Radiation is measured in sieverts (Sv) and accounts for both the amount absorbed and the sensitivity of different body parts to radiation. A millisievert (mSv) is one-thousandth of a sievert. Radiation leakage at the Fukushima nuclear power station after Japan’s tsunami earlier this year was up to 400mSv/h between reactor buildings and 0.6mSv/h at the main gate.
X-ray is the oldest, quickest and least expensive form of imaging. There is loss of fine detail in images of some soft tissues because various structures are overlaid within the image. Typical effective radiation dose: limbs 0.01mSv, chest 0.02mSv, abdomen 1mSv.
Computerised tomography (CT) uses X-rays to reveal tissues with greater precision because the image is a cross-section, not an overlay. There are now lower radiation dose options. Typical effective radiation dose: chest 8mSv, abdomen 10mSv.
Magnetic resonance imaging (MRI) also provides high-quality crosssections. Strong magnetic fields are used, avoiding the radiation associated with CT, but some medical devices such as pacemakers aren’t compatible with MRI. Radiation dose: Nil.
Positron emission tomography (PET) uses a scanning detector that shows the concentration of radioactive chemicals injected into the patient’s bloodstream. It may be used to identify areas of inadequate blood flow in the brain after a stroke. Typical effective radiation dose: 5mSv (when not done in conjunction with CT).
Ultrasound images are not high quality, but there is also no radiation. It is commonly used to study abdominal organs, pelvic organs, some joints and breast tissue. Radiation dose: Nil.
To CT or not CT?
- The idea that modern imaging equipment exposes patients only to modest radiation isn’t true of CT scans. Unnecessary chest CT scans could result in about 40 fatal cancers a year in Australia. To put the exposures in context:
- Exposure is cumulative, adding up over a lifetime and incrementally increasing the cancer risk.
- Normal background radiation dose is about 2.5mSv per year.
- A 13-hour flight could provide an additional 0.075mSv exposure.
- Airline crew may receive an additional 2mSv-5mSv exposure each year.
The general risk of radiation
- A 1Sv (1000mSv) dose (received all at one time) equals a 5% increase of fatal cancer over a lifetime, and causes temporary radiation sickness.
- Exposure as a foetus, child or adolescent increases and can double or triple this risk.
- Exposure over age 60 may reduce the 5% risk to about 1%, because potential cancer is less likely to manifest during the remainder of the person’s lifetime. Medical imaging risks include:
- An 8mSv chest CT has 400 times the radiation exposure of a 0.02mSv chest X-ray.
- A 10mSv abdomen CT may lead to an increased lifetime cancer risk of about 1 in 2000, and is called “low risk”.
- A 1mSv breast mammogram, associated with a lifetime cancer risk somewhere between 1 in 10,000 and 1 in 100,000, is called “very low risk”.
- A 0.01mSv limb X-ray has a “negligible risk” as it’s too small to quantify easily.
- The young are at greatest risk. CT scans should not be used where an X-ray is adequate, such as to confirm pneumonia has cleared in the lungs of a child.
- These risks represent small additions to the one in three chance we all have of getting cancer in our lifetimes.