Physics: Mri (Spatial Encoding Mri)

Front 1

What are the 3 things that are required to know where a signal is coming from

Back 1

First of all, the desired slice must be selected
Then, spatial information is encoded along the rows
Finally, spatial information is encoded along the columns

Front 2

How is decoding of the MR signal done

Back 2

Decoding of spatial information, included in the NMR signal as modifications of frequency and phase, is performed by an inverse Fourier Transform

Front 3

What is used to determine the location of a slice

Back 3

slice selection gradient (GSS)

Front 4

Which in each slice selection gradient what is done to further pinpoint where the signal is coming from

Back 4

Within this volume, the position of each point will be encoded vertically and horizontally by applying a phase encoding gradient (GPE), and a frequency-encoding gradient (GFE).

Front 5

What is spatial encoding referring to

Back 5

slice selection gradient
phase encoding gradient
frequency encoding gradient

Front 6

What gradient is usually done first

Back 6

First of all, a slice selection gradient (GSS) is used to select the anatomical volume of interest

Front 7

How are the 3 different gradients the same and different

Back 7

The different gradients used to perform spatial localization have identical properties but are applied at distinct moments and in different directions.

Front 8

What is an example of a gradient in the Z-axis

Back 8

Slice selection gradient

Front 9

What is an example of a gradient in the X-axis

Back 9

Left to Right

Front 10

What is an example of a gradient in the Y direction

Back 10

anterior posterior (relative to axial image)

Front 11

How are the different types of gradients shown on a MRI sequence diagram

Back 11

Front 12

How does a slice selection gradient work

Back 12

Notice how each slice proton spin is slightly off because of the gradient in the Z direction

Front 13

What happens when the an RF pulse is applied in the presence of the slice selection gradient

Back 13

An RF wave is simultaneously applied, with the same frequency as that of the protons in the desired slice plane. This causes a shift in the magnetization of only the protons on this plane. As none of the hydrogen nuclei located outside the slice plane are excited, they will not emit a signal.

Front 14

What is a selective pulse

Back 14

The RF wave associated with the slice selection gradient and the adapted resonance frequency, is called the selective pulse

Front 15

How is the thickness of a pulse determined

Back 15

The thickness of the slice can be varied by adjusting the bandwidth of the selective pulse and the amplitude of the slice selection gradien

Front 16

Does an RF pulse have only one frequency

Back 16

no, An RF pulse does not have one frequency only (for this, it would need to be of infinite duration). It covers a certain bandwith, which depends on the shape of the pulse and its duratio

Front 17

How is slice thickness determined in a fixed amplitude gradient

Back 17

For a fixed amplitude gradient, the wider the bandwidth, the greater the number of protons excited and the thicker the slice

Front 18

How is the slice thickness determined in a fixed bandwitdth gradient

Back 18

For a fixed bandwidth, the stronger the gradient, the greater the variation of precession frequency in space and the thinner the slice

Front 19

Does a RF pulse have only one frequency

Back 19

no, a bandwidth

Front 20

What determines the bandwidth of a RF pulse

Back 20

the shape and duration of the RF pulse

Front 21

What are 2 ways to change the slice thickness of an MR image

Back 21

by adjusting bandwidth of the RF pulse amplitude of the gradient

Front 22

If there is a fixed amplitude gradient what determines the slice thickeness

Back 22

the bandwidth. If the bandwidth is wider it means there will be a greater slice thickness

Front 23

What happens if there is a fixed bandwidth, then what will determine the slice thickness

Back 23

the amplitude of the gradient. The stronger the gradient the thicker the slice.

Front 24

When is the slice selection gradient turned on

Back 24

when the RF pulse is delivered

Front 25

What happens to protons as a result of the slice selection gradient being turned on during the RF pulse

Back 25

the slice selection gradient will have a spin dephasing effect due to the dispersion in the resonance frequency produced

Front 26

What is done to neutralize the effect of the slice selection gradient being turned on during the RF pulse

Back 26

o neutralize this effect, after applying the selective RF pulse (concomitant with the gradient) another gradient lobe is applied, along the same axis but in the opposite direction and with a surface (amplitude x time) equal to half the initial gradient lobe.

Front 27

Is a dephasing lobe always needed

Back 27

no, In the case of a 180° pulse, the dephasing effects neutralize symmetrically in relation to the centre of the RF pulse, so no rephasing lobe needs to be applied

Front 28

What is the second step in the spatial encoding

Back 28

the phase encoding gradient

Front 29

Does this have to be in the vertical direction

Back 29

no, but in this example it is

Front 30

What do all the protons within a slice look like before the phase encoding gradient (vertical direction)

Back 30

Front 31

What do they look like after the phase encoding gradient

Back 31

vertical direction they are slightly off

Front 32

Back 32

Front 33

What does the different rows of data look like

Back 33

Front 34

What is different in the dephased acqusitions

Back 34

strenght of the phase encoding gradient

Front 35

How is this represented in the MRI sequence chart thing

Back 35

on the right red lines

Front 36

What does each different phase encoding line fill up

Back 36

a Y direction in K-space

Front 37

Each phase encoding sequence will fill on line from top to bottom

Back 37

Front 38

What is the final step in spatial encoding

Back 38

The final step in spatial encoding consists in applying a frequency encoding gradient, when the signal is received, in the last direction (horizontal in our example)

Front 39

What is an easy way to remember what direction is columns

Back 39

up and down like a roman column

Front 40

What direction is the frequency encoding direction

Back 40

right to left

Front 41

What does the frequency encoding direction look like

Back 41

columns