Objective:
To compare multidetector CT (MDCT) radiation doses between default settings and a revised dose reduction protocol and to determine whether the diagnostic confidence can be maintained with imaging quality made under the revised protocol in paediatric head, chest and abdominal CT studies.
Methods:
The study retrospectively reviewed head, chest, abdominal and thoracoabdominal MDCT studies, comparing 231 CT studies taken before (Phase 1) and 195 CT studies taken after (Phase 2) the implemented revised protocol. Image quality was assessed using a five-point grading scale based on anatomical criteria, diagnostic confidence and overall quality. Image noise and dose–length product (DLP) were collected and compared.
Results:
The relative dose reductions between Phase 1 and Phase 2 were statistically significant in 35%, 51% and 54% (
p < 0.001) of head, chest and abdominal CT studies, respectively. There were no statistically significant differences in overall image quality score comparisons in the head (
p = 0.3), chest (
p = 0.7), abdominal (
p = 0.7) and contiguous thoracic (
p = 0.1) and abdominal (
p = 0.2) CT studies, with the exception of anatomical quality in definition of bronchial walls and delineation of intrahepatic portal branches in thoracoabdominal CTs, and diagnostic confidence in mass lesion in head CTs, liver lesion (>1 cm), splanchnic venous thrombosis, pancreatitis in abdominal CTs, and emphysema and aortic dissection in thoracoabdominal CTs.
Conclusion:
Paediatric CT radiation doses can be significantly reduced from manufacturer''s default protocol while still maintaining anatomical delineation, diagnostic confidence and overall imaging quality.
Advances in knowledge:
Revised paediatric CT protocol can provide a half DLP reduction while preserving overall imaging quality.The use of CT has been rapidly increasing all over the world during the past two decades, driven by advanced technology and the invention of the multidetector CT (MDCT). Use of MDCT has risen 12-fold in the UK and 20-fold in the USA during this time, and the mean effective dose from all medical X-rays in the USA has increased 7-fold during this period.
1–3 6–11% of all CT examinations in developed countries are performed on children aged from 0 to 15 years.
2,4–6 The organ-absorbed doses reported in adult and paediatric patients undergoing single CT examination are considerably lower than the threshold for initiation of a deterministic effect and the estimated effective doses are still within the annual exposure dose from natural background radiation.
7 The UK Radiation Protection Division of the Health Protection Agency, the US National Council on Radiological Protection and Measurement and the US National Academy of Sciences Biological Effects of Ionizing Radiation committees have proposed that, for doses <100 mSv, which is roughly equal to the dose range for multiple CT examinations, the radiation-induced cancers decrease linearly with decreasing dose with no threshold or a so called “linear no-threshold” model.
3,8,9 There was a linearly increasing risk for all solid cancers with increasing radiation dose and a higher radio sensitivity in children resulting in a larger attributable lifetime cancer risk in this patient group.
1,3Although the association of diagnostic medical radiation exposures in maternal pre-natal, children''s post natal and parental pre-conception periods with paediatric cancer risks are summarized in various studies, a CT scan-related cancer risk in children and adolescents has not been definitively proven.
6 A retrospective cohort study by Pearce et al
10 did, however, find a significant association between estimated cumulative radiation doses delivered by CT scan to the bone marrow and brain and subsequent increased risk of leukaemia and brain tumours in childhood.Diagnostic reference level (DRL) values are required for CT optimization, and these values are recommended by the International Commission on Radiological Protection; also each region or country is responsible for and authorized to enact details and implementation of their own DRLs.
11 Several age-based and weight-based DRLs for paediatric CT have been published.
12–17 General strategies for CT dose reduction in paediatric healthcare include such things as avoiding a CT scan if adequate clinical information can be obtained from ultrasound or MRI, avoiding multiphase examinations and designing CT protocols to minimize exposure time.
18 Nowadays, many professional societies, regulators and manufacturers have been trying innovative new technologies for reducing radiation dose while maintaining optimal image quality.Two of the most commonly used image quality parameters in diagnostic imaging are high-contrast (spatial) resolution and low-contrast resolution. Spatial resolution is the ability to distinguish small objects close to one another on an image and is influenced by various factors such as focal spot size, detector width and ray, pixel size and properties of the reconstruction filter. Low-contrast resolution refers to the visibility of an object against the background. In the absence of artefacts, the low-contrast resolution scan is affected mostly by noise.
19,20 Although noise derivative is a quality index that is more relevant to assess image quality than image noise, it is difficult to translate in clinical practice.
21 Image noise is measured by standard deviation (SD) of CT number, and it depends on milliamperes (mA), scan time, kilovoltage peak (kVp), patient size, pitch or table speed, slice thickness and reconstruction algorithm. If the milliampere–seconds value is reduced by 50%, the radiation dose will be reduced by the same amount, with an attendant noise increase of 41%, calculated by the equation (1/√2 = 1.41, a 41% increase). Tube voltage or beam energy has a direct influence on patient radiation dose. Reducing the peak kilovoltage results in a significant decrease in radiation dose owing to the square law relationship of these two values.
19,20,22–25 Thus, the image noise and tissue contrast will be affected by adjusting kilovoltage; however, reduced peak tube potential is useful for chest, airway and skeletal studies owing to a high contrast-to-noise ratio requirement in imaging evaluation.
18In our hospital (Songklanagarind Hospital, Hat Yai, Thailand), we began a revised CT dose reduction protocol in August 2010 that involved lowering kVp and mA, and using dose–length product (DLP) and DRLs based on the Nievelstein et al
23 protocol and national dose surveys from the UK and Canada
12,15 (