PET Imaging

Question 1

What is PET?

Answer 1

Positron emission tomography

== a type of nuclear medicine procedure that measures metabolic activity of the cells of body tissues

Question 2

What is NMR imaging?

Answer 2

Nuclear magnetic resonance imaging

Question 3

What is nuclear medicine?

Answer 3

The use of radioactive tracers (radiopharmaceuticals) to obtain diagnostic information or perform targeted radiotherapy

Question 4

What is a radiopharmaceutical?

Answer 4

A compound or drug which targets the organ or structure of interest

Pharmaceutical (traces physiology/localises in organs of interest)
+ (by biochemical bonding)
Radioactive nuclide (emits radiation)

Question 5

What is a radionuclide?

Answer 5

An unstable form of a chemical element that releases radiation as it breaks down and becomes more stable

The bit emitting the radiation that allows us to see where the drug has gone (for diagnosis) or emits the radiation which kills the cells we’re trying to treat

Question 6

What is the common radionuclide used in PET?

Answer 6

Fluorodeoxyglucose (FDG)
Radioactive atom combined with glucose

Glucose analogue - follows the same metabolic pathway as glucose until inside the cell at which point metabolism stops and FDG is trapped

Activity in the cell builds up in proportion to rate of glucose metabolism

Question 7

What is the structure of FDG?

Answer 7

A radioisotope which is an analog to glucose with the positron-emitting radionuclide fluorine-18 substituted for the normal hydroxyl group

Question 8

How is FDG used to detect tumours?

Answer 8

FDG is a modified form of glucose which can be absorbed by tissues

Cancerous tissues have high rates of metabolism and therefore will uptake lots of glucose

Once the glucose has entered the cancerous tissue, positrons will be emitted. They then collide with free electrons in annihilation near the decay event and give off gamma rays which are detected by the scanner indicating the location of the tumour

Question 9

What determines the distribution of the radiopharmaceutical?

Answer 9

Now whether this radiopharmaceutical goes to the bones, a specific tumour type, the lungs or brain obviously depends on what the drug or compound exactly is

Question 10

What is a positron?

Answer 10

Positively-charged electron

Question 11

What is the theory of PET?

Answer 11

Works by using a scanning/detection device to detect photons emitted by a radionuclide in the organ or tissue being imaged

Radionuclides used in PET scans are made by attaching a radioactive atom to chemical substances that are used naturally by the organ during metabolic processes

Positrons are emitted by the breakdown of the radionuclide as they are unstable

Gamma rays called annihilation photons are created when positrons collide with electrons near the decay event

The scanner then detects the annihilation photons, which arrive at the detectors in coincidence at 180 degrees apart from one another

Question 12

What is the line of coincidence?

Answer 12

PET is based on the detection, in coincidence, of 2 photons created upon annihilation of a positron

The line of coincidence is the 180 degrees plane where the 2 photons hit either side of the detector

Question 13

What is a true event?

Answer 13

A true event is recorded when the 2 annihilation photons are detected within a time coincidence window
i.e., the 2 photons hit either side of the detector at the same time

Question 14

What is a random event?

Answer 14

Within this scheme it is unavoidable that two uncorrelated photons might be detected sufficiently close in time to be mistakenly identified as a coincidence

This constitutes an accidental coincidence, also called random coincidence (or just random)

Question 15

What is the basis of PET imaging?

Answer 15

The basis behind PET imaging is being able to detect these simultaneous gamma rays

Question 16

What is the reconstruction of PET images based on?

Answer 16

Based on knowing that the annihilation happened somewhere along a line joining these 2 detection points
This is the sort of high-quality, high-resolution whole body image we can get with PET

Question 17

What is the structure of a PET scanner?

Answer 17

Based on a ring of independent scintillation detectors, all connected to each other via coincidence circuitry so that we can simultaneously detect the 2 back-to-back gammas produced when the positrons annihilate

Question 18

What are the type of images that can be produced with PET?

Answer 18

Static or dynamic
Dynamic = series of images
Static = dynamic images combined and smoothed

Question 19

What are recent/future advances of PET scanning?

Answer 19

Bigger FOV (field of view) (multi-ring) = means more of the body can be imaged at once which massively reduces imaging time

Design of detector block = a lot of work being done in terms of the design of the detector block
i.e., configuration of the individual scintillation crystal/PM tube units - design of these is critical in terms of the spatial resolution you can acheive

Question 20

What is FOV?

Answer 20

Field of view

Question 21

What is a problem with PET imaging?

Answer 21

Attenuation - loss of signal due to absorption or scatter of photons in tissue
Photons don’t leave the body without any sort of interaction

Question 22

How do we correct for attenuation?

Answer 22

X-rays from a CT scan are used to construct an attenuation map of density differences throughout the body that can then be used to correct for the absorption of the photons emitted from FDG decay

Compensates for the effect of signal loss due to scatter and absorption

Essential for quantitation
Vast majority of PET scanners are PET-CT –> attenuation correction is straightforward

PET-MR needs to use convoluted methods - is more challenging

Question 23

What are the PET acquisition parameters?

Answer 23

1. Required axial field of view (e.g., half-body, brain etc)
2. Scan duration per bed position
3. Bed overlap in subsequent bed positions

Question 24

What is a cyclotron?

Answer 24

A machine used to make short-lived radioactive isotopes that can be used for medical imaging or research

Question 25

What are the common isotopes used in PET and what are their half life?

Answer 25

15O - 2 min
13N - 10 min
11C - 20 min
18F - 110 min

Question 26

Why is fluorine (18F) the most used radionuclide in PET?

Answer 26

It has a half life of 110 min which is the longest out of the common isotopes in PET and therefore the signal isn’t reduced as quickly as other isotopes

Only one of the common isotopes that can feasibly be produced at a site remote from the PET scanner

Question 27

What has caused the recent explosion in PET imaging?

Answer 27

The introduction of FDG, an excellent tracer based on F-18 radionuclide caused the recent explosion

Question 28

How does a cyclotron produce PET isotopes?

Answer 28

The positron emitting nuclides used in PET are all cyclotron produced

A beam of positively charged particles are blasted into a stable target material to produce these proton-weighted nuclides which will tend to decay via positron emission so as to even up the balance of protons and neutrons in the nucleus

Question 29

What is a problem with cyclotron production?

Answer 29

They are very very expensive and very few cyclotrons

Question 30

How are tracers sourced without a cyclotron?

Answer 30

Commercially available tracers found at Alliance Medical and PETNET (commercial suppliers)

FDG is the most commonly used PET tracer supplied by commercial suppliers

Question 31

What are clinical uses of FDG?

Answer 31

Clinically can help with differential diagnosis of AD, epilepsy localisation, tumour characterisation

Can be used to map patterns of glucose metabolism for normal/diseased brain functions

Question 32

What other PET tracer can be useful for the diagnosis of AD?

Answer 32

Amyloid e.g., F18-florbetaben
Binds to beta amyloid plaques in the brain
Clinically can help with differential diagnosis of AD
In research can be used e.g., to assess the impact of anti-beta amyloid drugs