EM Notes – Radiation

CT Scan

  1. Axial scan data reconstructed into other views or 3D images without re-scanning.

Radiation Dose

  1. Increased by: overuse, multi-detectors (faster scans), high resolution scans, larger area.
  2. More paediatric scans done as faster (~1s) scans now so less need for sedation.
  3. Average background radiation = 2.5-3mSv/yr (~25-30 CXRs)

Typical scan doses

Modality Body Area Approx Ave Dose (mSv) Chest X-ray Equivalent Dose ~Equivalent Period of Background Radiation
Plain X-ray Limbs 0.005 0.25 <1d
Chest (PA) 0.02 1 3d
C-spine series 0.2 10 1mo
Hip 0.3 15 1.5mo
Pelvis or Abdomen 0.7 35 4mo
CT scan Head 2 100 1yr
C-Spine 6 300 3yr
Chest 8 400 4yr
CTPA 15 750 7.5yr
V/Q scan 2.2 110 1yr
Abdomen-Pelvis 10 500 5yr
CT angiogram of aorta 24 1200 12yr
Trauma pan scan 34 1700 17yr

Reducing radiation dose

  1. Is X-ray/CT really necessary
  2. Is there an alternative modality e.g. USS/MRI
  3. Focus scanning only on area of interest
  4. Adjust CT parameters (tube current and pitch) for body type & organ
  5. CT scans can be performed with lower exposure in children, if employed.
  6. Use of newer software that improves quality of low dose scan results

Cancer Risk

  1. Estimated lifetime cancer mortality risk attributable to a dose similar to an abdo CT: 1.1. 1yo child: ~0.05% (head) & 0.1% (abdominal), reducing to <0.01% & 0.02% in adults>35y. Overall ~1:1,000-1:10,000 CTs result in a cancer death.
  2. Risk F>M generally, sig (1.5-2.5x) if radiation of chest(breast) at all ages, or head <35y
  3. Additional risk is still low (1%) compared to background risk. (Lifetime risk of cancer in Aus ~25-33%, and lifetime cancer mortality ~10-15%).
  4. Survivors of atomic bombings @ Hiroshima and Nagasaki were exposed to ave 40mSv.