DDT 7: Diagnostics Therapy 3- Gamma Cameras Flashcards
radiation detectors
tools used to detect exposure of radiation - limited info on type of exposure or the kind of damage that has occured
simplest form of radiation detector and function
a Film Badge – these indicate cumulative exposure to radiation by the degree of blackening of photographic film!
what does film badge consist of?
It is typical for a single badge to contain a series of filters of different thicknesses and of different materials (the precise choice may be determined by the environment to be monitored).
2 common forms of radiation detectors
The Geiger-Muller Tube.
Scintillation Counters.
geiger muller tube
consists of a metal tube filled with gas (helium, neon, argon) and an electrode wire in it
encased in a glass tube and is sealed by a thin window at one end/
what happens when alpha particles beta particles or gamma rays enter the geiger muller tube
When a or b-particles or g-rays enter the tube, they cause ionization of some of the gas molecules
PET
Positron Emission Tomography
what is PET
imaging technique that uses gamma rays formed during positron-electron annihilation (pair production) to construct images of the form and function of various organs within the body.
specific purpose of PET scan
look at functionality of specific organs but not the anatomy
how are positrons produced
pair production
beta plus decay
other various ways
what happens when a positron encounters or comes in contact with an electron?
positron-electron annihilation occurs, producing TWO identical 511 keV g-rays travelling in exactly opposite directions (i.e. 180 degrees to each other).
what detects coincident gamma rays
These coincident g-rays are detected by scintillation counters at exactly (almost) the same time.
what direction are photons emitted
180* degrees to each other
how does emitting photons at a specific angle help with diagnostic imaging in PET scans
Since the photons are emitted at 180o to each other, it is possible to localise their source by projecting back along their line of coincidence
what does reconstruction surface determine?
Reconstruction software determines the angular and linear coincidence events.
How does positron-electron annihilation contribute to imaging?
By measuring the activity level (number of annihilation events) as a function of position, an image can be reconstructed – similar to that in CT
What radionuclides are used to generate the positrons used in PET?
a short-lived radioactive tracer isotope which decays by emitting a positron
how are radionuclides are being introduced to the body?
chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation).
why is there a waiting period in the PET scan?
while the metabolically active molecule become concentrated in tissues of interest; then the subject is placed in the imaging scanner
name radionuclide and waiting period
(most commonly fluorodeoxyglucose (FDG), waiting period of an hour
what is the average half life of the radionuclide in PET scan?
quite short
what compounds can be used to incorporate radionuclides in the body?
such as glucose, water or ammonia and then injected into the body to trace where they are distributed
what information are PET scanners good at getting?
information on metabolism or molecular biology processes
what other diagnostic imaging technique are they usually integrated with and why?
integrated with CT scan to get anatomical and metabolic information
how are images from PET scans and CT scans integrated?
The two sets of images are ‘co-registered’ or can be superimposed digitally so that areas of abnormality on the PET scan can be perfectly correlated with anatomy on the CT scan
how do geiger muller tubes work?
Thin mica window allows radiation to enter the tube
Causes ionisation to occur within the metal tube filled with inert gas.
positively charged high voltage wire electrode attracts electrons from ionisation
positively charged ions moved to walls of tube (cathode) - causing more ionisation
forms a short intense pulse of a current
what needs to occur after the detection of radiation and why?
quenching/dead time - 1 particle = 1 discharge
no further detection until discharge is stopped
scintillation counters
Defined as the production of small flashes of visible light from certain materials (scintillators) as a result of the absorption of high energy radiation (e.g. g-rays
what does scintillation counter consist of?
consists of a transparent crystal (e.g. sodium iodide (NaI)) which fluoresces when struck by ionising radiation.
what does the transparent crystal produce and how is it detected and amplified?
The visible flash (single or multiple photons) produced by the crystal is detected and amplified by a Photomultiplier tube (PMT).
why is the intensity of the scintillation flash can be used to determine the original photon energy?
Since the number of emitted photons per MeV of incident energy is fairly constant, the intensity of the scintillation flash can be used to determine the photon energy
series of events that occur in scintillation counters
incident photon enter scintillator and emits a visible photon
visible photon meets photocathode and releases photoelectron
photoelectron strikes first dynode - releases additional electrons
strikes another dynode of higher voltage generates more electrons - so on reach the anode, accumulation of charge is registered as a pulse of current
what is the dynode coated with and what does it form
This dynode is coated with a material which emits several additional electrons for every electron that strikes it.
where are scintillation counters commonly used
gamma cameras
what does gamma camera consist of?
A typical gamma camera might consist of a large flat crystal of Sodium Iodide
a bank of PMT tubes arranged in a hexagon fashion behind the crystal.
what does collimator consist of?
The collimator typically consists of a thick sheet of lead with thousands of adjacent holes drilled through it
purpose of components of collimator?
This allows spatial discrimination to the source of the detected photons (i.e. relative position of the source of the photons).
how are gamma cameras used?
A radiopharmaceutical is administered to the patient
is preferentially absorbed into the organ to be studied
The Gamma Camera detects the intensity of the radioactive activity within the organ under study and ‘plots’ the intensity as a function of position
function of gamma camera
Such imaging techniques allow for comprehensive studies of organ function
how are abnormalities detect in images?
Abnormalities lead to an increase in blood supply to the diseased region, which increases the amount of uptake of the tracer in the area - clearly visible as ‘hotspots’ in the above diagrams.
what does nuclear medicine studies used for specific organ function
Many diagnostic nuclear medicine studies use Radionuclides as tracers for specific organ function.
3 traits of an ideal radionuclide for nuclear medicine
Be short lived (i,e, short half lives).
Emit gamma-rays with little more energy than required for detection (low patient dose). Why not a and b?
Be non-toxic.
what is the coincident event in PET scans?
when the detectors opposite from each other detect gamma rays/photon at the same time
what do PET scans tell us and how are they used
cells that were the most active took up the most radionuclide substance and emitted more gamma rays and therefore appears brighter.
tumours and abnormalities have blood vessel that take up more nutrients thus this technique is used to detect tumours
