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Radiation Basics

Is there a difference between an x-ray exam and a nuclear medicine test?

Both of these tests are done with ionizing radiation—the x-ray exam is performed with machine-generated radiation while the nuclear medicine test is generally performed using injected liquid radioactive materials. X-ray exams generally show structure while nuclear medicine scans can show anatomy, function, and disease state.

X-ray machines can be turned off like a lightbulb (the radiation exposure ends when the machine is off) while the radiation in a patient’s body from a nuclear medicine test decays over time (it cannot be turned off with a switch).

What is radioactive material?

Radioactive material is just another name for a group of unstable atoms that emit ionizing radiation. These groups of unstable atoms emit radiation because they try to become stable. Radioactive materials emit radiation in a process called radioactive decay.

When you call it radioactive “decay,” do you mean it is rotting away like decay in a tooth?

In a way, yes. It is similar because some of the material is actually crumbling or fragmenting (called disintegrating). For example, when a radioactive atom decays, it usually loses a particle from its nucleus and changes into an atom of a different material. On the other hand, it is not the same as decay in a tooth because the material still looks the same physically—if a powder is radioactive and the radioactivity decays away, we still have the same amount of powder but the radiation is less.

How long does it take for the radioactivity to decay away completely?

Different types of radioactive materials have different decay times, varying from fractions of a second to millions of years. Each radioactive material has a decay rate. The time that it takes for half of the radioactive atoms to decay is called a half-life. For example, the previously mentioned technetium-99m has a half-life of six hours which means that, starting with 100 percent, after six hours, we will have 50 percent left. After six more hours, we’ll have 25 percent left (half of the 50 percent that remained after the first half-life). After six more hours, we have 12.5 percent remaining. After ten half-lives, only 0.1 percent of the radioactivity is left.

There are three types of half-life. One is the physical half-life. If you have a container of radioactive material sitting on a counter, the radioactivity decays according to its physical half-life. The second type is a biological half-life. If the radioactive material is in a human, for instance, it gets moved around inside our body just like nonradioactive materials. Sometimes our body will get rid of the material quickly, leading to a short biological half-life.  Sometimes the material might go to a spot in our body and stay there, leading to a long biological half-life. The third type of half-life is when you combine the first two. When a radioactive material is inside a biological system, like the human body, it is decaying by its physical half-life and being cleared from our body by its biological half-life. Combining these is called an effective half-life. This is important when we talk about health effects of radioactive materials that are inside our body.

So, after a lot of half-lives, is the radioactive material gone?

Essentially. For all practical purposes, there is only negligible activity left after 20 half-lives.

When the radioactive material is outside the human body and decaying by its physical half-life, two things can occur. The radioactive material might eventually contain too small an amount of radioactivity to be detected so, essentially, the radioactivity is gone and the atoms have become stable. Or, sometimes, the original radioactive material decays and becomes another radioactive material. That material could decay to another radioactive material or become stable.

If the radioactive material is inside the body, remember that it is decaying by its physical half-life and it is also being cleared by the body with its biological half-life. If the biological half-life is short, our bodies will get rid of the radioactive material so no radioactivity is left inside us. If the biological half-life is long and the physical half-life is short, the radioactive material will stay in the body but will no longer be radioactive in a short time—it will either become stable or decay into another radioactive material. If the biological and physical half-lives are long, the radioactive material will stay inside of us and expose us to its radiation for the duration of the biological or physical half-life, whichever is shorter.