Positron emission tomography

A positron is a particle of matter similar to an electron, except that it carries a positive charge. It is a form of antimatter because, when a positron comes across an electron, the two completely crush to yield energy.
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron is a type of beta particle (β+), the other beta particle being the electron (β−) emitted from the β− decay of a nucleus. The positron has an electric charge of +1 e, a spin of ½.
Positrons may be generated by positron emission radioactive decay (through weak interactions), or by pair production from a sufficiently energetic photon which is interacting with an atom in a material.
The short-lived positron emitting isotopes, 11C, 13N, 15O and 18F are used for positron emission tomography.

Positron emission tomography (PET)

Positron emission tomography (PET) involves introduction of a radioactive tracer into the human body through an intravenous injection.
A tracer is essentially a biological compound of interest labelled with a positron emitting isotope, such as 11C, 18F, and 15O. These isotopes are used because they have relatively short half-lives (minutes to less than two hours), allowing the tracers to reach equilibrium in the body, but without exposing the subjects to prolonged periods of radiation.
The emitted positron travels up to a range of a few millimetres in tissue before being beaten along with an electron from the surroundings.
This process produces two photons of equal energy (511 keV) travelling in opposite directions.
Positron emission tomography (PET) scanners create detailed three-dimensional images of metabolic activity within the human body.
They contain several rings of glittering detectors, usually made of bismuth germanate (BGO). These are converted into a tomographic image using standard reconstruction software.

Applications of PET

PET is an important research tool to map normal human brain and heart function, movement disorders such as parkinsonian syndromes, epilepsy, brain tumours, dementia, stroke and neuronal plasticity.
PET has also been widely used to study hyperkinetic movement disorders
Potential future applications include:
⦁ early diagnosis of brain metastasis; distinguishing local recurrences from radiotherapy induced changes; and detecting malignant transformation of low grade tumours
⦁ preoperative localization of seizure foci in potential candidates for epilepsy surgery, especially in those with equivocal MRI findings
⦁ as an adjunct to clinical diagnosis in atypical cases of parkinsonian syndromes and dementia
⦁ early and presymptomatic diagnosis of individuals at risk for neurodegenerative disorders.
Positron Emission Tomography (PET) scans measure emissions from positron-emitting molecules. Because many useful, common elements have positron emitting forms (carbon, nitrogen, and oxygen), valuable functional information can be obtained.  
The PET shows molecular function and activity and therefore can differentiate between normal and abnormal (cancerous / tumor) or live versus dead tissue.
The biggest advantage of a PET scan, compared with an MRI scan or X-ray, is that it can reveal how a part of the patient's body is functioning, and medical researchers find this aspect of PET scans mainly useful.
PET scans are commonly used to investigate the following conditions of brain

• To find changes in the brain that may cause epilepsy,
• Study the brain's blood flow and metabolic activity,
• To find nervous system problems such as Parkinson's disease, multiple sclerosis, transient ischemic attack (TIA), amyotrophic lateral sclerosis (ALS), Huntington's disease, stroke, and schizophrenia.
  PET scan pictures do not show as much detail as computed tomography (CT) scans or magnetic resonance imaging (MRI) because the pictures show only the location of the tracer. The PET picture may be matched with those from a CT scan to get more detailed information about where the tracer is located.
  PET also can produce three dimensional images, and is usually used to compliment rather than replace the information obtained from CT or MRI scans.