Positron emission tomography (PET) is a nuclear medicine imaging technique that produces 3D images of functional processes in the body. A small amount of radioactive tracer is injected into the body and detected by a PET scanner. As the tracer decays it emits positrons that collide with electrons, producing pairs of gamma rays. The scanner detects these gamma rays and uses the information to construct images that show metabolic activity in the body, useful for diagnosing diseases. PET scans provide information about organ function and can be used to diagnose cancer, heart disease, brain disorders, and to monitor treatment effectiveness over time.

1. Positron emission tomography

3. History
late 1950s, David E. Kuhl, Luke Chapman and Roy Edwards-They
introduced the concept of emission and transmission tomography

4. PET-keywords
Positron- is the antiparticle or the antimatter counterpart of
the electron
Tomography- refers to imaging by sections or sectioning, through the
use of any kind of penetrating wave.
Positron emission- is a particular type of radioactive decay and a
subtype of beta decay, in which a proton inside a radionuclide nucleus
is converted into a neutron while releasing a positron and an electron
neutrino.

5. PET-definition
PET stands for Positron Emission Tomography and is an imaging
technique which uses small amounts of radiolabeled biologically
active compounds (tracers) to help in the diagnosis of disease.
It is a nuclear medical imaging technique that produces a three-
dimensional image or picture of functional processes in the body.
The tracers are introduced into the body, by either injection or
inhalation of a gas, and a PET scanner is used to produce an image
showing the distribution of the tracer in the body.

6. PET-Whole body scan

7. PET-How it works

8. PET-Example Video

9. PET-Why the Test is Performed ?
• A PET scan can reveal the size, shape, position, and some function of
organs.
• Used to check brain function
• Used to diagnose cancer, heart problems, and brain disorders
• To see how far cancer has spread
• To show areas in which there is poor blood flow to the heart
• Several PET scans may be taken over time to check how well you are
responding to treatment for cancer or another illness.

10. PET-How to Prepare for the Test ?
• You may be asked not to eat anything for 4 - 6 hours before the scan.
You will be able to drink water.
PET-How the Test is Performed ?
• A PET scan uses a small amount of radioactive material (tracer).
The tracer is given through a vein (IV), most often on the inside of
your elbow.
• The tracer travels through your blood and collects in organs and
tissues. This helps the radiologist see certain areas of
concern more clearly.

11. • You will need to wait nearby as the tracer is absorbed by your body.
This takes about 1 hour.
• Then, you will lie on a narrow table that slides into a large tunnel-
shaped scanner. The PET picks up detects signals from the tracer. A
computer changes the signals into 3-D pictures. The images are
displayed on a monitor for your doctor to read.
• You must lie still during test. Too much movement can blur images
and cause errors.
• How long the test takes depends on what part of the body is being
scanned.

12. PET-How the Test Will Feel ?
• You may feel a sharp sting when the needle with the tracer is placed
into your vein.
• A PET scan causes no pain. The table may be hard or cold, but you can
request a blanket or pillow.
• An intercom in the room allows you to speak to someone at any time.
• There is no recovery time, unless you were given a medicine to relax

13. PET-Risks
• The amount of radiation used in a PET scan about the same amount
as for most CT scans.
• Short-lived tracers are used so the radiation is gone from your body in
about 2-10 hours.
• Tell your doctor before having this test if you are pregnant or breast
feeding. Infants and babies developing in the are more sensitive to
radiation because their organs are still growing.

14. Radiotracers
•Injected into the body
•Bonded to a radioactive atom called an isotope
•Consists of two components:
1. Pharmaceutical label– determines where the
tracer goes in the body and how it behaves.
2. Radioactive label– when attached to the pharmaceutical label,
the signal measured by the PET decays and emits positrons

15. Positrons
• The positrons interact with the patient’s tissues, gradually losing energy and slowing down
until their speed is low enough that they can be captured by an electron. The electron-
positron pair combines to form a transitory molecule called positronium.
• Positronium is very unstable and exists only for approximately 10^-10 seconds before the
positron and the electron mutually annihilate, generating two gamma rays (annihilation
photons)
• Each annihilation photon has exactly 511 keV
• the two photons travel away from the site of annihilation in almost exactly opposite
directions.

17. Radioisotopes
• The most common radioisotopes used in PET are F-18, C-
11, N-13, O-15, and Rb-82.
• All of these tracers have fairly short half-lives, ranging
from just more than a minute to just less than 2 hours.

19. Positron range
• refers to the distance that the positron travels before it slows
down enough to annihilate with an electron
• dependent on the kinetic energy of the positron
• The maximum kinetic energy depends on the radioisotope

20. PET Detectors
• The PET camera records positron decay events by
detecting the two annihilation photons that are
emitted. Both photons must be detected before an
event is recorded. To distinguish between annihilation
photons and photons detected from background
sources, the camera accepts only those photons that
arrive at close to the same time
Coincidence Detection

21. • The maximum amount of time apart that two photons can be
detected and still be considered to have come from the same
annihilation is determined by the coincidence timing window.
• The coincidence window is typically 5 to 10 ns and takes into account
the time the photons take to travel to the detector from the site of
annihilation and the variability in the time required to measure the
photon뭩 time of arrival.

22. • The path between 2 detectors is referred to as a line of response
(LOR).
• The simultaneous detection of 2 photons is referred to as a
“coincidence”

23. Scintillation Crystals
• Used for detecting annihilation photons
• The 511-keV gamma ray interacts with the crystal, exciting many
of the electrons in the crystal into a higher-energy state.
• As the electrons fall back to their ground state, they emit a photon of
visible or near-ultraviolet light. There are many electrons excited by
each gamma ray and so each gamma ray generates a shower of light
photons.
• The photon shower is detected by a photomultiplier tube (PMT),
which converts the light into an electrical signal and amplifies it. The
amplified electrical signal can then be processed and recorded in a
computer.

25. Important Detector Properties
- Spatial resolution
- Directly controls spatial resolution in reconstructed image
- Currently ~ 1 - 5 mm
- Depth-of-interaction?
- Reduces “parallax”

26. Important Detector Properties
- Detection efficiency (aka sensitivity, stopping power)
- Reduces noise from counting statistics
- Currently > ~ 30% (singles)
55M Events1M Events

27. Important Detector Properties
Random (accidental) coincidence
- Time resolution
- Affects acceptance of random coincidences
- Currently ~ 1 - 10 ns
- Time-of-flight (TOF)?
- c = ~ 1 ft/ns
- Need << 1 ns resolution

28. Important Detector Properties
Scatter and Attenuation
511 keV
- Energy resolution
- Scattered gammas change direction AND lose energy
- Affects acceptance of scattered coincidences
- Currently ~ 20%
- Deadtime
- Handle MHz count rates!
511 keV 400 keV

29. Scintillation
Crystal
PMT
Pre-Amplifier
+ Electronics
Gamma photon converts to
optical photons
(proportional to gamma
energy, typ. 1000’s)
photons are collected at the
end of the crystal
light is converted to an electrical signal & amplified
Front-end electronics condition
the signal for further
processing
Prototypical PET Detector
Gamma Ray
Optical reflector

30. New Developments
• Detectors
• Multimodality imaging
• Specialized applications

31. OPERATION
Positron Emission
Tomography
of
the

32. Steps of a PET Scan
Before
1) Radionuclide generation
2) Radiochemistry
During
3) Injection
4) Detection
After
5) Image construction
6) Examination of PET Images

33. Before
PET Scan
the

34. Fluorodeoxyglucose ( 𝟏𝟖
𝐅 − 𝐅𝐃𝐆)
• Standard radioactive tracer used for PET neuroimaging and
cancer patient management.
Positron Emission of Fluorine-18
𝟗
𝟏𝟖
𝑭 𝟖
𝟏𝟖
𝑶 + 𝒆+
• Fluorine-18 – radioactive isotope of Fluorine
• 109.7 mins – half-life of Fluorine-18
• 8-25 mSv – general dose of radiation from a PET scan

35. During
PET Scan
the

36. • Scintillator crystals –
convert gamma
radiation into light
• Photomultipliers –
convert light into
current
• ~2mm – max. travel dist.
of a positron

37. Coincidence detection events:
• Scattered coincidence – one or both photons undergo Compton scattering event
• Random coincidence – two photons (not from the same annihilation) are detected
• True coincidence – both photons are detected properly

38. • Line of response (LOR) – straight line of coincidence of two 511keV
gamma photons at almost 180 degrees with each other
• Annihilation – collision of a positron and an electron to produce
gamma ray

39. Time-of-flight PET

40. Block Diagram of a Positron Emission Tomography System

41. After
PET Scan
the

42. Examination of PET Images

43. Applications
Diagnosis of:
• Hodgkin’s disease
• Non-Hodgkin’s lymphoma
• Colorectal cancer
• Breast cancer
• Melanoma
• Lung cancer

44. References
• http://www.nlm.nih.gov/medlineplus/ency/article/
003827.htm
• http://www.nhs.uk/Conditions/PETscan/Pages/Intro
duction.aspx
• http://en.wikipedia.org/wiki/Positron_emission_to
mography