Nuclear Med Flashcards
SPECT - Compared to Gamma Camera
Better contrast
Worse spatial resolution than gamma
Higher noise than gamma
PET - Compared to SPECT
Better spatial resolution
Less noise
SPECT has better contrast
Decay Patterns
M -4 ALPHA DECAY (too many protons or neutrons)
A -2
M 0 BETA -VE DECAY (too many neutrons). Neutron converted to proton in nucleus and an electron and antineutrino released from the nucleus)
A +1
M 0 BETA +VE DECAY (too few neutrons) (positron produced)
A -1
M 0 ELECTRON CAPTURE (too few neutrons). A proton captures an electron from one of the shells
A -1 (starts with K) and converts to neutron. Iodine 123 decays this way producting a gamma wray and x-ray (given off as a result of hole in electron shell)
M 0 ISOMERIC TRANSITION (decay of radionuclide to ground state giving off gamma radiation as in Tn-99m)
A 0
Positrons
Same mass as electrons (1/1840)
Only exist while they have energy
Positive charge
PET
30,000 bismuth detectors
Gamma photons 511keV produced by annihilation between electron and positron
Fluorine decays by beta positive decay
The activity from each point source can be measured as a sine curve or sonogram
Spatial resolution 4-5mm
PET Detectors
Good energy resolution
High detection efficiency
Short scintillation decay time
High atomic number and physical density
High light output
SPECT
Camera moves in 3 or 6 degree increments
- can be continuous or step and shoot
Produces 3D images
Spatial resolution 15-20mm
Contrast good in SPECT
Cyclotron
Cyclotron
67Gallium
111Indium
123Iodine
201Tl
Gamma Camera
Gamma Camera
- 100 photomultiplier tubes that determine position and intensity of photons
- Technetium is a pure GAMMA EMITTER
- Lead collimator thickness varies:
- 0.3mm
- 1mm
- 2mm
Typical sensitivity is up to 2%
(Around 75-90% of gamma photons are detected)
Scattered photons can still be accepted if energy is within the photopeak
Photomultiplier tube contains a series of dynodes
Gamma Camera
Gamma Camera
-spatial resolution is made better by either
—making collimator holes smaller
—making lead septa thicker
—making collimator thicker
This however will reduce the number of gamma photons reaching the scintillation crystal (will hit collimator instead) and therefore sensitivity will decrease
Photomultipliers and count rate do NOT affect spatial resolution
Metal objects can cause artefact in Gamma cameras bu absorbing some of the gamma rays which result in appearance of a possible ‘cold lesion’
Photopeak
- Photopeak is produced by pulses that are a result of photoelectric interaction
- ‘Pulse pile up’ is caused by two or more photons being detected at same time. This peak occurs to the right of the main photopeak
Photomultiplier
Needs a photocathode for operation (converts light into electrons)
- Photocathode releases 1 electron for every 5-10 photons
- Photomultipler contains a series of dynodes than amplify electrons
- 10-12 dynodes, each more positive in potential than the last
- each dynode has potential difference of 1.2kV (total potential 1-1.2kV
- each dynode releases 4 electrons for every electron that hits it
Pulse Height Analyser
Reduces noise!!
Located between detector and counting system
Can detect more than one photo-peak
Wide window produces images faster but they are of lower quality due to scatter and lower energy photons
Narrow window produces better quality image
Collimator allows location of the source of gamma photons within patient (primary purpose is not to filter out scattered radiation. Pulse height analyser does this)
The use of collimators improves spatial resolution
Quality Control in Gamma
Field Uniformity (Test of detector)
-Field uniformity refers to the ability to produce a uniform distribution of activity
Can be tested using a flat, sealed plate of cobalt 57 called a flood field or sheet phantom
- Intrinsic floods performed WITHOUT collimator (addition of a collimator reduces intrinsic resolution)
- Extrinsic floods performed WITH collimator
Modern cameras have uniformity of at least 2%
Quality Control in Gamma
Spatial Resolution
Tested using a Line Source which results in creation of a graph called the Line Spread Function
- From this we can measure FWHM which is used to compare resolutions of different systems
- Gamma camera resolution generally 5-10mm
- Intrinsic resolution 3-4mm (intrinsic resolution is always the best)
- FWHM is a measure of spatial resolution of a system
Collimators in Gamma Camera
Collimator allows location of the source of gamma photons within patient (primary purpose is not to filter out scattered radiation. Pulse height analyser does this)
The use of collimators improves spatial resolution
There is a trade off between collimator sensitivity and spatial resolution
Increasing number of holes increases sensitivity!!
Increasing collimator sensitivity reduces patient dose (more gama rays passing through collimator measn less radionuclide needed hence lower patient dose. This results in lower spatial resolution however
Internal conversion
Gamma ray interacts with inner shell electrons via photoelectric effect releasing
Photoelectrons
Characteristic x rays
Examples: Iodine 125 and 123
Beta Particles
Beta Particles
- Are made up of electrons
- Negative electrons are negative beta particles
- Positive electrons are positrons
Beta particles follow a random path in matter like secondary electrons
Release as a continuous spectrum of energies (like bremmst)
NOT specific energies
Can only travel a few mm in tissue
Result in a trail of ionized atoms
Ideal Radiopharmaceutical Properties
Pure gamma emitter between 50 - 300keV
Half life suitable for examination
Half Lives
Krypton - 13 seconds
Fluorine - 110mins
Technetium 99 - 6 hours
Molybdenum 99 - 67 hours
Biological half life is dependent on the rate if ELIMINATION of the radionuclide from the body (NOT rate of radioactive decay)
The dose received by the organ is PROPORTIONAL to the effective dose
The pharmaceutical used does NOT affect the half life
Gamma Camera Colliamtors
Collimators locate radioactive source within patient along line of sight (primary function is NOT to reduce scatter)
In parallel hole - the field of view and in air sensitivity are the same at all distances
Divergent and Convergent collimators suffer from geometric distortion at edges and reduced spatial resolution
Fish tail collimators cause distortion
Gamma Camera Sodium Iodide Crysal
Sodium iodide crystal
- 500mm wide
- Approx 10mm thick
- emits 5000 light photons when struck by gamma photon
Absorbs gamma rays by photoelectric absorption
Fragile, can be damaged by damp and is susceptible to changes in temp
Dose in Radionuclide Imaging
Dependent on emitted gamma energy
Proportiontal to administered activity
Dependent on all half lives
Effective dose is measured in sieverts
Dose for different examinations
Renal studies - 2mSv
Lung scans - 2mSv
Bones Scan - 5mSv
Brain scan - 8mSv
Thalium heart scan - 18mSv
Creating Radioactivity
Nucleur Reactor - neutron added to a stable nucleus
Cyclotron - addition of a proton into a nucleus causing a neutron to be ejected out
Separating Radionuclides
Can be separated from carrier in Cyclotron as have different atomic numbers and therefore different properties
Cannot be separated in Nuclear Reactor as both have same atomic numbers and therefore same properties
Radioactive Decay
- Not an energy dependent process, occurs independently
- After decay can result in metastable, unstable or stable daughter nucleus
Radioactive decay of a radionuclide decreases by equal fractions in equal intervals of time
Number of radioactive atoms in a sample is proportional to rate of decay
The higher the rate of decay, the higher number of beta and gamma particles produced
Isomer
Isomer
Following beta decay in some radionuclides, gamma ray is not released immediately. Before and after the emission of the gamma ray, these are called isomers
- Different energy states and half lives
- Same atomic number
- Same mass number
Electron Capture
Where there is a a neutron deficit
Electron taken from inner shell and combined with proton to form a neutron
No net charge. Negative charge or electron with positive proton charge cancel eachother out
Characteristic radiation released
Ideal properties of a pharmaceutical agent (that attaches to radionuclide)
- Should accumulate in target tissue which is its function
- Should remain stable after labelling with radionuclide
Mo-99 Decay
Mo-99 decays to Tc-99m and releases a negative electron in the process
Technetium 99m then releases a gamma photon on its decay
Gamma Camera Sensitivity
It is a function of both intrinsic and collimator efficiency
Below 100keV the system sensitivity is almost 100%
Sensitivity tested using a known amount of activity in a small source
Expressed as total counts per second per megabecquerel
V/Q Imaging
Tc 99 MAA is used in CTPA
Microaggregated albumin particles are 20-40um
Ventilation and perfusion CANNOT be performed simultaneously
Background Radiation
- 85% are contributed from natural source
- Account for 10% of all cancers
- Air travel & watches contribute minimally
- More than 50% of the natural background radiation comes from radon
- Medical imaging contributes MORE than 10%
Bone Scan
Uses Tc99m-MDP
- Dose is approx 6mSv
- Are non-specific for fractures
- Dose is 555-750MBq dose
- For evaluation of mets, the time from injection to scan is approx 3-4 hours
Geiger Muller Counter
Requires a well stabilised voltage
Are used as part of quality control
NOT used for dose measurement
Usually coated in a metal such as lead (not glass)
Collimator Energy Types
Low energy - photon energy 150keV (Technetium)
- septal thickness of 0.3mm
Medium Energy - 300keV (Indium)
- septal thickness of 1mm
High energy - 400keV (Iodine 131)
- septal thickness of 2mm
Dynamic Imaging examples
Renal and cardiac imaging mostly performed dynamically
- MAG3 renal scan
- MAG3 is a compound with high renal excretion. It is labelled to Technetium. Images taken every 2 mins fir 20 mins to demonstrate uptak followed by excretion.
- A ROI over relavent organ needs to be obtained on first image and the number of counts present
- Images can be displayed as the time it takes for x number of counts to take place
SHORTER TIME OF ACQUISITION THE LOWER THE SPATIAL RESOLUTION
What is matrix size in SPECT?
Usually 64 x 64 or 128 x 128
- This is to reduce Noise
- Results in low resolution
PET - Data Correction
Attenuation correction
- A radioactive rod of gallium is rotated around inside the detector WITHOUT then WITH the patient
- Then allows for attenuation correction
Normalisation
- To ensure detectors are perfoming properly
- A radioactive source is rotated around without the patient and correction factor applied
Correction Factor = measured counts for line of response / average counts for all lines
What are the control rods in Nucleur reactors made of?
Cadium or Boron
What type of collimator used in SPECT?
High resolution collimator
Resolution in Gamma Cameras - what affects it?
Intrinsic made worse by:
- Lower energy gamma photons
- Thicker crystals
- Smaller number of PMT
Collimator resolution made better by:
- Longer thinner holes
- Shorter distance from patient
What increases spatial resolution?
- Smaller pixels
- Imaging superficial rather than deep structures
PET - inflammation imaging
Isotopes used are produced in Cyclotron
- Indium and Gallium
- Both produce gamma photons
- Indium labelled to autologous white cells
- Both decay by electron capture
TOF PET
Emerging technique
- Aimed to reduce the amount of random coincidences
- Allow more accurate localisation of the line of response
Advantages
- higher spatial resolution than PET
- gives more accurate location of LOR
Disadvantages
- Not all scintillation crystals can be used
- FDG cannot be used for TOF PET
Can isotopes only be used for ventilations scanning if they are gaseous?
Yes they have to be gaseous
Xenon must be used with rebreathe apparatus to avert bystander dose
- Xenon has half life of 5 days
- emits beta and gamma
- is gas at room temp
- produced in nucleur reactor
- Krypton has half life of 13secs
- gas at room temp
- produced in generator
- produces gamma rays
How does Technetium 99m decay?
Isomeric transition releasing gamma photons
How does Mo 99 decay?
Beta decay
Releases beta particle
What collimator used for technetium?
LEHR
Low energy high resolution
