positron emission tomography; physics and basic principles

Information about positron emission tomography; physics and basic principles

Published on July 8, 2010

Author: mrshami

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Positron Emission Tomography : Positron Emission Tomography Physics and Basic Principles Mujeeb ur Rahman MS Medical Physics Deptt. Of Physics and Applied Mathematics PIEAS Presentation Outlines : Presentation Outlines What is Positron Emission Tomography (PET) Basic Principle of PET Operation of PET Camera Detection Systems in PET Correction for quantitative PET Summary and Conclusion 8 July 2010 2 Mujeeb ur Rahman What is PET : What is PET PET is a nuclear imaging technology Produces a 2 or 3D image of the functional processes of the body 8 July 2010 3 Mujeeb ur Rahman Basic Principle of PET : Basic Principle of PET Positron-emitting tracer is injected into the body Radiotracers emits Positrons which travels 1-3mm Positrons collide with electrons (-) causing an “annihilation Results in the emission of two 511keV energy gamma rays at ~180 degrees 8 July 2010 4 Mujeeb ur Rahman Basic Principle of PET (Cont.) : Basic Principle of PET (Cont.) Gamma rays are detected by opposing detectors Energy window is set to ensure that each gamma is ~511 keV Coincidence time window is used for ensuring accurate localization of the tracer 8 July 2010 5 Mujeeb ur Rahman Basics Principle of PET (cont.) : Basics Principle of PET (cont.) The coincidence event is assigned to a line of response (LOR) Which give us positional information with out any physical collimator This is what is known as electronic collimation Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 6 Mujeeb ur Rahman Basic principle of PET (cont.) : Basic principle of PET (cont.) Types of Coincidence Events True events An event that results from single annihilation and the two photons are detected in opposing detectors in coincidence Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 7 Mujeeb ur Rahman Types of Coincidence Events (cont.) : Types of Coincidence Events (cont.) Scattered Events Events that arises when one or both photons of the same annihilation undergone scattering prior to detection in coincidence Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 8 Mujeeb ur Rahman Types of Coincidence Events (cont.) : Types of Coincidence Events (cont.) Random Events Events that results from the coincident detection of two photons from different annihilations Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 9 Mujeeb ur Rahman Types of Coincidence Events (cont.) : Types of Coincidence Events (cont.) Multiple Events Arises when multiple photons from two or more annihilations are detected in coincidence Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 10 Mujeeb ur Rahman Operation of PET Camera : Operation of PET Camera PET camera can be operated in both 2D and 3D In 2D mode there are collimators separating each detector ring In 3D collimators are absent Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 11 Mujeeb ur Rahman Operation of PET Camera (cont.) : Operation of PET Camera (cont.) The 3D configuration in the absence of collimators has an increased sensitivity to true events then the 2D But one of its disadvantage is increased sensitivity to the scattered and random events Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 12 Mujeeb ur Rahman Detection Systems in PET : Detection Systems in PET The detection system of the PET is composed of scintillations based block detectors 4 PMT,s are connected to a detector block of hundreds of detecting crystals (detector block) Detector module PMT Detector block Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 13 Mujeeb ur Rahman Detection Systems in PET (cont.) : Detection Systems in PET (cont.) Basic Principle of Detection The incident photon is converted into visible light due to interaction with scintillator medium The visible photon is then converted into electrical signals by PMT 8 July 2010 14 Mujeeb ur Rahman Basic Principle of Detection (cont.) : Basic Principle of Detection (cont.) The PMT also amplify the signal and transfer it to outer electronics circuitry for processing URL: www.telatomic.com/images/nuclear/spa38.jpg 8 July 2010 15 Mujeeb ur Rahman Pulse Processing : Pulse Processing The signal or the pulse from the PMT passes through the coincidence circuit which ensures that the events are from same annihilation The signal is also allowed to pass through energy window to avoid any scatter 8 July 2010 16 Mujeeb ur Rahman Pulse Processing (cont.) : Pulse Processing (cont.) M. E. Phelps eds., PET Physics, Instrumentation, and Scanners 8 July 2010 17 Mujeeb ur Rahman Correction for Quantitative PET : Correction for Quantitative PET PET offers the possibility of quantitative measurements of tracer concentration in vivo. However, there are several factors which needs to be corrected 8 July 2010 18 Mujeeb ur Rahman Correction in PET Imaging : Correction in PET Imaging Attenuation Correction Attenuation is the loss of true events due to their absorption or scattering from FOV by the patient body Loss of counts due to attenuation increases image noise, image artifacts, and image distortion therefore must be corrected After correction Before correction 8 July 2010 19 Mujeeb ur Rahman Correction Factors (cont.) : Correction Factors (cont.) Scatter Correction Scatter results in distorted image therefore there should be correction for scattering Random Correction Random events also contributes to image distortion and needs correction Scatter Random Positron Emission Tomography; Basic Science by Dale L. Bailey et al., 2004 8 July 2010 20 Mujeeb ur Rahman Correction Factors (cont.) : Correction Factors (cont.) Detector Normalization Practically the detector sensitivity is not uniform and results in artifacts There should be correction for this non-uniform sensitivity termed as detector normalization Dead Time Correction Many of the true events are lost due to dead time of the detector For quantitative image there should be correction for dead time losses 8 July 2010 21 Mujeeb ur Rahman Summary and Conclusion : Summary and Conclusion PET is a radiotracer imaging technique which produce 2 or 3D image of the patient body Make use of annihilation photons from the patient body Only those events are processed which fall with in a specified time interval and energy range in order to get correct spatial information The PET can be operated both in 2D and 3D configuration The detecting system is based on scintillation type Main components of processing circuit are; coincidence and energy discriminating window Several correction factors are required to get quantitative images 8 July 2010 22 Mujeeb ur Rahman Slide 23: Thanks to All

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