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Course: MCAT > Unit 3

Lesson 1: Chemical and physical sciences practice passage questions

Understanding the properties of radioactive tracers


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 99mTc is generated through the natural decay of 99Mo, 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.
Into what substance does 99mTc decay when it emits gamma radiation?
Choose 1 answer: