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
In-111: Primary output: 173 and 237 keV; 67 hour t1/2
Affordable and readily available
IMAGING ANATOMY
General Anatomic Considerations
CLINICAL IMPLICATIONS
Clinical Importance
Selected References
Mettler et al. Essentials of Nuclear Medicine and Molecular Imaging. Elsevier, 2018
Cherry SR et al. Physics in Nuclear Medicine. Saunders, 2012
Christian PE et al. Nuclear Medicine and PET: Technology and Techniques, 5th Edition. Mosby, 2004
Karesh SM et al. Questions and Answers in Nuclear Medicine. Mosby, 1999
Chandra. Nuclear Medicine Physics: The Basics, 5th Edition. Lippincott Williams & Wilkins, 1998
Early et al. Principles and Practice of Nuclear Medicine, 2nd Edition. Mosby, 1995.
Related Anatomy
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Related Differential Diagnoses
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References
Tables
Tables
KEY FACTS
Terminology
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