Radioactive tracer Technetium-99
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Radioactive tracer Technetium-99
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[post_date] => 2025-01-09 21:39:51
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
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 
is generated through the natural decay of 
, 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.
[post_title] => Radioactive tracer Technetium-99
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[questions] => Array
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[quiz_unique_key] => 578908434
[question] => Into what substance does decay when it emits gamma radiation?
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[answer] => 2
[description] => Reason for the Correct Answer:
The decay process described here results in the emission of a gamma ray photon.
Gamma emission occurs only due to changes in nuclear energy not to changes in the specific nucleons present.
Gamma radiation does not change the mass number of an isotope.
The gamma ray emission does not change the isotope, and so it remains
)
[answers] => Array
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[0] => Array
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[each_answer] => A.
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[1] => Array
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[each_answer] => B. 
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[2] => Array
(
[each_answer] => C.
)
[3] => Array
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[each_answer] => D. 
)
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => The mass of a Tc isotope of mass M is observed to change by a very small amount m right as it emits a gamma ray. Which of the following gives the wavelength of the observed gamma ray? *(h is Planck’s constant and c is the speed of light)
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
The total mass of the isotope does not affect the wavelength of emission, only the energy level structure of the nucleus.
The energy of the photon is related to the mass change by 
The wavelength and energy of the emitted photon are related by 
By substitution, 
By rearrangement, 
The wavelength of the emitted photon is related to the mass change by 
)
[answers] => Array
(
[0] => Array
(
[each_answer] => A. 
)
[1] => Array
(
[each_answer] => B. 
)
[2] => Array
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[each_answer] => C. 
)
[3] => Array
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[each_answer] => D.
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Which of the following processes is likely used to make 99Tc from 99Mo?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
Note that the atomic mass remains the same before and after the reaction.
The mass would not remain the same if the particle were undergoing neutron capture or alpha emission.
Note that the atomic number must change because the element name changes.
The isotope must undergo beta decay, which converts a neutron to a proton (and releases an electron of negligible mass).
)
[answers] => Array
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[0] => Array
(
[each_answer] => A. Beta decay
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[1] => Array
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[each_answer] => B. Alpha emission
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[2] => Array
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[each_answer] => C. Gamma ray emission
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[3] => Array
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[each_answer] => D. Neutron capture
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[3] => Array
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[quiz_unique_key] => 2377279144
[question] => Which of the following mechanisms is responsible for the peak in the graph?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
The tracer first must reach the site being monitored, and so the radioactivity counts start at zero and increase from there.
Once the tracer has mixed into the bloodstream, the only process limiting the total number of counts it generates is its natural decay.
The peak is the joint result of diffusion of the tracer into the bloodstream, and its natural decay.
)
[answers] => Array
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[0] => Array
(
[each_answer] => A. The rate of decay of the isotope reaches its peak value
)
[1] => Array
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[each_answer] => B. The competing effects of diffusion of the tracer into the blood vessel, and its natural decay
)
[2] => Array
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[each_answer] => C. Once the radiation level reaches a certain threshold, it saturates and the tracer becomes less effective
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[3] => Array
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[each_answer] => D. The tracer first diffuses into the bloodstream, but its effectiveness degrades due to natural shielding by bone and other dense tissues
)
)
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[4] => Array
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[quiz_unique_key] => 2261298308
[question] => After t = 3 hours, the curve displays a best-fit to an exponential with a time constant of 4.8 hours, which is shorter than expected based on the decay constant of Technetium-99m. Which of the following provides the best explanation for this effect?
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
Magnetic and electric fields do not greatly affect nuclear decay.
Nuclear chain reactions rely on neutron-induced splitting, not gamma rays.
Radioisotopes, like non-radioactive chemicals and drugs, display a biological half-life determined by the rate at which the body processes and excretes them.
The additional loss due to natural excretion diminishes the apparent time constant for Technetium-99m.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. The body is excreting the radioisotopes
)
[1] => Array
(
[each_answer] => B. Free electrons cause Technetium to decay more rapidly than it would in the lab
)
[2] => Array
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[each_answer] => C. Decay radiation induces other nuclei to decay, resulting in a chain reaction
)
[3] => Array
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[each_answer] => D. Miniscule magnetic fields in the body are accelerating Technetium decay
)
)
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[559921|1] => B
[559921|2] => D
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[559921|5] => A
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