Particles/radioactivity: Radioactive tracer Technetium-99

Technetium-99m is a commonly-used radioactive tracer isotope. The isotope undergoes gamma decay with a half-life of less than a day, making it feasible for deep-tissue applications like imaging the circulatory system. In such experiments, the tracer is injected into the patient’s tissue and allowed to circulate while a radiation detector is held outside the body. Careful imaging allows the shape and flow characteristics of various parts of the circulatory system to be imaged.

The majority of ${}^{99m}Tc$‍ is generated through the natural decay of ${}^{99}Mo$‍, which can be generated in a laboratory. Once the $Tc$‍ isotope is generated, its short half-life requires that it be used immediately for imaging.

In order to calculate the body’s response to the tracer, scientists place a high-resolution radioactivity monitor right beside a blood vessel near the heart of a patient. At $t=0$‍, they inject the radioactive tracer into the arm of the patient. The count of gamma rays recorded by the monitor are plotted as a function of time in Figure 1.

Figure 1: The counts versus time for a radiation detector placed above the blood vessel of a patient. The patient is injected with a radioactive tracer at $t=0$‍.