Scans explained

Medical imaging is described by the UK Health Security Agency in its guide published on 22 September 2022 as "one of the biggest advances in modern medicine", used to help diagnose and treat patients. The guide covers five broad families of examination: X-ray, CT, ultrasound, MRI and nuclear medicine imaging. Behind that list sits a fundamental divide, and understanding it is the clearest way into the whole subject.

Two families, divided by physics

According to the UK Health Security Agency, the X-ray family and nuclear medicine both use ionising radiation. The X-ray family is wide: general radiography, which produces 2D images; dental radiography, described by the same source as the most common type of X-ray examination; fluoroscopy, which produces moving images of parts of the body; interventional radiology, which uses X-rays to produce moving images that can be used to guide and deliver treatment; and CT, which produces 3D images of the body. Nuclear medicine, including PET-CT and PET-MRI, uses a small amount of a radioactive substance that is usually injected into a vein, as the UK Health Security Agency explains.

On the other side of the divide, ultrasound and MRI are described by the UK Health Security Agency as "types of imaging that do not use X-rays or radioactive substances". Ultrasound works with sound waves, and MRI is carried out in "a large machine with a short tunnel through it, and a flat bed that moves through the scanner" (NHS, 11 September 2025).

Different machines, different questions

The NHS describes an X-ray as a test that takes pictures of the inside of the body using a small amount of radiation, with bones, teeth, lungs and the digestive system among the targets it names. A CT scan, the NHS says, takes detailed pictures of the inside of the body, using a large ring that moves around part of the body as the person lies on a flat bed. An MRI scanner is described by the NHS as a large machine with a short tunnel through it and a flat bed that moves through the scanner, and the examination usually takes between 15 minutes and 1 hour. Ultrasound, again according to the NHS, is used to see images of the inside of the body, such as muscles, organs, or a baby in the womb, and the scan lasts around 10 to 45 minutes.

Because the underlying measurement differs, the choice between methods depends on what is being looked at, and that comparison is easier to see on the modalities table, which sets the main techniques side by side on a single grid.

What this section covers

This section of Inside the Scanner contains four pages, one for each of the principal methods:

• Plain X-rays, covering general and dental radiography, fluoroscopy and interventional radiology.
• CT scans, the technique that uses X-rays to build 3D images of the body.
• Ultrasound, the method based on sound waves rather than radiation.
• Having an MRI scan, the method that relies on magnetic resonance rather than X-rays or radioactive substances.

Each page describes how the machine works, what the NHS says the examination involves, and how long it typically lasts, so that the physics and the practical experience of the scan can be read together.

The other doors of the publication: Having a scan, Radiation and safety and People and history. Every one of them points back to modalities at a glance.