1. 1. Positron emission tomography
2. 2. History late 1950s, David E. Kuhl, Luke Chapman and Roy Edwards-They introduced the concept of emission and transmission tomography
3. 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. 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. 5. PET-Whole body scan
6. 6. PET-How it works
7. 7. PET-Example Video
8. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 21. Important Detector Properties - Spatial resolution - Directly controls spatial resolution in reconstructed image - Currently ~ 1 - 5 mm - Depth-of-interaction? - Reduces parallax
22. 22. Important Detector Properties - Detection efficiency (aka sensitivity, stopping power) - Reduces noise from counting statistics - Currently > ~ 30% (singles) 55M Events1M Events
23. 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