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P=Positron E= Emission T=Tomography

Positron emission or beta plus decay (+ ) is a particular type of radioactive decay, in which a proton inside a radionuclide nucleus is converted into a neutrino while releasing a positron.

Tomography is derived from two Greek words, tome ( cut ) or tomos ( part or section ) and graphein ( to write ). Tomography refers to imaging by sections through use of any kind of penetrating waves.

A positron emission tomography is a nuclear medical imaging technique which produces an image of functional processes in the body. Radioactive glucose

PET is based on the annihilation coincidence detection of the two collinear 511- kev -rays resulting from the mutual annihilation of a positron and a negatron, its antiparticle.

Patient injected with drug having radioactive isotope that emits positron (+ ). Radioactive isotopes are chemically incorporated into metabolically active molecule. Isotopes decays, emitting positron (+ ). Positron (+ ) annihilates with electron from tissue, forming back to back 511 kev photon pair.

Positron lies on line defined by detector pair (a chord). Reconstruction of 2D map.

Some of the positive beta emitters prepared in cyclotron are: 1. Carbon (C11), half life(20.3min). 2. Nitrogen (N13), half life (9.98min). 3. Oxygen (O15), half life( 2.05min). 4. Fluorine (F18), half life (110min ).

PET scan machine consists of: Positron camera a) Detectors ring (cylindrical, hexagon) b) Detectors block each composed of i. Scintillator crystal ii. Photomultiplier tubes (PMT s) Approx. 24-32 rings of detector crystals Hundred of crystals/ring

A positron camera comprises a ring, surrounding the patient. To conduct a scan, a radioactive tracer is injected into the blood circulation. There is a time period while the active molecule becomes concentrated in the tissues. The radioisotope undergoes positron emission decay. After decaying, it interacts with the electron, producing a pair of ɣ- photons (511keV).

The detector pair of PET are installed in ring like pattern, which allow measurement of radioactivity. The camera is composed of scintillators & photomultiplier tubes coupled to bismuth germinate detectors. The annihilation photons enters the detectors. They produce coincident pulses. These photons are then detected in the scintillator creating a burst of light which is detected by photomultiplier tube.

The detectors measures the sum of activity. The coincident events b/w rings of camera generate the cross-data which minimizes gap b/w the imaging planes.

Detect cancer. Determine effectiveness of treatment such as cancer therapy. Detect return of a cancer. Determine blood flow to heart muscles. Determine the effects of heart attack on areas of hearts. To mapnormal human brain and heart function. Evaluate brainabnormalities; tumors; memory disorder and other nervous system disoder.ss

Image information unique, high sensitivity. High spatial resolution. More précised, cheaper. Can detect disease at an early stage than CT scan or MRI.

Injection of radioisotopes may cause pain and redness. Expensive due to cyclotrons need to produce short-lived radionuclide's. Low accessibility Take time

Herman. G. T. Fundamentals of Computerized Tomography Bailey, D. L. Positron Emission Tomography A. C. Kak and M. Slaney Principles of Computerized Tomographic Imaging