Physics: Spatial Encoding (In Practice)

Front 1

How does the MR computer know where the signal is coming from

Back 1

Front 2

What is the magnetic field strength at the isocentre

Back 2

same as Bo

Front 3

What determines the slope of the gradient magnetic field

Back 3

the amplitude of the magnetic field gradient

Front 4

What does the slope of the gradient magnetic field determin

Back 4

it will determine the rate of change along the gradient axis

Front 5

What is the difference between as gradient with a steep slop and a shallow slope

Back 5

the steep slope have a greater change in gradient between to points

Front 6

What causes a greater alteration in the precession of 2 protons; shhallow or steep slope

Back 6

steep

Front 7

Sleep stop Vs shallow slope

Back 7

Front 8

NOTE THE GRADIENTS ARE CHANGED DEPENDENDING ON THE IMAGE BEING OBTAINED

Back 8

Front 9

What is the are the axis of the gradients of the head

Back 9

Front 10

How is a slice selection gradient aligned

Back 10

the same direction as the main magnetic field

Front 11

How is a particular slice chosen

Back 11

since there is slightly different precession along the gradient just give a RF pulse that will only react to that pulse (according to the lamor equation)

Front 12

How do you know which direction is going to be the slice selection gradient

Back 12

by the imaging plane being imaged

Front 13

In axial imaging what plane is the slice selecting gradient

Back 13

z-plane

Front 14

In coronal imaging what axis is the slice selection gradient

Back 14

Y axis

Front 15

What happens if you want to image oblique images

Back 15

a combination of 2 magnetic field gradients will have to be used for the slice selection (not shown but use your imagination)

Front 16

How is the size of the slice selected

Back 16

depends on the steepness of the magnetic field gradient being used. But steeper is thinner

Front 17

How is the frequency bandwidth that is recieved in thin slices compared to thick slices.

Back 17

The same

Front 18

Error previous page. This slide is misleading becauses either the size of the bandwith being recieved or the steepness of the gradient can be adjusted to change the slice thickness.

Back 18

Front 19

How is the bandwidth or steepness of the magnetic gradient changed.

Back 19

It is automaticaly done by selecting the slice size in the MRI scanner

Front 20

What is the bandwidth in these images the RF pulse or the bandwitdth that is recieved by the machine when the signal occurs

Back 20

The RF pulse bandwidth

Front 21

What determines the size of the gap between the gradient

Back 21

The gap between the slices is determined by the gradient slope and by
the thickness of the slice.
(NOT SHOWN IN THIS IMAGE AND THIS EXPLANATION DOES NOT FULLY EXPLAIN WHY)

Front 22

Why is the slice selection gradient turned on during RF pulse administration.

Back 22

Front 23

The slice selection gradient is turned on because the RF pulse will be tuned to that precessional frequency and needs to know where to go.

Back 23

k

Front 24

Which axis is typically the frequency encoding direction

Back 24

long axis

Front 25

What is the direction of the frequency encoding (long axis) of the body

Back 25

X

Front 26

What is meant by the long axis

Back 26

the longer of the two remaining dimensions once the slice selection gradient is chosen (IE look at the frequency encoding direction of the head (nose to occiput)

Front 27

When is the frequency encoding (readout gradient) turned on

Back 27

When the signal is going to be recieved.

Front 28

how does the turning on the frequency encoding gradient effect the rephasing of the protons

Back 28

I am assuming it will make them rephase quicker then dephase

Front 29

What determines the field of view (FOV) when doing a scan

Back 29

Front 30

What happens to the protons as the readout gradient occurs

Back 30

continued rephasing

Front 31

What happens to the protons as the readout gradient occurs

Back 31

continued rephasing

Front 32

What does the steepness of the frequency encoding gradient determine

Back 32

he steepness of the slope of the frequency encoding
gradient determines the size of the anatomy covered along the frequency
encoding axis during the scan. This is called the field of view (FOV). The steeper the less anatomy that will be covered.

Front 33

What happens to the nuclei at the isocentre of the phase encoding gradient

Back 33

No change

Front 34

Back 34

Front 35

How are these frequencies mapped

Back 35

Each frequency is allocated a
frequency column that is mapped onto the FOV along the frequency axis

Front 36

What does 1 cycle look like

Back 36

Front 37

What must occur to the bottom for nyquist thereom to be followed

Back 37

increased sampling time. (nyquist only says 2 samples per cycle not at least 2 samples)

Front 38

What does reducing the frequency bandwidth do to the sampling time

Back 38

Reducing the receive bandwidth affects the minimum TE because the
echo is usually centred on the middle of the frequency encoding gradien

Front 39

What does increasing the recieve bandwidth do

Back 39

increases TE time