[Cat 1] Express the radioactive decay law in mathematical terms.
[Cat 1] Perform simple calculations using the concepts of physical, biological and effective half-lives.
[Cat 1] Describe the construction and mode of operation of scintillation detectors.
[Cat 1] Describe the main features and mode of operation of a gamma camera.
[Cat 1] Describe the main features and mode of operation of a SPECT camera.
[Cat 1] Discuss the performance characteristics of SPECT & gamma cameras.
[Cat 1] Describe the physical, biological and chemical characteristics of radionuclides which are suitable for nuclear imaging.
[Cat 1] Discuss major indicators of the physical quality of SPECT images.
[Cat 1] Describe the main features and mode of operation of a PET scanner.
[Cat 1] Discuss issues that limit the performance of PET scanners.
Positron emission tomography (PET) is a functional imaging technique that uses radiotracers to visualize and measure changes in metabolic processes, blood flow, regional chemical composition, and absorption. Different tracers are used for various imaging purposes, depending on the target process within the body. 18F-FDG is commonly used to detect cancer, NaF-F18 is widely used for detecting bone formation, and oxygen-15 is used to measure blood flow.
Scintillation detectors are most commonly made up of bismuth germanate or lutetium yttrium orthosilicate (LYSO), which have a high detection efficiency and are able to absorb and convert photons with 511 keV into light.
Ideal Radionuclides
- Decay chain
- Proportion of decay events that result in this form of radiation being emitted
- daughter products should be stable
- Type/energy of emission
- want to image not treat (gamma, not alpha or beta)
- Single gamma energy preferrable with no higher b (so no down scatter)
- detection efficiency
- transmission through imaged object (patient)
- Half-life
- still present in the area of uptake, after background has been excreted
- permit transport of reconstituted radiopharmaceuticals
- Chemical
- must able to be produced in pure
- stable
Principle of Operation
A radiotracer produced in a cyclotron is injected intravenously into the patient
The unstable proton-rich radionuclide emits a positrons via beta + decay It accumulates
When the positron combines with an electron it is an event call annihilation
Following the collision of a positron with an electron, two gamma photons
(511 keV) are produced, which travel in opposite directions. The simultaneous
detection of these photons along the line of response will lead to the registration of
the event.
Coincidence detection of pairs of 511 keV photons by 3D ring of pet detectors determines a unique detection line (LOR: line of response) which is used to forming tomographic images with PET
In PET imaging, two gamma rays emitted from a positron-electron annihilation event travel in opposite directions to one another are detected by a ring of detectors. A line of response is then calculated from photons detected in coincidence.
If the measured line of response places the annihilation event along an artefactual projection, the coincidence is rejected. This is how collimation is achieved in PET imaging.
Coincidence timing window: 1 – 20 seconds
Iteractive reconstruction
Ideal characteristics include:
time resolution <2ns
Energy resolution <14%
Dead time as low as possible <200ns
Small pixels 2-3mm
High light output scintillator, high density
Factors affecting image quality
Contrast
- tracer uptake (lesion to background)
- resolution, motion (PV effectS)
- background (scatter and randoms)
Noise
- Administered activity, acquisition time
- sensitivity
- count rate performance
- corrections for attenuation scatter and randoms
Artifacts
- streaks
- attenuation issues
Spatial resolution
- size and design of detectors contributed greatest lost in resolution
Intrinsic properties affecting resolution
- Annihilation range
- Positron annihilates at a distance away from where it was emitted 18F has a short range hence its favourability
SPECT
Data acquisition
- Count photons detected in a particular part of the detector that meet energy criteria
- The counts are deemed to have originate somewhere along the line or response for that part of the detector
- The counts are the sum from the entire line of response
- Detector positoined opposite
- Non circulate rotation
- Set energy window (140kev +/- 10%
- images for 20s
- rotate 3 deg
- reconstruct
It produces a sinogram as the detector is rotated around the object.
Reconstruction
Backprojection
- Counts equally allocated to pixels along the LOR
FBP
- Filters the acquired data with ramp filter – removes the (1/r) blur effect
Iterative reconstruction
Improve signal by
- increase administer activity
- reduce matrix size
- more efficient collimator
- accept the noise