Lecture 19: Positron Emission Tomography (PET) Flashcards
Summarise the general process of PET
1) A biologically active molecule labelled with a positron emitter is injected.
2) The emitted positron will then travel a short distance in tissue before annihilating with an electron.
3) This generates two back-to-back photons which are detected.
Give 7 example of positron emitting isotopes
O-15
N-13
C-11
F-18
Ga-68
Cu-63
Rb-82
What are 2 main characteristics of positron emitting isotopes?
- They have fewer neutrons than their respective stable isotopes
- They have relatively short half-lives
Compare the graphs of counts per second for PET tracers with SPECT tracers
Do PET or SPECT radioisotopes have a greater useful dose? Why?
PET: They have a shorter half-life but the same overall dose is given, meaning that more emissions occur whilst inside the scanner.
Why does PET have an increased signal to noise ratio (SNR)?
- Short half-life of radioisotopes, so more useful counts are used.
- Coincidence detection.
How are images constructed using PET?
Back projection reconstruction: computes the spatial distribution of annihilation events.
Spatial resolution for PET is impacted by the ____________ of gamma rays.
Non-collinearity
What is the practical separation of gamma rays after annihilation?
180 ± 0.25º
What does the resolution depend on for PET?
The diameter of the patient, the detector sizes, and the distance travelled by the positron in tissue (~2mm).
What is the result of a lower spatial resolution in PET?
A slight blurring of the image
What is the equation for the FWHM (full width half maximum) for the blurring of the image generated by PET?
D = patient diameter
What is the problem with photons being attenuated in tissue in PET?
The deeper sources appear to have a lower uptake of tracer.
How can the attenuation of photons emitted in the body be corrected?
The probability of detecting both photons is the same as the probability of detecting a single photon that travels through the body on the same path. This means that attenuation is only dependent on µ and path length so can be corrected for by determining µ at each position. A PET-CT scanner is required for this.
How can PET be used to treat cancer?
Fluorodeoxyglucose (FDG) is an analog of glucose that contains a radiotracer. It can be given to a patient and will be uptaken primarily by cancerous cells. This method works because glucose is linked to metabolism and cancers have high metabolism.
