Positron Emission Tomography

Brief explanation of what is PET, the main components for a PET system along with their basic functions. The principle behind PET inclusive of positron emission and emission detection. Acquisition and reconstruction of the collected data to produce the final image. Finally the pros and cons of Positron emission tomography.

1. Positron Emission Tomography… _ Ramzee Small

2. Outline Positron Emission Tomography Definition Clinical Application Main System Components Principle of PET Image Acquisition Image Reconstruction Pros and Cons Basic Configuration of PET Facility References

3. What is PET??.. Positron Emission Tomography is an imaging technique which uses small amounts of radiolabeled biologically active compounds to help in the diagnosis of disease. The tracers are introduced into the body, by either injection or inhalation of a gas. PET scanner is used to produce an image showing the distribution of the tracer in the body.

4. Clinical Applications of PET Oncology Role in lesion detection, lesion characterization, staging of malignant lesions and assessment of the therapeutic response Brain PET Study the brain's blood flow and metabolic activity. It aid in discovery of nervous system problems, such as Alzheimer's disease, Parkinson's disease etc. Heart PET can help find damaged heart tissue especially after a heart attack and can help choose the best treatment such as coronary bypass heart surgery for a person with heart disease.

5. Main System Components Scanner Gantry Detector Septa Coincidence Circuit Table Computer Cyclotron

6. Main System Component… Detector C omprised of an 8 x 8 scintillation, inorganic crystals which emits light photons after the interaction of photons 4 photomultiplier tubes (PMTs) arranged in a circular pattern around the patient

7. Main System Components… Detector Materials

8. Main System Component Septa Lead or tungsten circular shield mounted between the detector rings Limits scattered radiation from the object reaching the detector ( scattered out the transverse plane)

9. Main System Component Coincidence circuit Specific electronic circuits "coincidence" circuits pick up gamma pairs due to the two gamma rays emitted during the positron annihilation almost simultaneously. This coincidence is a very strong signature that distinguishes them from other photons. On the image it is requested that the signals coming from the scintillators A and B coincide within 12 billionths of a second (nanosecond).

10. Main System Component Cyclotron A machine used to produce the radioisotopes (radioactive chemical elements) which are used to synthesize the radiopharmaceuticals. The most frequently used radioisotopes in PET are: Carbon-11 Nitrogen-13 Oxygen-15 Fluorine-18 18FDG ( Fluorodeoxyglucose ) is the most widely used PET tracer.

11. Main System Component Table The bed is capable of moving in and out of the scanner to measure the distribution of PET radiopharmaceuticals throughout the body, and it adjusts to a very low position for easy patient access Computer A computer analyzes the gamma rays and uses the information to create an image map of the organ or tissue being studied .

12. Principle of PET Positron Emission Positron Emission occurs when the isotope decays and a proton decays to a Neutron, a Positron and a Neutrino. After traveling a short distance (3-5mm), the positron emitted encounters an electron from the surrounding environment. The two particles combine and "annihilate" each other, resulting in the emission of two gamma rays in opposite directions of 0.511 MeV each.

13. Positron Emission…

14. Principle of PET Emission Detection As positron annihilation occurs, the detector detects the isotope's location and concentration The resultant light photons are converted to electrical signals that are registered by the system electronics almost instantly The reconstruction software then takes the coincidence events measured at all angular and linear positions to reconstruct an image

15. Image Acquisition

16. Image Acquisition…

17. Image Acquisition… The image acquisition is based on the external detection in coincidence of the emitted Gamma rays. V alid annihilation event requires a coincidence within 12 nanoseconds between two detectors on opposite sides of the scanner. For accepted coincidences, lines of response connecting the coincidence detectors are drawn through the object and used in the image reconstruction.

18. Image Reconstruction Images are created from raw data collected as rays corresponding to each detected annihilation event The basic principles of tomographic image reconstruction from projections of an object is common to CT .

19. Image Reconstruction… Process… The detectors collect a series of lines of responses LORs, A profile of counts versus distance is produced for each angle of LORs Each profile maps the location of the source in the direction parallel to the scan profile (however , the source can lie at any depth along the line perpendicular to that profile) The source distribution can be obtained by projecting the data from each scan profile back across the entire image grid. [backprojection]

20. Image Reconstruction… Back projections of scan profiles at different angles are then added together (linear superposition), to produce an approximation of the original radioactivity distribution. This operation is called linear superposition of back projections (LSBP ). There is an inherent blurring in this technique, which is removed by performing a filtering operation on the projections or scan profiles The entire reconstruction process is known as linear superposition of filtered back projections (LSFBP).

21. Image Reconstruction PET Reconstruction Algorithms Filtered Back Projection Simple Quick Streak artifacts Iterative Reconstruction Need fast computer

22. Pros and Cons Advantages Uniquely shows the chemical functioning of organs and tissues Detect functional changes Study metabolic functions- may be an alternative to biopsy. Distinguish between benign and malignant tumors- reducing unnecessary surgeries due to misdiagnosis Determine the spread of disease (cancer) and function of the heart. Diagnose early stages of neurological illness, e.g. Epilepsy, Alzheimer's disease. Disadvantages Ionizing radiation Radioactive compound is short lived. Radioisotope must be produced in a laboratory near the PET scanner.

23. PET Facility Configuration

24. Finally!!! A Scan that is available for all PETs… and not just CAT…

25. References Alessio AM, Kinahan PE, Cheng PM, Vesselle H, Karp JS. PET/CT scanner instrumentation, challenges, and solutions. Radiol Clin N Am. 2004;42(6): 1017-32. Phelps ME, HotTman EJ, Mullani NA, Ter-Pogossian MM. Applications of annihilation coincidence to transaxial reconstruction tomography. J Nucl Med 1975;16:210-224. Valk PE, Bailey DE, Townsend DW, Maisey MN. Positron Emission Tomography: Basic Science and Clinical Practice. London: Springer- Verlag ; 2003. Fahey H. F. Data Acquisition in PET Imaging, Journal of Nuclear Medical Technology; June 2002;30(2): 39-49 Daghighian F., Sumida. R, Phelps E. M. PET imaging: An overview and instrumentation. J. Nucl . Med. Technol. 1990;18:5-13. ShuklaK . A, Kumar U. Positron emission tomography Scan; an overview J Med Phys. 2006 Jan-Mar; 31(1): 13–21. doi : 10.4103/0971-6203.25665 Ranger NT, Thompson CJ, Evans AC. The Application of a masked orbiting transmission source for attenuation correction in PET. J Nucl Med. 1989;30:1056–68.