3.10.6 Radionuclide Imaging and Therapy

Question 1

What are radionuclides?

Answer 1

Atoms with unstable nuclei that undergo radioactive decay.

Question 2

What are radiopharmaceuticals (tracers)?

Answer 2

Radionuclides are chemically combined with substances to form compounds that will be absorbed by specific parts of the body.

Question 3

Why are radiopharmaceuticals ideally only gamma emitters?

Answer 3

So radiation can be detected outside of the body.

Question 4

How are radiopharmaceuticals used?

Answer 4

They enter the patient [swallowed/ injected] and gets absorbed by the target tissue.
- amount of absorption an indicate issues in that tissue
(more absorption = more gamma emissions from that tissue)

Question 5

What are the main radionuclides used?

Answer 5

Tc - 99(m)
Iodine - 131

Question 6

What is Tc - 99m?

Answer 6

A metastable state of Tc - 99.
=excited state, formed from decay of Mo - 99
Doesn’t decay immediately.
Has half-life of 6h (short enough to limit exposure, but long enough to carry out the scan)

Question 7

What is Iodine - 131?

Answer 7

Used for: investigating thyroid problems
Beta-minus emitter, so patient receives a dose of ionising radiation.
Risk of radiation/damage is outweighed by benefits of correct diagnosis.
Half-life = 8 days

Question 8

In bone scanning, what does lots of absorption detected imply?

Answer 8

High emission rate due to:
bone turnover
fractures
infections
arthritis
bone cancer

Question 9

In bone scanning, what does low absorption detected imply?

Answer 9

Low emission rate due to:
reduced blood flow
bone death

Question 10

What are the 5 main components of a gamma camera?

Answer 10

Collimator
NaI crystal
Photomultiplier tubes
Computer
Display

Question 11

What does the collimator do in a gamma camera?

Answer 11

Usually made from lead.
Only gamma rays travelling straight through will be detected, to avoid any scattered rays reaching the detector, that would reduce the clarity and accuracy of the image.

Question 12

What does the detector/ Scintillation NaI crystal do in a gamma camera?

Answer 12

Converts gamma rays that reach it into light energy.

Question 13

What does the electronic systems/ photomultiplier tubes do in a gamma camera?

Answer 13

Detect light energy received from detector crystal and converts it into electric signals.

Question 14

What does PET scan stand for?

Answer 14

Positron emission tomography

Question 15

What does tomography mean?

Answer 15

Produce images of slides through the body (using a 3D detector)

Question 16

What types of emitters are the radiopharmaceuticals used in PET scans?

Answer 16

Beta plus emitters (positron)

Question 17

What happens to the positrons emitted, in a PET scan?

Answer 17

Collide with any electron close by and annihilate.
- their masses are converted into the energy of 2 gamma photons (in opposite directions to conserve momentum)

Question 18

Why are PET scans used?

Answer 18

To show up cancerous tissue.
Shows brain activity - allows us to study which part of brain is used for what.

Question 19

What are the 2 factors affecting the decrease of the amount of radiation emitted inside the body from a radioactive source overtime?

Answer 19

Natural physical decay of isotope.
Body’s biological processes that remove the material.

Question 20

What are some of the body’s biological processes that remove the material?

Answer 20

Breathing
Sweating
Crying
Urinating
Defecating
Skin loss
Lactation

Question 21

What is the physical half-life [Tp]?

Answer 21

The time taken for half of the original radiative nuclei in a sample to decay.

Question 22

What is the biological half-life [Tb]?

Answer 22

The time taken for the body to eliminate half of the radioactive material through biological processes.

Question 23

What is the effective half-life [Te]?

Answer 23

The time taken for the rate of decay within the body to decay to half, due to the initial sample.

Question 24

What is a type of radiotherapy?

Answer 24

X-ray therapy

Question 25

Why is radiotherapy used?

Answer 25

To destroy or control malignant tumours.

Question 26

Why are x-rays used when a tumour is located deep within the body?

Answer 26

Because treatment requires high energy x-rays which can pass through tissue with minimal absorption at the surface.

Question 27

What are 4 precautions taken to reduce ionising damage to healthy surrounding tissues?

Answer 27

1. Accurate scans to locate the tumour 2. Selection of correct x-ray energy 3. Use of multiple beams 4. fine, collimated x-ray beams

Question 28

How does selection of x-ray energy reduce damage to healthy cells?

Answer 28

Choosing the right energy level ensures that x-rays penetrate deep enough to reach the tumour, but avoid excessive scattering/ absorption that could damage surrounding healthy cells.

Question 29

How does use of multiple beams reduce damage to healthy cells?

Answer 29

Multiple beams from different angles converge on the tumour; each individual beam delivers only a small dose along its path, but where they intersect, the tumour receives the full intended dose.

Question 30

How does fine, collimated x-ray beams reduce damage to healthy cells?

Answer 30

Collimation narrows and shapes the x-ray beam to match the size and shape of the tumour, reducing dose delivered to adjacent healthy cells and ensuing radiation is as focussed as possible.

Question 31

What emitters are radioactive implants?

Answer 31

Beta radiation

Question 32

Why is beta radiation used for radioactive implants?

Answer 32

Since beta radiation loses most of its energy over a short distance, it effectively kills cells near the implant, while causing minimal damage to surrounding healthy tissue.

Question 33

What are radioactive implants commonly made from and why?

Answer 33

Iridium - 192 half-life = 74 days - because implants need to remain active for a longer period

Question 34

When is iodine - 131 used?

Answer 34

Absorbed by the thyroid gland - useful for diagnosing thyroid conditions and for targeting tumours in the gland itself.