Radionuclides for radiopharmaceuticals
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Radionuclides for radiopharmaceuticals
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[post_date] => 2025-01-09 21:51:16
[post_date_gmt] => 2025-01-10 02:51:16
[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.
One of the benefits of using radiopharmaceuticals is the non-invasive diagnosis and therapy of cancer. Correspondingly, there are therapeutic or diagnostic radiopharmaceuticals, each of which possesses certain characteristics that allow for their optimal performance. A radiopharmaceutical preparation contains a radionuclide in the following forms: as an element in atomic or molecular form, as an anion, or included in or attached to organic molecules.
There are three methods of producing radionuclides. The first method is through irradiation of a target element with neutrons in nuclear reactors. Either neutron beams are used to produce radioisotopes with excessive numbers of neutrons or the fission products are isolated and purified. The second method involves placing a parent radionuclide into a generator which facilitates the transport over long distances into hospitals and the extraction of the daughter nuclide. Their use has proven to be generally cost-efficient, especially in the case of Technetium-99m. The final method is through irradiation with positive charged baryons in a cyclotron. Radionuclides prepared via this route are often beta-positive emitters, but some decay by electron capture.
There are a multitude of existing radionuclides, which are radioactive isotopes of existing elements, but only a select few offer any therapeutic value. To be of any therapeutic value, the radionuclide must have an optimal half-life, an emission with an energy appropriate to its use, and a cost-effective method of synthesis. The optimal half-life should be short enough to minimize the radiation dose to the patient but long enough to perform the diagnostic procedure. When a radionuclide undergoes decay, it can release an alpha, beta, or gamma particle, and the decay mode, along with energy production, are two key factors in determining the scope of the radionuclide’s usage.
[post_title] => Radionuclides for radiopharmaceuticals
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[question] => Which of the following is equal to the total number of nucleons and electrons in technetium-99?
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[answer] => 4
[description] => Reason for the Correct Answer:
Subatomic particles include not only the protons, electrons, and neutrons, but also technically the quarks, so the total would far exceed what we are looking for.
The total number of nucleons is equal to the number of protons and neutrons, which is equal to the mass number (A).
The number of electrons is equal to the number of protons, which is equal to the atomic number (Z).
Therefore, the total number of nucleons and electrons is equal to the sum of the atomic and mass numbers.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. Total number of protons and electrons
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[each_answer] => B. Total number of subatomic particles
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[2] => Array
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[each_answer] => C. Total number of neutrons and electrons
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[each_answer] => D. Sum of the atomic and mass numbers
)
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Diagnostic procedures like single photon emission computed tomography (SPECT) use radionuclides to take cross-sectional snapshots of various parts of the body. Why are gamma emitters particularly suited for diagnostic purposes?
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[answer] => 3
[description] => Reason for the Correct Answer:
SPECT is a procedure where the emissions from the radionuclide indicates area of capillary blood flow.
Gamma rays indeed have the most energy of the decay products, but its energy would not impact imaging.
Technetium-99 is the most readily available radionuclide in hospitals because of its optimal half-life and cost-effectiveness in production through generators. While Tc-99 is a gamma emitter, we cannot deduce that of all gamma emitters.
In order for a snapshot to be taken of the tissue, the radionuclide must emit radiation that is able to penetrate tissue to reach the cameras, so gamma rays is the best because of its penetrating ability.
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[0] => Array
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[each_answer] => A. Gamma emitters are generally produced in reactors or generators which are cheaper to use.
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[each_answer] => B. Gamma rays are the most energetic of all the decay products.
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[2] => Array
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[each_answer] => C. Gamma rays are able to penetrate human tissue more easily.
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[3] => Array
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[each_answer] => D. Gamma emitters are much more readily available in hospitals than the others.
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[2] => Array
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[quiz_unique_key] => 83407773
[question] => Beta-positive emitters are used in positron emission tomography or PET. When a particle interacts with its antiparticle, they both are annihilated entirely to create one or more particles. When a slow-moving positron is emitted, it instantly annihilates with an electron and gamma radiation is produced. What happens as a result of this pair annihilation?
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[answer] => 3
[description] => Reason for the Correct Answer:
If two particles are annihilated, we can assume that their velocities are brought to zero, as well as their momenta.
We are looking for an answer that would give a scenario with zero momentum.
If the energy becomes one gamma ray with some velocity, this would violate the conservation of momentum law.
Electromagnetic radiation is always emitted as quantized packets of energy, the energy cannot be spread in all directions.
Two gamma rays of the same energy traveling in opposite directions to each other would satisfy the conservation of momentum law.
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[0] => Array
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[each_answer] => A. One gamma ray is produced and continues in the same direction as the positron.
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[1] => Array
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[each_answer] => B. Two gamma rays are produced and continue in the same direction at 90 to each other.
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[2] => Array
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[each_answer] => C. Two gamma rays of the same energy are produced and travel in opposite directions to each other.
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[each_answer] => D. The energy is spread into many gamma rays which travel in all directions.
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[quiz_unique_key] => 2377279144
[question] => A large percentage of the 99mTc is produced in the first 3 half-lives of the technetium-99m generator, which needs to be replaced after that time period. How often will a hospital need a new generator for 99mTc production?
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[answer] => 3
[description] => Reason for the Correct Answer:
The technetium-99m generator produces 99mTc in the hospital for diagnostic use in imaging. There is a parent and daughter radionuclide, which are 99Mo and 99mTc, respectively.
Since we are producing technetium-99m, the half-lives concerned are for the parent nuclide molybdenum-99 and not 99mTc.
Each half-life is 2.74 days, so 3 half-lives would be around 8 days.
If the hospital waits 10 days, there will be a time period of just over 1 day when there may be insufficient amounts of 99mTc. So the hospital needs a new generator every week.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. Every day
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[1] => Array
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[each_answer] => B. Every other day
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[2] => Array
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[each_answer] => C. Every week
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[3] => Array
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[each_answer] => D. Every 10 days
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[4] => Array
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[quiz_unique_key] => 2261298308
[question] => Radon-223 emits 4 alpha particles and undergoes 4 beta decays to produce its final decay product. What is this decay product?
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[answer] => 1
[description] => Reason for the Correct Answer:
Radon-223 tells us that radon has a mass number of 223, which may be written as
223Rn. The mass number (A) indicates that there are 223 nucleons, aka protons and neutrons.
We look to the periodic table to find out that radon has 86 protons since atomic number (Z) is equal to the number of protons (86Rn), which is what defines an element. We can deduce that radon-223 has 137 neutrons.
The question stems tells us that radon emits 4 alpha particles, which are synonymous with helium nuclei. Each alpha particle comprises 2 protons and 2 neutrons, so 4 would comprise 8 protons and 8 neutrons.
We would subtract 8 from the atomic number and 16 from the mass number. 207 is the mass number, and 78 is the atomic number.
Each beta decay produces a proton and emitted electron from a neutron, so 4 would produce 4 protons with a loss of 4 neutrons. The mass number stays the same, but the atomic number increases by 4.
207 is the mass number, and 82 is the atomic number, which is lead.
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[each_answer] => A. 207Pb
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[each_answer] => B. 199Re
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[each_answer] => C. 215Tl
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[each_answer] => D. 207Pt
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