MRI Spatial Encoding

Spatial Encoding in MRI

• Spatial encoding in MRI involves localizing signals in 3D space by manipulating the magnetic properties of nuclei in a predictable way.
• Signals are returned with a particular frequency and phase, which are then slotted into their respective locations.
• The brightness of the pixel is determined by the amplitude of the signal returned.

Key Concepts

• The use of gradients is a key concept in spatial encoding.
• There are three steps involved in identifying where in a 3D location a signal is arising from:
  • Slice selection along the z-axis
  • Segment selection along the x-axis using frequency encoding
  • Part of segment selection along the y-axis using phase encoding

Slice Selection

• Slice selection involves localizing the location of the axial slice within the object being imaged.
• This is achieved by using the RF pulse to select which slice to activate, i.e., which slice will have the magnetic vector of its nuclei flipped to the transverse plane to return a signal.

Gradient Echo Sequences (GRE)

• Gradient Echo Sequences (GRE) are an alternative technique to spin-echo sequences.
• GRE differs from spin-echo sequences in two principal points:
  • Utilization of gradient fields to generate transverse magnetization
  • Flip angles of less than 90°
• The flip angle is an MRI phenomenon where the axis of the hydrogen proton shifts from its longitudinal plane (static magnetic field B0) Z axis to its transverse plane XY axis by excitation with the help of radiofrequency (RF) pulses.
• Flip angles are usually at or close to 90 degrees for spin-echo sequences, but less for gradient-echo sequences (10-80 degrees, usually denoted by α).
• Larger flip angles give more T1 weighting to the image, while smaller flip angles give more T2 or T2* weighting to the images.

Spatial Encoding in MRI

• Spatial encoding in MRI involves localizing signals in 3D space by manipulating the magnetic properties of nuclei in a predictable way.
• Signals are returned with a particular frequency and phase, which are then slotted into their respective locations.
• The brightness of the pixel is determined by the amplitude of the signal returned.

Key Concepts

• The use of gradients is a key concept in spatial encoding.
• There are three steps involved in identifying where in a 3D location a signal is arising from:
  • Slice selection along the z-axis
  • Segment selection along the x-axis using frequency encoding
  • Part of segment selection along the y-axis using phase encoding

Slice Selection

• Slice selection involves localizing the location of the axial slice within the object being imaged.
• This is achieved by using the RF pulse to select which slice to activate, i.e., which slice will have the magnetic vector of its nuclei flipped to the transverse plane to return a signal.

Gradient Echo Sequences (GRE)

• Gradient Echo Sequences (GRE) are an alternative technique to spin-echo sequences.
• GRE differs from spin-echo sequences in two principal points:
  • Utilization of gradient fields to generate transverse magnetization
  • Flip angles of less than 90°
• The flip angle is an MRI phenomenon where the axis of the hydrogen proton shifts from its longitudinal plane (static magnetic field B0) Z axis to its transverse plane XY axis by excitation with the help of radiofrequency (RF) pulses.
• Flip angles are usually at or close to 90 degrees for spin-echo sequences, but less for gradient-echo sequences (10-80 degrees, usually denoted by α).
• Larger flip angles give more T1 weighting to the image, while smaller flip angles give more T2 or T2* weighting to the images.