Physics Week3 Mri In Practice

Front 1

What is necessary for resonance to occur:

Back 1

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

Front 2

Gradients are -

Back 2

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

Front 3

What do gradients do?

Back 3

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

Front 4

When a gradient is applied the following occur:

Back 4

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)

Front 5

How do gradients work?

Back 5

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

Front 6

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).

Back 6

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).

Front 7

Z axis = ?
Y axis = ?
X axis = ?

Back 7

Z axis = (longitudinal).
Y axis = (vertical)
X axis = (horizontal)

Front 8

What is the centre of all THREE gradients.

Back 8

The isocentre.

Front 9

Then ________ is the centre of all three (3) gradients.

Back 9

The isocentre is the centre of all three gradients.

Front 10

formula for finding the "final magnetic field strength" is -

Back 10

B = Bo (+/-) (DxG / 1000)

Front 11

B =
Bo =
D =
G =

Back 11

B = final magnetic field strength
Bo = Main magnetic field strength
D = distance from magnet center
G = gradient strength

Front 12

There are only three gradients and they are used to perform which important functions during a pulse sequence?
1.
2.
3.

Back 12

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

Front 13

locating a slice in the scan plane selected is -

Back 13

Slice selection (SS)

Front 14

Spatially locating a signal along the long axis of the image is called _______ ______ .

Back 14

frequency encoding. (FE)

Front 15

Spatially locating a signal along the short axis of the image. This is called _____ ________ .

Back 15

phase encoding.(PE)

Front 16

spatial encoding IS -

Back 16

spatially locating a signal in three dimensions using gradients.

Front 17

Spatial encoding is accomplished by ______ _____ ____ .

Back 17

superimposing gradient fields.

Front 18

There are three gradient fields in the _ ,_ and _
directions.
• Gradients alter the magnetic field resulting in a change in _____ _____ or a change in _____ .

Back 18

x, y, and z
resonance frequency
phase.

Front 19

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 -

Back 19

1. a unique precessional frequency when the gradient is on.
2.transmitting RF at that unique precessional frequency.

Front 20

The __ gradient selects axial slices, so that nuclei in the patient’s head spin at a different frequency from those in the feet.

Back 20

Z gradient =axial slices

Front 21

the range of the frequencies we wish to sample is called -

Back 21

• Receive bandwidth

Front 22

Enough frequencies must occur during readout to achieve sufficient data points. This is determined by

Back 22

the receive bandwidth.

Front 23

• Sampling frequency is :

Back 23

32000 Hz = 32KHz

Front 24

• Hz =

Back 24

how many cycles per second

Front 25

1 cycle in 0.00003125 s =

Back 25

• 32 000 cycles in 1 s

Front 26

• The __ gradient selects coronal slices, so that nuclei at the back of the patient spin at a different frequency from those at the front.

Back 26

Y

Front 27

•The _ gradient selects axial slices, so that nuclei in the patient’s head spin at a different frequency from those in the feet.

Back 27

Z

Front 28

• 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.

Back 28

X

Front 29

-Spatial encoding is accomplished by

Back 29

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.

Front 30

-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?

Back 30

have a unique precessional frequency when the gradient is on.

Front 31

- A slice can therefore be selectively excited by ----

Back 31

transmitting RF at that unique precessional frequency.

Front 32

•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 __ ___ ____

Back 32

head - feet. z
back of the patient - the front. y
right-hand side - on the left. x

Front 33

•To give each slice a thickness, a ____ of nuclei must be excited by the excitation pulse.

Back 33

“band”

Front 34

•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 ____ ____

Back 34

large, thin slice
small ,thick slice

Front 35

•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 ______ _______

Back 35

bandwidth
transmit bandwidth