PET and SPECT Flashcards
What do PET and SPECT stand for?
PET = Positron Emission Tomography
SPECT = Single Photon Emission Computed Tomography
Describe main physical principles, i.e. how PET/SPECT images are generated.
Physical Principles:
PET:
- Radioactive tracer is injected into the subject
- Tracers then emit positron
- Positron collide with electron (total annihilation)
- 2 gamma rays in opposite directions are emitted
- Detectors capture these rays and calculate even location
- Computer reconstructs projections to form PET image
* Different reconstruction algorithms are used such as ART, FBP, SART, LM…
SPECT:
- Different (than PET) tracers are injected
- These tracers (unlike PET tracers) emit gamma-rays directly
- Gamma rays are captured by detector
- Projections are then reconstructed by computer
Compare PET vs SPECT with respect to cost (money), image resolution, equipment and tracers.
Which are the main clinical applications for PET?
- Main clinical applications is whole body oncology.
- Neuorology (dementia) is emerging.
Is it commonplace to use only a PET?
All new scanners are PET/CT (i.e. combine PET and CT in one system).
Structures with high density appear bright – CT is very good at depicting bone.
PET gives functional information (metabolism, perfusion etc.).
Which are the main clinical application for SPECT?
Main clinical applications:
- Cardiology
- Neuorology
- Oncology
SPECT gives functional information
What is a tracer?
A radioactive tracer is a chemical compound in which one or more atoms have been replaced by a radioisotope. It is applied in minimal amounts, therefore, it has no pharmacologic effect in vivo. Different physiological or pathological processes in the body can be imaged by having suitable tracers.
Which is the most commonly used PET tracer?
FDG (the most commonly used PET tracer)
What is needed for a PET scan?
Cyclotron + clean room with hot cells are needed
Explain Annihilation
A positron is emitted during radioactive decay. It will collide with an electron (antiparticle) resulting in an annihilation where two gamma rays are sent out in opposite directions
What does simultaneous detection means? What is LOR?
“Simultaneous” detection of two photons means that an event occurred along the line connecting the two detectors. This line is called line of response (LOR)
Both PET and SPECT detects photon. What are the differences?
As for PET, photons are detected. The difference is that it is single photons and a collimator is used to determine projection angle
Which are the error sources for PET?
- Random (accidental) coincidence
- Important parameter: coincidence window (which depends on scintillator material)
- Scatter - gammas change direction AND lose energy
- Important parameter: energy resolution of the detector
Name the three types of coincidences related to PET.
- True coincidence is the simultaneous detection of the two emissions resulting from a single decay event.
- Scatter coincidence is when one or both photons from a single event are scattered and both are detected.
- Random coincidence is the simultaneous detection of emission from more than one decay event
Explain Attenuation Correction.
Attenuation corrects for the diminishing levels of the signal coming from deep-seated tissues. Photons coming e.g., from the center of the brain, must pass through more tissue than those from the cortex. As they travel, some are absorbed, leading to an apparent loss of activity. To correct for this effect, a short transmission scan (about 10 minutes) is taken where the patient remains in the scanner with an external source of radiation, stronger than the injected activity if the scanner is PET only. For PET/CT scanners, the CT is used for attenuation correction.
Explain Time-of-flight (TOF).
Conventional PET detects coincidence photons and records individual lines of response (LOR) between the crystals. The actual location where the annihilation occurred along the LOR is not measured.
Time of Flight (TOF) measures the actual time difference between the detection of each coincidence photon. This timing information is used to better localize the event along each LOR.
What happens to the tracer when it enters the body?
Explain the data collection during dynamic PET-scanning.
Data is collected in separate time windows, frames, in order to study what happens with the tracer concentration in the tissue over time giving the data its temporal component
Detailed description of tracer [18F]FDG
The irreversible tracer [18F]FDG
- 2-[18F]-Fluoro-2-deoxy-D-glucose - a glucose analogue • Labelled with 18F (half life 110 min)
- Most commonly used PET tracer, commercially available and distributed to PET centers without cyclotron and lab capacity
- Used to estimate the metabolic rate of glucose in a tissue (MRglu)
- MRglu is increased in tumor tissue, decreased in neurodegeneration
