16 Questions
What is the primary concept in spatial encoding?
Gradient encoding
How many steps are involved in identifying the location of a signal in a 3D space?
3
What is the purpose of slice selection in MRI?
To localise the location of the axial slice within the object being imaged
What is the main difference between gradient echo sequences and spin-echo sequences?
The use of gradients to generate transverse magnetisation and flip angles of less than 90°
What is the flip angle in a spin-echo sequence?
Usually at or close to 90 degrees
What is the effect of larger flip angles in a gradient-echo sequence?
More T1 weighting to the image
What is the purpose of the RF pulse in MRI?
To flip the axis of the hydrogen proton
What is the weighting of the image when the flip angle is small in a gradient-echo sequence?
T2* weighting
What determines the brightness of the pixel in MRI?
The amplitude of the signal returned
What is the purpose of the gradient fields in MRI?
To encode the spatial location of the signal
What is the axis along which the segment of the slice is selected by frequency encoding?
x-axis
What is the range of flip angles typically used in gradient-echo sequences?
10 - 80 degrees
What is the effect of smaller flip angles in a gradient-echo sequence?
More T2* weighting to the image
What determines the spatial location of the signal in a 3D space?
The frequency and phase of the signal returned
What is the role of the RF pulse in slice selection?
To select the slice to be imaged
What is the difference between the transverse plane and the longitudinal plane in MRI?
The transverse plane is XY axis and the longitudinal plane is Z axis
Study Notes
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.
Learn about spatial encoding in MRI, including how signals are localized in 3D space, the role of gradients, and the process of identifying signal locations. Understand the key concepts of slice selection, phase encoding, and frequency encoding.
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