What is the risk of ionising radiation in children?
Children are more sensitive to ionising radiation compared to adults due to:
1. Higher number of pluripotent cells, with significantly more fragile DNA / RNA structures.
2. Longer life, therefore longer lead time for developing cancer
3. Increased risk of higher accumulative dose based on longer life span
4. Increased risk in females than males (Sun, Ng Sarji, Singapore Med J 2010)
Therefore it is important to minimize their radiation exposure.
Risk of developing a radiation induced cancer = 5% per Sv at any age.
Risk of developing a radiation induced cancer = 15% per Sv for 0-10 y.
10mSv exposure gives a 1 in 1,000 risk of leukaemia or solid organ cancer (Sun et al) for children under 4, risk of thyroid cancer is 200 x higher for CT neck than for plain film (Negus et al, Clin Rad 2014)
What is the risk of injury in paediatric trauma?
Paediatric anatomy and physiology are different from adults:
1. Bone & cartilage, proportion and composition are different.
2. Fat-muscle ratio is different
3. Different body proportions
4. Neurological status, reflexes and reactions.
Hence the same mechanism of trauma can cause different injury to adults, and different injury in children of different ages or sizes (Negus et al).
In some cases the risk of injury is significantly higher eg Head Injury:
Children <2 y are at increased risk of intracranial injury because skull is relatively thin, brain is more prone to axonal injuries and the neck muscles are less protective in acceleration deceleration trauma. Therefore CT is justified as first line examination for head trauma if there is an adequate mechanism of injury in a young child.
However there are other cases, where the risk of injury is rare in comparison to adults eg Cervical Spine, and should be dealt with caution.
Severe injuries occur in less than 2% of small children and can usually be identified on Xray (90% sensitivity) (Nigrovic, Rogers, Adelgais, Ped Emerg Care 2012) therefore plain radiographs may be a sensible first step. However significant injuries should be taken seriously and if the clinical findings or mechanism of injury are indicating, CT scan should be performed.
How can we mitigate the risk of Ionising Radiation in Children?
Dose reduction in paediatric trauma is crucial but should not compromise diagnostic quality. Dose reduction can be implemented before, during or after the scan:
Before the CT
1. Reducing unnecessary scans where possible
CT may not be indicated at all or limited body part CT may be feasible. (Royal College of Radiologist Paediatric trauma protocols 2014).
For example,
Table: Imaging modality recommended (assuming imaging is indicated clinically)
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Site of injury
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Primary imaging modality (if clinically indicated)
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Head
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CT
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Neck
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Xray (lat, AP, peg) * CT neck should not automatically accompany CT Head
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Chest
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XR for blunt trauma
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Abdomen – look for certain clinical features eg lapbelt or handle bar injury, abdominal wall bruising and tenderness, abdominal distension, persistent hypovolaemia, blood per rectum or via nasogastric tube (RCR)
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Dual phase single acquisition CTfor blunt trauma
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2. Scanning less body parts.
3. Following the Trauma protocols and predictive algorithm whilst considering the individual differences in Trauma situations are crucial in keeping the radiation dose under control.
For example, just the CT Head, not CT Head and Neck together.
The tables below show the relatively high radiation dose delivered to paediatric trauma patients at different trauma centres, with a relatively high number of ‘normal’ diagnoses. We suggest this implies less body parts and less CTs should be performed overall.
Table: Effective dose in a paediatric polytrauma patients: (Munk et al, Freiburg, n=51, 2004-2006) or (Livingston et al, Birmingham 2007-2008)
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Location
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Average Effective Dose
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Munk et al,2004-6
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Freiburg, DE
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20.8mSV (n=51
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Livingston et al (2007-8)
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Birmingham, UK
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13.5mSV
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|
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No of ‘normal’ CTs
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|
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Livingston et al (UK)
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56%
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*mean no of scans per pt = 2.6
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Fenton et al (USA)
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54%
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*67% of abdo CTs were normal
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|
|
|
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During the CT
1 Altering mA and kV. The mA can be altered depending on the weight of the child. For CT head, mA should be decreased depending on the age of the child in children under 6 years old.
Peak kVP can be adjusted. For example 120kV adult dose can be altered to 80-100kV for a child without compromising diagnostic quality and resulting in effective dose of 2.5mSV (Sun et al).
Attenuation based online modulation can be used to reduce the dose delivered by up to 40% without compromising image quality (Grees et al. Eur Radiol 2004).
2. Altering collimation and pitch. A wider slice thickness (eg 3mm) will decrease radiation dose. Pitch is inversely proportion to radiation dose, pitch of 1.0 to 1.5 should be ideal (Sun).
3. The scanners’ automated intervention/adjustment software can be disabled to avoid unnecessary modification of the protocol,
4. All external devices can be removed from the FOV
5. Appropriate positioning of the patient
6. Avoiding radiation sensitive organs if possible (for example orbits in a CT head)
7. Sedation can be used to reduce motion artefact if possible.
After the CT
i) Post processing is generally a successful way to reduce radiation dose in a Trauma scan
ii) Adaptive statistical interactive reconstruction algorithm (Brady, Yee, Kaufman, Radiology 2014) reduces dose while compensating for increased noise on CT.
iii) Size Specific dose estimation (SSDE) provides a simple method for calculating organ dose (Moore, Brady, Mirro Medical Physics 2014)
These can improve the quality of imaging and avoid additional imaging
