The four main imaging tests answer different questions, and the right one depends on what is being looked for rather than which is “best”. In short: X-ray for bone and chest, ultrasound for soft tissue, organs and anything that needs watching in motion, CT for speed and detail across the whole body, and MRI for the finest soft-tissue and neurological detail. We offer ultrasound only, which is why this page also tells you plainly when ultrasound is the wrong test.
Contents
| Ultrasound | X-ray | CT | MRI | |
|---|---|---|---|---|
| How it works | Sound waves | Ionising radiation | Ionising radiation, many angles | Magnetic field and radio waves |
| Ionising radiation | None | Low | Higher | None |
| Best for | Soft tissue, organs, blood flow, tendons, pregnancy | Bone, chest, foreign bodies | Trauma, chest, abdomen, lungs, bone detail | Brain, spine, joints internally, soft-tissue detail |
| Real-time / moving | Yes | No | No | No |
| Typical duration | 15–30 min | Minutes | 5–10 min | 20–60 min |
| Enclosed space | No | No | Short tunnel | Long tunnel, noisy |
| Metal implants an issue | No | No | No | Sometimes — must be checked |
| Safe in pregnancy | Yes — first choice | Avoided where possible | Avoided where possible | Usually, with caution |
| Operator dependent | Yes, significantly | No | No | No |
| Available here | Yes, from £235 | No | No | No |
| Your question | Right test |
|---|---|
| Have I broken a bone? | X-ray |
| Is this lump solid or fluid? | Ultrasound |
| Do I have gallstones? | Ultrasound |
| Is there a clot in my leg? | Ultrasound |
| Have I torn my rotator cuff? | Ultrasound (MRI if surgery is planned) |
| Have I torn my meniscus or cruciate ligament? | MRI |
| Are my coronary arteries narrowed? | CT coronary angiography |
| How strong is my heart pumping? | Ultrasound (echocardiogram) |
| Is my baby developing normally? | Ultrasound |
| Is there something wrong in my brain or spinal cord? | MRI |
| How advanced is my arthritis? | Weight-bearing X-ray |
| Is my liver scarred? | Neither — FibroScan or a fibrosis blood panel |
Radiation is the most common worry, and the figures are less alarming in context than they sound in isolation. Dose is measured in millisieverts (mSv). The UK average background exposure is around 2.2 mSv a year from natural sources, varying regionally from roughly 1.5 to 7.5 mSv.
| Test | Approximate dose | Roughly equivalent to |
|---|---|---|
| Ultrasound | None | — |
| MRI | None | — |
| Chest X-ray | About 0.02 mSv | A few days of background radiation |
| CT head | About 2 mSv | Around a year of background radiation |
| CT chest | About 7 mSv | Around three years |
| CT abdomen and pelvis | About 8–10 mSv | Around three to four years |
These are approximations — actual dose varies with the scanner, the protocol and your build. The relevant point is that a CT is not ordered casually, and where ultrasound or MRI can answer the same question, they are generally preferred. Under UK regulations every exposure has to be clinically justified.
Sound does not travel through bone or gas. That single physical fact defines most of the limits:
Ultrasound is also more operator-dependent than the other three. A CT scanner produces the same images whoever presses the button; an ultrasound is only as good as the person holding the probe and the time they spend.
Neither — they answer different questions. Ultrasound is superior for superficial tendons, blood flow, dynamic assessment and guiding injections. MRI is required for the inside of joints, the brain and spine, and bone marrow. For a shoulder, ultrasound is often the better first test; for a knee, MRI usually is.
Yes. It uses sound waves, not ionising radiation, and there is no known harm from diagnostic ultrasound at the levels used. It is the standard imaging test in pregnancy for that reason.
A CT of the abdomen and pelvis is roughly 8–10 mSv, about three to four years of natural background exposure in the UK. A chest X-ray is about 0.02 mSv, a few days’ worth. Every exposure has to be clinically justified.
It can detect many masses and characterise whether they are solid or fluid-filled, which is often the first step. It cannot exclude cancer everywhere in the body, and a definitive diagnosis usually needs a biopsy. Some cancers need CT, MRI or mammography to find at all.
Usually bowel gas or body habitus restricting the view. It describes image quality rather than your health. Fasting beforehand reduces gas and improves the view of the upper abdomen.
No. We are an ultrasound clinic. If your question needs MRI or CT we will tell you, and the report will say so, so you can take it to your GP or a specialist.
It depends on the implant. Many modern implants are MRI-safe but every case has to be checked in advance by the MRI unit. Ultrasound and CT have no such restriction.
Related: do I need a GP referral, understanding your ultrasound report, and MSK ultrasound compared with MRI.
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