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1. Positron emission tomography
2. History late 1950s, David E. Kuhl, Luke Chapman and Roy
Edwards-They introduced the concept of emission and transmission
tomography
3. 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.
4. 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.
5. PET-Whole body scan
6. PET-How it works
7. PET-Example Video
8. 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.
9. 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.
10. 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.
11. 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
12. 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.
13. 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
14. Positrons The positrons interact with the patients 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.
15. 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.
16. 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
17. 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
18. 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 photon time of arrival.
19. 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
20. 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.
21. Important Detector Properties - Spatial resolution -
Directly controls spatial resolution in reconstructed image -
Currently ~ 1 - 5 mm - Depth-of-interaction? - Reduces
parallax
22. Important Detector Properties - Detection efficiency (aka
sensitivity, stopping power) - Reduces noise from counting
statistics - Currently > ~ 30% (singles) 55M Events1M
Events
23. 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