The British Journal of Radiology, 77 (2004), 472–478 E 2004 The British Institute of Radiology
Effect of multislice scanners on patient dose from routine CT
examinations in East Anglia
S J YATES, MSc, L C PIKE, BSc and K E GOLDSTONE, MSc, FIPEM
East Anglian Regional Radiation Protection Service, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, UK
Abstract. As part of the dose optimization process, the Ionising Radiation (Medical Exposure) Regulations 2000
include requirements relating to the assessment of patient dose, and the setting and subsequent review of
diagnostic reference levels. In East Anglia, audits of effective dose in CT have been carried out in 1996, 1999
and 2002. In the 2002 audit, nine of the 14 scanners assessed had been replaced since the previous audit. Eight
of the new scanners were multislice scanners, acquiring up to 16 slices in a single rotation. The objective of the
2002 audit was to investigate the effect of the introduction of these multislice scanners on patient doses from
routine CT examinations. Exposure parameters were collected for 10 different types of routine CT examination.
In excess of 550 sets of patient data were obtained. For each of these, effective doses were calculated using the
results of Monte Carlo simulations published by the National Radiological Protection Board. Averaged across
all 10 examinations, regional mean effective doses are 34% higher than in 1999. The multislice scanners in the
region give, on average, 35% more effective dose than the single-slice scanners. The effect of collimation in
multislice scanners makes these effective dose differences most notable for examinations that use narrow slice
widths. Further optimization of exposures on multislice scanners has the potential to reduce the differences
observed between single-slice and multislice doses. However, when taken in combination with the increased use
of CT in many hospitals, the effective dose increases observed are likely to result in a signiﬁcant increase in the
already substantial collective radiation dose from CT.
X-ray CT is the technique whereby tomographic images survey of CT practice in the UK, which established mean
of a patient are obtained from a mathematical reconstruc- effective doses for a range of examinations .
tion of X-ray attenuation measurements made through There have however been substantial changes in CT
a thin axial slice of the patient. Notwithstanding the technology since that time. Most recently, the late 1990s
undoubted clinical beneﬁts of CT, it is a relatively high saw the introduction into the UK market of scanners with
dose technique when compared with other imaging moda- multislice capabilities. These scanners allow the acquisition
lities. Indeed, whilst CT accounts for only about 3% of of several images in a single rotation of the X-ray tube. At
all examinations performed using X-rays in the UK, radia- a simplistic level, except for having multiple detectors in
tion doses from CT account for approximately 40% of the axial direction, multislice scanners are physically very
the collective radiation dose arising from these medical similar to single-slice scanners. However, to avoid prob-
exposures . lems associated with the beam penumbra, it is necessary in
In Great Britain, the use of ionizing radiation for multislice CT to irradiate more of the patient than is
medical exposures is subject to the Ionising Radiations actually imaged (Figure 1). This effect is of particular
Regulations 1999 (IRR99)  and the Ionising Radiation signiﬁcance for narrow slices, where it is estimated that
(Medical Exposure) Regulations 2000 (IR(ME)R) . doses could be up to 40% higher than for well-collimated
With the aim of keeping medical exposures as low as single-slice systems .
reasonably practicable, IRR99 requires that ‘‘such measure- In 1999, a European Commission document  pro-
ments [are made] as are necessary to enable the assessment posed reference dose values for nine common CT exami-
of representative doses from any radiation equipment to nations. However, many of the reference values are based
persons undergoing medical exposures’’ (Reg. 32). Similarly, solely on doses from the 1991 UK audit, and as such do
IR(ME)R requires the employer to establish diagnostic not represent any more recent data. The most recent
reference levels (DRLs) for standard radiodiagnostic exami- NRPB summary of medical radiation exposures of the UK
nations. Employer’s procedures should specify that these population  also bases the majority of its CT data on the
DRLs ‘‘are expected not to be exceeded for standard 1991 audit.
procedures when good and normal practice regarding diag- At a local level, regional audits of effective dose in CT
nostic and technical performance is applied’’ (Schedule 1). have been carried out in 1996 and 1999. However, the
There have been a number of assessments of patient dose introduction of multislice technology, combined with
from CT in the past. In 1991 the National Radiological signiﬁcant funding from the National Lottery’s New
Protection Board (NRPB) published the results of a national Opportunities Fund, has led to a substantial change in
regional CT equipment since 1999, as is shown in Table 1.
Received 22 July 2003 and in revised form 4 November 2003, accepted This substantial change in equipment meant that many
12 November 2003. hospitals in the region no longer had effective dose infor-
This work was funded in part by Access to Learning for the Public
mation relevant to their current scanner. Consequently, a
Health Agenda (ALPHA), formerly known as the Anglia Clinical further regional audit of patient dose in CT was carried
Audit and Effectiveness Team (ACET). out in 2002. The methods used in this audit were chosen to
472 The British Journal of Radiology, June 2004
Effect of multislice scanners on patient dose in CT
Figure 1. Beam collimation in single-slice and multislice CT. (a) In a well-collimated single-slice system, a 5 mm slice width is
achieved by irradiating approximately 5 mm of the single 10 mm wide detector. (b) In a multislice system used to acquire four
1.25 mm slices, the shape of the beam proﬁle means that more than 5 mm of the detector must be irradiated in order to ensure that
the four detectors are irradiated to uniform intensity.
Table 1. CT scanners in East Anglia in 1999 and 2002 reﬂect closely methods used previously, hence allowing
meaningful comparison between successive audits, and
Manufacturer Model n Number allowing the changes resulting from the introduction of
multislice technology to be assessed.
GE 9000 1 1 —
GE HiSpeed Advantage 1 2 1 Method
GE HiSpeed CT/i 1 1 1
GE HiSpeed LX/i 1 — 1 Previous work has shown that, when auditing patient
Philips Tomoscan AV 1 1 1 doses in CT, standard protocols can be of limited use in
Picker PQ 7000 1 1 — assessing actual patient dose . For this reason actual
Siemens CR 512 1 1 — patient exposure data was sought from each scanner, for a
Siemens HiQ 1 1 — minimum of 10 patients for each of 10 common categories of
Siemens Somatom AR-HP 1 1 1
Siemens Somatom Plus 4 1 1 1
CT examination (head, neck, routine chest, high-resolution
GE HiSpeed NX/i 2 — 1 chest, chest-abdomen, chest-abdomen-pelvis, abdomen-
GE LightSpeed Plus 4 — 4 pelvis, abdomen, pancreas and lumbar spine). Audit forms
Siemens Somatom Sensation 4b 4 — 2 were distributed to CT superintendent radiographers at
Siemens Somatom Sensation 16 16 — 1 each hospital, requesting information relating to the expo-
sures carried out, as well as information relating to patient
Total 10 14
gender, height and weight. Following recommendations
n is the maximum number of slices that can be acquired in a
made for patient dose measurements in radiography ,
single rotation. patients with weights outside the range 70¡20 kg were
GE, Buc Cedex, France; Philips, Reigate, UK; Picker (now part excluded from the ﬁnal analysis.
of Philips); Siemens, Bracknell, UK. Effective doses were calculated using the results of
Previously marketed as the Somatom Volume Zoom. Monte Carlo simulations carried out in the early 1990s by
The British Journal of Radiology, June 2004 473
S J Yates, L C Pike and K E Goldstone
the NRPB . These results allow equivalent doses to the appropriate, participating staff were therefore asked to
major organs in the body to be calculated for the irradia- report acquisitions in sections, with each section represent-
tion of a speciﬁed 5 mm wide slab of an anthropomorphic ing a region of the body where the mAs remained
phantom. Each organ dose is calculated as the product of reasonably constant. Typical mAs values were reported for
the X-ray tube current, the tube rotation time, the nor- each section, based on the mAs values reported on each
malized CT dose index measured free-in-air (nCTDI100,air) image. Where available, other information, such as maxi-
for the beam collimation, tube potential and beam ﬁltra- mum mAs, minimum mAs, total mAs and dose–length
tion used, and the appropriate normalized organ dose pro- product (DLP) were also reported. These values allowed for
vided by the NRPB. For a complete acquisition, consisting some veriﬁcation of the estimates of ‘‘typical mAs’’ made.
of the irradiation of several slabs of the phantom, organ In addition to calculations of effective dose, DLPs were
doses resulting from the irradiation of each slab are summed, also calculated for each procedure. The DLP is deﬁned as
and a ﬁnal effective dose is calculated by applying tissue the pitch corrected weighted CT dose index (CTDIw)
weighting factors according to ICRP60 . multiplied by the scanned length. The CTDIw is an
Data analysis was carried out using in-house spread- estimate of the average dose to a standard CT dosimetry
sheets linked to the ImPACT CT patient dosimetry phantom from a single axial CT slice. It is calculated as
calculator , which provides a regularly updated the weighted average of CTDI measurements made at the
convenient user interface for the NRPB’s data. The centre (CTDI100,c) and periphery (CTDI100,p) of a 16 cm
ImPACT dosimetry calculator also identiﬁes which of diameter (head) or 32 cm diameter (body) polymethyl-
the NRPB data-sets is most appropriate for use with methacrylate phantom, according to Equation (1).
modern scanners that did not exist at the time of the
original Monte Carlo simulations. This scanner matching CTDIw ~( CTDI100,c z CTDI100,p ) ð1Þ
is based on the concept of the ImPACT factor, which is 3 3
related to the effective beam energy, and has been shown Whilst modern scanners normally report DLP for
to correlate well with the effective dose from a scanner examinations, routine quality control measurements on
. The ImPACT dosimetry calculator also provides scanners in the region have shown that discrepancies often
generic values of nCTDI100,air for a range of scanner exist between reported and measured values. Calculated
models. However, in this work, measured values of DLPs, based on measured values of CTDIw, were there-
nCTDI100,air were used in the dose calculation procedure. fore used in this audit, avoiding the issue of these
These results were collected as a part of routine quality discrepancies, and allowing older scanners not reporting
control measurements, using a 100 mm pencil ionization DLP to be included in the analysis. CTDIw measurements
chamber with an air-kerma calibration traceable to were made on each scanner using the pencil ionization
national standards via a therapy-level secondary standard. chamber and standard CT dosimetry phantoms described
Translating an individual patient exposure onto the previously.
anthropomorphic phantom is relatively straightforward in
most situations. However, where a patient’s height is
signiﬁcantly different from the size of the phantom, use of Results and discussion
the actual scanned length can result in the irradiation of Audit forms were returned for a total of over 550
too few, or too many, of the organs in the mathematical patient examinations from the 12 scanners assessed.
phantom. To overcome this potential discrepancy, our (Although there were 14 scanners in the region in 2002,
audit forms included a diagram of the human skeleton, on at 2 hospitals there were pairs of identical scanners. No
which radiographers reported the upper and lower extent attempt was made to distinguish between the two scanners
of each scan. This information was used in the selection of at each of these hospitals, as in both cases the two
the part of the phantom irradiated, improving the scanners were set up and used identically.)
correspondence between the organs irradiated in the
patient and the phantom.
Accurately modelling head examinations using the Monte Effective doses
Carlo data is also hindered because head scans are often
Table 2 summarizes the overall results of the 2002 audit,
performed with a tilted gantry, whereas the NRPB data only
and provides results from the 1999 audit  for com-
allow for the calculation of doses from slices acquired
parison. These results show that on average, mean effec-
perpendicular to the long axis of the patient. Whilst the effect
tive doses from CT in the region are 34% higher in 2002
of gantry tilt on the equivalent dose to the lens of the eye will
than in 1999.
be appreciable, the effect of tilt on the effective dose will be
To demonstrate whether or not the observed increase in
much less signiﬁcant. These scans were therefore modelled effective dose can be attributed to the introduction of
using standard slices, perpendicular to the long axis of the multislice scanners, it is necessary to look at the distri-
patient, with the selection of slices being made so as to best bution of effective doses between scanners (Figure 2). To
represent the scan actually performed. compare scanners across all examinations, relative effective
In several cases the scanners assessed were capable of doses were calculated for each scanner, for each examina-
some form of automatic dose optimization. This is tion category, according to Equation (2).
normally achieved by modulating the mAs during a
spiral acquisition, based either on a previously acquired Relative effective dose~
scan projection radiograph (SPR), or on the preceding Effective dose for scanner (mSv)
rotation in the spiral acquisition. Such facilities make dose ð2Þ
Mean of effective doses for all scanners (mSv)
calculations more complicated, because a single mAs
cannot be assumed for the whole acquisition. Where The relative effective doses for each scanner were then
474 The British Journal of Radiology, June 2004
Effect of multislice scanners on patient dose in CT
Table 2. Regional mean effective doses for each examination type in 2002 and 1999. Figures in parentheses represent the range of
individual scanner means. Also shown is the percentage increase in regional mean effective dose between 1999 and 2002
Examination 2002 Audit 1999 Audit Increase in dose (%)
No. of cases Effective dose (mSv) No. of cases Effective dose (mSv)
Head 41, 43 1.7 (0.9–2.4) 99 1.3 (0.6–2.1) 31
Neck 12, 17 3.2 (1.7–5.4) 38 1.5 (0.3–3.1) 113
Routine chest 10, 26 3.5 (1.9–5.2) 45 4.2 (1.7–7.0) 217
High resolution chest 38, 37 2.2 (0.7–3.7) 61 1.4 (0.3–3.1) 57
Chest-abdomen 39, 30 7.9 (5.0–10.6) 66 7.6 (3.1–15.8) 4
Chest-abdomen-pelvis 39, 22 10.9 (8.1–16.6) 57 10.2 (4.7–16.4) 7
Abdomen-pelvis 47, 35 9.2 (6.2–11.3) 88 7.2 (3.7–10.0) 28
Abdomen 35, 22 7.0 (6.1–8.5) 53 6.6 (2.7–12.5) 6
Pancreas 25, 12 10.3 (5.1–14.6) 29 6.4 (2.4–14.5) 61
Lumbar spine 17, 4 6.4 (3.3–11.6) 38 4.4 (2.2–5.9) 45
Case numbers for 2002 are presented as number of single-slice cases, number of multislice cases.
averaged across all examination categories, to give a mea- For example, irradiating 7 mm of patient to acquire four
sure of the overall performance of each scanner (Figure 3). 1.25 mm slices results in a geometric efﬁciency of about 70%,
It is apparent from Figure 3 that on the whole, the use whereas irradiating 22 mm of patient to acquire four 5 mm
of multislice scanners results in higher patient doses than slices maintains a geometric efﬁciency of about 90%.
those from single-slice scanners, with ﬁve of the six scan- It is of note that, in the majority of cases where scanners
ners giving highest effective doses being multislice. It is of have remained unchanged since 1999, the effective doses
note that the only multislice scanner appearing in the assessed in 2002 were very similar to those in 1999. On the
lower half of the scanner distribution (scanner D) is the assumption that clinical practice has not changed sig-
one dual-slice scanner in the audit. Beam collimation niﬁcantly, this provides conﬁdence in the reliability of the
effects can be less signiﬁcant for such scanners, because assessment method. For example, for scanner C, 2002
two detectors can be irradiated to equal intensity without effective doses were within ¡10% of 1999 values for seven
having to widen the beam to avoid utilizing the penumbra of the 10 examination types. Discrepancies were found to
(c.f. Figure 1). Consequently, in dosimetric terms, the be greater in situations where the number of cases returned
scanner is seen to behave more like the single-slice scan- in either 1999 or 2002 was small, and so there are cor-
ners in the region. respondingly larger uncertainties in the quoted values.
The single-slice scanner with apparently high effective
doses (scanner I) is one of the oldest single-slice scanners
in the audit. This scanner is restricted to operate at 130 kV Dose–length products and diagnostic reference levels
and compared with other scanners, has a relatively In addition to effective doses, DLPs were calculated for
small amount of beam ﬁltration. Consequently values of each examination in the 2002 audit. Results are sum-
nCTDI100,air, i.e. tube output in mGy/mAs, are relatively marized in Table 3. In the majority of cases, relatively high
high for this scanner. It is noted however that the mAs effective doses at individual hospitals are predictably
values used on this scanner are very similar to those used reﬂected by relatively high DLPs, and vice versa. The
for similar examinations on other single-slice scanners. The value of including DLP results, however, is that modern
result is relatively high patient doses, as observed. scanners normally quote the DLP for an examination, and
The average relative effective dose for all single-slice as such it is the most useful quantity for a DRL. Working
scanners is 0.85¡0.10 (mean¡standard error. The uncer- Parties on DRLs have recommended a clear distinction
tainty quoted here is representative of the variation bet- between Local and National DRLs [14, 15]. Separate
ween scanners; the uncertainty in the underlying Monte Local DRLs should be set for each piece of equipment,
Carlo simulation and effective dose calculation has not and may be calculated as the mean ‘‘dose’’ received by a
been quantiﬁed in this study). Similarly, the average relative set of standard patients on that equipment. As such, the
effective dose for all multislice scanners is 1.15¡0.06. The mean DLPs established for each examination for each
multislice scanners in the region therefore give, on average, scanner in this audit represent appropriate Local DRLs
35% more effective dose than the single-slice scanners. This for each scanner.
difference is not seen uniformly across all examinations As yet there are no National DRLs for CT, and so there
however. As demonstrated in Figure 2, the distinction are no formal values with which to compare the local
between single-slice and multislice effective doses is generally values derived. The NRPB, CT Users Group and ImPACT
seen to be greatest for examinations using narrow slices, e.g. are however in the process of carrying out a national
head, high resolution chest, but is less apparent for other survey of CT practice, from which it is intended to derive
examinations, e.g. abdomen-pelvis. This can again be National DRLs. The DLP results from this audit will at
explained by reference to the beam collimation effect in that point be of value in assessing each hospital’s com-
multislice CT. Assuming that it is necessary to irradiate a pliance with National DRLs. Regulation 2 of IR(ME)R
constant amount more of the patient than is actually used for deﬁnes (National) DRLs as being ‘‘dose’’ levels for
imaging, the geometric efﬁciency (the ratio of total imaged broadly deﬁned types of equipment. Given the differences
width to irradiated width) will be poorest where a small observed in this audit between single-slice and multislice
number of narrow slices are acquired in a single rotation. doses, it is suggested that separate National DRLs might
The British Journal of Radiology, June 2004 475
S J Yates, L C Pike and K E Goldstone
Figure 2. Mean effective doses for each scanner, for each examination category. Scanners are anonymously identiﬁed by the letters
A to L, according to the order in which they appear in Figure 3. Varying numbers of scanners contributed data to the different
examination categories. Error bars represent the standard error of each mean.
476 The British Journal of Radiology, June 2004
Effect of multislice scanners on patient dose in CT
European reference levels recommended in 1999 , and
mean effective doses from the NRPB’s dose audit in 1991 .
These data are summarized in Table 4. DLPs all comply
with European Reference Levels for routine chest, high
resolution chest and abdomen examinations. There are
however two scanners for which the mean head DLP
exceeds the European reference value of 1050 mGy.cm.
These European reference values pre-date multislice CT
however, and exceeding this value is perhaps further evi-
dence of the need for more up-to-date National and
European CT dose information. Again, it should be noted
that head examinations are one of those examinations
where signiﬁcant effective dose differences have been observed
between single-slice and multislice scanners.
There is good agreement between our results and NRPB
data for head, neck (cervical spine) and abdomen exami-
Figure 3. Mean relative effective doses (averaged across all nations. This reverses a previous trend observed in our
examinations) for each scanner in the 2002 audit. Error bars audits, where mean results have fallen below the NRPB
represent the standard error of each mean. data. The agreement is poorer for routine chest examina-
tions, although it should be noted that our result, which is
less than 50% of the NRPB effective dose, is consistent
Table 3. 2002 regional mean dose–length products (DLPs) for with our previous ﬁndings (see Table 2). Our 2002 results
each examination category. Figures in parentheses represent the
for pancreas and lumbar spine examinations fall substan-
range of individual scanner means
tially above the effective doses quoted by NRPB, and are
Examination DLP (mGy.cm) also greater than our 1999 results for these examinations.
Again, it is apparent from Figure 2 that these are both
Head 760 (360–1180) examinations where signiﬁcant effective dose differences
Neck 330 (190–540)
have been observed between single-slice and multislice
Routine chest 190 (70–270)
High resolution chest 110 (35–240) scanners. It should be noted, however, that a relatively
Chest-abdomen 430 (200–680) small amount of data were submitted for these examina-
Chest-abdomen-pelvis 580 (320–750) tions, leading to larger uncertainties in our results.
Abdomen-pelvis 470 (240–570)
Abdomen 400 (250–440)
Pancreas 560 (300–910) Dose optimization
Lumbar spine 300 (220–570)
In this audit we have focused solely on CT doses, and
have made no attempt to compare image quality, or the
be required for single-slice and multislice scanners, treating diagnostic value of images, between scanners. It is of note
these as different broadly deﬁned types of equipment. that at the time of this audit many of the multislice
Alternatively, National DRLs for CT will need to be scanners had been in clinical use for less than a year. In
set with sufﬁcient ﬂexibility to allow for the potentially many cases examination protocols were therefore still very
higher doses from multislice CT. More stringent limits strongly inﬂuenced by settings recommended by the manu-
could however still be applied to individual scanners in the facturer, as there had been insufﬁcient time for major
form of the Local DRL. optimization of protocols.
Since the data for this audit were collected, one of the
highest dose multislice scanners (scanner K) has received a
software upgrade, and is now capable of automatic tube
Comparison with European and National data
current modulation. Initial results from the introduction of
Whilst there are no formal National DRLs with which this feature into clinical use suggest that tube currents are
to compare our results, comparison can be made with typically 20% lower than those used in standard protocols.
Table 4. Regional effective doses and dose–length products (DLPs) [expressed as mean (range)], compared with National
Radiological Protection Board (NRPB) mean effective doses  and European reference levels of DLP . Only those examinations
for which there are comparable National or European data are shown
Examination 2002 Audit NRPB mean European reference
effective dose (mSv) DLP (mGy.cm)
Effective dose (mSv) DLP (mGy.cm)
Head 1.7 (0.9–2.4) 760 (360–1180) 1.8 1050
Neck (C spine) 3.2 (1.7–5.4) 330 (190–540) 2.9 —
Routine chest 3.5 (1.9–5.2) 190 (70–270) 8.3 650
High res. chest 2.2 (0.7–3.7) 110 (35–240) — 280
Abdomen 7.0 (6.1–8.5) 400 (250–440) 7.2 780
Pancreas 10.3 (5.1–14.6) 560 (300–910) 4.6 —
Lumbar spine 6.4 (3.3–11.6) 300 (220–570) 3.6 —
The British Journal of Radiology, June 2004 477
S J Yates, L C Pike and K E Goldstone
The introduction of this and further dose reduction tech- Acknowledgments
nology, together with smaller scale improvements to indi-
vidual examination protocols, will reduce some of the We are grateful to Mr M J White and Dr J P Eatough
doses reported in this audit. It is therefore predicted that, for their work in carrying out the 1999 dose audit. We
in time, some of the distinction between single-slice and would also like to thank all the radiographers at hospitals
multislice doses reported here will be lost, as successive in the region who contributed data toward the 2002 audit.
improvements reduce doses from multislice scanners.
The importance of dose optimization is highlighted
further if, rather than individual doses, collective doses
from CT are considered. No attempt has been made here
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2002, and 16 currently) suggests however that there has 2. The Ionising Radiations Regulations 1999 (SI 1999/3232).
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478 The British Journal of Radiology, June 2004