Lecture 19: Positron Emission Tomography (PET)

Question 1

Summarise the general process of PET

Answer 1

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.

Question 2

Give 7 example of positron emitting isotopes

Answer 2

O-15
N-13
C-11
F-18
Ga-68
Cu-63
Rb-82

Question 3

What are 2 main characteristics of positron emitting isotopes?

Answer 3

• They have fewer neutrons than their respective stable isotopes
• They have relatively short half-lives

Question 4

Compare the graphs of counts per second for PET tracers with SPECT tracers

Answer 4

Question 5

Do PET or SPECT radioisotopes have a greater useful dose? Why?

Answer 5

PET: They have a shorter half-life but the same overall dose is given, meaning that more emissions occur whilst inside the scanner.

Question 6

Why does PET have an increased signal to noise ratio (SNR)?

Answer 6

• Short half-life of radioisotopes, so more useful counts are used.
• Coincidence detection.

Question 7

How are images constructed using PET?

Answer 7

Back projection reconstruction: computes the spatial distribution of annihilation events.

Question 8

Spatial resolution for PET is impacted by the ____________ of gamma rays.

Answer 8

Non-collinearity

Question 9

What is the practical separation of gamma rays after annihilation?

Answer 9

180 ± 0.25º

Question 10

What does the resolution depend on for PET?

Answer 10

The diameter of the patient, the detector sizes, and the distance travelled by the positron in tissue (~2mm).

Question 11

What is the result of a lower spatial resolution in PET?

Answer 11

A slight blurring of the image

Question 12

What is the equation for the FWHM (full width half maximum) for the blurring of the image generated by PET?

Answer 12

D = patient diameter

Question 13

What is the problem with photons being attenuated in tissue in PET?

Answer 13

The deeper sources appear to have a lower uptake of tracer.

Question 14

How can the attenuation of photons emitted in the body be corrected?

Answer 14

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.

Question 15

How can PET be used to treat cancer?

Answer 15

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.