Radiation dose in CT are we meeting the challenge by joq12180

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									The British Journal of Radiology, 75 (2002), 1–4   E   2002 The British Institute of Radiology



Commentary
Radiation dose in CT: are we meeting the challenge?
1
    S J GOLDING, MA, FRCR and 2P C SHRIMPTON, MA, PhD
1
 Department of Radiology, University of Oxford and 2National Radiological Protection Board, Chilton, Didcot,
Oxon OX11 0RQ, UK



What challenge?                                               factor of 20 in Norway [5]. Overall, the evidence
                                                              indicates a strong trend of increasing population
   CT has developed dramatically: scanning is
                                                              dose owing to rising use of CT and to increased
faster, images are better, applications have grown
                                                              dose per examination. It now seems clear that
and, of course, radiation doses have come down,
                                                              variations in practice have become more impor-
have they not? Well, no, actually; quite the con-
                                                              tant than scanner technology in determining the
trary in fact.
                                                              dose to the patient [6].
   When CT was new it was appreciated that it
                                                                 Ease of use may be contributing to this syn-
was a relatively high dose technique but there was            drome. Early CT scanners were rigid tools and
overriding clinical justification for using it; in the         extending an examination implied a time penalty.
brain no technique could approach it and when                 The introduction of spiral CT reduced this dra-
body CT began it concentrated on patients with                matically and probably contributed significantly
malignant disease, where radiation dose was of                to new variations in practice, as few technique
less concern. Now circumstances are different.                guidelines were available when the new tech-
The technique is widely used, extensively in                  nology was introduced. For example, contrast
benign disease as well as in young patients for               enhanced studies have become more widespread
whom radiation protection considerations are                  and multiphase enhancement has become com-
paramount. Awareness dawned of the magnitude                  mon [7]. Although this latter technique has
of the challenge following a national dose survey             extended the application of CT, its use when a
in the UK. In 1989 the National Radiological                  smaller number of phases would suffice cannot be
Protection Board showed that despite comprising               justified. There is also anecdotal evidence that
only 2% of all examinations, CT contributed                   workload pressure may be adding to the problem.
around 20% of the collective dose to the popu-                In the face of rising demand, radiologists may rely
lation from diagnostic imaging [1]. Subsequent                on CT examinations being performed with stan-
analyses for the UK suggest that this latter figure            dard ‘‘catch all’’ protocols while they concentrate
may have risen to 40% [2]. One department in the              on other work; retrospective reporting of a
USA has claimed that CT now represents 67% of                 comprehensive examination is efficient for the
the collective dose that it delivers [3]. Patient             radiologist. Inexperienced radiologists in parti-
doses from CT are among the highest in diag-                  cular are likely to feel more confident the more
nostic radiology; an abdominal examination in an              sections they have available to read. However,
adult with an effective dose of 10 mSv has been               this removes one of the key elements of radiation
estimated to increase the lifetime risk of fatal              protection in CT, namely supervision by the
cancer by 1 in 2000 [4]. Radiation exposure from              radiologist, who should terminate the examina-
CT is rising, not falling.                                    tion at the point at which it has delivered the
   This increase is not simply owing to CT                    information required for clinical management.
replacing other techniques. We know that large                The approach of ‘‘imaging overkill’’ may be
variations in CT practice exist; experience from              tempting in current circumstances but cannot
review clinics suggests volumes of exposure, num-             withstand serious enquiry.
ber of slices and repeat exposures can be widely                 A new urgency exists in the form of multislice
different within the same clinical application, with          CT [8]. This technique offers almost no resistance
little apparent clinical justification. Surveys of             to extending the examination, introduces new
practice and dose suggest that (effective) dose for           applications and will be widely used despite the
a given examination may vary by a factor of 40                fact that, examination for examination, absorbed
between departments for a UK patient [1], or a                dose may be up to 40% higher [9, 10]. Inter-
                                                              ventional CT and CT fluoroscopy pose a par-
Received 27 March 2001 and in revised form 31 May             ticular problem. The latter may use an exposure
2001, accepted 18 June 2001.                                  rate 10 times that of conventional fluoroscopy

The British Journal of Radiology, January 2002                                                                 1
                                                                              S J Golding and P C Shrimpton

and it is not difficult for the hands of the operator    clearly required in view of the propensity of CT to
to reach the annual occupational dose limits [11].      develop new uses.
If procedures under CT fluoroscopy become estab-            Optimization of CT is more problematic. In
lished, a particularly intense approach to protect-     conventional radiography it is usually clear when
ing both patient and operator is indicated [12].        over exposure has taken place. This is not true
                                                        in CT, as the technology compensates for wide
                                                        variations in exposure parameters and it is pos-
                                                        sible for the difference between an adequate image
What should be done?                                    and a high quality one obtained at much higher
   Our situation is given new impetus by new            exposure not to be immediately evident. The ‘‘as
European and national regulations [13] that re-         low as reasonably achievable’’ (ALARA) principle
quire departments to introduce robust procedures        applies just as much to CT as it does to con-
for the protection of the patient, including the        ventional radiography. We know that by manip-
twin elements of ensuring clinical justification of      ulating exposure factors the radiologist can
the examination and optimization of the tech-           readily alter the dose to the patient by a sig-
nique. In CT this translates into strong considera-     nificant amount [11]. However, these factors also
tion of other techniques, notably ultrasound and        determine image quality and therefore clinical
MRI. Where these are a practical alternative,           efficacy. Contrast resolution, which may be cri-
rigorous adoption of clinical guidelines for vetting    tical in some common examinations such as
and accepting requests, as well as ensuring that        detecting soft tissue lesions in the liver, is
examination parameters are appropriate to the           detracted by image noise and therefore improved
indication is required. UK departments are cur-         by increasing exposure. Ideally we would know
rently adapting their practice to the new regula-       what represents acceptable image quality (and
tions, but a major challenge faces the task in          therefore exposure) for each application, but
hand. Common experience suggests that clinicians        unfortunately we suffer from a serious lack of
have come to rely progressively on imaging where        evidence in this area [6]. In areas of high natural
hitherto clinical examination would have been           contrast, such as the chest or skeleton, image
regarded as sufficient. This is especially true for      noise is less critical and clinically acceptable
doctors in training, who find an authorita-              images can be obtained with limited exposure
tive result reassuring in the face of inexperience      factors, especially reduced tube current [18]. A
in clinical skills. Some requests are not easy to       dose reduction of 50% is possible in the chest
decline; for example it is not easy to exclude pul-     without diagnostic loss [19], and in orbital trauma
monary embolism clinically in patients presenting       a low dose approach may produce a reduction by
with chest pain, and CT pulmonary arteriography         an order of magnitude [20]. In fact, where the
threatens to become a ubiquitous screening tool         clinical objective is very limited and the image
unless precise clinical criteria for examination are    quality not critical, a 25-fold reduction is possible
defined [14, 15]. Similar constraints may need to        [21]. In most other major areas of use we have a
be applied to ‘‘exciting’’ new applications such as     serious need for studies indicating the threshold
CT colonography or CT bronchography [8]. In             exposure factors that will deliver clinically effec-
that sense, attempts to conform to the new regu-        tive image quality, including pitch factors and
lations are swimming against a tide. The need for       section thickness with the new technology [6, 22,
effective justification in children is even more         23]. National regulations [13] to promote optimi-
important, as a higher radiosensitivity owing to        zation of patient protection now require the
the longer opportunity for delayed effects to           formal use in CT departments of diagnostic refer-
appear is compounded by the fact that effective         ence levels as investigation levels to help identify
doses may also be higher [16].                          unusually high doses.
   It is a golden rule of investigative medicine that      Mounting concern about radiation dose in CT
the benefit of the procedure should outweigh the         is reflected in international efforts to raise the
risk. In CT, justification depends on the prob-          profile of the issue and to advise on practice. In
ability that clinical management will be positively     1994 a European Commission (EC) Working
influenced by the results. Practitioners are in fact     Party began work on European guidelines for
quite fortunate in the amount of research that has      good practice in CT [24], including the develop-
been carried out into the clinical applications of      ment of reference dosimetry based on the
CT, and the evidence base is now strong. Clinical       practical dose quantities of weighted CT dose
guidelines such as those produced in the UK by          index (CTDIw) and dose–length product (DLP)
the Royal College of Radiologists [17] provide          [25]. The European Co-ordination Committee of
strong guidance for radiologists’ consideration of      the Radiological and Electromedical Industries
whether clinical requests are sufficiently justified      has recently published a handbook on radiation
to merit acceptance, although continued update is       exposure in CT [18], and the International

2                                                              The British Journal of Radiology, January 2002
Commentary: Radiation dose in CT

Commission for Radiological Protection (ICRP)             In view of the magnitude of its task, the EC
set up a task group in 1999 to report on the           Working Party has continued under Euratom
management of patient dose in CT. This latter          Framework Programmes 4 and 5. One of the
document has been through its consultation phase       tasks completed by the group is a pilot study
and publication is imminent [11]. The report           regarding application of the criteria, covering five
concentrates on the magnitude of doses, the            types of examination over four countries [32].
practical steps to reduce them and recommenda-         This study was instrumental in providing refer-
tions to manufacturers on technological develop-       ence doses to complement those available from
ments.                                                 previous surveys and also supported the conten-
   Although containment of dose has not been as        tion that diagnostic criteria could be used to
prominent in CT development as other features,         optimize CT to achieve dose reduction. The
manufacturers have an important role in ensuring       Working Party is currently planning a European
that patient dose is kept to a minimum. One            field survey focusing on evaluating examination
recommendation of the EC Working Group was             protocols and assessing patient dose, with a
that scanner consoles should give an indication        further survey intended to target image quality
of patient dose, and the International Electro-        and dose in a selected patient group. Also planned
technical Commission now recommends the dis-           are refinements to methods for calculating effec-
play of CTDIw [26], albeit modified by the pitch        tive doses from CT, as well as an Internet data-
setting, although some manufacturers also help-        base for dosimetric data for CT, together with
fully include values of DLP. Another develop-          revision and extension of the European guidelines.
ment that is strongly recommended by the ICRP          It is intended that this work, together with that of
is on-line tube current control; this adapts the       other centres, should provide a scientific founda-
exposure to the shape of the body and may              tion for effective examination guidance in the
achieve a dose saving of 15–50% [27, 28]. The use      future, particularly in relation to reference dose
of solid-state detectors may also reduce dose by       levels and image quality.
30% [29].                                                 For the present it behoves all involved in CT to
                                                       observe the following:
The European guidelines on quality                     (1) There must be clear justification for CT. This
criteria for CT                                            means active consideration of whether the
                                                           examination is required, whether it could be
   The quality criteria concept has been developed
                                                           replaced by ultrasound or MRI and, if
by the EC as an effective method for optimization
                                                           accepted, whether it conforms to current
in medical imaging. The definition of quality
                                                           clinical guidelines.
criteria for CT was carried out by a multinational
                                                       (2) The examination technique must be targeted
group composed of radiologists and physicists.
                                                           to the clinical application and the exposure
Prescribing quality criteria is necessarily more
                                                           parameters must be adjusted to those settings
complex in CT than in conventional radiography
                                                           delivering the minimum dose necessary, where
in view of the specific requirements of multiple
                                                           these are known.
applications. The Working Party therefore con-
                                                       (3) A single spiral exposure or sequence of serial
centrated on common applications of CT and the
                                                           scans should be used where this alone will
resulting guidelines cover six major areas of use
                                                           satisfy clinical need.
[30]. For each of these areas, recommendations
                                                       (4) Additional scans with contrast enhancement
are made on preparatory steps before investiga-
                                                           should only be used when there is clear clini-
tion, on the criteria for acceptable images and for
                                                           cal evidence to support their application.
good imaging practice, and on clinical conditions
                                                       (5) Tube current should be reduced to a mini-
that impact on good imaging performance. The
                                                           mum where possible, especially in high resolu-
proposed guidelines underwent extensive consulta-
                                                           tion studies.
tion, including an open workshop in Aarhus, and
                                                       (6) The literature should be observed constantly
were published early in 2000. They are also avail-
                                                           and practice should be adapted as more
able on a website (http://www.drs.dk/guidelines/ct/
                                                           evidence becomes available.
quality/). Integral to the Working Party’s appr-
                                                       (7) Centres should participate in further surveys
oach was the concept of reference dose levels. These
                                                           of dose and national initiatives to refine
are relatively easy to define in conventional radiog-
                                                           diagnostic reference levels for CT.
raphy but less readily provided in CT owing to the
complexity of dosimetry and the variability              Radiation protection in CT must not become
of examinations [18]. Initial values of CTDIw          subject to paranoia or a witch-hunt, but equally
and DLP have been suggested for some common            there is no place for complacency. Both patients
examinations on adult [24] and paediatric patients     and purchasers have the right to expect that staff
[31].                                                  conform to best practice principles. As far as staff

The British Journal of Radiology, January 2002                                                           3
                                                                                   S J Golding and P C Shrimpton

and departments are concerned, this means an                 16. Huda W, Atherton JV, Ware DE, Cumming WA.
awareness of emerging evidence and the implied                   An approach for the estimation of radiation dose
                                                                 at CT in pediatric patients. Radiology 1997;203:
changes in practice, with revision of protocols to               417–22.
take account of advances. It also means establish-           17. Royal College of Radiologists. Making the best use
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where there is clinical justification for exceeding           18. Nagel HD, editor. Radiation exposure in computed
                                                                 tomography (2nd edn). Frankfurt: European Co-
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                                                             19. Mayo JR, Hartman TE, Lee KS, Primack SL,
                                                                 Vedal S, Muller NL. CT of the chest: minimal tube
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4                                                                   The British Journal of Radiology, January 2002

								
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