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35 Cards in this Set

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What is necessary for resonance to occur:
RF must be applied at 90 degree to Bo
RF frequency must match precessional frequency of hydrogen
Transverse magnetization is created
Magnetic moments of the hydrogen are in phase
A voltage or signal is therefore induced in the receiver coil
MR signal has a frequency equal to the Larmor frequency of the hydrogen
The system must be able to locate signal spatially in three dimensions
Each signal – correct point on the image
Gradients are -
Gradients are coils of wire that, when a current is passed through them, alter the magnetic field strength of the magnet in a controlled and predictable way
What do gradients do?
Gradients alter the magnetic field strength of the magnet in a controlled and predictable way
They add or subtract from the existing field in a linear fashion so that the magnetic field strength at any point along the gradient is known
When a gradient is applied the following occur:
At isocentre the field strength remains unchanged
At a certain distance away from isocentre the field strength either increases or decreases.
The slope of the gradient signifies the rate of change of the magnetic field strength along its length.
The strength or amplitude of the gradient is determined by how much current is applied to the gradient coil.

Larger currents create steeper gradients so that the change in field strength over distance is greater.
The polarity of the gradient determines which end of the gradient produces a higher field strength than isocentre (positive) and which a lower field strength than isocentre (negative)
How do gradients work?
The precessional frequency of the magnetic moments of nuclei is proportional to the magnetic field strength experienced by them (as stated by the Larmor equation). The frequency of the signal received from the patient can be changed according to its position along the gradient.

Therefore the position of a nucleus along a gradient can be indentified according to its percessional frequency.
In order to do this there must be three orthogonal sets of gradients situated within the bore of the magnet.
They are named according to the axis along which they work
The _ gradient alters the magnetic field strength along the _ axis (longitudinal).
The _ gradient alters the magnetic field strength along the _ axis (vertical).
The _ gradient alters the magnetic field strength along the _ axis (horizontal).
The Z gradient alters the magnetic field strength along the Z axis (long).
The Y gradient alters the magnetic field strength along the Y axis (vertical).
The X gradient alters the magnetic field strength along the X axis (horizontal).
Z axis = ?
Y axis = ?
X axis = ?
Z axis = (longitudinal).
Y axis = (vertical)
X axis = (horizontal)
What is the centre of all THREE gradients.
The isocentre.
Then ________ is the centre of all three (3) gradients.
The isocentre is the centre of all three gradients.
formula for finding the "final magnetic field strength" is -
B = Bo (+/-) (DxG / 1000)
B =
Bo =
D =
G =
B = final magnetic field strength
Bo = Main magnetic field strength
D = distance from magnet center
G = gradient strength
There are only three gradients and they are used to perform which important functions during a pulse sequence?
1.
2.
3.
1 Slice selection – locating a slice in the scan plane selected.

2 Spatially locating a signal along the long axis of the image. This is called frequency encoding.

3 Spatially locating a signal along the short axis of the image. This is called phase encoding
locating a slice in the scan plane selected is -
Slice selection (SS)
Spatially locating a signal along the long axis of the image is called _______ ______ .
frequency encoding. (FE)
Spatially locating a signal along the short axis of the image. This is called _____ ________ .
phase encoding.(PE)
spatial encoding IS -
spatially locating a signal in three dimensions using gradients.
Spatial encoding is accomplished by ______ _____ ____ .
superimposing gradient fields.
There are three gradient fields in the _ ,_ and _
directions.
• Gradients alter the magnetic field resulting in a change in _____ _____ or a change in _____ .
x, y, and z
resonance frequency
phase.
As a gradient alters the magnetic field strength of the magnet linearly, the magnetic moments of nuclei within a specific slice location along the gradient have ?- 1 -
A slice can therefore be selectively excited by ?- 2 -
1. a unique precessional frequency when the gradient is on.
2.transmitting RF at that unique precessional frequency.
The __ gradient selects axial slices, so that nuclei in the patient’s head spin at a different frequency from those in the feet.
Z gradient =axial slices
the range of the frequencies we wish to sample is called -
• Receive bandwidth
Enough frequencies must occur during readout to achieve sufficient data points. This is determined by
the receive bandwidth.
• Sampling frequency is :
32000 Hz = 32KHz
• Hz =
how many cycles per second
1 cycle in 0.00003125 s =
• 32 000 cycles in 1 s
• The __ gradient selects coronal slices, so that nuclei at the back of the patient spin at a different frequency from those at the front.
Y
•The _ gradient selects axial slices, so that nuclei in the patient’s head spin at a different frequency from those in the feet.
Z
• The __ gradient selects sagittal slices, so that nuclei on the right-hand side of the patient spin at a different frequency from those on the left.
X
-Spatial encoding is accomplished by
superimposing gradient fields. There are three gradient fields in the
x, y, and z directions.
• Gradients alter the magnetic field resulting in a change in resonance frequency or a change in phase.
-As a gradient alters the magnetic field strength of the magnet linearly, the magnetic moments of nuclei within a specific slice location along the gradient ----Have what?
have a unique precessional frequency when the gradient is on.
- A slice can therefore be selectively excited by ----
transmitting RF at that unique precessional frequency.
•The Z gradient selects axial slices, so that nuclei in the patient’s___ spin at a different frequency from those in the ____
• The Y gradient selects coronal slices, so that nuclei at the __ __ ___ ___ spin at a different frequency from those at ___ _____
• The X gradient selects sagittal slices, so that nuclei on the ___ ___ ____ of the patient spin at a different frequency from those __ ___ ____
head - feet. z
back of the patient - the front. y
right-hand side - on the left. x
•To give each slice a thickness, a ____ of nuclei must be excited by the excitation pulse.
“band”
•The slope of the slice select gradient determines the difference in precessional frequency between two points on the gradient.
•Steep gradient slope – results in _____ difference in precessional frequency between to point – results in ___ ____
•Shallow gradient slopes result in ____ difference in precessional frequency between the same two point – results in ____ ____
large, thin slice
small ,thick slice
•In order to attain slice thickness, a range of frequencies must be transmitted to produce resonance across the whole slice (and therefore to excite the whole slice). This range of frequencies is called a ______ and because RF is being transmitted at this instant, it is specifically called the ______ _______
bandwidth
transmit bandwidth