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Commentary |
ImPACT, Department of Medical Physics & Bioengineering, St. George's Hospital, London SW17 0QT, UK
Increasing dose from CT?
CT is undoubtedly a powerful diagnostic tool, but data from surveys of CT practice show that, over the 30 years of clinical use, CT has contributed an ever-increasing percentage to the population dose from medical X-ray examinations [1, 2]. Although some regard the radiation dose levels from CT as beneficial [3], the generally held view is one of alarm at the increasing dose contribution, and the consensus amongst the radiology professions is that steps should be taken to reverse, or at least arrest, this trend. However, the reasons for this increasing dose contribution should be examined. Is it only as a result of increased numbers of examinations, or also as a result of an increase in dose per examination? Increased numbers of examinations, if correctly justified, must be viewed as resulting in a net benefit to patient management. However, if the doses to individual patients are increasing, the reasons must be carefully examined. Monitoring of trends in CT patient doses is currently particularly important as the technology is evolving rapidly. Multislice CT scanners, capable of simultaneously acquiring four slices in a single rotation, were introduced into clinical practice in 1999 and, as a result of targeted government funding to replace older scanner models, their adoption in the UK has been rapid. After only 6 years since their launch, multislice systems constitute around 70% of the CT scanners operating in England [4].
Multislice scanning has led to a major revolution in CT practice. Four slice scanners have been followed by 8-slice, 16-slice and up to 64-slice scanners. The greatly reduced examination times on these systems and their capacity to scan long lengths with narrow slices have further increased the scope of CT as a diagnostic tool. The use of sub-millimetre slice widths increases the spatial resolution in the scan axis direction, allowing a high spatial resolution in all planes. This isotropic resolution capability results in high quality three-dimensional (3D) and multiplanar reconstructions which are of particular benefit in angiography and endoscopy studies. The 16- and 64-slice scanners are also making an impact in the field of cardiology, and CT cardiac angiography in particular is now rivalling conventional diagnostic techniques, and has the advantage of being non-invasive.
The trend in technology is generally towards more dose efficient equipment, with solid-state detectors, an emphasis on high geometric dose efficiency and the introduction of dose-saving and dose-awareness features. However, the ever increasing capabilities of modern CT scanners, in terms of fast examination time and more powerful X-ray tubes, allow for the scanning of longer patient lengths and the use of higher tube currents. Unfortunately, there is still little published data available on trends in doses following the introduction of multislice CT. Two recent studies suggest average increases in effective dose per patient of 10% and 34% for multislice compared with single slice scanning [5, 6]. The recently published results of the 2003 UK CT dose survey [7] show that there has been a reduction in average patient doses from CT examinations since the last national UK CT dose survey published in 1991 [8]. However, they also show that doses from multislice are generally slightly higher than current dose levels from single slice CT scanners.
Dose implications of multislice scanners
In terms of hardware, multislice CT scanners share many design features with their single slice counterparts. Manufacturers often use the same detector material, scanner geometry and scan plane detector configuration on both their single and multislice systems. Despite this, concerns have been raised that multislice scanners are intrinsically higher dose systems and lead to increased patient doses. This perception may partly stem from the fact that one of the early multislice scanner models had significantly reduced dose efficiencies due to a large proportion of the X-ray beam width not being utilized for imaging [9]. These systems have now been upgraded, so the extent of the beam width not used for imaging is at most 2–3 mm on all multislice scanners. This factor results in absorbed dose increases of just a few percent for beam widths of 20 mm and above, but a doubling or more of dose for beams of less than about 4 mm. The advantage of scanners simultaneously acquiring 16 slices and above is that the narrow X-ray beam widths seldom need to be used, as narrow slices can be acquired with wider beam widths. Another factor that can lead to reduced dose efficiency on multislice scanners is the greater number of septa with increasing numbers of detector banks, as this can result in a reduced proportion of active detector area. However, this is a relatively small effect on current systems [10].
When scanning in helical mode, all CT scanners acquire additional rotations at each end of the scan length in order to obtain sufficient data to reconstruct the full imaged volume. Reconstruction methods on multislice systems sometimes require a greater number of additional rotations. This, together with the greater X-ray beam widths used, can result in a significant increase in effective dose, particularly for short scan lengths [11]. In these situations it may be appropriate to revert to conventional slice by slice scanning.
Apart from the scanner design aspects, there are important dose issues arising from the clinical application of multislice scanners. On single slice scanners, scan length and tube current are often limited by the X-ray tube heat capacity. This also places constraints on the imaged slice width used, as a higher tube current (mA) is required with narrow slices to maintain image noise at an acceptable level. The patient breath-hold time may add additional limitations on scan length and imaged slice width used. Multislice scanners are not restricted by these factors. As a result of high heat capacity tubes and use of larger X-ray beam widths, it is possible to scan long lengths, with narrow slices, at high tube current–time product (mAs) levels. The high gantry rotation speeds and increased length covered per rotation lead to shorter examination times, and so scan length and slice width are rarely limited by patient breath-hold time. Multiphase contrast studies can also be easily performed due to the decreased examination times.
The reduction in constraints on scan parameters for multislice scanners offers the potential to improve diagnostic quality, but the flexibility can lead to unnecessarily high doses if care is not taken to optimize scanning protocols. The increased capabilities also present the possibility for a greater range of applications, including CT screening, a controversial but expanding area, particularly in the private healthcare sector.
Is there scope for dose reduction?
The most powerful dose reduction tool in radiation protection is the process of formal justification of a study, i.e. the avoidance of unnecessary procedures through risk-benefit assessment. However, where an examination is undertaken, the emphasis must be on dose optimization, i.e. achieving the required image quality at the lowest possible dose level. This can be approached in two ways; the first is through the design of dose efficient equipment, and the second, through the optimization of scan protocols.
Equipment design is primarily the responsibility of manufacturers, and in recent years they have put a major effort into dose efficient design and dose optimization features on their scanners. One of the most recent and promising techniques is the automatic adaptation of tube current (mA) to account for varying patient attenuation or, in other words, automatic exposure control (AEC) for CT. The tube current can be automatically adjusted from patient to patient to achieve the required image quality regardless of patient size. Additionally, the tube current can be varied between rotations to compensate for the changing attenuation along the patient's length. A further level of AEC can be achieved by varying the tube current during the course of a rotation to account for changing attenuation through different projections around the patient, and this can lead to average absorbed dose reductions (CTDIvol) of up to 50% in some anatomical regions without any loss of image quality [12]. Automatic adaptation of tube current for patient size can be particularly beneficial in reducing absorbed doses in paediatric patients. AEC is also being employed for dose reduction in ECG-gated cardiac CT examinations by reducing the tube current for the cardiac phases from which data is not utilized in image reconstruction. Reductions in CTDIvol of the order of 50% can be achieved by this method [13].
Noise-reducing reconstruction algorithms are another approach being explored by manufacturers for potential dose reduction. Often referred to as "adaptive filters", they operate either in the raw data domain or use post-processing methods to reduce noise whilst maintaining the resolution in specific regions of the image, thereby offering the potential to use lower mAs values [14, 15].
The second approach to dose reduction is through the optimization of scan protocols. The 1991 NRPB UK CT dose survey showed that effective doses for the same examination could vary by up to a factor of 40 between different institutions [8]. Data from the 2003 UK CT dose survey shows that effective dose variations, for specific examinations on adults, are still an order of magnitude in range [7]. Differences in dose efficiency of equipment generally contribute at most a factor of three to this [16], and, for more recent scanners, less than a factor of two for routine examinations [17–20]. Much of the dose variation can therefore be attributed to differences in scan protocol, so there is great scope for optimization in this area.
As a general rule, patient scan lengths should be minimized and the need for multiphase contrast studies justified. AEC in CT will help in adjusting the tube current to patient size, but determining the mAs level, or required image noise level, to be used as a baseline is not a trivial matter. Studies suggest that higher mAs values are generally used in CT than are required to achieve diagnostic images [21]. One approach being investigated to determine the acceptable level of image noise for diagnosis is through the addition of varying amounts of computer-simulated noise to an original low noise (high mAs) image [21, 22]. The groups of images with different noise levels are then scored by radiologists, and in this way the minimum mAs level that enables accurate diagnosis can be established. This is particularly important now that narrower slice widths are being used routinely, in order to avoid high mAs values that may be used to compensate for the increased noise levels. When increasing mAs values for this purpose, it should be noted that use of narrow slices results in a reduction of partial volume effect and therefore increased contrast levels. It is not always necessary, therefore, to double the mAs when the imaged slice width is halved as visualization of a given structure may be achieved at a higher image noise level.
Dose optimization in paediatric patients is a particular area where large dose savings can be made through the use of appropriate protocols. Selecting 80 kVp as opposed to the standard 120 kVp in iodine contrast studies can lead to reductions in absorbed dose of around 50% for the same image quality [23]. In-plane bismuth shields, although not widely used, have been shown, in phantom studies, to reduce absorbed doses to the breast and eyes by around 30% and 60%, respectively, without adversely affecting image quality [24, 25].
Adequate education of CT users as to the effect of scan parameter setting on dose and image quality is important due to the increased complexity of multislice scanners. In some instances, the impact of scan parameter setting on image quality differs between multislice and single slice scanners. For example, the image noise on single slice scanners remains constant with helical pitch, whereas on most multislice scanner models noise would increase with pitch if the mAs remained constant. Often with multislice scanners, to maintain a constant level of image noise with changing pitch, the tube current is automatically increased as pitch increases. With this approach, the CTDIvol does not vary with pitch, and so increasing the pitch cannot be used as a dose saving measure in the same way as on single slice systems.
A feature that should increase dose awareness and help CT users in dose optimization is the prospective display on the scanner console of CTDIvol (volume-averaged CT dose index) and, on some scanners, the dose–length product (DLP) [26]. These give an indication of the average absorbed dose and relative radiation risk to a standard patient. The user is thereby alerted to scan parameter settings that may lead to high doses, and can adjust the protocol if appropriate. It should be noted that CTDIvol and DLP do not take patient size into account, and therefore, relative to a standard patient, these will give overestimates and underestimates for large and small patients, respectively.
The displayed dose descriptors can also be used to monitor trends in patient doses and compare local doses to national reference doses. The requirement of the Ionising Radiation (Medical Exposure) Regulations 2000 [27] to set local diagnostic reference levels (DRLs) for common CT examinations should identify centres using high dose techniques and lead them to modify protocols where necessary. The recently published NRPB report of the 2003 UK CT dose survey presents new national reference dose levels for adult and paediatric CT examinations [7]. These new levels should now be adopted into clinical practice.
Summary
Multislice scanners have further expanded the role of CT as a valuable diagnostic tool and, when appropriately used, bring benefits to patient management. The risk is that their flexibility in terms of long scan lengths and use of narrow imaged slices with high mAs values can lead to unnecessarily high doses if diagnostic requirements are not adequately considered.
The focus on dose efficient design and inclusion of dose reduction features on scanners is an important aspect of optimizing doses in CT. AEC in CT has the potential for reducing absorbed doses by around 50% in some anatomical regions for standard sized patients alone. For children and smaller patients, the tailoring of tube current to patient size can lead to even greater absorbed dose savings.
CT dose surveys have shown that the range of effective doses for a given examination can vary by more than an order of magnitude between different institutions. This is a much larger variation than can be explained by differences in dose efficiency between scanner models, and therefore leads to the conclusion that there is much scope for dose optimization through use of appropriate protocols.
The establishment of acceptable levels of image quality for different examination types utilizing computer-simulated noise addition, together with the setting of appropriate reference dose levels for CT, show a promising way forward towards dose optimization in CT scanning.
Received for publication November 24, 2004. Revision received April 4, 2005. Accepted for publication May 4, 2005.
References
This article has been cited by other articles:
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  P C Shrimpton, M C Hillier, M A Lewis, and M Dunn National survey of doses from CT in the UK: 2003 Br. J. Radiol., December 1, 2006; 79(948): 968 - 980. [Abstract] [Full Text] [PDF]
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BJR Review of the Year - 2005. Br. J. Radiol., March 1, 2006; 79(939): 183 - 187. [Full Text] [PDF]
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