MRI, CT or ultrasound, which scan do I need?

MRI suits soft tissues such as the brain, spine, joints, prostate and liver. CT suits the lungs, bones, bleeding and fast emergency imaging. Ultrasound suits the gallbladder, thyroid, pelvis, kidneys and lumps under the skin. The right scan depends on the question a doctor needs answered. The Online GP by The Wellness assesses you first, then arranges the scan that fits.

Key points

• MRI and ultrasound use no ionising radiation. CT uses X-rays, which carry a small radiation dose.

• MRI gives the most detailed images of soft tissue but takes longer, typically 15 to 60 minutes, and is not suitable for everyone with metal implants.

• CT is fast, often under a few minutes in the scanner, and is the preferred test for many lung, bone and emergency problems.

• Ultrasound is quick, real-time and well suited to the abdomen, pelvis, thyroid, blood vessels and lumps.

• A single-area private MRI in London typically costs £580 to more than £950 in 2026, according to WeCovr.

What is the difference between MRI, CT and ultrasound?

MRI uses a strong magnet and radio waves to create detailed images of soft tissue without radiation. CT uses X-rays and a computer to create fast cross-sectional images, particularly good for lungs and bones. Ultrasound uses sound waves to create live images of organs and blood flow, also without radiation.

• MRI. Best for the brain, spinal cord, discs, joints, ligaments, prostate, liver and pelvis. No radiation. Longer scan, noisy, enclosed space.

• CT. Best for lungs, bones, kidney stones, bleeding and emergencies. Very fast. Uses ionising radiation.

• Ultrasound. Best for the gallbladder, liver, kidneys, thyroid, ovaries, uterus, testes, blood vessels and lumps. No radiation. Quick and widely available.

Ask a doctor on WhatsApp which scan answers your question.

When is MRI the right choice?

MRI is the right choice when a doctor needs fine detail of soft tissue. Common reasons include back pain with nerve symptoms, persistent joint injuries, headaches with worrying features, a raised PSA, liver changes seen on ultrasound, and pelvic conditions such as endometriosis or fibroids.
MRI is also the first scan after a raised PSA. The PROMIS trial, published in The Lancet in 2017, found prostate MRI detected significant cancer with 93% sensitivity, compared with 48% for standard biopsy. Our guide to PSA testing and prostate MRI explains the pathway.
MRI is not suitable for everyone. Some pacemakers, cochlear implants and metal fragments are unsafe in the magnet, so every patient completes a safety questionnaire first.

When is CT the right choice?

CT is the right choice for lung problems, suspected kidney stones, complex fractures, some abdominal emergencies and situations where speed matters. CT shows lung tissue and bone in detail that MRI cannot match, and a scan takes only minutes.
CT uses ionising radiation. The dose is higher than a standard X-ray, so doctors use CT when the clinical benefit clearly outweighs the small risk. The average person in the UK receives about 2.7 millisieverts of natural background radiation each year, according to UKHSA, which helps put scan doses in context.
"The scan should follow the question, never the other way round," says Dr Adel Elalfy, Head of Primary Care at The Online GP by The Wellness. "Choosing the right test first saves time, avoids unnecessary radiation and gives a clearer answer."

When is ultrasound the right choice?

Ultrasound is the right choice for gallstones, fatty liver, kidney problems, thyroid nodules, ovarian and uterine conditions, testicular lumps, blood clots in the legs and lumps under the skin. It is quick, painless and uses no radiation, and the person performing it can watch organs moving in real time.
Ultrasound is often the first scan, with MRI or CT used if more detail is needed. It works less well through air and bone, so it is not used for the lungs or brain in adults.
Book an assessment for the right scan on WhatsApp.

Should I have a scan without symptoms?

A scan without symptoms is worth considering only when you have a specific risk, such as an inherited cancer syndrome, a strong family history, or a previous abnormal result that needs monitoring. For most healthy adults, blood tests and established screening programmes give better value.
The Royal College of Radiologists does not advise whole-body scans without a clear medical indication, its president told Medscape News UK in 2026. Our guide to whether a whole-body MRI is worth it covers this in detail.

How much do private scans cost in London?

A single-area private MRI in London typically costs £580 to more than £950 in 2026, and full-body MRI £1,500 to more than £3,500, according to WeCovr's 2026 MRI cost guide. Prices depend on the area, the use of contrast and who reports the images.
At The Online GP by The Wellness, MRI, CT and ultrasound are quoted per patient from the clinic's imaging partner list, and the team confirms fees on WhatsApp first. The doctor's assessment starts with a classic GP appointment from £150, or a same-day urgent appointment from £220. Ask about scan fees on WhatsApp.

Can a doctor tell me which scan I need?

Yes. The Online GP by The Wellness offers a free suitability review. Send your symptoms, how long they have lasted and any previous scans on WhatsApp. A doctor either recommends a specific scan or tells you plainly that a scan is not needed right now.
Start your free suitability review on WhatsApp.

Frequently asked questions

Which is better, MRI or CT?

Neither is better overall. MRI shows soft tissue in more detail without radiation, while CT is faster and better for lungs, bones and emergencies. The right choice depends on the question.

Is ultrasound as accurate as MRI?

For some areas, such as the gallbladder, thyroid and many pelvic conditions, ultrasound is the preferred first test. For the brain, spine and joints, MRI gives more detail.

Do I need a referral for a private scan?

Yes. Private MRI, CT and most ultrasound scans need a referral from a doctor who states the clinical question.

Is MRI contrast dye safe?

MRI contrast is generally safe. The team checks kidney function and allergy history before giving it.

Sources

• Ahmed HU and colleagues, PROMIS trial, The Lancet, 2017

• Medscape News UK, When full-body scans trigger a medical cascade, 2026

• UK Health Security Agency, background radiation dose guidance

• WeCovr, Private MRI scan costs in the UK, 2026
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