PET Scan | Summary

PET (Positron Emission Tomography) scan is used to detect cancer and cardiovascular or neurological abnormalities.
Show whether a tumor is benign or malignant.
Help doctors determine the stage of disease.
See if the cancer is metastasizing.
Help doctors evaluate the effectiveness of chemotherapy.
Help do differentiate Alzheimer’s from other forms of dementia such as Pick’s disease.
Test blood flow and cardiovascular disease.
Description of instrument:
Tracer: a radioisotope with short half live is connected with glucose; there is a list of some of them:
Labeling agent
Half-life
carbon-11
20.3 minutes
oxygen-15
2.03 minutes
fluorine-18
109.8 minutes
bromine-75
98.0 minutes




Gantry: compose of several rings of radiation detectors. When a photon get inside the detector a pulse of light is emitted, it gets to the photocathode and amplified by the photomultiplier and from there to the computer.
The computer: reconstructs the exact places where each pulse of radiation came from. It also counts the number of pulses per second coming from each point of the image.
Reconstruction of the image: The number of radiation pulses counted by the computer during a fixed interval of time is displayed in the screen as a dot, with its intensity shown in shades of gray. Black means no activity, and pure white the highest count level. The same image can be displayed in false color, which is able to show in a better contrast the "hot" regions. The false color scale converts each level of gray into a shade of color, like in a rainbow.
The computer shows an image which is like a cross section.
Taking several adjacent slices at a time, a special computer program can be used to make a three-dimensional reconstruction of the brain.





Scientific principle:
A positron emitter is connected to glucose and injected to the body after few minutes of circulation in the blood it gets into the organ that is examined, if there is cancer there, it will be lots of metabolism in the area and lots of sugar will accumulate there, and more photons will be emitted from there.
When a positron meets an electron, the collision produces two gamma rays having the same energy, but going in opposite directions (1800). The gamma rays leave the patient’s body and are detected by the PET scanner.
Only photons that were detected by two detectors that are 1800 (±30) in location from each other and in the same time are computed.

Operation:
The patient will be injected with an appropriate dose of radiotracer. After few minutes the technologist will position you on a "bed" (also called a "couch") that is attached to the PET scanner. The scanner itself looks like a large box with a central, round opening. Gamma ray detectors are located all around the opening. The bed will slide into the opening, and the detectors will record the gamma rays being released from the body from your head to your feet depending on the type of the examination. The test is taken about 45 minutes, the patient must not move so the picture will not be ruined.
After the scan is complete, the patient is able to resume normal daily activities. There will be no restrictions on eating or drinking.

Safety features:
· Because the radioactivity is very short-lived, your radiation exposure is extremely low. The substance amount is so small that it does not affect the normal processes of the body.
· The test may not be appropriate for pregnant women or those who are breastfeeding.
· If you are breastfeeding, you should not nurse your baby for approximately 36 hours after the radiotracer injection, since radiation can be passed through the breast milk.
· The radiation injected during a nuclear medicine study is eliminated from you body through the kidneys. For that reason, the patient should drink plenty of fluids and urinate frequently after the examination.



http://www.brighamandwomens.org/nuclearmedicine/Patient/PETScan.asp
http://www.triumf.ca/welcome/petscan.html
http://www.radiologyinfo.org/content/petomography.htm
http://www.epub.org.br/cm/n01/pet/petworks.htm