Diagnostic Methods in Medicine (OCR A Level Physics): Flashcards
Exam code: H556
1/230Still learning
Know0
- FrontMedical Tracers
Define radioactive tracer.
Was this flashcard helpful?
Cards in this collection (23)
Define radioactive tracer.
A radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body.
What are the half-lives of technetium-99m and fluorine-18?
Technetium-99m has a half-life of about six hours; fluorine-18 has a shorter half-life of about 110 minutes.
State two properties that make technetium-99m and fluorine-18 suitable medical tracers.
They both bind to organic molecules (such as glucose or water) that are readily available in the body, and they both emit gamma radiation and decay into stable isotopes.
Define fluorodeoxyglucose (FDG).
A glucose molecule with radioactive fluorine (fluorine-18) attached, used as a tracer in PET scanning.
Fluorine-18 undergoes .......... decay, emitting a positron.
Fluorine-18 undergoes β+ (beta-plus) decay, emitting a positron.
Give two medical uses of radioactive tracers besides diagnosing cancer.
Detecting areas of decreased blood flow in the heart, and identifying brain injuries such as Alzheimer's disease and dementia.
True or False?
A beta-emitting tracer would be preferred over a gamma-emitting tracer for imaging inside the body.
False.
Gamma radiation is only weakly ionising and passes through the patient, so it can be easily detected outside the body with little cell damage. Beta particles are highly ionising and are absorbed within the patient, so cannot be detected.
Define collimator (in a gamma camera).
A device made of thin lead tubes that absorbs gamma photons not travelling parallel to the tubes, so only parallel photons reach the scintillator crystal.
List the four major components of a gamma camera.
Collimator, scintillator, photomultiplier tubes, and computer and display.
How does collimator design affect the resolution of a gamma camera image?
The narrower and longer the collimators, the more effectively they filter out scattered gamma rays, which improves the resolution of the image.
Describe the role of the scintillator crystal in a gamma camera.
It absorbs an incident gamma photon, exciting an electron to a higher energy state. As this electron travels through the crystal it excites further electrons, and when these return to their original state the energy is released as visible light photons.
In a photomultiplier tube, photons from the scintillator release electrons from the .........., which then accelerate through a series of dynodes at increasing potential difference.
In a photomultiplier tube, photons from the scintillator release electrons from the photocathode, which then accelerate through a series of dynodes at increasing potential difference.
Define technetium-99m.
A metastable gamma-emitting isotope with a half-life of about six hours, commonly used as a medical tracer; it decays by emitting a gamma photon of exactly 140 keV.
Why is technetium-99m considered an ideal medical tracer?
It has a short half-life, so it remains active long enough to be imaged while reducing harm to the patient, and its chemical properties allow it to be incorporated into several tissues, so multiple organs can be imaged at once.
True or False?
Photons travelling parallel to a collimator's lead tubes are absorbed, while scattered photons pass through.
False.
It is the opposite: photons travelling parallel to the collimator pass through to the scintillator, while scattered photons travelling in other directions are absorbed by the lead tubes.
Define Positron Emission Tomography (PET).
A type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues.
Define annihilation.
When a particle meets its equivalent antiparticle, they are both destroyed and their mass is converted into energy.
What is meant by the "line of response" in PET scanning?
The straight line joining the two detectors that register a pair of gamma photons from the same annihilation event, along which the annihilation occurred.
Why are the two gamma photons from a positron-electron annihilation emitted in exactly opposite directions?
To conserve momentum, since the total momentum of the positron-electron system is close to zero before annihilation.
How does a PET scanner determine the location of an annihilation event within the body?
It measures the delay time between the arrival of the two gamma photons at the ring of detectors; the shorter the delay, the more precisely the annihilation point along the line of response can be found.
Before a PET scan, the patient is injected with a ..........-emitting isotope, usually fluorine-18.
Before a PET scan, the patient is injected with a beta-plus (β+)-emitting isotope, usually fluorine-18.
State one advantage and one disadvantage of using a short half-life tracer in PET scanning.
Advantage: it reduces the patient's exposure time to radiation. Disadvantage: the patient must be scanned quickly, and PET scanners are expensive, so not all hospitals have access to one.
True or False?
Any two gamma photons arriving at the detector ring within a few microseconds of each other are assumed to come from the same annihilation event.
False.
Only photons arriving within about a nanosecond of each other are counted as a valid pair; photons arriving further apart than this are ignored, as they cannot have come from the same annihilation event.
Sign up to unlock flashcards
or