Detailed pictures without X-rays
MRI (magnetic resonance imaging) creates detailed pictures of the inside of the body using a strong magnet and radio waves. It’s especially good at showing soft tissues like the brain, spinal cord, muscles, ligaments and organs, which are hard to see on X-rays. And unlike X-rays and CT scans, MRI doesn’t use ionizing radiation.
History
| Year | Event |
|---|---|
| 1938–1946 | Isidor Rabi, then Felix Bloch and Edward Purcell, discover nuclear magnetic resonance (NMR) |
| 1971 | Raymond Damadian shows that tumours and healthy tissue give different NMR signals |
| 1973 | Paul Lauterbur publishes the first MRI image, of two tubes of water |
| Mid-1970s | Peter Mansfield develops faster imaging methods |
| 1977 | First MRI scan of a human body |
| 1980s | MRI scanners enter hospitals |
| 1990 | Functional MRI (fMRI) developed, showing brain activity |
| 2003 | Lauterbur and Mansfield win the Nobel Prize in Physiology or Medicine |
How MRI works
The human body is about 60% water, and water contains hydrogen atoms. The nucleus of each hydrogen atom acts like a tiny spinning magnet.
- Lining up: The patient lies inside a huge magnet. Its magnetic field makes the hydrogen nuclei line up with it, like compass needles.
- Knocking them over: The machine sends pulses of radio waves. The hydrogen nuclei absorb this energy and tip out of line.
- Relaxing: When the radio pulse stops, the nuclei relax back into line, giving off faint radio signals.
- Listening: Coils around the body detect these signals.
- Building the picture: Different tissues contain different amounts of water and fat, and relax at different speeds. A computer uses these differences to build detailed images, slice by slice.
Extra coils called gradient coils vary the magnetic field slightly across the body, so the computer can tell exactly where each signal comes from. Switching them on and off rapidly causes the loud banging and buzzing noises MRI scanners make.
The magnet
Most MRI scanners use a superconducting magnet: coils of special wire cooled with liquid helium to about −269 °C, just 4 degrees above absolute zero. At this temperature, electricity flows with no resistance, creating a very strong, steady magnetic field.
| Magnetic field | Strength (tesla) |
|---|---|
| Earth’s magnetic field | About 0.00005 T |
| Fridge magnet | About 0.005 T |
| Common hospital MRI | 1.5–3 T |
| Research MRI | 7 T or more |
A 3-tesla MRI is about 60 000 times stronger than Earth’s magnetic field.
What MRI is used for
| Body area | What MRI can show |
|---|---|
| Brain and spinal cord | Tumours, strokes, multiple sclerosis, injuries |
| Joints | Torn ligaments and cartilage (common in sports injuries) |
| Heart and blood vessels | Heart damage, blood flow |
| Organs | Liver, kidney and other organ problems |
| Cancer | Finding and tracking tumours |
Functional MRI
Functional MRI (fMRI) detects changes in blood oxygen to show which parts of the brain are active while a person thinks, talks or moves. Scientists use it to study memory, language, emotion and learning.
MRI vs. other imaging
| X-ray | CT scan | MRI | Ultrasound | |
|---|---|---|---|---|
| Uses | X-rays | X-rays from many angles | Magnet and radio waves | Sound waves |
| Ionizing radiation? | Yes (low) | Yes (higher) | No | No |
| Best for | Bones | Detailed cross-sections, emergencies | Soft tissue, brain, joints | Pregnancy, organs, real-time images |
| Time | Seconds | Minutes | 15–60 minutes | Minutes |
Safety
MRI is very safe, but the powerful magnet requires precautions:
- No metal: the magnet can pull metal objects like keys, scissors or oxygen tanks across the room at high speed.
- People with some pacemakers, metal implants or metal fragments may not be able to have an MRI.
- Patients must stay still, sometimes in a narrow tube, which can feel uncomfortable or loud. Earplugs or headphones are provided.
- Sometimes a contrast dye is injected to make certain tissues show up better.
MRI in Canada
MRI scanners are expensive, often costing several million dollars, plus operating costs. Wait times for non-urgent MRI scans are a challenge in parts of Canada, and provinces have been adding scanners to reduce waits.
Common mistakes
- Thinking MRI uses X-rays or radiation. It uses magnets and radio waves.
- Thinking the magnet turns off between patients. Superconducting magnets stay on all the time.
- Thinking MRI shows bones best. X-rays and CT are usually better for bones; MRI excels at soft tissue.
Try it yourself
- Put several compasses on a table. Bring a magnet near and watch them line up. Remove it. How does this relate to hydrogen atoms in MRI?
- Compare an X-ray image and an MRI image of a knee online. Which shows ligaments better?
- Make a chart of MRI safety rules.
Quick recap
- MRI uses a strong magnet and radio waves to create detailed images.
- Hydrogen nuclei in water line up, absorb radio energy and give off signals as they relax.
- Superconducting magnets are cooled with liquid helium.
- MRI is excellent for soft tissues and uses no ionizing radiation.
- Metal is dangerous near an MRI magnet.