Medical imaging explained: X-ray, CT, MRI and ultrasound

X-rays: quick pictures of bones

The X-ray is the oldest and most familiar scan. It uses a small burst of radiation that passes through the body onto a detector. Dense tissues such as bone absorb more of the beam and show up white, while softer tissues let more through and appear darker, creating a shadow picture. X-rays are fast, widely available and excellent for looking at bones, so they are the go-to test for suspected fractures. They can also show some chest and lung problems, such as pneumonia, and are used in dentistry. The radiation dose from a single X-ray is very low, comparable to a short period of natural background radiation, but doctors still use them only when the information will genuinely help.

CT scans: detailed cross-sections

A CT (computed tomography) scan also uses X-rays, but instead of one flat picture it takes many images from different angles as you pass through a ring-shaped scanner. A computer combines these into detailed cross-sectional slices, and can build three-dimensional views. This makes CT far more detailed than a plain X-ray and excellent for seeing organs, blood vessels, bleeding, and complex injuries quickly, which is why it is heavily used in emergencies such as major trauma and suspected stroke. Scans are fast, often taking only minutes. Because CT uses more radiation than a single X-ray, doctors weigh the benefit against the dose, especially in younger people, and sometimes a contrast dye is injected to make certain structures show up more clearly.

MRI: powerful magnets, no radiation

MRI (magnetic resonance imaging) uses a strong magnetic field and radio waves rather than radiation. It works by nudging the water molecules in the body and reading the signals they give off, producing extremely detailed pictures of soft tissues. MRI is outstanding for the brain and spinal cord, joints, muscles and many internal organs, often showing things that other scans cannot. The trade-offs are that scans take longer, can be noisy, and involve lying still inside a tunnel, which some people find uncomfortable. Because of the powerful magnet, metal objects and some implants can be unsafe, so staff carefully check for these first. MRI uses no ionising radiation, which is one reason it is often preferred for detailed soft-tissue questions.

Ultrasound: sound waves in real time

Ultrasound uses high-pitched sound waves, far above what we can hear, sent into the body from a small handheld probe. The waves bounce back off tissues and are turned into moving images on a screen in real time. It uses no radiation, is painless and portable, and is very good for looking at soft tissues, the abdomen, blood flow, and the developing baby in pregnancy, which is why it is the standard scan in antenatal care. Because it shows movement live, it is also used to guide procedures such as taking a sample of tissue. A gel is applied to the skin so the probe makes good contact. Its main limits are that sound waves do not pass well through bone or gas.

How doctors choose and what to expect

No single scan is best for everything, so the choice depends on the question being asked, how urgent it is, and safety. A suspected broken wrist needs a quick X-ray; a possible stroke needs a fast CT; a detailed look at a knee ligament or the brain often needs MRI; and a pregnancy or gallstone check suits ultrasound. Doctors also consider radiation, so scans using it are used thoughtfully, particularly in children and pregnancy. Most scans are straightforward: you may be asked to remove metal or jewellery, wear a gown, or avoid eating beforehand for some tests. If a contrast dye is needed, staff will explain it. Results are reviewed by a radiologist, a doctor who specialises in reading scans, and shared with your team.