PET SCAN

By sreeremya.sAsst professor,sree narayana guru

college,cbe

Positron emission tomography (PET), as its nameimplies, is a three-dimensional (3D) medicalimaging technology based on the detection ofpositrons. The positron is an elementary particlethat is identical to an electron but possessing apositive charge.

• PET RADIOTRACERS• PET images are based on the detection of a tracer• that is typically injected into the body. By comparing• the distribution of the tracer in a patient to• normal templates, a physician is able to evaluate• how well different organs and systems in the body• are functioning. The tracer consists of two

components:• a pharmaceutical and a radioactive label.

• The radiopharmaceutical is given in very low concentrations,

• usually on the order of nano- to picomolar• (10−9–0−12 mol/L), so that there is typically• no pharmacological effect; that is, the tracer• does not disturb the physiology that it is measuring.• The pharmaceutical component determines• where the tracer goes in the body and how it

behaves.

• PET DETECTORS• Coincidence Detection• 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—this• is referred to as coincidence detection

• Block Detector• Unlike the gamma camera, which uses a single• large scintillation crystal, the detector in a PET• camera is made up of a large number of small distinct• crystals. Each crystal element is a few millimeters• wide (typically 4–6 mm) and 2 to 3 cm in• length. The crystals are then coupled to a PMT array• that collects and amplifies the signal from the• crystals.

• TYPES OF EVENTS• The signals collected with a PET camera can be• categorized into one of several types.• A single refers to the detection of a photon• whose measured energy falls within the photopeak• energy window. The photopeak energy window• is wide compared to single-photon imaging,• typically 400 to 600 keV. The width is caused• by the generally poorer energy resolution of• the PET scintillation crystals, the higher energy of• the photons detected, and a desire to maximize the• sensitivity of the system.