P.E.T. Positron Emission Tomography

Presentation Outline What is PET? Schema of a PET acquisition process Basic Principle of PET Image reconstruction using coincidence statistics 2D/3D reconstruction Advantages and Disadvantages Applications Conclusion References

What is PET? nuclear imaging technique Produces 3D/2D images of functional processes(glucose metabolism,blood flow) in the body. radiolabeled biologically active compounds (tracers) are introduced into the body, by injection through vein. PET scanner is used to produce an image showing the distribution of the tracer in the body.

Schema of a PET acquisition process

Basic Principle of PET (Positron Emission) Positron Emission occurs when the Proton rich isotope decays. Protron neutron + positron + neutrino. positron travels a short distance and encounters with electron from the surrounding environment. combine and "annihilate" each other,produce two anti-parallel gamma ray of 0.511 MeV each.

Positron Emission (continued)

Positron Emission(continued) The distance a positron travels depends on its energy, and varies between isotopes.

Basic Principle of PET (Emission Detection) Shown here in schematic form, the light photons are converted to electrical signals that are registered by the tomograph's electronics almost instantly.

Detector block diagram Optical reflector light is converted to an electrical signal & amplified Gamma Ray Scintillation Crystal PMT Pre-Amplifier + Electronics Gamma photon converts to optical photons photons are collected at the end of the crystal Front-end electronics condition the signal for further processing

Emission Detection(continued) The ring of squares schematically represents one ring of detectors in a PET scanner, which may, for example, have fifteen such rings for simultaneous tomography of many transaxial slices

Emission Detection(continued) Each detector can be operated in multiple coincidence with many detectors across from it, thereby defining coincidence sampling paths over many angles.

Emission Detection(continued) The tomograph's reconstruction software then takes the coincidence events measured at all angular and linear positions to reconstruct an image that depicts the localization and concentration of the radioisotope within a plane of the organ that was scanned.

IMAGE RECONSTRUCTION USING COINCIDENCE STATISTICS Coincidence events are grouped into projection image called “sinograms”. Sinograms is formed in coincidence processing unit. Each LOR is plotted as function of angular veiw & distance from center.

Sinogram formation Sinogram of brain scan

Image reconstruction Sinogram FBP/IEM Reconstructed image Filtered back projection/Iterative expectation maximization- Algorithms used to reconstruct images from projections. It made – Random coicidence correction,detector dead time correction,& dectector sensitivity correction.

2D/3D reconstruction Two approache- 2D- treat each ring as separate entity,coincidences within a ring is detected. 3D-coincidences b/w rings as well as within ring is detected.

Advantages PET imaging is unique it shows the chemical functioning of organs and tissues while other imaging techniques such as X-ray, CT and MRI show structure. It can detect initial stage of cancer.

Disadvantages Time-consuming. PET scanning can give false results if chemical balances within the body are not normal.

Applications of PET Cancer: Heart Disease: Brain Disorders: Lung Cancer Colorectal Cancer Breast Cancer Prostate Cancer Heart Disease: Coronary Artery Disease Brain Disorders: Alzheimer’s Parkinson’s

Conclusion Not so costly & accurate than traditional techniques. detection and curing of diseases like cancer,alzheimer’s,brain disorders,cardic Further researches in this technology are going on.

References http://depts.washington.edu/nucmed/IRL/pet_intro/ http://www.petnm.unimelb.edu.au/pet/detail/principle.html http://www.wikipedia.com http://www.authorstrem.com

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