What's the difference between a PET scan and a CT scan?

A CT scan shows structure. A PET scan shows activity. CT tells your team where things are and what shape they are in; PET tells them which tissue is chemically busy.

That is why the two are so often done together, in one machine and one appointment, as a PET/CT.

How each one makes its picture

A CT scan uses an x-ray machine linked to a computer to take pictures from many angles, then builds them into detailed 3-D images. You lie still on a table that slides through a donut-shaped scanner while the machine moves around you.

You may be given a contrast agent, a dye that highlights certain areas and makes the pictures easier to read. Iodine and barium are the common ones, given by mouth, by injection into a vein, or by enema.

A PET scan is a nuclear scan. You receive an injection of a radioactive tracer, and then lie still while the scanner detects where it collected. The most common tracer is FDG, a radioactive form of glucose. Cancerous tumors usually burn glucose faster than normal tissue, so they take up more FDG and stand out.

Other PET tracers exist for other questions, including oxygen levels in tissue, new blood vessel formation, bone growth, and whether tumor cells are actively dividing. For suspected prostate cancer spread, PSMA-targeted agents such as piflufolastat F-18 and Ga-68 gozetotide are used.

Why they get combined

In a PET/CT, the CT runs first to map the anatomy, then the PET adds the functional layer on top. NCI notes this combination may improve the ability to diagnose cancer, judge how far it has spread, plan treatment, and monitor response. It can also cut the number of extra imaging tests you need.

The practical upshot: a PET finding without a CT is hard to place, and a CT finding without a PET can be hard to interpret. Together they answer "where" and "is it active" at once.

Radiation, in numbers you can compare

Both use ionizing radiation, and NCI gives figures worth having.

The average person in the United States receives about 3 millisieverts, or mSv, per year from natural background radiation. A low-dose chest CT is about 1.5 mSv, comparable to 6 months of background. A typical chest CT is about 6.1 mSv, roughly 2 years of background. A PSMA PET procedure is about 3 mSv, and a whole-body PSMA PET/CT ranges from 4 to 23 mSv depending on the protocol.

NCI's judgment is that the increase in cancer risk from one CT is small, and that the benefit greatly outweighs the risk when the scan is being used to diagnose cancer or another serious condition in someone with symptoms. Women carry a somewhat higher risk than men from the same exposure at the same age.

Children are more sensitive, because their cells divide faster and they have more years ahead. NCI cites a large multinational study in which people who had at least one CT before age 22 had a higher risk of blood cancers, at about 1 to 2 extra cases per 10,000 scans in the 12 years afterward.

Practical points before your appointment

Tell the technologist immediately if you feel itching, hives, shortness of breath, or swelling of the throat after contrast, since serious reactions are rare but treatable when caught fast. Contrast can also affect the kidneys in some people, and a simple blood test beforehand checks kidney function.

Tell your provider if there is any chance you are pregnant.

Keeping your own record of imaging is worth the effort, especially if you see doctors in different systems. Our guide to what to expect during a PET scan covers the day itself, and the piece on how cancer is diagnosed covers the other tests.

Sources

• https://www.cancer.gov/about-cancer/diagnosis-staging/diagnosis
• https://www.cancer.gov/about-cancer/diagnosis-staging/ct-scans-fact-sheet