Published results indicate a large variation of patient doses for the same type of CT scan examination. This is unsurprising considering the long time period that this review covered during which CT scanning technology has evolved and examination protocols have been optimised.
Median CTDIw and DLP values are below the proposed DRLs for all examinations ( and ); however, there are individual studies for which the DRLs are exceeded (). Variation of results among studies (maximum/minimum reported value) is evident for both dose descriptors; however, it is more profound for DLP and CT examinations of the trunk. CTDIw varies by a factor of 4.2/3.8/6.0/7.2 for studies concerning the head/chest/abdomen/pelvis, respectively. DLP varies by a factor of 3.2/15.1/9.3/8.2 for the head/chest/abdomen/pelvis, respectively.
It is anticipated that variation of CTDIw
among studies can be attributed to intrinsic differences among the makes and models of CT scanners such as beam geometry, radiation quality, number of detector rows, absorption and scattered X-rays [24
]. An additional significant factor of variation is the selected examination protocol and in particular the milliampere seconds selection because the dose is directly proportional to this [13
]. Variations in examination protocols are attributed to different routine protocols among institutions [25
] but also to different clinical indications, which can lead to variation in scanning techniques [26
The greater variability of results for DLP is the result of an additional parameter i.e.
scan length. Longer scan lengths can be attributed in part to the use of contrast media since these procedures involve a repeated scan of the same region (i.e.
with and without the contrast media) [24
]. Two obvious measures to reduce patient dose consequent to the above are reducing the milliampere seconds whilst maintaining diagnostic image quality, especially for thinner than standard patients, and reducing scan length as much as possible without missing any vital anatomical regions.
An effect that contributes to increased patient exposure is “overscan” which arises from the requirement in spiral reconstruction algorithms for data beyond the actual volume to be reconstructed [27
]. Thus, at the beginning and the end of the scan volume areas are exposed that are not part of the area medically in question [28
Regarding ED, results indicate that in CT examinations of the head and neck ED is close to 2 mSv but increases 2- to 4-fold for examinations of the trunk (). Thus, ED for head examinations is lower than that of the trunk although individual organ doses for the head are considerably greater than for other parts of the body (). This is owing to the uneven distribution of radiosensitive organs in the human body and the lower weighting factors for the head organs [18
]. Accordingly, the correlation between DLP and ED must be separated into two parts: one for CT examinations of the head and one for CT examinations of the trunk [29
]. As the relationship between DLP and ED is linear and the relationship between ED and stochastic risk is also assumed to be linear, DLP, which is normally quoted by modern scanners, can be used for comparing the stochastic risk for different CT examinations [29
The investigation of ED variation over different time periods () revealed that ED for CT examinations of the head, chest and abdomen prior to 1995 were significantly higher than for the later studies whereas over the period between 1996 and 2009 the ED remained virtually unchanged. The finding that ED for abdomen and pelvis examinations are increased during the most recent period (2006–2009) is most likely attributable to studies from developing countries which are in the initial stages of protocol optimisation [24,30,31
Significant dose reduction in more recent studies is attributed to the implementation of dose management procedures that correspond to the ALARA principle [32
]. The mechanisms for dose reduction in CT equipment include [33
- X-ray beam filtration;
- X-ray beam collimation;
- X-ray current modulation and adaptation for patient body habitus;
- Peak kilovoltage optimisation;
- Improved detection system efficiency; and
- Noise reduction algorithms.
Tube current modulation and patient size dependent tube current adaptation are jointly referred to as automatic exposure control (AEC). The first commercially available tube current modulation systems were introduced in 1994 and provided dose reduction of up to 20% without considerable degradation of image quality [34
]. Subsequent studies validated that dose reduction is achievable and that in elliptical body regions anatomically based modulation of the tube current results in up to 40% dose reduction [35
Multislice CT technology, which is currently common in clinical practice, increases the efficacy of CT procedures and offers new promising applications such as multiphase exams, vascular and cardiac exams, perfusion imaging and screening exams of the heart, chest and colon [36
]. However, the expanding use of multislice CT systems may result in a considerable increase in both the frequency of CT procedures and patient exposure levels. Regarding patient exposure, a certain dose increase compared with single-slice CT is unavoidable owing to underlying physical principles and in particular dose efficiency [39
]. The dose efficiency of a CT scanner refers to the fraction of the total X-rays emitted from the X-ray source that are captured by the detectors and contribute to image formation. Dose efficiency has two components: the absorption efficiency, which is the fraction of X-rays that are captured by active detector area; and geometric efficiency, which is the fraction of X-rays that exit the patient and enter the active detector area [40
]. The absorption efficiencies of single-slice CT and multislice CT are similar since the same type of detectors is used in both cases. However, geometric efficiency for multislice CT is reduced owing to the requirement for dividers between individual detector elements along the z
-axis, which create dead spaces [40
]. Additionally, with multislice CT only the plateau of the trapezoid dose profile may be used to ensure equal signal level for all detector slices while the penumbra region has to be discarded. This represents a wasted dose to the patient in contrast to single-slice CT where the entire dose profile contributes to detector signal [39,41
]. Since the penumbra accounts for a larger percentage of the beam when a narrow detector configuration is used, dose efficiency is decreased with the use of a narrow collimation. This principle applies to CT scans performed with any multislice CT scanner, but the effect on radiation dose is greater with four slice scanners (because of the small beam size) than with scanners capable of acquiring more slices per rotation [42
]. Thus, the relative contribution of the penumbra region to patient dose decreases with increasing slice width and also decreases with increasing number of simultaneously acquired slices [39,40
To evaluate the effect of multislice technology for the most common types of CT examinations (head, chest, abdomen and pelvis), the ED were extracted from studies that reported results for single-slice CT and multislice CT. The number of studies that report ED for non-cardiac multislice CT examinations is currently limited and the results for multislice CT refer to various types of scanners that acquire from 2 to 256 slices per rotation. As previously mentioned, median effective doses for multislice CT scans are equal or lower than single-slice CT scans but the statistical power is not adequate to detect any significant differences between them. A few comparative studies have evaluated multislice CT against single-slice CT technology. A study of single-slice CT, dual-slice and quad-slice systems concluded that average ED over all examinations is increased by 10% for quad-slice systems but decreased by 26% for dual-slice systems [43
]. Two subsequent studies indicate that doses are increased for multislice CT with four or more detector rows compared with single-slice CT [26,44
] and that on average multislice CT scanners deliver 35% more ED than single-slice CT scanners, although this difference is not uniformly spread across all examinations [44
]. The distinction between single-slice CT and multislice CT ED is generally greatest for examinations using narrow slices (e.g.
head or high resolution chest) but is less apparent for other examinations (e.g.
abdomen or pelvis) [44
]. This characteristic is owing to the necessity of multislice CT scanners to irradiate more of the patient than is actually imaged particularly for acquisition of narrow slices and it can result to doses up to 40% higher compared with well-collimated single-slice CT systems [45
Regarding comparison of delivered dose among multislice CT with varying number of detector rows, a study comparing radiation dose at routine chest examination between scanners with 4, 8 and 16 rows revealed a trend towards decreasing radiation dose with increasing number of detector rows [46
]. Another study comparing doses associated with 16-slice and 256-slice scanners also concluded that dose reduction is achieved for all types of CT examinations with the 256-slice scanner [47
]. A recent study comparing organ and ED in chest and abdominal CT examinations concluded that doses associated with 64-slice scanners are similar to those with 4-, 8- and 16-slice scanners [48
A development in multislice CT technology is the introduction of scanners with two X-ray tubes, for which patient doses could be up to a factor of two lower than a single source CT scanner of the same number of acquired slices [49
A concurrent finding of the above comparative studies is that multislice CT scanners with more that 4-detector rows deliver higher patient dose than single-slice CT and that patient dose is decreased with increasing number of detector rows. However, it has to be noted that most comparative studies evaluated single-slice CT scanners that had heavily optimised protocols of exposure settings already available and implemented compared with multislice CT scanners which were the beginning of their optimisation process [43
]. It is therefore possible that with modern multislice CT scanners, capable of acquiring 16 or more slices per rotation in combination with sophisticated exposure reduction techniques, radiation doses to patients during CT examinations could be substantially reduced. However, the improved clinical efficacy and new applications available with multislice CT are likely to lead to rising examination frequency and thorough justification of exams and an effort to minimise patient irradiation should always be undertaken.