Basic MRI I

1
Chapter 9

• Basic MRI I
• Mark D. Herbst, MD, PhD

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Notice

• This lecture contained many drawings on the
  whiteboard, so get these from one of the other
  students if you missed the class.

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Spatial characteristics of the MR image

• The body is 3-dimensional
• The images we look at are 2-dimensional
• Pixels have 2 dimensions
• Voxels have 3 dimensions
• The third dimension is slice thickness
• Slicing and dicing the body for MR imaging is
  like going from whole potato to chips to fries to
  home fries.

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K-space

• Signals from the patient are stored in the
  computer in a place called k-space and when it
  is filled, we use Fourier Transform to transform
  the information from k-space to image space,
  that is, an image.

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Gradients

• We use gradients in 3 directions to get images
• Slice selection gradient
• Phase encoding gradient
• Frequency encoding gradient
• All these are orthogonal (at right angles) to
  each other

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Slice select gradient

• Selective excitation
• Apply a gradient in the slice selection (CC for
  axial slices, RL for sagittal slices, AP for
  coronal slices)
• Two determinants of slice thickness
• Strength of gradient
• Transmit bandwidth
• Remember the previous discussion about multislice
  imaging? We use the gradient to select the slice
  we want to excite with RF.

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Slice Selection

• 3D volume acquisition
• Uses two phase encoding gradient pulses, one in
  the usual phase encoding way, to be discussed
  below, and the other in the slice select
  direction. We need to collect a full set of
  echoes in a separate k-space for each slice, so
  we have to multiply the time for one slice by the
  number of slices to get total time. This is why
  we only use very fast GE methods to get 3D volume
  acquisitions.

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Phase Encoding Gradient

• Applied after the excitation pulse (90 degree or
  alpha pulse)
• Changed to different level for each echo
  collected
• If 256 x 256 image, need 256 echoes, therefore
  256 different phase encoding pulses are applied,
  each at a different strength.
• Acquisition of each of these echoes takes one TR
  if conventional spin echo, less if fast spin echo
  or EPI

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Frequency Encoding Gradient

• Applied during the echo, can be seen on some
  pulse diagrams to be applied also during the FID.
• Changes the frequency of precession of the net
  mag vector in each row of voxels in the frequency
  encoding direction