Define radioactive tracer .

A radioactive tracer is a radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body.

Why do gamma emitters make good radioactive tracers, rather than alpha or beta emitters?

Gamma radiation can leave the body to be detected externally, and it does not ionise tissue as much as alpha or beta radiation.

Name the three common radioactive tracers that emit gamma radiation, discussed in this topic.

Technetium-99m , iodine-131 and indium-111 .

Define radiopharmaceutical .

A radiopharmaceutical is a molecule labelled with a radioactive isotope .

The 'm' in Technetium-99m indicates that the nucleus is in a .......... excited state.

The 'm' in Technetium-99m indicates that the nucleus is in a metastable excited state.

State the three essential properties a radioactive isotope must have to be a good medical tracer.

It must be a gamma emitter , so radiation can be detected outside the body Its gamma rays must be as low-energy as possible, to reduce ionisation damage It must have a short half-life , to reduce total dose to the patient while still giving a high enough gamma intensity to detect

True or False? Iodine-131 is a pure gamma emitter, like Technetium-99m.

False. Iodine-131 emits both beta-minus particles and gamma photons, unlike Technetium-99m, which emits gamma only.

Why is indium-111 particularly well-suited to labelling blood cells and locating infection?

It emits gamma photons (170 keV and 250 keV) with a half-life of 68 hours, and can label red blood cells, white blood cells, bone marrow and spinal fluid, allowing infection, inflammation and blood disorders to be located.

What is the purpose of a molybdenum-technetium generator ?

It is a device that produces technetium-99m from molybdenum-99 , allowing Tc-99m to be produced in the hospital when needed.

Technetium-99m has a half-life of .......... hours.

Technetium-99m has a half-life of 6.0 hours.

Why is molybdenum-99 transported to hospitals while technetium-99m must be produced on-site?

Mo-99 has a longer half-life ( 66 hours ), so it can survive transport and be delivered weekly. Tc-99m's half-life ( 6.0 hours ) is too short to maintain useful radioactivity during transport.

How is Tc-99m extracted from a molybdenum-technetium generator?

A saline solution is passed over the radioactive materials in the generator. Tc-99m dissolves into the solution, which is then extracted and injected into patients.

True or False? Molybdenum-99 has a shorter half-life than technetium-99m.

False. Mo-99 has a longer half-life (66 hours) than Tc-99m (6.0 hours) — this is why Mo-99 can be transported to hospitals while Tc-99m cannot.

Once Mo-99 arrives at a hospital, for how long can the generator continue producing usable Tc-99m?

Up to a week , with Tc-99m extracted from the generator every few days.

Define Positron Emission Tomography (PET) .

PET is a type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues.

Describe what happens when a positron emitted by F-18 meets an electron in the body.

The positron annihilates with the electron, producing two identical gamma photons that travel in opposite directions.

Why are two gamma-ray photons of equal energy produced when a positron and electron annihilate, rather than one?

The total energy released (from the mass-energy equivalence of the electron-positron pair) is shared equally between two identical photons, which travel in opposite directions to conserve momentum .

Each gamma photon produced by positron-electron annihilation has an energy of .......... keV.

Each gamma photon produced by positron-electron annihilation has an energy of 512 keV.

Why are gamma photons that arrive at the detectors more than a nanosecond apart ignored in PET scanning?

They cannot have come from the same annihilation event , since true coincident photons are produced simultaneously and travel in opposite directions along the same line of response .

True or False? A positron travels several centimetres through body tissue before annihilating with an electron.

False. A positron travels less than a millimetre before annihilating with an atomic electron.

How is an image of tracer concentration formed from the signals detected in a PET scan?

By processing the arrival times of the gamma-ray photons; a greater number of photons detected from a point indicates a higher tracer concentration , shown as a brighter point on the image.

Define physical half-life .

Physical half-life is the time taken for the number of radioactive nuclei to halve.

Define biological half-life .

Biological half-life is the time taken for the concentration of a substance in the body to decrease by half, due to processes such as excretion and respiration.

Define effective half-life .

Effective half-life is the combined half-life of the physical and biological half-life, accounting for both radioactive decay and biological excretion.

Biological half-life depends on many factors, such as the patient's .......... and .......... .

Biological half-life depends on many factors, such as the patient's health and metabolism .

True or False? Effective half-life is always longer than both the physical and biological half-lives.

False. Effective half-life is always shorter than the shortest of the physical and biological half-lives.

Name the four major components of a gamma camera .

Collimator Scintillator Photomultiplier tubes Computer and display

A collimator is made of thin lead tubes that absorb gamma photons unless they travel parallel to the tubes, ensuring only parallel photons reach the scintillator to produce sharp, high-resolution images.

How does making the collimators narrower and longer affect the image produced by a gamma camera?

It filters out more scattered gamma rays , improving the image resolution .

Describe what happens when a gamma photon strikes the scintillator crystal.

An electron is excited to a higher energy state and excites further electrons as it travels through the crystal. When these electrons fall back to their original state, the energy is released as visible light photons , which travel to the photomultiplier tubes.

Why are photomultiplier tubes needed after the scintillator?

The light photons produced by the scintillator are very faint , so they must be converted into an electrical signal and amplified .

In a photomultiplier tube, electrons released from the photocathode accelerate through a series of .......... , each at a progressively higher potential difference.

In a photomultiplier tube, electrons released from the photocathode accelerate through a series of dynodes , each at a progressively higher potential difference.

True or False? A brighter point on a gamma camera image indicates a lower concentration of tracer in that tissue.

False. A brighter point indicates more photons detected from that location, meaning a higher concentration of tracer is present.

How is an image of tracer concentration formed on the computer of a gamma camera?

By processing the arrival times of the gamma-ray photons detected by the photomultiplier tube array.

Why do X-rays destroy cancer cells at a greater rate than healthy cells?

Cancerous cells divide more frequently than healthy cells, and dividing cells are more sensitive to X-ray radiation.

Why are low-energy X-rays used to treat skin cancer, rather than high-energy X-rays?

Low-energy X-rays do not penetrate deep into the body, so they can treat surface tumours while reducing the risk of damage to deeper, healthy tissue.

Define external beam radiotherapy (EBRT) .

EBRT is the use of high-energy X-rays directed at a tumour deep inside the body from an external source .

Describe two ways healthy tissue exposure is reduced during high-energy X-ray radiotherapy.

Metal (aluminium) filters remove low-energy X-rays, which would otherwise damage tissue near the surface The X-ray source is rotated and directed from different directions, so the tumour at the centre of rotation receives a much greater dose than surrounding tissue

In EBRT, an aluminium filter removes .......... -energy X-rays, as these are more likely to damage tissue near the surface.

In EBRT, an aluminium filter removes low -energy X-rays, as these are more likely to damage tissue near the surface.

State two properties a radioisotope used in a radioactive implant should have.

A short half-life (as long as activity is also high) A short range (as long as it can pass through the implant casing) Be highly ionising

Why is beta radiation the most common choice for radioactive implants, rather than alpha radiation?

Beta radiation can penetrate the seed's metal casing but does not travel far beyond the implant site. Alpha radiation cannot penetrate the casing, and even if it could, would only ionise tissue immediately next to the seed and could not penetrate through the whole tumour.

True or False? The main advantage of internal radiotherapy is that no healthy tissue is ever exposed to ionising radiation.

False. The main advantage is that the source can be placed as close to the cancer as possible, but a small amount of healthy tissue is still exposed to ionising radiation.

Why is an X-ray scan too risky for imaging a foetus, when other scans give sufficient resolution?

X-rays carry a radiation risk , and comparable resolution can be achieved safely with alternatives such as ultrasound , so the risk is not justified.

Which imaging method is most appropriate for a lower-body injury , and why?

External X-ray imaging ; it is quick and easy, and gives sufficient resolution to show bone damage without the unnecessary radiation dose of a CT scan.

Which imaging method is best for detecting a lung tumour , and why not ultrasound or external X-ray?

A CT scan ; it highlights cancerous tissue against healthy tissue in cross-section. Ultrasound is absorbed by air in the lungs, and X-rays may not differentiate tissues, with the ribcage potentially obscuring the tumour.

Why is an ingested gamma source appropriate for imaging a hyperactive thyroid, rather than a CT, X-ray or ultrasound scan?

It shows the function of tissue, revealing whether the thyroid is performing at a higher than normal rate, whereas the other methods show only structure.

An ingested gamma source has the lowest .......... of all the imaging methods listed, at around 6 mm.

An ingested gamma source has the lowest resolution of all the imaging methods listed, at around 6 mm.

True or False? CT scans carry no radiation risk.

False. CT scans involve a large radiation dose , despite their high resolution.

Name two advantages and two disadvantages of using an MRI scan.

Advantages: distinguishes types of tissue; no radiation risk Disadvantages: expensive and time-consuming; uncomfortable for some patients

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Radionuclide Imaging & Therapy (AQA A Level Physics): Flashcards

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Define radioactive tracer.
A radioactive tracer is a radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body.
Why do gamma emitters make good radioactive tracers, rather than alpha or beta emitters?
Gamma radiation can leave the body to be detected externally, and it does not ionise tissue as much as alpha or beta radiation.
Name the three common radioactive tracers that emit gamma radiation, discussed in this topic.
Technetium-99m, iodine-131 and indium-111.
Define radiopharmaceutical.
A radiopharmaceutical is a molecule labelled with a radioactive isotope.
The 'm' in Technetium-99m indicates that the nucleus is in a .......... excited state.
The 'm' in Technetium-99m indicates that the nucleus is in a metastable excited state.
State the three essential properties a radioactive isotope must have to be a good medical tracer.
- It must be a gamma emitter, so radiation can be detected outside the body
- Its gamma rays must be as low-energy as possible, to reduce ionisation damage
- It must have a short half-life, to reduce total dose to the patient while still giving a high enough gamma intensity to detect
True or False?
Iodine-131 is a pure gamma emitter, like Technetium-99m.
False.
Iodine-131 emits both beta-minus particles and gamma photons, unlike Technetium-99m, which emits gamma only.
Why is indium-111 particularly well-suited to labelling blood cells and locating infection?
It emits gamma photons (170 keV and 250 keV) with a half-life of 68 hours, and can label red blood cells, white blood cells, bone marrow and spinal fluid, allowing infection, inflammation and blood disorders to be located.
What is the purpose of a molybdenum-technetium generator?
It is a device that produces technetium-99m from molybdenum-99, allowing Tc-99m to be produced in the hospital when needed.
Technetium-99m has a half-life of .......... hours.
Technetium-99m has a half-life of 6.0 hours.
Why is molybdenum-99 transported to hospitals while technetium-99m must be produced on-site?
Mo-99 has a longer half-life (66 hours), so it can survive transport and be delivered weekly. Tc-99m's half-life (6.0 hours) is too short to maintain useful radioactivity during transport.
How is Tc-99m extracted from a molybdenum-technetium generator?
A saline solution is passed over the radioactive materials in the generator. Tc-99m dissolves into the solution, which is then extracted and injected into patients.
True or False?
Molybdenum-99 has a shorter half-life than technetium-99m.
False.
Mo-99 has a longer half-life (66 hours) than Tc-99m (6.0 hours) — this is why Mo-99 can be transported to hospitals while Tc-99m cannot.
Once Mo-99 arrives at a hospital, for how long can the generator continue producing usable Tc-99m?
Up to a week, with Tc-99m extracted from the generator every few days.
Define Positron Emission Tomography (PET).
PET is a type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues.
Which radioisotope is commonly used as a beta-plus emitting tracer in PET scanning?
Fluorine-18 (F-18).
Describe what happens when a positron emitted by F-18 meets an electron in the body.
The positron annihilates with the electron, producing two identical gamma photons that travel in opposite directions.
Why are two gamma-ray photons of equal energy produced when a positron and electron annihilate, rather than one?
The total energy released (from the mass-energy equivalence of the electron-positron pair) is shared equally between two identical photons, which travel in opposite directions to conserve momentum.
Each gamma photon produced by positron-electron annihilation has an energy of .......... keV.
Each gamma photon produced by positron-electron annihilation has an energy of 512 keV.
Why are gamma photons that arrive at the detectors more than a nanosecond apart ignored in PET scanning?
They cannot have come from the same annihilation event, since true coincident photons are produced simultaneously and travel in opposite directions along the same line of response.
True or False?
A positron travels several centimetres through body tissue before annihilating with an electron.
False.
A positron travels less than a millimetre before annihilating with an atomic electron.
How is an image of tracer concentration formed from the signals detected in a PET scan?
By processing the arrival times of the gamma-ray photons; a greater number of photons detected from a point indicates a higher tracer concentration, shown as a brighter point on the image.
Define physical half-life.
Physical half-life is the time taken for the number of radioactive nuclei to halve.
Define biological half-life.
Biological half-life is the time taken for the concentration of a substance in the body to decrease by half, due to processes such as excretion and respiration.
Define effective half-life.
Effective half-life is the combined half-life of the physical and biological half-life, accounting for both radioactive decay and biological excretion.
Biological half-life depends on many factors, such as the patient's .......... and ...........
Biological half-life depends on many factors, such as the patient's health and metabolism.
State the equation linking effective half-life ( $T_{E}$ ), physical half-life ( $T_{P}$ ) and biological half-life ( $T_{B}$ ).
$\frac{1}{T_{E}} = \frac{1}{T_{P}} + \frac{1}{T_{B}}$
True or False?
Effective half-life is always longer than both the physical and biological half-lives.
False.
Effective half-life is always shorter than the shortest of the physical and biological half-lives.
Iodine-131 has a physical half-life of 8.05 days and a biological half-life of 138 days. Calculate its effective half-life.
$\frac{1}{T_{E}} = \frac{1}{8.05} + \frac{1}{138} = 0.1315$
$T_{E} = \frac{1}{0.1315} = 7.60 days$
Name the four major components of a gamma camera.
- Collimator
- Scintillator
- Photomultiplier tubes
- Computer and display
Define collimator.
A collimator is made of thin lead tubes that absorb gamma photons unless they travel parallel to the tubes, ensuring only parallel photons reach the scintillator to produce sharp, high-resolution images.
How does making the collimators narrower and longer affect the image produced by a gamma camera?
It filters out more scattered gamma rays, improving the image resolution.
Describe what happens when a gamma photon strikes the scintillator crystal.
An electron is excited to a higher energy state and excites further electrons as it travels through the crystal. When these electrons fall back to their original state, the energy is released as visible light photons, which travel to the photomultiplier tubes.
Why are photomultiplier tubes needed after the scintillator?
The light photons produced by the scintillator are very faint, so they must be converted into an electrical signal and amplified.
In a photomultiplier tube, electrons released from the photocathode accelerate through a series of .........., each at a progressively higher potential difference.
In a photomultiplier tube, electrons released from the photocathode accelerate through a series of dynodes, each at a progressively higher potential difference.
True or False?
A brighter point on a gamma camera image indicates a lower concentration of tracer in that tissue.
False.
A brighter point indicates more photons detected from that location, meaning a higher concentration of tracer is present.
How is an image of tracer concentration formed on the computer of a gamma camera?
By processing the arrival times of the gamma-ray photons detected by the photomultiplier tube array.
Why do X-rays destroy cancer cells at a greater rate than healthy cells?
Cancerous cells divide more frequently than healthy cells, and dividing cells are more sensitive to X-ray radiation.
Why are low-energy X-rays used to treat skin cancer, rather than high-energy X-rays?
Low-energy X-rays do not penetrate deep into the body, so they can treat surface tumours while reducing the risk of damage to deeper, healthy tissue.
Define external beam radiotherapy (EBRT).
EBRT is the use of high-energy X-rays directed at a tumour deep inside the body from an external source.
Describe two ways healthy tissue exposure is reduced during high-energy X-ray radiotherapy.
- Metal (aluminium) filters remove low-energy X-rays, which would otherwise damage tissue near the surface
- The X-ray source is rotated and directed from different directions, so the tumour at the centre of rotation receives a much greater dose than surrounding tissue
In EBRT, an aluminium filter removes ..........-energy X-rays, as these are more likely to damage tissue near the surface.
In EBRT, an aluminium filter removes low-energy X-rays, as these are more likely to damage tissue near the surface.
State two properties a radioisotope used in a radioactive implant should have.
- A short half-life (as long as activity is also high)
- A short range (as long as it can pass through the implant casing)
- Be highly ionising
Why is beta radiation the most common choice for radioactive implants, rather than alpha radiation?
Beta radiation can penetrate the seed's metal casing but does not travel far beyond the implant site. Alpha radiation cannot penetrate the casing, and even if it could, would only ionise tissue immediately next to the seed and could not penetrate through the whole tumour.
True or False?
The main advantage of internal radiotherapy is that no healthy tissue is ever exposed to ionising radiation.
False.
The main advantage is that the source can be placed as close to the cancer as possible, but a small amount of healthy tissue is still exposed to ionising radiation.
Why is an X-ray scan too risky for imaging a foetus, when other scans give sufficient resolution?
X-rays carry a radiation risk, and comparable resolution can be achieved safely with alternatives such as ultrasound, so the risk is not justified.
Which two imaging methods discussed carry no radiation risk?
MRI and ultrasound.
Which imaging method is most appropriate for a lower-body injury, and why?
External X-ray imaging; it is quick and easy, and gives sufficient resolution to show bone damage without the unnecessary radiation dose of a CT scan.
Which imaging method is best for detecting a lung tumour, and why not ultrasound or external X-ray?
A CT scan; it highlights cancerous tissue against healthy tissue in cross-section. Ultrasound is absorbed by air in the lungs, and X-rays may not differentiate tissues, with the ribcage potentially obscuring the tumour.
Why is an ingested gamma source appropriate for imaging a hyperactive thyroid, rather than a CT, X-ray or ultrasound scan?
It shows the function of tissue, revealing whether the thyroid is performing at a higher than normal rate, whereas the other methods show only structure.
An ingested gamma source has the lowest .......... of all the imaging methods listed, at around 6 mm.
An ingested gamma source has the lowest resolution of all the imaging methods listed, at around 6 mm.
True or False?
CT scans carry no radiation risk.
False.
CT scans involve a large radiation dose, despite their high resolution.
Name two advantages and two disadvantages of using an MRI scan.
- Advantages: distinguishes types of tissue; no radiation risk
- Disadvantages: expensive and time-consuming; uncomfortable for some patients

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