
Figure 2. Fraction of adult mAs–slice
width product per slice required for a
paediatric abdomen CT image with
the same noise level plotted against
phantom age, using the standard
dimensions from Table 2. The curves
are spline fits to points for 80 kV,
100 kV, 120 kV and 140 kV at ages
neonate, 1 year, 5 years, 10 years and
15 years, and adult placed at age 30
years.
results obtained from phantom body sections of realistic findings are consistent with those of Rogella et al [12]
dimensions illustrate that the choice of mAs–slice width who have reported no loss of diagnostic information for
product value based on these relationships does produce paediatric chest CT performed at 25 mAs per slice, and
images with similar noise content to the adult case. Huda et al [13] who found that the dose per slice for head
Figure 2 serves to demonstrate the relationship between scans of newborns could be reduced to 40% of the adult
mAs–slice width product and age resulting from the use value.
of Equation 1 for the standard phantoms used. It is not Difficulty was experienced in obtaining the low mAs
recommended that this kind of relationship be used as the values required for the experimental validation on one of
basis of a clinical scheme to select CT technique factors the CT scanners used, and changes in slice width had to be
dependent on age alone, as the variation in patient size for employed to simulate the effect of applying lower doses to
any given age could result in non-diagnostic images for the phantoms. For reduction in paediatric technique
larger children. factors to the low levels proposed, an extended lower
Additional factors that could affect the noise content in range of tube current settings would be of value.
the paediatric image include choice of a different image
reconstruction filter, use of zoom reconstruction and use
of different choices of beam hardening correction. The Conclusion
variations offered will change depending on the model of
CT scanner used, but experimental tests on choice of adult A method for the systematic selection of paediatric
and paediatric filters (with the same ‘‘sharpness’’), zoom CT technique factors has been described using a simple
reconstruction and a range of fields of view showed no attenuation model based on measured patient equivalent
significant difference in image noise content. Some diameter. Images produced for paediatric patients scanned
scanners change beam filtration for small fields of view. using technique factors selected in this manner will be
Since the rates of change of meff with total filtration are similar, in terms of signal-to-noise ratio, to the images
small (Table 2), differences in filtration between the adult produced for the equivalent adult examination, the
and paediatric cases within the range 3–12 mm Al will technique factors for which will be much better established
produce a maximum error on the estimated mAs–slice in most centres. Using this method, a significant reduction
width product of 12%. from present patient dose levels for paediatric CT would
All experimental work described employed single slice be expected.
axial scanning. The method derived can, however, be
applied directly to spiral scanning protocols, as the
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