CT accounts for approximately 9% of all radiological examinations but is responsible for 47% of medical radiation dose.32,33,34,35
Approximately 11% of CT examinations performed are in the paediatric population. The continuing evolution of new CT technology with faster scanning times and high resolution means that the number of CT examinations in children, already in the millions worldwide per year, will likely continue to increase. For example, cardiac CT examinations, previously seldom indicated in paediatric patients, are now widely carried out.
CT image contrast is directly related to the mean photon energy. The most significant determinant of the latter is x
ray tube voltage. Kilovoltage has an exponential relationship with dose,36
so any reduction has the potential to greatly reduce dose. Patient size is also a modest determinant of image contrast, as increasing patient size means increasing mean photon energy due to preferential loss of low energy photons (beam hardening). Patient size is, however, a major determinant of CT image noise, with increasing size leading to increasing noise. Nonetheless, the most important determinant of image noise is the number of x
ray photons used, that is tube current. In comparison to normal‐sized adults, image contrast‐to‐noise ratio (CNR) for neonates is a factor of four higher if the same kV and mAs are used. To maintain the same CNR in infants as in adults, both mAs and kV should be modified.
Unfortunately higher dose adult techniques have often been applied to children,37
which can lead to excessive and unnecessary radiation dose. Indeed, 2 years after an article in the American Journal of Roentgenology
, which pointed out the high radiation burden inherent in all CT examinations, had provoked much controversy, only 43% of imaging departments indicated that they had made any adjustment in technique for paediatric patients.33
In one survey of members of the Society of Pediatric Radiology, only 33% of respondents indicated that they performed helical CT of the chest in children 4 years old and younger with a tube current of less than 100 mAs.38
Many CT examinations in the paediatric population may not actually be necessary. It has been estimated that up to 30–40% of paediatric CT studies performed in the USA are not indicated.39
These must be eliminated. If CT examination is justified, then techniques specific to and appropriate for the child's age, size (ranging from premature neonates to oversized adult proportions), region to be imaged and the particular indication for the examination should be used. Sedation may be required to ensure acquisition of images of adequate quality at the first attempt.
Multiphase examinations should only rarely be necessary. It is tempting to perform scans through the liver, for example, at the arterial, venous equilibrium and portal venous phases but these additional CT runs seldom add useful extra information in paediatric practice (the importance of ultrasound scanning should not be overlooked). Performing two phases using the same parameters doubles the radiation dose. Consequently, if an extra phase of examination is performed, parameters should be adjusted to reduce dose. For example, a pre‐contrast scan to detect calcification in a lesion may be done adequately with lower dose, noisier images.40
It must be stressed here that non‐ionising techniques such as ultrasound and MRI should be used before CT whenever possible in the paediatric setting. MR scanning may require anaesthesia or sedation, but this may be preferable to repeated CT (for example in a child with a malignancy).
Lower dose, noisier CT images may also be acceptable and diagnostic where there is intrinsic high contrast, for example, in paediatric chest and musculoskeletal CT, and with CT angiography.40,41,42,43,44,45
Factors that will reduce dose include lowering kilovoltage, lowering tube current, decreasing gantry rotation time and increasing pitch.
Gantry rotation time (s) affects radiation dose. This is a linear relationship.46
Decreasing the gantry rotation cycle time from 1.0 to 0.5 s decreases the radiation dose by 50%. As fast a gantry rotation time as possible should be used. This decreases movement artefact and is important because of a child's limited capacity for co‐operation.
In a single‐slice CT (SSCT), pitch is the ratio of table movement per gantry rotation (mm) to collimation (mm): the higher the pitch, the lower the radiation dose. However, higher pitch results in more scan artefact and lower resolution. In spiral CT, if pitch is greater than unity, then a reduction in dose is achieved in comparison with contiguous axial scans. SSCT pitches of 1.5 have been shown to reduce radiation dose by approximately 33% compared with a pitch of 1, with no loss in diagnostic accuracy.47
In multi‐slice CT (MSCT), pitch is independent of gantry rotation time.40
For MSCT, collimation equals the total width of all the detector channels. Dose can be reduced by using thicker detector configurations, resulting in fewer rotations to cover the same distance.
There are many differing views on how best to reduce dose in CT while not losing diagnostic accuracy. One dose reduction regimen is based on a colour‐coded chart of patient weight.48
Others have championed the use of image‐noise measurements to adjust chest, abdomen and pelvis CT techniques, claiming a decrease in measured entrance dose of 60–90%.49
Others believe body diameter is a better parameter as patients with different body habitus may have the same weight.50
Until recently, our institution used the child's weight to set the mAs for a chest CT,51
with no more than 20 mAs per slice for children weighing less than 15 kg and no more than 55 mAs for those weighing up to 44 kg.
Traditionally, a single tube current was used to scan an entire region of varying attenuations. The latest CT scanners have automatic exposure control, where the x
ray tube current is automatically adjusted, based on the attenuation of the section being scanned, providing for a decreased overall dose. Dose reduction has at last become a priority for many manufacturers.52,53,54,55