Nuclear Medicine

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Nuclear Medicine

  1. 1. 1 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Nuclear Medicine ■ Make images of the distribution of radioactive elements that have been “inserted” into body ● Elements “decay” to create gamma- ray photons ● Patient’s organs “emit” gamma-ray photons © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Radioactive Decay Unstable Nucleus Stable Nucleus Stable Nucleus Energetic Particle (e.g., electron) or Photon © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Energetic Photons ■ Byproducts of radioactive decay ● Technetium (atomic wgt. = 99) ► E = 140 keV ► 1 keV = 1 “kilo-electron-volt” ► 1 eV = energy of 1 electron after acceleration by a voltage of 1 volt ● Rubidium (a.w. = 82) ► E = 777 keV ● Tantalum (a.w. = 178) ► E = 93 keV
  2. 2. 2 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Meaning of “keV” “kilo ■ 140 keV = 140,000 electron volts ● unit of “energy” E = hν ■ 1 eV = 1.6 × 10-19 joule ■ 140 keV ⇒ ν @ 2.4 × 10+19 Hz ⇒ λ @ 8.9 × 10-12 m @ 0.01 nm VERY SHORT WAVELENGTHS!! NO LENSES EXIST! © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals “Tagging” ■ Radioactive element is transported by body to organ of interest ■ Radioactive decay of elements produces high-energy radiation ● Gamma ray photons ● Beta particles (electrons) ■ Escaping radiation is imaged © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Making the Radioactive Tracers ■ Technetium-99 made from radioactive decay of Molybdenum-99 ● Mo-99 delivered to hospital and processed ■ Many new tracers made in atomic accelerators at the hospital
  3. 3. 3 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Desirable Characteristics of Tracers 1. tag to elements that “go” to desired spot ■ e.g., iodine to thyroid gland 2. photons must be energetic enough to escape 3. radioactivity should have a very short “half-life” ■ want radiation to decrease quickly ■ minimize radioactive dose to patient © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Gamma Camera ■ “Misnomer” because only the detector of escaping radiation ■ Create an “image” of distribution ■ also called the “Anger Camera” ● after inventor Hal Anger ■ Scintillator + Array of Photomultiplier Tubes (PMTs) + Electronics © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Scintillator ■ Typically Cesium Iodide (CsI) or Sodium Iodide (NaI) (doped with Thallium) ■ Absorbs gamma rays and creates “shower” of lower-energy photons
  4. 4. 4 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Photon Selector + Gamma Camera Gamma Camera Pinhole (diameter 2 - 6 mm) 250-500 mm © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Photomultiplier Tube (PMT) ■ Amplifies (“multiplies”) photons: ● 105-107 electrons per photon ■ Counts electrons by measuring current Photon e- +- photoelectron Big Signal Out © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals PMT PMT PMT PMT PMT PMT PMT PMT PMT PMT PMT PMT PMT PMT PMT PMTPMT PMT PMT PMT PMT Gamma Camera Scintillator PMT PMT
  5. 5. 5 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Scintillator How Gamma Camera Works PMT PMT PMT PMT Scintillation creates “shower” of visible photons Position of scintillation determined from ratios of signals in PMTs measured at same time © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Electroncs Calculate “Smooth Curve” of Scintillation to Estimate Position Scintillator PMT PMT PMT PMT x Number ofCounts measuredat eachPMT Estimated Position x0 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Gamma Camera Constraints ■ Want to see ONLY one absorption of a gamma ray at any one time ● To measure position signal ■ Number of signals can measure per second depends on the scintillator ● maximum of 100,000 = 105 absorptions per second
  6. 6. 6 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Photon Selectors ■ Pinhole (diameter 2 – 6 mm) in sheet of lead ■ “Parallel-Hole Collimator” ■ “Focused Collimator” ● Slatted Grid of lead © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Gamma Camera + “Focused Collimator” “Focuser” made from Lead Camera “sees” only photons from only one depth Camera is scanned across to make image of one plane PMT PMT PMT PMT Blocked Passed © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Focused Collimator = “Magnifying” Collimator Images are farther apart on scintillator PMT PMT PMT PMT
  7. 7. 7 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals “Minifying Collimator Images are “closer together” on scintillator PMT PMT PMT PMT © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Gamma Camera Layout http://rst.gsfc.nasa.gov/Intro/Part2_26d.html © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Commercial Gamma Cameras http://rst.gsfc.nasa.gov/Intro/Part2_26d.html
  8. 8. 8 © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Body Scan with Gamma Camera http://rst.gsfc.nasa.gov/Intro/Part2_26d.html © 2006 Carlson Center for Imaging Science / RITImaging Science Fundamentals Image of Thyroid Gland of Cat http://rst.gsfc.nasa.gov/Intro/Part2_26d.html

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