X-ray, Ultrasound, CT or MRI: What Is the Difference?
X-rays use a small dose of radiation to show bones. Ultrasound uses sound waves and is best for soft-tissue structures close to the surface. CT combines many X-ray images into detailed cross-sections. MRI uses a strong magnetic field to produce the clearest soft-tissue pictures. Each scan answers a different clinical question, so the right choice depends on what your clinician needs to see.
Key takeaways
- X-ray and CT use ionising radiation. Ultrasound and MRI do not.
- MRI is generally the best tool for soft tissue such as discs, ligaments and nerves.
- CT gives fast, highly detailed cross-sectional bone images.
- All four standard scans are done lying down, which can hide alignment problems that only appear when you stand.
- Standing, weight-bearing imaging (such as EOS) captures your skeleton under real load.
If you have been told you need a scan, or you are trying to work out which type of imaging will help, it is easy to feel confused. The names get thrown around, but nobody explains what each one actually does or why it matters. This guide breaks it down in plain language.
How does each type of medical scan work?
Each scan uses a different kind of energy to create an image. That energy determines what the scan shows well and what it misses.
A quick beam of ionising radiation passes through the body. Dense structures like bone block more of the beam and show up white on the image. Fast, widely available and low cost.
High-frequency sound waves bounce off tissues and are converted into a live image. No radiation at all. Best for muscles, tendons and fluid-filled structures near the skin surface.
An X-ray tube rotates around you, taking hundreds of images from different angles. A computer builds detailed cross-sectional slices. Higher radiation dose than a standard X-ray, but superb bone detail.
A powerful magnet and radio waves cause hydrogen atoms in your tissues to emit signals. The result is highly detailed images of soft tissue, including discs, cartilage, ligaments and nerves. No ionising radiation.
What is the difference between CT and MRI, X-ray and ultrasound?
The main difference between CT and MRI is the type of energy used and what it reveals. CT uses X-rays and excels at showing bone and bleeding. MRI uses magnetism and excels at showing soft tissue. The table below puts all four side by side.
| X-ray | Ultrasound | CT | MRI | |
|---|---|---|---|---|
| How it works | Single radiation beam | Sound waves | Rotating X-ray beam | Magnetic field + radio waves |
| Ionising radiation | Low dose | None | Higher dose | None |
| Best for | Bones, fractures, chest | Muscles, tendons, surface soft tissue | Complex fractures, bleeding, organs | Discs, cartilage, ligaments, nerves, brain |
| Bone detail | Good | Limited | Excellent | Moderate |
| Soft-tissue detail | Poor | Good (superficial) | Good | Excellent |
| Contrast dye | Rarely | No | Often | Sometimes |
| Claustrophobia risk | None | None | Mild (short tunnel) | Higher (enclosed, noisy) |
| Patient position | Usually standing or lying | Lying or sitting | Lying | Lying |
Why does it matter whether a scan is done standing or lying down?
It matters because your skeleton behaves differently under load. When you lie down for a CT, MRI or most X-rays, gravity is removed and your muscles relax. Joints decompress. A disc bulge may shrink. A tilt in your pelvis may flatten out. The image looks reassuring, but it does not reflect how your body actually works when you are upright and walking.
Your body is a connected chain from your feet to your head. A slight shift at one level, say your pelvis, can make your spine, hip or knee compensate above or below. That compensation is invisible on a lying-down scan because the load has been taken away. It is one reason why many people get scans that come back "normal" yet still feel pain. The problem is real. The imaging just did not capture it.
Standing, weight-bearing imaging captures your alignment under the same forces you experience all day. Learn more about weight-bearing versus lying imaging and how it applies to back, hip and knee problems.
Not sure which scan would help you? A short video call with our team can clarify whether standing imaging is the right next step, or whether MRI, CT or another scan is a better fit.
Book a free video consultationWhich scan should I choose for my problem?
The right scan depends on the clinical question, not just the body part. Here is a simplified guide.
- You have a suspected fracture
- You want a quick bone check
- You need to see full-body alignment under load
- You are planning or following up joint surgery
- You have disc, cartilage or ligament symptoms
- You have nerve pain, numbness or tingling
- A tendon or muscle tear is suspected
- You need a scan with no radiation at all
Sometimes more than one scan is needed. A standing X-ray or EOS scan can reveal the alignment picture, and an MRI can then show the soft-tissue detail. They answer different questions and often work best together. See how EOS compared with x-ray, CT and MRI fits into the picture.
A note on MRI. EOS and standing X-ray show bones, joints and whole-body alignment. For soft tissue such as discs, nerves, cartilage or ligaments, MRI is usually the better tool. If your clinician suspects a soft-tissue problem, MRI is likely the right starting point. ScanAlign does not offer MRI, but we are always honest about which imaging suits you best.
Frequently asked questions about types of medical scans
CT uses ionising radiation (X-rays) and is excellent at showing bone detail, bleeding and organ injuries quickly. MRI uses a magnetic field with no radiation and gives far better images of soft tissue such as discs, ligaments, cartilage and nerves. The right choice depends on what your clinician needs to see.
MRI does not use ionising radiation, so there is no radiation dose. CT does deliver a radiation dose, though modern scanners keep it as low as reasonably possible. Both are considered safe when used appropriately. Your clinician will recommend whichever one answers the clinical question best.
Ultrasound can show the surface of bone, but it cannot see through it. It is not a good tool for fractures or joint alignment. It is best suited to soft tissue near the skin surface, such as tendons, muscles and fluid collections.
At ScanAlign, private standing imaging in London, you do not need a GP referral. You can self-refer for a standing EOS scan. For MRI or CT at other clinics, referral requirements vary. Check with the provider directly.
A CT scan produces detailed cross-sectional images. It shows bones in high detail, including complex fractures, as well as organs, blood vessels and some soft-tissue structures. Contrast dye is often used to highlight blood flow or tumours.
X-rays are fast, widely available and the standard first step for suspected fractures, chest problems or basic joint checks. If the X-ray does not explain your symptoms, your doctor may then request an MRI for a closer look at the soft tissue.
EOS is a standing, weight-bearing X-ray system that captures your full body from head to feet in a single image, using about 90% less radiation than a standard X-ray, around ten times less. Because you stand, it shows your real alignment under load. Standard X-rays, CT and MRI are almost always done lying down.
Costs vary by scan type and provider. For EOS scan pricing at ScanAlign, visit our frequently asked questions page.
Some metal implants are safe in an MRI scanner and some are not. Older pacemakers, certain aneurysm clips and some cochlear implants are usually not compatible. Your MRI provider will screen you carefully before the scan. If MRI is not suitable, CT or EOS may be an alternative depending on the clinical question.
The best scan depends on your symptoms and what your clinician suspects. If you are unsure, you can check which scan suits you with a free video consultation. Our radiographer can advise whether standing imaging, MRI or another option would be most helpful.
Not sure which scan you need?
Talk to our specialist radiographer in a short, free video call. No referral needed, no obligation.
Book a free video consultationSources
- NHS. "CT scan." nhs.uk, 2023.
- NHS. "MRI scan." nhs.uk, 2023.
- The Royal College of Radiologists. iRefer: Making the best use of clinical radiology, 8th edition.
Written by Abbas Dhami (Specialist Diagnostic Radiographer)
This page is general information and not a diagnosis.
ScanAlign is the trading name of CoreMed Solutions Ltd and operates under The Harley Street Hospital's CQC licence.