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Patient dose in multislice CT: why is it increasing and does it matter?

UCL Hospitals, Mortimer Street, London W1T 3AA, UK

	Abstract

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
A brief review is presented of the reasons why multislice spiral/helical CT is associated with a higher radiation dose burden to the patient even than incremental CT. These include both intrinsic technological and geometric factors as well as simply a growing use of CT in an increasing number of applications. The typical magnitude of this dose burden is indicated and the basis for the anxiety that underpins it, namely the linear no-threshold (LNT) hypothesis, is discussed, together with the countervailing hypothesis that there is indeed a threshold for radiation harm in man and that the radiation doses associated with CT may lie below this threshold and may even be beneficial (radiation hormesis). There are as yet no certainties in this important area but it is argued that it is not a given that the doses associated with CT are harmful.

	Introduction

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
With two exceptions, radiation doses from diagnostic X-ray procedures have been decreasing over recent years as a consequence both of better equipment and of better training in its use [1]. The two exceptions are interventional radiology, which is a special case, being therapeutic rather than purely diagnostic, and CT.

CT was always considered a "high dose" technique, but recent technological developments have conspired to make it more so as a consequence both of changes in the technology and the consequent changes in practice that it has allowed. The dose burden, both to individuals and to the general population, associated with CT has now become a source of concern to radiologists and to regulatory authorities around the world [1–4]. These anxieties may perhaps be summed up by quoting the UK National Radiological Protection Board (NRPB), who state that the increased lifetime risk of death from a malignancy induced by a typical CT examination of the abdomen is of the order of 1 in 2000 [5]. For more complex or repeated examinations it is presumably higher in their assessment.

We will examine the reasons for the higher dose in CT in general and for the yet higher dose in multislice helical CT, and will seek briefly to examine the basis for such claims as that of the NRPB.

	Why is CT a "high dose" examination?

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
The information content of a medical X-ray image is a function of several factors and their complex interaction, but there is one overarching fact, namely that information is purchased with photons. CT technology in its various guises utilizes many photons in what amounts to a series of exposures rather than the single exposure of conventional projection radiography. The price paid by the patient is a greater radiation dose. For example, the effective dose E to the patient of a conventional chest X-ray is approximately 0.02 mSv*, whereas the E associated with a CT scan of the thorax is of the order of 8 mSv [1]. All other things being equal, we may even say that, broadly speaking, image quality, an admittedly complex issue, improves with increasing dose.

There is another important factor at work, namely the loss of self-regulation inherent in conventional radiography afforded by the use of film. Thus, if too high a dose is used to obtain a plain film, the loss of quality (e.g. a black film) will be noticeable; if too high/unnecessarily high dose is used in CT the result will be admirable, high quality images.

	Spiral and multislice spiral systems

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
Technology-related factors
All spiral/helical systems are associated with a somewhat higher dose burden than incremental scanners because a larger volume of body is in the event scanned than that which is selected [6]. The reason for this lies in the interpolation technique used to reconstruct the image. To reconstruct the highest slice, a data set of a higher slice yet must be available; similarly, to obtain the lowest slice, a lower slice data set must be available. This factor may contribute up to 10% increase in E.

Multislice systems also pay this penalty, but they also pay the price of "penumbral" effects [6]. In early machines this could contribute anything from 10–100% increase in dose [7].

Then there is a geometrical efficiency decrease associated with the interdetector gaps and the fact that greater scatter is associated with cone beams [1]. Interestingly, the importance of penumbral effects decreases with the number of detector rows and so will become less of an issue with 8, 16 and 32 slice machines [1].

All other things being equal, pitch of course plays a considerable part in determining radiation dose.

Practice-related factors
As scanners improved in speed of image acquisition, so did their versatility, which was exploited in clinical practice. The technological development first of spiral/helical scanners and, latterly, of multislice scanners introduced enormous power and flexibility. Thus, true multiphase scanning with well defined phases is now possible: larger body volumes may be scanned with little relevant time penalty, and rapid screening in a variety of guises has been made possible. Because things are possible and interesting there is a tendency for them to be done. Multiphase liver scanning, for example, is undoubtedly done more often than can be justified by diagnostic yield, and the ethics of lung cancer [8], colon cancer [9] and coronary artery calcium screening [10] seem at this time much in doubt, the radiation burden being difficult to justify under the Ionising Radiation (Medical Exposure) Regulations (IRMER) [11] in situations in which benefits to patients and to society are difficult to demonstrate. The core problem may be seen to lie in the little resistance now offered by the technology to any demands placed upon it.

	The dose burden

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
In 1989 the NRPB estimated that CT represented some 2% of radiological examinations but contributed approximately 20% to the annual collective (effective) dose to the population from all X-ray examinations. Less than a decade later in 1998 the corresponding figures were 5% and 40%, respectively [12]. One American radiology department has claimed that CT contributes some 67% of its collective patient dose [13].

As regards the absolute values of dose, a number of units may be chosen from [6], but perhaps the best universal currency for comparative purposes is the effective dose E. This takes account of the dose received by key organs, weights them for radiosensitivity and takes the weighted sum of effects. A handful of typical values for CT examinations is given in Table 1 [1].

	Regulatory initiatives

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

	Dose reduction measures

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
On the assumption that CT is at least a relatively high dose procedure and that there are, in general, dangers in radiation exposure, it is reasonable to pursue all dose reduction measures as are consistent with the aim of obtaining images of diagnostic quality. A number of these may be seen to be the responsibility of the radiologist and a number that of the manufacturers.

Radiologists' responsibilities
It is for radiologists to ensure that every examination is justified, that a suitably targeted technique is used and that the procedure is carried out efficiently and effectively to avoid repetition of images. In short, he must work within the constraints of IRMER [11]. Parameters should be modified for patient size – this is particularly the case with children where mA should be reduced where possible. No greater volume of body than is necessary should be imaged and a pitch of greater than 1 should generally be used. Multiphase scans should be avoided unless they are likely to yield useful and relevant information. "Catch-all" protocols should be eschewed.

Manufacturers' responsibilities
Manufacturers may reduce dose by the universal use of solid-state detectors, increasing beam pre-filtration, modulating the mA during gantry rotation and by providing suitable pre-sets of low dose protocols, for children for example, and by installing a variety of automatic exposure devices and dose indications [6]. Further development of algorithms for z- and adaptive-filtering and noise-reducing image construction will allow the best to be made of data sets obtained with modest dose [6].

It should be noted that modulation of tube current during gantry rotation may have its limitations in that the cathode filament cannot react instantly and gantry rotation times are now down to as little as 0.375 s.

CT in paediatrics
Both manufacturers and radiologists have a particular responsibility in paediatrics. In the USA, some 11% of CT examinations are performed on children and, in the year 2000, 2.7 million CT studies were performed on the under 15 year old group, and the evidence would appear to be that CT techniques are often not suitably modified from those used in adults. There is evidence of increased sensitivity to radiation by up to a factor of 10 times (girls to a greater extent than boys) [15, 16]. There is, of course, a longer lifetime in which the radiation effects may be manifest. It is said that there is a statistically significant risk of cancer development with effective doses below 100 mSv. There is an unfortunate tendency to use the catch-all protocols in all patients, including children.

The Food and Drug Administration (FDA) issued recommendations in 2002 [15] to reduce risks in children and small adults, namely that all requests should be vetted, all parameters should be optimized, mA should be reduced, pitch should be increased and the number of scans should be reduced (e.g. avoid pre- and post-contrast and multiphase scans). Diagnostic reference levels should be locally developed and should be specific for children [1, 11, 15].

	CT fluoroscopy

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
In the UK, CT fluoroscopy [17] has not been very widely adopted. This technique was originally pioneered by Toshiba in 1993. It involves acquiring some 3–12 frames per second at a 256 x 256 matrix with some processing subtleties abandoned. Because there is scanning in one position, skin dose in particular is an issue. Erythema and epilation skin doses may be reached in some cases [17].

	Is all the anxiety justified?

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
While caution in the use of all radiation should be the watchword, it should be said for completion that the basis on which we traditionally base consideration of these matters is no longer entirely secure. The idea that there is no threshold for radiation injury (the linear no-threshold (LNT) hypothesis), long taken as an article of faith in the radiobiology community, is contradicted by a number of studies [18–31] and evidence has been emerging for some years now that, as has been established with a number of chemical and other agents, radiation too is capable of a hormetic effect; in short, some amount of radiation may actually be positively beneficial rather than likely to be harmful. This has been established at various dose levels in a variety of animal and plant models but is not experimentally established in man. Nevertheless, some epidemiological studies do provide some evidence for it. Feinendegen et al [32] have attempted to summarize the overall evidence, which indicates that below a threshold of some 200 mSv there is evidence of beneficial effects of radiation but that above a threshold of this order there is evidence of increasing harm with increasing dose.

This is an important and active area of research in which these ideas are developing momentum. It would not be possible, or appropriate, to seek to review the subject in depth here. It is raised only to indicate that it is by no means self-evident that CT, even in its modern multislice guise and as often used today, does represent a serious danger to patients.

In short, we may say that while it would be irresponsible to take the discussion too far on the basis of present evidence in man and to suggest that CT is positively beneficial in terms of a hormetic effect, it should not be taken as a given that the widespread anxieties about the radiation burden of modern CT are incontrovertibly well based.

	Conclusions

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 
In the UK, CT represents some 5% of all radiological examinations but contributes approximately 40% to the annual collective (effective) dose. Its use is increasing steadily, a fact causing widespread concern.

As regards ionising radiation injury and carcinogenesis, the prevailing paradigm, based on the LNT hypothesis, would have it that there is a definable/calculable risk of cancer induction associated with radiation doses of the magnitudes of those associated with modern CT. However, some modern thinking and data indicate that the LNT model may not be valid and that there may, in fact, be hormetic effects of radiation doses at these levels.

	Footnotes

 
*Any doses quoted in this article are intended to be illustrative, general order doses and will vary from machine to machine and with the parameters used with that machine.

Received for publication July 7, 2003. Revision received February 5, 2004. Accepted for publication February 16, 2004.

	References

Top Abstract Introduction Why is CT a... Spiral and multislice spiral... The dose burden Regulatory initiatives Dose reduction measures CT fluoroscopy Is all the anxiety... Conclusions References

 

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This Article Abstract Full Text (PDF) Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Dawson, P Search for Related Content PubMed PubMed Citation Articles by Dawson, P