Chapter 11 Lecture

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Chapter 11 Lecture

• Mark D. Herbst, M.D., Ph.D.

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Acquistion Time and Protocol Optimization

• Time of scan ( phase encoding steps xTR x
  NEX)/ETL

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Ways to save time

• Reduced matrix
• always done in phase direction (why do it if it
  doesnt save time?)
• No change in frequency matrix
• Reduces number of phase encoding steps

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Ways to save time

• Rectangular FOV
• Done in phase enc direction, to match reduced
  matrix size to make square pixels

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Ways to save time

• Half acquistions
• Half scan or half Fourier
• Uses the phenomenon of Hermitian Symmetry to make
  up the empty part of k-space with known info
• Equal to ½ NEX
• Results in decreased S/N
• Incorrect in book---says increases noisenot
  really, it decreases S

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Signal Averaging

• Decreases noise, but increases time (NEX)

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Protocol Design

• Usually all are optimized by manufacturer or
  techs that are working at that center
• Consider contrast sensitivity
• For example, uterus is shown well with T2WI, not
  so well with T1WI
• Image detail rule of thumb is to make the
  slices half the size of the thing you want to
  see, or less.

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Protocol Design

• Consider rectangular FOV and matrix
• Dont overdo detail, since small voxels will give
  you decreased S/N
• Consider 3D MR, which gives more signal
• Artifact reduction (see ch 14)
• Speed factor turbo factor ETL for Fast Spin
  Echo (FSE, TurboSE)