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Introduction to Nuclear Medicine Physics Jerry Allison, Ph.D. Department of Radiology Medical College of Georgia Augusta University Augusta, GA Welcome to new residents 2016 GRU Medical Physics Resource • Medical Physics Course Calendar Handouts • http://www.gru.edu/mcg/radiology/residency/resphysicscourse.php What to learn? • AAPM Diagnostic Radiology Residents Physics Curriculum Module 17: Nuclear Medicine • • http://www.aapm.org/education/documents/Curriculum.pdf ABR Core Exam Study Guide Physics Section: Part 17) Nuclear Medicine and Instrumentation Radioisotope Safety Examination (RISE) Section • http://www.theabr.org/sites/all/themes/abrmedia/pdf/CORE_Exam_Study_Guide_FINAL%28V10 %29.pdf Wonderful new books (FREE)! Nuclear Medicine Physics : A Handbook for Teachers and Students Diagnostic Radiology Physics: A Handbook for Teachers and Students International Atomic Energy Agency, 2014 http://www-pub.iaea.org/books/IAEABooks/Subject_Areas/0103/Medical-physicsincluding-dosimetry- A note of thanks to Sameer Tipnis, Ph.D. G. Donald Frey, Ph.D. Medical University of South Carolina And Z. J. Cao, Ph.D. for Sharing nuclear medicine presentation content Outline for Today I. II. III. IV. V. Radioactivity Gamma Camera SPECT PET Hybrids I. II. III. SPECT/CT PET/CT PET/MR Radioactivity Atoms • Neutrons and Protons (+) In Nucleus • Electrons (-) Line of Stability • Naturally occurring (stabile) nuclides • These nuclei have a stable configuration of protons and neutrons in the nucleus Radionuclides • Many nuclei are not stable (radioactive) • Radionuclides close to line of stability Return by beta decay and electron capture Radionuclides below the line b- (e.g. I-131) Radionuclides above the line b+ (e.g. F-18) and EC (e.g. Tl-201) https://commons.wikimedia.org/wiki/File%3ATable_isotopes_en.svg Radioactive Decay A = lN A(t) = A0e-lt • • • • N = number of radioactive nuclei A = radioactivity (decays per second) A(t) = radioactivity (disintegrations per second) at time t l = decay constant (fraction of N that decays per unit time) Radioactivity • unit in SI: 1 Bq = 1 dps (Becquerel) • unit in traditional: 1 Ci = 3.7×1010 dps • (1g of Ra-226, extracted first by Mme. Curie) • 1 mCi = 37 MBq • NM imaging: ~ 1 to 30 mCi (30 – 1100 MBq) Half-Life (Physical Half-Life) • The physical half life T1/2 is the time it takes for 50% of the radioactive nuclei to decay T1/2 = 0.693/l T1/2 for Tc-99m is 6 hours l for Tc-99m is 0.1155/hr Biologic Half-life (Tb) Tb = Time taken to reduce the amount of radiopharmaceutical in the body by one half due to functional clearance 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR Effective Half-Life Te = Time to reduce radiopharmaceutical in the body by one half due to functional clearance and radioactive decay if Tp >> Tb, Te ≈ Tb if Tp << Tb, Te ≈ Tp Radionuclides used in nuclear medicine Less than 20 radionuclides but hundreds of labeled compounds © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps, 4th edition, 2012 17 Most radionuclides used in nuclear medicine are produced Radionuclide Production Radionuclide Production Method P-32 I-125 I-131 F-18 Ga-67 In-111 I-123 Tl-201 Neutron Activation (Reactor) Neutron Activation (Reactor) Neutron Activation (Reactor) Cyclotron Cyclotron Cyclotron Cyclotron Cyclotron Rb-82 Sr82/Rb82 Radionuclide Generator Tc-99m Mo99/Tc99m Radionuclide Generator 99mTc – a workhorse in NM! • T1/2 = 6 hr – ideally suited to study metabolic processes in patients • 140 keV emission - low patient dose & ideal for gamma cameras • No high-energy b- radiation – low pt. dose • Versatile chemistry - can form tracers by being incorporated into a range of biologically-active substances to target tissue or organ of interest 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR Dose Definition • Effective dose E (Sv): measure of absorbed dose to whole body, the product of equivalent dose and organ specific weighting factors Whole body dose equivalent to the nonuniform dose delivered 20 Effective dose of NM procedures 21 Nuclear Medicine Gamma Camera • The dual head is the most common design • Most cameras use rectangular heads • Most cameras are designed to do SPECT imaging How to obtain a NM image? • Administer radiopharmaceutical (a radionuclide labeled to a pharmaceutical) • The radiopharmaceutical is concentrated in the desired locations. • Nucleus of the radionuclide decays to emit g photons • Detect the g photons using a “gamma camera” Gamma Camera Basics Pulse Height Analysis position analysis X Y Z c o m p ut e r amplify & sum p r e - a m p P M T d et e ct or displa y c o lli m a t o r p a t ie n t Why collimator? – image formation w/o collimator with collimator images image detector collimator sources Image of a point source is the whole detector. Image of a point source is a point. Why collimator? – image formation • to establish geometric relationship between the source and image • The collimator has a major affect on gamma camera count rate and spatial resolution parallel-hole collimator Modern Camera Design © GE: Discovery © Siemens: Symbia © Philips: Brightview SPECT (Single Photon Emission Computed Tomography) • We can produce tomographic images by acquiring conventional gamma camera projection data at several angles around the patient Similar to CT Sinogram © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps Filtered Back Projection • Attenuates streaks by filtering the projections © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps Iterative Reconstruction • Quantitatively more accurate Can model various corrections • • • • Collimator Scatter System geometry Detector resolution Attenuation Correction • Like all radionuclide imaging there is a problem due to attenuation. • Correction can be important for quantiifying the activity of lesions Modeling attenuation (Chang) Measuring attenuation with transmission sources Attenuation correction: Transmission measurements • Radionuclide source (e.g. Gd-153) • X-ray source (SPECT/CT) SPECT/CT PET (Positron Emission Tomography) The Positron Positron is an elementary particle Has same mass as electron The charge is equal but opposite to electron + - Positron Electron Positron is an Anti-particle • When a particle and and antiparticle interact they annihilate Both particles are destroyed Two photons(Gammarays) are created Two photons are emitted in ~opposite directions (± 0.25 degrees for F-18) Gamma 1 + - Gamma 2 Photon Energy • For positron-electron annihilation 511 keV (for each gamma-ray) • This is much higher than the 140 keV of Tc-99m Where was the event? ? Coincidence Where was the event? Annihilation Detection In coincidence counting an event is ONLY registered if a signal is received from two detectors within a narrow window of time. A few nanoseconds is usually used. Coincidence Time-of-Flight PET In “Time-of-Flight” pet, use of a very small time window (<100 picoseconds) can localize an annihilation event to within a few cm along the line of coincidence. Time-of-Flight PET can improve SNR. Coincidence PET/CT Events in PET Scanners Trues Scatter RRnd = CTW Rtrue Rtrue Random CTW = timing window Attenuation Correction • Like all radionuclide imaging there is a problem due to attenuation. • It is much less for PET than for Tc-99m imaging • Correction can be important for quantifying the metabolic activity of lesions Attenuation Correction • Radioactive rod sources Ge -68 or Cs-137 • CT data reconstructed to make a attenuation map of the body Attenuation map information is used in image reconstruction Positron Emitting Radionuclides • • • • Usually cyclotron produced Most are very short lived F-18 (110 min) can be shipped Sr-82 (25 day)/Rb-82 (1.2min) is a generator system • Others have very short half-lives PET Radionuclides Radionuclide Half-life Positron Energy Production C-11 N-13 O-15 F-18 Rb-82 20.5 min 10.0 min 2.0 min 110 min 1.2 min 960 keV 1198 keV 1732 keV 634 keV 3356 keV Cyclotron Cyclotron Cyclotron Cyclotron Sr-82/Rb-82 Hybrids: Incorporation of CT (& MRI) SPECT/CT PET/CT • CT images can be used for • Attenuation Correction • Scatter correction • Diagnosis (# CT slices varies widely) • Fusion of physiological map on anatomical map PET/CT SPECT/CT •Enables SPECT and PET images having spatially registered/fused CT images •Spatial resolutions •SPECT ~10mm •PET ~5mm •CT ~1mm PET Registration and Fusion CT PET + CT PET/MRI MRI improves soft tissue contrast More complex to integrate with PET MRI has some inherent spatial distortion More expensive Difficult if PET PMTs are involved Primary Reference • Physics in Nuclear Medicine: Simon Cherry, James Sorenson and Michael Phelps, 4th Edition, Elsevier, 2012