Nuclear medicine in plain terms: scans that follow a tracer

Most scans show what the body looks like. Nuclear medicine is mainly about what the body is doing. A tiny amount of radioactive material is put into the body, a special camera follows where it goes, and the pictures reveal how an organ or tissue is working. That makes the experience different from an X-ray or MRI, with more waiting and a few extra safety points. This page explains it in plain terms, as public bodies describe it. Which scan, if any, is right for someone is for their doctor to say. For the wider picture, see types of medical imaging.

What nuclear medicine is

RadiologyInfo.org says nuclear medicine uses small amounts of radioactive material combined with a carrier molecule. The compound is called a radiotracer or radiopharmaceutical. Once it is in the body it builds up in certain areas, such as a particular organ or an inflamed area, and the energy it gives off is picked up by an imaging device. The source stresses that nuclear medicine determines how the body is functioning at a cellular level, and that the tests are non-invasive and usually painless.

The National Institute of Biomedical Imaging and Bioengineering (NIBIB) gives an example of the idea: to find the exact source of intestinal bleeding, doctors may attach a radioactive atom to a sample of the patient's own red blood cells, reinject them and follow their path with gamma camera detectors, so that any build-up of radioactivity in the intestines shows where the problem lies. Tracers differ with the purpose of the scan, and the nuclear medicine doctor chooses the one that will give the most specific and reliable information for the problem.

The cameras

The most basic device is the gamma camera. RadiologyInfo.org notes that the camera itself does not emit any radiation; it detects the energy coming from the tracer in your body and converts it into an image. Its detector heads are often box shaped and mounted on a round, donut-shaped gantry, and you lie on a table that slides between two heads above and below you.

In SPECT (single photon emission computed tomography) the camera heads rotate around the body to produce detailed three-dimensional images. Many centres also combine nuclear medicine images with CT or MRI, a technique called image fusion, so the doctor sees information from two exams in one picture. The source adds that fusion leads to more precise information and a more exact diagnosis, and it lists as a limitation that the resolution of nuclear medicine images may not be as high as that of CT or MRI.

One kind of nuclear medicine scan: PET

PET, or positron emission tomography, is one of the two most common nuclear medicine imaging methods alongside SPECT, according to NIBIB. RadiologyInfo.org describes a PET scanner as a large machine with a round, donut-shaped hole in the middle that looks like a CT or MRI unit, with rings inside that detect energy from the tracer. The most common tracer is a sugar-like compound called FDG, and many centres combine PET with CT in one machine. The visit follows the same pattern described on this page, and your own imaging team will explain any details specific to the scan you have been offered.

What a visit involves

According to RadiologyInfo.org, nuclear medicine tests are done on an outpatient basis, although you may also have one while in hospital. A nurse or technologist places an intravenous line in a vein in your hand or arm and you receive the tracer by injection, or you may swallow it or inhale it as a gas. You may feel a slight pin prick when the needle goes in and a cold sensation moving up your arm during the injection. The source says radiotracers have little or no taste and that inhaling one feels no different from breathing normal air.

Then comes the waiting that sets these scans apart. The tracer travels through the body and builds up in the area being studied, which, the source says, may take anywhere from several seconds to several days. Your doctor will tell you when imaging begins. The actual imaging time ranges from 20 minutes to several hours, and a test may be spread over several days. You will need to stay still for brief periods, and in some cases the camera may move very close to the body, which is necessary for the best quality images. The page advises telling the technologist if you have a fear of closed spaces.

Young children may need gentle wrapping or sedation to hold still, and RadiologyInfo.org suggests asking whether a child life specialist is available to make a child comfortable and less anxious without sedation. For adults, it advises that you can resume normal activities afterwards unless your doctor says otherwise. The small amount of tracer loses its radioactivity over time through natural decay and much of it passes out in urine or stool over the first hours and days, so the source recommends drinking plenty of water and following the safety precautions you are given to avoid exposing other people to radiation.

What it is used for

RadiologyInfo.org lists a wide range of uses, grouped by body system. In the heart, nuclear medicine can look at blood flow and function, as in a myocardial perfusion scan. In the lungs, it can check breathing and blood flow problems. In bones, it can check for fractures, infection and arthritis. It can also measure thyroid function, look at gallbladder function, evaluate how the kidneys work and how the stomach empties, and help find infection. In children, the same page lists uses such as looking at tear ducts and blood flow in some heart conditions.

The same technology is also used for treatment, which is a separate topic: RadiologyInfo.org explains that therapy uses a radiopharmaceutical that attaches to specific cells and delivers a high dose of radiation. This page is about diagnostic scans, and the safety statements below apply to those.

Radiation safety in nuclear medicine

All of these scans involve radiation from the tracer. RadiologyInfo.org states that nuclear medicine exams use only a small dose of radiotracer acceptable for diagnostic exams, that the potential benefits of an exam outweigh the very low radiation risk, and that doctors have used diagnostic nuclear medicine procedures for more than six decades with no known long-term adverse effects from such low-dose exposure. NIBIB says the total dose from the majority of radiopharmaceuticals used in most diagnostic studies is no more than what is conferred during routine chest X-rays or CT exams, and that nuclear medicine physicians are committed to giving the least amount of radiopharmaceutical needed. How doses are compared across scan types is on the page about radiation, contrast and imaging safety.

The source adds that allergic reactions to radiotracers are extremely rare and usually mild, and that the injection may cause slight pain and redness that should rapidly resolve. Women should always tell their doctor and technologist if they might be pregnant or are breastfeeding, preferably when scheduling, because some of the tracer can pass into breast milk and special instructions apply. Pregnancy and children are covered in more detail on the page about imaging, children and pregnancy.

The short version

Nuclear medicine scans follow a small amount of radioactive tracer, using a camera that detects energy given off inside the body, to show how organs and tissues are working. The visits can involve waiting and a long time on the table, and the radiation dose from diagnostic tracers is described by public bodies as small. Tell the team about pregnancy, breastfeeding, allergies and medicines, and follow the instructions you are given afterwards.