MEDRADSC 3DE3 Lecture Notes - Lecture 6: Larmor Precession, Dephasing, Transverse Plane
18 Jun 2018
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Introduction to Spatial Encoding
ď‚·Resonance requires RF applied at 90 degrees to Bo at precessional frequency of H
ď‚·RF pulse gives H enough energy so that Transverse magnetization occurs
ď‚·Voltage induced at transverse plane (signal frequency= Larmor frequency of H)
ď‚·
ď‚·The system must be able to locate signal spatially in three dimensions, so that it can position
each signal at the correct point on the image. To do this:
1. Locates a slice in any plane (this is where its different from CT we can prospectively
acquire in the x y or z plane).
2. the signal is located/encoded along both axes of the image.
oThese tasks are performed by gradients.
Gradients:
ď‚·Create spatial linear variation along Bo
oAdd or subtract to/from magnetization
oGradients are alterations to B0 and are generated by coils of wire located within the
bore of the magnet through which current is passed… passage of current through a
gradient coil induces a gradient (magnetic) field around it, which either subtracts from
or adds to B0
oThe magnitude of Bo is altered in a linear fashion by the gradient coils so that the
magnetic field strength and therefore the precessional frequency experienced by the
nuclei situated along the axis of the gradient can be predicted (this is called spatial
encoding)
ď‚·
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oA gradient has been applied that increases the magnetic field strength (and causes
nuclei spins to speed up) towards the right-hand side of the magnet and decreases it
(spins slow down) towards the left
The position of a nucleus along a gradient can be identified according to its
precessional frequency
Faster or slower spin will void signal from that area bc there will not be an
efficient transfer of energy to that region
oAt point A: a nucleus experiences a field of 0.9995 T
Gradient fields changing field strength – localization is performed
ď‚·Magnetic isocenter: the center point of the axis of all three gradients and the bore of the
magnet. The magnetic field strength/precessional field strengths not altered here even when
gradients applied
oIsocentre: Larmor frequency
ď‚·Three main functions of gradients in encoding (signal localization):
oSlice selection
Gradient coil turned on/applied
Specific point along axis will have specific frequency so slice can be selectively
excited, by transmitting RF with a band of frequencies coinciding with the
Larmor frequencies of spins in a particular slice as defined by the slice select
gradient
Resonance occurs, signal captured
ď‚·Nuclei situated in other slices along gradient will not resonate,
precessional frequency different than that of applied gradient
Slice select on the x: sagittal
ď‚·Alters field strength along left-right direction (horizonal of B0)
Slice select on the Y: coronal
ď‚·Alters field strength along vertical axis of bore
Slice select on the Z: axial
ď‚·Alters field strength along the long axis bore
Obliques: two gradients together
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oFrequency encoding: spatially locating (encoding) signal along the long axis of the
anatomy
oPhase encoding: spatially locating (encoding) signal along the short axis of the anatomy
o
ď‚·
oSteep vs gradual
oDefines slice thickness – slope will impact the rate of change and therefore the ROI that
is susceptible to energy transfer
oPrecessional frequency is altered in a linear fashion
ď‚·Dephasing and Rephasing: Recap
oSpins dephase/precess slower due to position along gradient
Faster precessing magnetic moments dephase due to stronger magnet
Speed of dephasing: gradient amplitude or strength and duration
Gradient moment
Can reverse application: ie. bring back to zero

oRephase
Apply gradient of reverse polarity
Spins become equal
Measurable signal
Echo

Slice Thickness
ď‚·To give each slice a thickness a band of nuclei must be excited by the excitation pulse
ď‚·the slope of the selected slice gradient will determine the precessional frequency difference
between two points on the gradient
osteep slopes= greater difference in precessional frequency between two points on the
gradient
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Document Summary
Resonance requires rf applied at 90 degrees to bo at precessional frequency of h. Rf pulse gives h enough energy so that transverse magnetization occurs. Voltage induced at transverse plane (signal frequency= larmor frequency of h) The system must be able to locate signal spatially in three dimensions, so that it can position each signal at the correct point on the image. Locates a slice in any plane (this is where its different from ct we can prospectively acquire in the x y or z plane). 2. the signal is located/encoded along both axes of the image: these tasks are performed by gradients. The position of a nucleus along a gradient can be identified according to its precessional frequency. Faster or slower spin will void signal from that area bc there will not be an efficient transfer of energy to that region: at point a: a nucleus experiences a field of 0. 9995 t.
