ReviewDose reduction in paediatric MDCT: general principles
Introduction
In 1989, computed tomography (CT) accounted for 4% of diagnostic radiology examinations performed in the UK, contributing 40% of the collective population dose from medical radiation.1 By 1999, one North American institution quoted that 11.1% of the department workload was due to CT examinations; a contribution of 67% to the collective dose.2 CT can now be responsible for up to 17% of the departmental workload accounting for 70–75% of the collective dose from medical radiation.3, 4, 5 With reference to the paediatric population, the British survey of 1989 stated that 4% of CT examinations were performed in children less than 15 years of age.1 Mettler showed this figure to have risen to 11.2% by 1999.2 Coren and colleagues reported a 63% increase in requests for paediatric CT between 1991–1994,6 and McAllister a 92% increase in paediatric abdomino-pelvic CT examinations between 1996–1999.7
These often-quoted figures mirror the introduction of the first single detector helical CT machines (SDCT) in the late 1980s and multi-detector array CT (MDCT) machines in the late 1990s. The technological advances in commercially available CT machines have allowed the radiologist to increase the range of studies they perform using CT: peripheral and cardiac angiography; virtual endoscopy, including bronchoscopy and colonoscopy; multiplanar and volume reformats from isotropic data sets for skeletal examinations; and the more mundane evaluation of appendicitis and renal calculi are all now being performed using CT. However, such technical advances, although producing increased diagnostic accuracy (and some would argue beautiful, aesthetically stunning images) do not come without cost. One potential cost is the amount of radiation that can result from MDCT examinations.
Section snippets
Radiation dose measurements
To interpret the radiation risks from CT, it is necessary to be familiar with descriptors of dose and their units of measurement.8, 9, 10, 11, 12 The absorbed dose (measured in Grays) is the amount of energy absorbed per unit mass of an organ or tissue. The absorbed dose cannot be practically measured in patients. The equivalent dose (measured in Sieverts) takes into account the type or quality of radiation an organ is exposed to. It is numerically equal to the absorbed dose, when x-rays are
Radiation risks
Bio-effects associated with radiation exposure, can be divided into two main groups: deterministic risk relates to cell death and can be quantified in terms of the radiation dose an organ or body region has received. Above a certain threshold dose, the effects of radiation are seen, and the higher the dose, the more severe the effect. Deterministic effects are rarely seen in diagnostic radiology, but may become a problem with angiographic procedures, including CT fluoroscopy.23 In addition,
Image quality and radiation dose in CT: basic principles
There is a complex relationship between image quality and the radiation dose imparted to the patient. Image quality in CT is determined by spatial resolution and contrast. The tube current (measured in milliAmperes) milliAmperes primarily affects spatial resolution and peak kilovoltage (kVp) affects both spatial and contrast resolution. The principle determinants of the dose a patient receives during a CT examination are due to these same factors: x-ray beam energy (related to the peak
General considerations
The biggest dose saving in MDCT is when the examination is simply not performed. In children, if the clinical question can be answered by ultrasound or MRI (with no additional patient risks, such as sedation or anaesthesia to consider), then these methods should be used if available. Vetting of CT request forms by a consultant radiologist (and preferably by a consultant with a designated interest and specialist training in paediatric radiology) is potentially important in this regard, given
Dose reduction and the manufacturers
In recent years, all of the major MDCT machine manufacturers have made alterations to their equipment, and attempt to have age or size-adjusted protocols, aimed at controlling the radiation dose, whilst maintaining image quality.71, 72, 73, 74 Most manufacturers now program paediatric protocols into their machines, and these act as a useful guide for paediatric dose reduction. More significant dose savings can be achieved when these protocols are modified by the radiologist, with the help of
Conclusion
With the continued development of MDCT, the use of CT technology and its contribution to the collective population dose from medical radiation have increased. It has been shown that the effective dose delivered during some CT examinations overlaps with those doses reported to increase cancer rates.7, 85 There are unique considerations with dose in children, as well. It must be recognized that children are more radiosensitive than adults and have a longer life ahead of them, in which radiation
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