Chapter 8

Self-assessment Questions

 

  1. Define “signal” and “noise”.
  2. What are the differences between “raw signal-noise-ratio”, “contrast-noise-ratio”, and “experimental signal-noise-ratio”?
  3. Over what range does experimental SNR usually vary in fMRI studies? Is it usually less than or greater than 1.0?
  4. How does spatial smoothing affect SNR?
  5. What theoretical effect should field strength have upon SNR, and why?
  6. What is the effect of field strength upon SNR, in practice?
  7. How do T1 and T2* change with increasing field strength?
  8. What are potential disadvantages of high field strength for fMRI studies?
  9. Why is manipulation of temperature not a feasible method for increasing SNR?
  10. What causes thermal noise in a MRI scanner? How does thermal noise vary across the brain?
  11. What is the difference between thermal noise and system noise? What are common types of system noise?
  12. What are common forms of physiological noise?
  13. What is “intersubject variability” in the hemodynamic response? How do researchers account for this variability?
  14. What are some sources of behavioral or task-related variability in many fMRI experiments?
  15. What is signal averaging? How should SNR increase as one averages more and more data (e.g., has a larger sample size)?
  16. How does signal averaging affect the spatial extent of activation? Why does spatial extent change with increased sample size?
  17. What are the basic principles of fMRI power analyses? Why are they important?