Signature explainer
Which scan is which?
X-ray, CT, MRI and ultrasound answer different questions. This explainer shows what each one is best for, which involve radiation and which involve none, and what the examination actually feels like, so a referral never feels like a black box.
Find your scan
The four exams, side by side
| Question | X-ray | CT scan | MRI scan | Ultrasound |
|---|---|---|---|---|
| Makes the picture with | X-ray beam | Rotating X-ray beam | Magnet and radio waves | Sound waves |
| Ionising radiation | Small dose | Moderate dose | No | No |
| Strongest at | Bones, chest | Emergencies, fine body detail | Brain, spine, joints, soft tissue | Pregnancy, organs, blood flow |
| Time in the room | Minutes | 5 to 15 min | 15 to 90 min | 15 to 45 min |
| Live moving pictures | No | No | No | Yes |
| Enclosed space | No | Open ring | Tunnel | No |
| Contrast dye | Rare | Often | Sometimes | Rare |
X-ray
Small radiation dosePlain radiograph
A brief beam of X-rays passes through the body onto a digital detector.
Dense tissue such as bone absorbs more of the beam than soft tissue, so bones appear white and air appears dark. Modern digital detectors need far less radiation than older film systems.
Best for
- Suspected broken bones and checking how fractures heal
- The chest: infections such as pneumonia, fluid, an enlarged heart
- Joint problems such as arthritis, and bone alignment
- Some dental and abdominal checks
Honest limits
- Shows soft tissues (muscles, ligaments, brain, organs) only in outline
- Small or early problems can be invisible on a plain film
- How long
- A few minutes; each exposure takes a fraction of a second.
- Contrast dye
- Rarely; barium or iodine studies are a special case.
- What it feels like
- You stand or lie against the detector, hold still, and sometimes hold a breath. Painless and quick.
Uses a small dose of ionising radiation. A chest X-ray is roughly the same exposure as about 10 days of natural background radiation.
CT scan
Moderate radiation doseComputed tomography
An X-ray tube rotates around you and a computer rebuilds cross-section slices.
Because it collects images from many angles, CT separates structures that overlap on a plain X-ray and shows bone, blood, organs and lungs in fine detail. Scans are fast, which makes CT the workhorse of emergency imaging.
Best for
- Emergencies: serious injury, suspected stroke, sudden severe pain
- Detailed pictures of the lungs, abdomen and pelvis
- Complex fractures and surgical planning
- Finding and following many cancers, and guiding biopsies
Honest limits
- Higher radiation dose than a plain X-ray, so the benefit must justify it
- Fine soft-tissue contrast (for example spinal cord detail) is better on MRI
- How long
- Usually 5 to 15 minutes in the room; the scan itself takes seconds.
- Contrast dye
- Often; iodine-based contrast by injection or drink for many studies.
- What it feels like
- You lie on a table that slides through a short, open ring. If contrast is injected you may feel briefly warm all over. Not enclosed like an MRI tunnel.
Uses more radiation than a plain X-ray: an abdomen and pelvis CT is roughly comparable to about 3 years of natural background radiation. Doctors weigh this small exposure against the value of the answer.
MRI scan
No ionising radiationMagnetic resonance imaging
A strong magnetic field and radio waves map the water in your tissues. No X-rays at all.
MRI gives the best pictures of soft tissue: brain, spinal cord, ligaments, tendons, cartilage and organs. Different sequences highlight different tissue properties, which is why one exam contains many separate runs.
Best for
- The brain and spinal cord: tumours, multiple sclerosis, disc problems pressing on nerves
- Joints: ligament, meniscus and cartilage injuries
- Soft-tissue and many organ studies where fine detail matters
- Situations where avoiding ionising radiation matters most
Honest limits
- Slow: exams run 15 to 90 minutes, and you must keep very still
- The tunnel is narrow and noisy; some people feel claustrophobic
- Strict metal safety rules: some implants cannot go in the magnet
- How long
- 15 to 90 minutes depending on the body part.
- Contrast dye
- Sometimes; gadolinium-based contrast by injection.
- What it feels like
- You lie inside a tunnel that knocks and buzzes loudly; you get earplugs or headphones and a call bell. Painless, but keeping still is the hard part.
Uses no ionising radiation. Safety screening is about metal and implants instead: the magnet is always on.
Ultrasound
No ionising radiationSonography
A handheld probe sends high-frequency sound waves and listens for the echoes.
The echoes build a live, moving picture, so ultrasound can watch blood flow, a beating heart or a baby in real time. There is no radiation, which is why it is the standard imaging in pregnancy.
Best for
- Pregnancy: dating, growth and wellbeing checks
- Abdominal organs: gallstones, liver, kidneys
- Blood vessels: clots in leg veins, narrowed arteries (Doppler)
- Thyroid, neck lumps, breast lumps, muscles and tendons near the surface
Honest limits
- Sound does not travel through bone or gas, so some areas are hidden
- Image quality depends on the operator and on body build
- How long
- Usually 15 to 45 minutes.
- Contrast dye
- Rarely; microbubble contrast is a special case.
- What it feels like
- Warm gel on the skin and gentle probe pressure. Painless; you can usually watch the screen.
Uses no ionising radiation at all. Sound waves only.
Sources
- X-ray NHS, 2022
- CT scan NHS, 2023
- MRI scan NHS, 2022
- Ultrasound scan NHS, 2023
- Patient information on imaging exams RadiologyInfo.org (ACR / RSNA), 2024Typical uses and exam descriptions follow this public patient guidance.
Last reviewed: 2026-07-16
Reviewed against public NHS and RadiologyInfo.org (ACR / RSNA) patient guidance. Educational content only: the choice of examination always belongs to your doctor and the radiology team.