Terminology

Definitions

• Gamma radiation
  • Electromagnetic radiation that has at least enough energy to knock electron out of orbit (ionizing radiation)
  • Gamma ray and x-ray of same energy will appear alike but have different origin
    • Gamma: Originates from nucleus of atom, usually from radioactive decay
    • X-ray: Originates from interactions with atom's electron r
• Gamma camera (Anger camera): Detection device capable of capturing and counting gamma rays
  • 2 types: Crystal + photomultiplier tubes (PMTs) (2-step process) or solid state detector (1-step process)
  • Sodium iodide crystal (scintillator) + PMTs
    • Thallium-doped sodium iodide crystal produces light photons when stimulated by gamma or x-ray photons
      • Thicker crystal is more sensitive (more counts stopped) but has lower resolution than thinner crystal
      • Thickness typically 3/8" based on primary use of Tc-99m (140 keV)
      • As electron falls back to lower energy level, low-energy photon (typically in visible spectrum) is emitted
    • PMTs collect light photons and convert them into electronic signals; sets of dynodes amplify signal
  • Solid state detector [e.g., CZT: Cadmium zinc telluride (CdZnTe)]: Collects gamma or x-ray photons and converts them directly into electronic signals
• Collimator
  • Device attached to gamma camera image receptor that only allows gamma photons perpendicular to face of camera to reach camera crystal or solid state detector
  • Designed as series of holes that allow gamma rays through inside metal honeycomb pattern
  • Designed to eliminate photons that would degrade image (e.g., scattered or off-angle)
  • Protects crystal face; typically designed with pressure sensor to prevent impact with patient or other objects
  • Collimators by shape
    • Parallel hole (most common type): Hole columns are all parallel to each other and perpendicular to gamma camera image receptor
      • Only allows gamma photons perpendicular to face of camera to reach crystal or solid state detector
    • Diverging: Hole columns are arranged in "V" with camera at narrow end and body at wider end; resulting images on camera are smaller than actual size
    • Converging: Hole columns are arranged in "V" with camera at wide end and body at narrow end; resulting images on camera are larger than actual size
    • Pinhole: Exaggerated version of converging collimator where there is only 1 hole that converges out to width of camera to capture enlarged image
  • Collimators by energy and resolution (LEHR, MEGP, etc.)
    • Low energy (LE): Thin; used with Tc-99m or Tl-201
    • Medium energy (ME): Medium thickness; used with Ga-67 or In-111
    • High energy (HE): Thick; used with I-131
    • High resolution (HR): Many holes and thick walls between hole columns; lower counts but increased image quality
    • Ultra high resolution (UHR): Many holes and thickest walls between hole columns; lower counts but increased image quality
    • General purpose (GP): Medium number of holes and medium thickness of walls; average between high resolution and high sensitivity
    • High sensitivity (HS): Thin walled between hole columns to allow for higher counts

Planar Imaging

• Camera is placed in fixed position or single view over patient
  • Acquires image of radionuclide position in body from one orientation

Radionuclides Used for Gamma Camera Imaging

• t1/2 short enough to see biological function
• Primarily gamma-only emitter
  • Beta or alpha emitters better for radiation therapy than imaging
• Preferably single energy or only a few energies between 100-300 keV
• Best example: Tc-99m 140 keV, 6 hour t1/2
• Other commonly used gamma camera imaging radionuclides
  • Ga-67: Primary output: 94, 184, and 296 keV; 79 hour t1/2
  • Tl-201: Primary output: 71, 135, and 167 keV; 73 hour t1/2
  • Xe-133: Primary output: 81 keV; 5.3 day t1/2
  • In-111: Primary output: 173 and 237 keV; 67 hour t1/2
• Affordable and readily available

General Anatomic Considerations

Clinical Importance