You’ve had a computerized tomography (CT) scan or magnetic resonance imaging (MRI) scan, but now your doctor says you need to have a positron emission tomography (PET) scan. Why? What will the PET scan show that the others did not, or could not?
Unlike radiation, magnetic and sound-wave imaging tests (see below), which show anatomy — the structures inside your body — a PET scan is a nuclear scan. It uses radioactive substances to show changes in the organs and tissues and how those structures are working. It can also show if disease is present. The images are 3D and in color.
The most common use of a PET scan is to stage cancer — to see where it has spread. It may also be used to see how well treatment is working or to locate the best place to take a biopsy or give radiation treatments.
PET scans can detect areas of cancer that may not be large enough to show up on a CT or MRI scan. However, PET scan images are not as sharp as those produced by CT or MRI. PET scans identify areas where the cells are highly active (more about that later), but since they don’t show all structures (anatomy) clearly, it is difficult to identify the exact structure where the suspicious cells are located.
To solve this problem, Roswell Park uses a combination PET and CT scan that’s more accurate than either test on its own. The CT images clearly show the structures/anatomy. The PET images are combined with the CT images. Using this technique, when cancer cells "light up" on the PET scan, the physician can tell where they are located — in bone, an organ or another structure. (Although both tests are done, you may still hear your doctors and nurses call it a “PET scan.”)
Preparing for a PET Scan
It’s important to tell your doctor about:
- All the medications you take (prescription, over-the-counter, herbs, supplements, vitamins)
- Any allergies you have
- Whether you are — or might be — pregnant, or if you are breastfeeding
- Whether you have diabetes
Depending on the type of PET-CT ordered, you will be given instructions about when to stop eating and drinking before the test. The most common type of PET uses the radiotracer 18FDG (fluorodeoxyglucose). FDG is a type of glucose (sugar) combined with radioactive material. You will need to stop eating and drinking about 4-6 hours before the test, because sugar and calories can affect the test results. You may drink water.
There are a few newer types of radiotracers used at Roswell Park today. These do not have the eating/drinking restrictions of FDG:
- Ga-68 dotate (NETSPOT®). This is used for neuroendocrine tumors, which don't show up well with FDG.
- F-18 sodium fluoride, which is used for bone tumors.
- F-18 fluciclovine (Axumin®), which is used for recurrent prostate cancer.
Other radiotracers are under review by the Food & Drug Administration (FDA) and will likely find their way to the clinic soon.
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What Happens During the Scan?
- Before the test, you will be given detailed instructions about eating and activity.
- On the day of testing, you will get an injection of a small dose of a radiotracer, such as FDG or one of the newer ones described above.
- To give the radiotracer time to build up in the body part being scanned, you will have to wait an hour between getting the tracer and having the scan.
- The more “active” the cell, the more radiotracer it will absorb. Cancer cells grow and divide (reproduce) very fast, so they usually absorb more than most normal cells.
- As the radiotracer breaks down, it gives off positively charged particles called positrons. They stay in the area and interact with local, negatively charged particles. This interaction gives off electromagnetic radiation.
- You'll lie on a bed that will slide you into the camera.
- The special scanner rotates around you, collects the electromagnetic radiation and uses it to create 3D images in black-and-white or color.
- The cancer cells that have picked up more of the radiotracer will show up on the images and will look different from the normal cells surrounding them.
There are no restrictions once the test has been completed. The radiotracer will continue to break down and leave your body in your urine. Drinking a lot of water will help flush it out.
For More Information
Positron Emission Tomography (PET) and Computed Tomography (CT): Patient site of the Radiological Society of North America (RSNA®) and the American College of Radiology (ACR®)
Nuclear Scans: National Institutes of Health: U.S. National Library of Medicine
Other Imaging Tests: How Do They Work?
X-rays use electromagnetic radiation to take pictures of the inside of your body. The X-ray machine sends out a radiation beam, which creates a black-and-white image. Different types of tissue absorb different amounts of radiation, so dense tissue, such as bone, appears white; less-dense tissues appear gray; and empty spaces are black. (Fluoroscopy uses the same technology, but instead of a single, still image, it makes a moving picture that is more like a movie.)
A CT scan also uses radiation, but it produces a full 360-degree view by sending a series of narrow beams through the body as the machine rotates around you. This produces many highly detailed pictures. The machine’s computer puts these cross-sectional images together to show “slices” of your anatomy, like the slices in a loaf of bread. The images are usually two-dimensional (2D), but 3D is possible. Specific types of CT scans include spiral (helical) scans and EBCT (electron beam CT) scans, which produce images more quickly. You may be given contrast (dye) through an IV to help create clearer pictures.
An MRI produces a cross-sectional, 3D image, but it works differently from CT scans, X-rays and MRI. Instead of using ionizing radiation, such as X-rays and CT scans, it creates a temporary magnetic field around you. Radio waves are sent to and from a transmitter/receiver in the machine. The radio waves are used to make digital images of the part(s) of your body being scanned. Each type of tissue (bone, organs, muscle, etc.) looks different because the signals vary depending on the type of tissue — how much water it contains, its magnetic properties, etc. You may be given a gadolinium contrast through an IV to help create clearer pictures.
Ultrasound creates images of soft tissues, such as muscles and internal organs. A small device called a transducer is moved across the skin. It sends out high-frequency sound waves that bounce off organs and tissues. The computer uses the returning sound waves, or echoes, to create images. The images may be flat, 3D or 4D (3D images in motion). They may be black-and-white or color.