PET & Spect Flashcards
What does PET stand for?
Positron Emission Tomography
How are positron-emitting radionuclides produced? (3)
- Produced in cyclotron
- Tend to be useful nuclides such as OXygen, Nitrogen or Fluoride
- O - 2mins
- F - 110 mins
- Radionuclides need to be delivered to hospital before half-life is too weak
Basis of PET scanning (5)
- Positron is emitted within the body, where the electron & positron anihilate to produce two photons.
- Two photons travel in opposite directions
- Two photons detected within a few billionths of a second
- A line is drawn where two photons are recorded
- Gives rise to high-resolution image
What is Iterative Reconstruction?
Uses many different iterations to work out the image
What does FDG stand for?
FluoroDeoxyGlucose
Benefits of FDG (2)
- Provides a more accurate staging than other imaging procedures alone
- FDG can be shipped from site with 2 hour half-life
What does SUV stand for?
Standardised Uptake Value
What does SUV do? (2)
- Useful semi-quantitive index of FDG uptake
- Monitor progress of disease during therapy
Calculation of SUV
SUV = counts / (injected dose x body weight)
What value of SUV is good?
A low SUV value
How are radionuclides produced?
- Accelerate charged particles (protons) to high energy
- Make them collide with a target e.g. H2O with 18O
- Nuclear rection sinside target
- 18O plus a proton = 18F plus a neutron
- Original 18O becomes 18F- / [18O]H2O
- 18F has too much +’ve charge so emits e+
- Extract radionuclides (18F) from target mixture
What equipment is used to accelerate a charged particle?
Cyclotron
How much energy do charged particles need to have?
10MeV
Which two fields does a cyclotron use to accelerate a charged particle and their uses
- Magnetic - causes particles to follow circular path
- Electric - causes acceleration
Cyclotron Operation (4)
- D’s connected to AC
- Ions released into gap between D’s
- Electric discharge in gas
- Accelerated right into D by E-field
- No E-field in D
- Steered back by M-field
- Emerge into Dee as e-field switches direction
Two types of Cyclotron Developments (2)
- Negative Ion technology
- Targets
What is Negative Ion technology? (2)
- H- ions accelerated to 10-50MeV
- Pass through carbon foils that strips electrons to produce H+ (protons) beam to collide with target
How does the proton beam & H- ion beam diverge?
Through magnetic field depending on the sign of the ion
What is the target made out of?
A stable target nucleus
Example of FDG production (3)
- Irradiate [18O]H2O liquid –> 18F/[18O]H2O mixture washing down transfer line to hot cell
- Can be up to 10-20 metres
- Approx takes 20mins for new equipment
What does LOR stand for?
Line Of Response
What does a PET scanner do? (3)
- Patient surrounded by full/partial ring of detectors
- Two photons detected in coincidence (LOR)
- Want to maximise A, minimise B
PET Camera Design (2)
- Ring of detectors around patient
- BGO - good stopping power, long decay time
- Each ring separated by septa 2D mode
What is the resolution of a PET system determined by?
Detector size
Sensitivity for a PET camera @ low count rates is determined by (5)
- Geometry
- Total volume of detector material
- Stopping power of detectors
- Characteristics of septa
- Low Compton Scatter
What do the Septa on a PET scanner do?
Minimise scatter
What does the removal of Septa allow?
Take account of 3D imaging by allowing more scattering which allows more counts
Factors affecting PET camera performance (5)
- 2D or 3D
- Geometrical design
- Choice of scintillator
- Electronics
- Attenuation Correction
- Scatter Correction
What does IQ stand for?
Image Quality
Benefits of 3D PET scanning (3)
- Improved IQ
- Shorter scan for same IQ
- Reduce dose
Disadvantage of 3D over 2D scanning
More scatter but can be corrected
How does scatter correction work in 3D PET scanning? (3)
- Energy window based
- Convolution / deconvolution based
- Direct estimate of scatter (through Monte-Carlo methods), however these are computationally intensive
What factor affects the geometry of PET scanner?
Whether or not the axis is aligned
What is a scintillator?
A material that fluoresces when struck by a charged particle or high-energy photon
What are required of Scintillators in the new generation of 3D scanners? (2)
- Faster coincidence detection
- Ideal Scintillators
What makes up an ideal scintillator? (6)
- Short decay time
- High light output
- High atomic number
- Rugged
- Low cost
- High Availability
The reduction in detected photons is only dependent on…
… the total attenuation path, which is independent of the source.
What type of correction is used to find out the attenuation co-efficient along each LOR?
Transmission attenuation
What angles are the LORs for 2D & 3D?
- 2D - LORs nearly all the same angle
- 3D - LORs all different angles
What is determines the resolution of State of the Art PET systems?
The size of individual crystals
What do new LSO (Lutetium Oxyorthoscillate) crystals have over older formations?
A higher light output
What sets the theoretical resolution limit?
Positron range in tissue ~ 1mm
Depth of interaction problem (2)
- Causes parallax error or radial elongation
- Effect worse for small-ring systems
What does TOF stand for?
Time-of-Flight
Why is TOF useful? (2)
- Calculate the difference in time travelled by photos
- Use timing information to restrict the LOR
What does TOF info improve? (2)
SNR
- Reduces noise, therefore the ‘noise equivalent count rate’ (NEC) increases
- Also known as sensitivity increase
Reduces overlapping of LORs
- Overlapping causes blurring & noise
- As patient size increases, # overlapping LORs within patient increases
What does SPECT stand for?
Single Photon Emission Computed Tomography
Advantages of PET over SPECT (2)
-
Sensitivity
- 2D PET 0.5% of all photon pairs
- 3D PET 3 -5%
- Gamma cameras 0.01 - 0.02% per detector head
-
Radio-pharmaceuticals
- FDG is very flexible
- Generally easier to label with positron emitters
Disadvantages of PET (4)
- Logistically challenging due to short tracer half-life
- 11C - 20.4mins & 18F - 109.5 mins
- High capital cost due to large volume of detector material
- Extrememly dense detectors required due to high energy emissions
- High radiation doses to both patients (particulate) & staff (high energy)
Advantages of SPECT (6)
- Half-life better suited for logistics
- Require less dense scintillators
- Lower radiation dose to patients
- Better spatial resolution than PET
- Less signal loss due to attenuation & compton scattering
- Single photon detected
- Radiopharmaceutical production simpler
Gamma Camera: Design & Construction (5)
- Collimator
- Scintillation Detector Crystal
- Photomultiplier Tubes
- Positron/Eneergy Circuit
- Computer
What is a Collimator? (3)
- A device that narrows a beam or particles or waves.
- Cuts out any photons outside of a narrow window angle.
- Resolution varies with distance
Gamma Camera Inner workings (6)
- Ray parallel to hole, detected
- Penetrates collimator, detected
- Ray trapped by collimator
- Ray scatters once in patient, then detected
- Ray Scatters, not detected
- Multiple scatter, then detected
How does a Gamma Camera Work? (4)
- Collimator lets some photons through
- Crystal converts this to light, photomultiplier tubes to electrical pulses
- Locations determined by electronics
- Image is always square
What provides a better resolution for a collimator?
Smaller/longer holes
What is Voxel-based Morphometry (VBM) & method?
A neuro-imaging technique that allows investigation of the brain
- Take patient scan
- Smooth images
- Compare patient scan to database
