Nuclear Medicine Intro & Physics from Medical Imaging Signals and Systems, Chapter 7, by Prince and Links NM - introduction Relies on EMISSION of photons from body (versus transmission of photons through body like x-ray, CT) Radiotracers (unstable radioactive nuclides) are attached to each molecule of a substance. Image localization occurs due to natural biodistribution in the body of the body of the substance (inject, inhale, ingest) and measurement (with scintillation camera) of ionizing radiation emitted when radioactive atom undergoes decay => FUNCTIONAL IMAGING MODALITY Planar, 2D images (scintigraphy) produced or tomographic images produced (PET, SPECT) 1
NM - introduction Functional vs. Anatomic Bone scan ( planar scintigraphy) vs. Projection x-ray Myocardial perfusion scan (PET) (distribution of blood flow) vs. Projection x-ray (flouroscopy) (anatomy of coronary arteries) vs. Tomographic x-ray (CT) (anatomy of heart muscle) NM - physics On your own: define nucleon, atomic number, mass number, nuclide, radionuclide, isotope, isobar, isotone, isomer. Radioactive isotopes serve as the source of ionizing radiation we image in NM Mass defect: sum of masses of parts of atoms is more than mass of atom => there is missing energy acc to E=mc 2 Binding energy: missing energy. Applies to protons and neutrons in nucleus and to orbiting electrons (nuclear binding energy and electron binding energy) Radioactive decay: process by which an atom rearranges its protons and neutrons to end up with lower energy. Occurs spontaneously. Releases energy. 2
NM - physics In general, ratio of protons to neutrons determines stable/unstable For stability at higher atomic numbers, have more neutrons than protons. One way of conceptualizing radioactive decay is the attempt of an atom that is off the line of stability to reach the line.. Changes the proton to neutron ratio in the process of decay Red = line of stability NM - physics Radioactivity : how many radioactive atoms are undergoing decay per second. Has nothing to do with type/energy of radiation Units: curie, [Ci], 1Ci=3.7x10 10 disintegrations per second (dps) SI Units, becquerel, [Bq], 1Ci=3.7x10 10 Bq NM imaging range: mci, MBq 3
NM - physics Radioactive Decay Law dn N dt N = number of radioactive atoms in source (constant) λ = decay constant. Units: inverse time, sec -1 Assume N 0 atoms at t=0, then number of atoms at time, t, is Nt t N e 0 also the radioactive decay law NM - physics Also, if radioactivity is A=number of atoms disintegrating per unit time dn A N dt t A A e t 0 (also the radioactive decay law) Big difference in detection: In CT, x-ray, measure total energy of beam, intensity In NM, measure dps => you must be able to count and separate γ-rays, cannot just use measure of intensity 4
NM - physics Decay factor, DF, = Half life: e t A t t A 1/ 2 1/ 2 t 0 1/ 2 1 e 2 0.693 Half life and decay constant have fixed relationship for all radionuclides. Constant numbers per radionuclide => different applications choose different radionuclides NM physics - ionizing radiation Modes of radioactive decay govern types of ionizing radiation produced alpha decay: results in emission of alpha particle (2 protons + 2 neutrons) beta decay: results in emission of a beta particle (like electrons) positron decay*: results in emission of a positron (antimatter electrons), which rapidly annihilate with electron to produce 2 gamma rays isomeric transition*: results in emission of a gamma ray (remember: x-rays and gamma rays are indistinguishable after emission. Difference is in origin nucleus vs. electron cloud) * Used in PET *Used in planar scintigraphy and SPECT Particulate ionizing radiation electromagnetic ionizing radiation 5
NM physics - radiotracers 1500 known radionuclides 200 can be purchased ~dozen suitable for NM 1. clean gamma ray emitters (don t also emit alpha and beta particles that contribute to dose and not image formation) or positron emitters. 2. Want correct energy gamma rays, 70-511keV Unlike CT, x-ray, attenuation CONFOUNDS our image formation. Contributes to dose, reduces detected signal => want higher energy gamma rays BUT, higher energy = less likely to interact with detector 3. Half life: need to form images in minutes, not seconds or hours NM physics - radiotracers Want radionuclides that are useful and safe to trace Want monoenergetic decay => energysensitive detection can filter out Compton scatter Technetium-99m (Tc-99m) is most commonly used gamma ray producing radionuclide in NM. ½ life of 6 hrs. easily produced on site. Can be tagged to variety of molecules. Keep it in mind as example. 6
KEY POINTS - physics NM produces images that depict the distribution of a radiotracer; the distribution is governed by body function, not structure. NM makes use of radionuclides; these are unique nuclear species representing radioactive atoms, which emit ionizing radiation upon spontaneous decay. A given radionuclide is characterized by its decay mode (indicates type of ionizing radiation) and half-life (time for half the radioactive atoms to decay, on average Radioactive decay is a random process (governed by Poisson distribution); described by non-exponential relation and is a function of time and the decay constant (probability of decay), which is a characteristic of a given radionuclide. Radiotracers make use of radionuclides that emit radiation of appropriate type and energy, have half-lives that are appropriate, and are chemically inert. Overview NM Scintigraphy, SPECT, PET 7
Overview NM Scintigraphy, SPECT, PET Emission Detection Planar Imaging (Scintigraphy) Single gamma photon (produced by isomeric transition) Anger camera Projection imaging SPECT PET Single gamma photon (produced by isomeric transition) Positron Annihilated 2 gamma photons (511keV photons) emmitted [cons. of energy] back to back [cons. of momentum] Tomographic due to emission No corresponding projection mode Rotating Anger camera Tomographic due to detection/reconstruction Annihilation Coincidence Detection based on Anger camera principle Planar scintigraphy : SPECT :: Projection x-ray : CT nothing: PET :: Projection x-ray : CT Overview NM Scintigraphy, SPECT, PET Planar scintigraphy SPECT PET 8
Overview Planar Scintigraphy Overview SPECT 9
Overview SPECT Planar scintigraphy images at different angles. Same line of detectors in each image is pulled to form the sinogram for that slice. Overview PET https://www.inkling.com/read/nuclear-medicine-ziessman-4th/chapter-5/positron-emission-tomography 10
3/30/2015 Overview PET Each detector pair, and thereby each line of response (LOR) corresponds to a particular pixel in the sinogram depending on its orientation angle and distance from the center of the gantry. Therefore, for each coincidence detection, the LOR for that detection is determined, the pixel in the sinogram associated with that LOR is located, and the value in the pixel is incremented. Fahey et al., Data Acquistion in PET Imaging, Journal of Nuclear Medicine Technology, 2002. Overview PET versus SPECT PET, May 2012 Lower resolution More sensitive SPECT, April 2012 Higher resolution Less sensitive (used on anatomies with less attenuation) 11
3/30/2015 Overview PET versus SPECT Overview functional + anatomic Functional (PET) + anatomic (CT) Functional (PET) + anatomic (MRI) 12
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