Questions | Answers |
1. What is a gamma camera crystal made of? | 1. sodium iodide activated with thallium |
2. What is the purpose of the crystal in a gamma camera? | 2. to absorb gamma (γ) rays and convert them into visible-light photons |
3. What is the mechanism by which gamma (γ) rays interact with a gamma camera crystal? | 3. photoelectric effect or Compton scattering |
4. What is the relationship between the energy of an absorbed gamma (γ) ray and the amount of light produced by a gamma camera crystal? | 4. The amount of light produced is directly proportional to the amount of energy lost by the absorbed gamma (γ) ray. |
5. What is the purpose of the photo-multiplier tube (PMT) array? | 5. to detect the photons produced in the crystal and produce a proportional pulse |
6. How does increasing crystal thickness influence camera sensitivity and resolution? | 6. Thick crystals (i.e., thicker than one-half inch or 1 cm) have higher sensitivity but lower resolution. |
7. What is the output of a single PMT? | 7. a pulse (very small signal) that represents amplification of the light detected |
8. The PMTs are __________ to the crystal by optical grease or a light pipe. | 8. optically coupled |
9. What is the output of the PMT array decoder circuit? | 9. spatial coordinates (x+, x-, y+, y-) of the scintillation event |
10. How does increasing the number of PMTs affect the resolution? | 10. It improves the resolution. |
11. How many PMTs are in a modern gamma camera? | 11. 75 or 91 per head |
12. a pulse height analyzer | |
13. Why do scattered photons from within the patient cause major imaging difficulties? | 13. The scattered photons lead to incorrectly positioned radioactive events. |
14. What is the “window” in a pulse height analyzer (PHA)? | 14. the range of acceptable energies around a photopeak |
15. What are the energy limits of a 20% window centered at 140 keV? | 15. 126 to 154 keV |
16. Why do some gamma cameras have multiple PHAs? | 16. to acquire several peaks simultaneously (e.g., 67Ga) |
17. What are the advantages of a digital camera? | 17. It has better energy and spatial resolution, as well as less dead time. |
18. Digital gamma cameras use a __________ to apply prestored energy and position corrections to each scintillation event. | 18. computer |
19. What is a gamma camera collimator made of? | 19. lead or tungsten |
20. What is the purpose of a gamma camera collimator? | 20. to project an image of the source distribution directly onto the crystal |
21. What does the term “collimator sensitivity” mean? | 21. the ratio of gamma (γ) rays that pass through the collimator to those incident upon it |
22. When is it appropriate to use a pinhole collimator? | 22. when imaging small organs that lie close to the skin (e.g., thyroid gland) |
