MRI versus CT: soft tissue contrast, radiation and contraindications
MRI is chosen over CT for its soft tissue contrast (it separates tissues that are almost the same on CT) in a patient who is not acutely unwell.
Remember
Choose MRI for detail of soft tissue without ionising radiation (the type used in X-rays and CT) when there is no contraindication, and choose CT for speed, bone and acute bleeding.
Magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves to form images, and the tissues give off signals. Different tissues give different signals, so MRI separates normal brain from a low-grade tumour (a slow-growing tumour whose cells look close to normal). That is its soft tissue contrast.
This is why MRI suits a brain tumour, where the margins (edges) are defined before surgery, and focal epilepsy (seizures that start in one area of the brain), where it can show small tumours that CT misses. MRI also uses no ionising radiation, the type used in X-rays and CT that can damage DNA and slightly raises cancer risk over a lifetime. That is helpful for younger patients and anyone who needs repeat imaging.
Choose CT for speed, bone and acute bleeding. MRI is slower, usually taking from 15 minutes to 1 hour, and it is more expensive and less widely available. It is therefore unsuitable for an unstable patient who needs a quick scan. CT also shows bone and calcium better, because dense bone gives little MRI signal.
The strong magnet pulls on ferromagnetic metal (metal containing iron), which can move or heat up and injure nearby tissue. Magnetically activated or electronically controlled devices can also malfunction. Non-MRI-compatible metal is a contraindication to MRI, but some devices are only relative contraindications (they do not automatically rule MRI out), so patients complete a safety form, and the radiology team checks the make and model.
Key facts
- Soft tissue contrast
- MRI separates normal brain from a low-grade tumour and shows the margins of the tumour before surgery.
- Epilepsy
- MRI can show small tumours that CT misses, and any small abnormality that may be causing seizures can be seen in far greater detail than on CT.
- Radiation
- MRI uses a magnetic field and radio waves, so there is no ionising radiation.
- Time and cost
- MRI is slower and more expensive than CT, so CT is preferred in an emergency.
- Metal
- The strong magnet can move or heat up ferromagnetic metal and injure nearby tissue, so non-MRI-compatible metal is a contraindication.
- CT is better for
- Speed, dense bone detail, fracture lines and calcium; CT is also usually the first test for acute bleeding.
Exam traps
- Detail of dense bone and fracture lines is not an MRI advantage, because dense bone gives little MRI signal, so CT is better for them.
- Ionising radiation does not explain why MRI shows tissues well, because MRI uses a magnetic field and radio waves, not ionising radiation.
- A history of earlier exposure to ionising radiation does not make MRI unsuitable, because MRI uses no ionising radiation, so such a history favours MRI.
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