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QUESTIONS
» Image Creation

How is the MRI image created? This section explores slice selection, spatial encoding and the FFT.

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Creating a MRI Image Q&A

These questions are concerned with the localisation of the MRI signal, and the subsequent creation of an image. The generation and evolution of the MRI signal is not covered in this section.

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  1. What is an RF pulse?
  2. What is excitation?
  3. What's the simplest pulse sequence diagram you can draw?
  4. How is slice selection achieved?
  5. How is the slice thickness increased?
  6. How is the slice location varied?
  7. Should the slice-selection gradient be applied before, during or after the RF excitation pulse?
  8. What imaging slice orientations are allowed in MRI?
  9. How does increasing the transmitted frequency bandwidth of the RF excitation pulse affect the imaging slice?
  10. How does the increasing the slice-selection gradient magnitude affect the imaging slice?
  11. When a slice is selected, what happens in the rest of the patient?
  12. What does the frequency encoding gradient do?
  13. When is the frequency encoding gradient applied?
  14. What does the Fourier transform do to a signal waveform?
  15. Why is the Fourier transform appropriate for use in MRI?
  16. How does the frequency encoding gradient destroy the MRI signal?
  17. Why can't we perform frequency encoding in both directions within an imaging slice?
  18. How many MRI signal measurements are required to produce a MRI image?
  19. What is a phase difference?
  20. What is a rate of change of phase?
  21. What is k-space?
  22. What is digitisation of the MRI signal?
  23. What is the difference between applying a 1D Fourier transform in (i) the frequency encoding direction and then the phase encoding direction, and (ii) the phase encoding direction and then the frequency encoding direction?
  24. Can the phase encoding gradient be applied at the same time as the slice selection gradient?
  25. Can the phase encoding gradient be applied at the same time as the frequency encoding gradient?
  26. What is a flip angle?
  27. How long should we allow for "full" recovery of Mz after a 90° RF excitation pulse?
  28. For a tissue which has a T1 relaxation time of 900 ms, how long will a 256×256 pixel image take to acquire if 90° RF excitation pulses are used?
  29. A shorter repetition time (which shortens overall image acquisition time) is allowed if the flip angle is smaller. Why is this?
  30. Draw a gradient echo pulse sequence diagram.

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