3.10.6 Radionuclide Imaging and Therapy Flashcards
What are radionuclides?
Atoms with unstable nuclei that undergo radioactive decay.
What are radiopharmaceuticals (tracers)?
Radionuclides are chemically combined with substances to form compounds that will be absorbed by specific parts of the body.
Why are radiopharmaceuticals ideally only gamma emitters?
So radiation can be detected outside of the body.
How are radiopharmaceuticals used?
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)
What are the main radionuclides used?
Tc - 99(m)
Iodine - 131
What is Tc - 99m?
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)
What is Iodine - 131?
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
In bone scanning, what does lots of absorption detected imply?
High emission rate due to:
bone turnover
fractures
infections
arthritis
bone cancer
In bone scanning, what does low absorption detected imply?
Low emission rate due to:
reduced blood flow
bone death
What are the 5 main components of a gamma camera?
Collimator
NaI crystal
Photomultiplier tubes
Computer
Display
What does the collimator do in a gamma camera?
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.
What does the detector/ Scintillation NaI crystal do in a gamma camera?
Converts gamma rays that reach it into light energy.
What does the electronic systems/ photomultiplier tubes do in a gamma camera?
Detect light energy received from detector crystal and converts it into electric signals.
What does PET scan stand for?
Positron emission tomography
What does tomography mean?
Produce images of slides through the body (using a 3D detector)
What types of emitters are the radiopharmaceuticals used in PET scans?
Beta plus emitters (positron)
What happens to the positrons emitted, in a PET scan?
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)
Why are PET scans used?
To show up cancerous tissue.
Shows brain activity - allows us to study which part of brain is used for what.
What are the 2 factors affecting the decrease of the amount of radiation emitted inside the body from a radioactive source overtime?
Natural physical decay of isotope.
Body’s biological processes that remove the material.
What are some of the body’s biological processes that remove the material?
Breathing
Sweating
Crying
Urinating
Defecating
Skin loss
Lactation
What is the physical half-life [Tp]?
The time taken for half of the original radiative nuclei in a sample to decay.
What is the biological half-life [Tb]?
The time taken for the body to eliminate half of the radioactive material through biological processes.
What is the effective half-life [Te]?
The time taken for the rate of decay within the body to decay to half, due to the initial sample.
What is a type of radiotherapy?
X-ray therapy
Why is radiotherapy used?
To destroy or control malignant tumours.
Why are x-rays used when a tumour is located deep within the body?
Because treatment requires high energy x-rays which can pass through tissue with minimal absorption at the surface.
What are 4 precautions taken to reduce ionising damage to healthy surrounding tissues?
- Accurate scans to locate the tumour
- Selection of correct x-ray energy
- Use of multiple beams
- fine, collimated x-ray beams
How does selection of x-ray energy reduce damage to healthy cells?
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.
How does use of multiple beams reduce damage to healthy cells?
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.
How does fine, collimated x-ray beams reduce damage to healthy cells?
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.
What emitters are radioactive implants?
Beta radiation
Why is beta radiation used for radioactive implants?
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.
What are radioactive implants commonly made from and why?
Iridium - 192
half-life = 74 days
- because implants need to remain active for a longer period
When is iodine - 131 used?
Absorbed by the thyroid gland
- useful for diagnosing thyroid conditions and for targeting tumours in the gland itself.
