Instrumentation NMTCB Practice with Certified Solutions

Instrumentation NMTCB Practice with Certified Solutions What is a gamma camera crystal made of? Sodium iodide activated with thallium What is the purpose of the crystal in a gamma camera? To absorb gamma (γ) rays and convert them into visible-light photons What is the mechanism by which gamma (γ) rays interact with a gamma camera crystal? Photoelectric effect or Compton scattering. What is the relationship between the energy of an absorbed gamma (γ) ray and the amount of light produced by a gamma camera crystal? The amount of light produced is directly proportional to the amount of energy lost by the absorbed gamma (γ) ray. What is the purpose of the photomultiplier tube (PMT) array? To detect the photons produced in the crystal and produce a proportional pulse How does increasing crystal thickness influence camera sensitivity and resolution? Thick crystals have higher sensitivity but lower resolution (i.e., thicker than 0.5 inch or 1 cm) What is the output of a single PMT? A pulse (very small signal) that represents amplification of the light detected. The PMTs are __________ to the crystal by optical grease or a light pipe. optically coupled What is the output of the PMT array decoder circuit? Spatial coordinates (x+ , x- , y+ , y- ) of the scintillation event. How does increasing the number of PMTs affect the resolution? It improves the resolution. How many PMTs are in a modern gamma camera? 75 or 91 per head The electronic component in a gamma camera that is used to disregard unwanted photons is called __________. a pulse height analyzer Why do scattered photons from within the patient cause major imaging difficulties? The scattered photons lead to incorrectly positioned radioactive events. What is the "window" in a pulse height analyzer (PHA)? The range of acceptable energies around a photopeak. What are the energy limits of a 20% window centered at 140 keV? 126 to 154 keV ******** 140kev(0.20) = 28 28/2 = 14 140 +14 = 154 kev 140 -14 = 126kev Why do some gamma cameras have multiple PHAs? To acquire several peaks simultaneously (e.g., 67Ga). What is a gamma camera collimator made of? lead or tungsten What is the purpose of a gamma camera collimator? Project an image of the source distribution directly onto the crystal What does the term "collimator sensitivity" mean? The ratio of gamma (γ) rays that pass through the collimator to those incident upon it. When is it appropriate to use a pinhole collimator? When imaging small organs that lie close to the skin (e.g., thyroid gland) . What are the principal disadvantages of a pinhole collimator? Image distortion and poor sensitivity Walls within the collimator that separate adjacent holes are called _________ collimator septa What is the purpose of collimator septa? To block gamma (γ) rays traveling obliquely toward the crystal. The presence of a starburst-type appearance when imaging a hot object with a parallel-hole collimator is due to __________. septal penetration What defines the energy range of a collimato