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Effect of multislice scanners on patient dose from routine CT

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Effect of multislice scanners on patient dose from routine CT Powered By Docstoc
					The British Journal of Radiology, 77 (2004), 472–478        E   2004 The British Institute of Radiology
DOI: 10.1259/bjr/21927258



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 [4].
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 [1].                                                          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) [2] and the Ionising Radiation                 significance for narrow slices, where it is estimated that
(Medical Exposure) Regulations 2000 (IR(ME)R) [3].                      doses could be up to 40% higher than for well-collimated
With the aim of keeping medical exposures as low as                     single-slice systems [5].
reasonably practicable, IRR99 requires that ‘‘such measure-                In 1999, a European Commission document [6] 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 [1] 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
               a
                                                                   meaningful comparison between successive audits, and
Manufacturer       Model                  n     Number             allowing the changes resulting from the introduction of
                                                of scanners
                                                                   multislice technology to be assessed.
                                                1999    2002
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 [7]. 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 [8],
  single rotation.                                                 patients with weights outside the range 70¡20 kg were
a
 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
b
 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 [9]. 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 [10].                         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 [11], 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
                                                                                       1           2
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
[12]. 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 [13] 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 (%)
                                         a
                          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

a
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 [6], and
                                                                       mean effective doses from the NRPB’s dose audit in 1991 [4].
                                                                       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 [4] and European reference levels of DLP [6]. 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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average, 35% more effective dose than the single-slice               (NRPB-SR250). Chilton: National Radiological Protection
scanners. However, at the time of the audit, there had been          Board, 1993.
                                                                 10. 1990 Recommendations of the International Commission on
relatively little opportunity for the detailed optimization of
                                                                     Radiological Protection (ICRP 60). Oxford: Pergamon Press,
exposure protocols on many of the multislice scanners. It
                                                                     1991.
is therefore anticipated that further optimization of pro-       11. Keat N. ImPACT CT patient dosimetry calculator (version
tocols, together with an increase in the availability and use        0.99 m). London: ImPACT, 2002. (www.impactscan.org/
of automatic tube current modulation, will help to reduce            ctdosimetry.htm)
doses from multislice CT in the future. Despite this, when       12. Lewis MA, Edyvean S, Sassi SA, Kiremidjian H, Keat N,
combined with the increasing number of CT scanners                   Britten AJ. Estimating patient dose on current CT scanners:
across the region, and in the UK as a whole, it would                Results of the ImPACT CT dose survey. London: ImPACT,
appear that the collective radiation dose from CT to the             1999. (www.impactscan.org/dosesurveysummary.htm).
UK population is likely to continue to rise beyond current       13. White MJ, Eatough JP, Goldstone KE. Audit of effective
estimates. As would be expected from a simplistic con-               dose for patients undergoing X-ray computed tomography.
sideration of geometric efficiencies, the distinction between         Cambridge: East Anglian Regional Radiation Protection
single-slice and multislice effective doses was found to be          Service, 1999.
                                                                 14. Workman A, Kotre J, Shaw A, Fong R, Wall B, Bury R, et al.
greatest for examinations using narrow slice widths, where
                                                                     IPEM/NRPB/RCR/CoR/BIR diagnostic reference levels
the effect of beam collimation in multislice CT is most              working party. IPEM Newsletter 2000;67:2–4.
severe. Given the differences observed between single-slice      15. Workman A. Reference doses and DRLs: the background
and multislice scanners, it is suggested that separate               and the publication. In: Proceedings of UK Radiological
National DRLs might need to be set for single-slice and              Congress 2003; 2003 June 15–17; Birmingham. London:
multislice CT. As a minimum however, National DRLs                   British Institute of Radiology, 2003.
will need to be set with sufficient flexibility to allow for the
potentially higher doses from multislice CT.




478                                                                                 The British Journal of Radiology, June 2004

				
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