Computed tomography (CT) and positron emission tomography (PET) are two types of medical scans. They are used for different purposes, but are often used together.

A CT scan shows the structure of the body, such as organs, bones and tissues.
A PET scan shows how parts of the body are working at a cellular level.

Both are commonly used to help diagnose and monitor a wide range of conditions, including cancer. In many cases, they are combined into a PET-CT scan to give a more complete picture.

What is a CT scan?

A CT scan uses multiple X-ray beams taken from different angles to create detailed images of the inside of the body.

These images are shown as thin “slices”, which can also be combined into a 3D view.

CT scans involve a small amount of ionising radiation. The scanner itself is a large, ring-shaped machine with a table that moves through it.

Common uses include:

• Detecting internal injuries or bleeding
• Diagnosing bone and muscle problems, such as fractures or tumours
• Identifying blood vessel issues, such as clots or blockages
• Locating infections, abscesses or bleeding
• Diagnosing and monitoring conditions such as stroke, heart disease, lung disease and cancer
• Guiding procedures such as biopsies or surgery
• Monitoring how well treatment is working

Sometimes a contrast dye is used (either as a drink or via a vein) to make certain areas show up more clearly.

What is a PET scan?

A PET scan looks at how your body is functioning by measuring metabolic activity.

It uses a small amount of radioactive material, called a radiotracer, which is usually given through a vein. This helps highlight areas of the body that are more active.

PET scans are commonly used for:

• Diagnosing and monitoring cancer, including spread to other areas
• Assessing brain conditions such as epilepsy or Alzheimer’s disease
• Evaluating heart conditions such as coronary artery disease or heart failure

Areas that are more active, such as fast-growing cancer cells, appear brighter on the scan.

Because of this, PET scans can sometimes detect disease earlier than a CT scan.

Key differences

CT scan:

• Shows structure (bones, organs, tissues)
• Uses X-rays
• Quick, usually 10 to 30 minutes
• Radiation leaves the body immediately
• Better for physical abnormalities
• Often used to guide procedures
• Generally lower cost

PET scan:

• Shows function (how tissues are working)
• Uses a small amount of radioactive tracer
• Can take longer depending on the scan
• Small amount of radiation remains briefly after
• Better for detecting early or active disease
• Not used to guide procedures
• More expensive

What they have in common

• Both are non-invasive and usually painless
• Both are often done as outpatient procedures
• Both are used in cancer diagnosis and monitoring
• Both can reduce the need for exploratory surgery

What is a PET-CT scan?

A PET-CT scan combines both techniques into one machine.

This allows doctors to see both:

• The structure of the body (from CT)
• The activity within it (from PET)

This is particularly useful in cancer care, as it can:

• Detect smaller tumours
• Identify spread earlier
• Help decide the best treatment
• Monitor response to treatment
• Check for recurrence

The main drawback is cost, as PET-CT scans are more expensive than CT scans alone.