Chapter 5 Spatial Encoding

Question 1

RF pulse is applied at

Answer 1

90 degree angle from B0
- at the processional frequency of the magnetic moments of hydrogen nuclei (within tissue)

Question 2

magnetization is created in

Answer 2

the transverse plane and the individual magnetic moment of hydrogen line up into phase

Question 3

voltage/ signal is created in

Answer 3

transverse plane of the receiver coil

Question 4

signal in an alternating voltage equal to

Answer 4

Larmor frequency

Question 5

magnetic moments precess at the same frequency

Answer 5

so all their signals are oscillating at the same frequency
- so the system cannot spatially locate them

Question 6

to create an image the system needs to

Answer 6

calculate how much signal is coming from each #d location in the paitent

Question 7

voxel

Answer 7

3D

Question 8

1st step is to

Answer 8

select a slice within the tissues that is gonna be examined

Question 9

pixel

Answer 9

2D
- there is a signal within

Question 10

spatial encoding

Answer 10

process of locating the signal

Question 11

spatial encoding is a function of

Answer 11

gradients

Question 12

all hydrogen precesses at the same frequency when

Answer 12

exposed to the same magnetic field strength

Question 13

within the main magnetic field there are

Answer 13

inhomogenities

Question 14

to locate the individual signals

Answer 14

the B0 is altered so it slopes in a linear and predictable way

Question 15

graded (sloped) magnetic fields are generated by

Answer 15

cylinder electro magnets that are in the warm bore of the cryostat
- coils called gradient coils

Question 16

warm bore

Answer 16

inside the cylinder of cryostat
- contains patient bore and the components of MR system
- operates at room temp

Question 17

gradient coil

Answer 17

current is passed through each of the coils

Question 18

current is passed through each coil at

Answer 18

a specific time during the pulse sequence

Question 19

Faraday’s law

Answer 19

when a current is passed through a gradient, a magnetic field is created

Question 20

polarity depends on

Answer 20

which end of the gradient magnetic field is higher than B0 and which ones is lower

Question 21

Isocenter

Answer 21

center of the bore of the magnet in all planes
- this means the middle axis of the gradient stay at the same field strength as B0 even when they are switched on

Question 22

Larmor equation

Answer 22

magnetic moments increase or decrease based on strength of the magnet

Question 23

magnetic field strength increases

Answer 23

hydrogen nuclei speed up

Question 24

magnetic field strength decreases

Answer 24

hydrogen nuclei slow down

Question 25

change in the magnetic field caused by gradient is

Answer 25

small - frequency of the magnetic moment spin located in the gradient is less than 1%

Question 26

gradient amplitude determines

Answer 26

rate of change of the magnetic field strength along the gradient axis

Question 27

steep gradient slopes=

Answer 27

large differences in precessional frequency

Question 28

shallow gradient slopes

Answer 28

small differences in precessional frequency

Question 29

what happens in the gradient coil that determines the polarity of the gradient?

Answer 29

the direction of the current through the coil

Question 30

what happens in a gradient that determines the amplitude of a gradient?

Answer 30

amplitude of the current applied through the gradient coil.

Question 31

to perform spatial localization

Answer 31

MR signal must be spatially encoded in 3 directions - use 3 gradient magnetic fields

Question 32

3 axes

Answer 32

X, Y , Z

Question 33

the order that these gradients are activated

Answer 33

- slice select - phase encoding - frequency/ readout encoding

Question 34

any one of these 3 gradients can perform

Answer 34

- slice select - phase encoding - frequency/ readout encoding

Question 35

each one of these gradient magnetic fields are assigned

Answer 35

- one orthogonal direction - one function of spatial encoding

Question 36

Z axis

Answer 36

always parallel to B0

Question 37

Y axis

Answer 37

always perpendicular (90 degrees) to B0 - Ceiling to floor

Question 38

X gradient

Answer 38

left to right

Question 39

slice planes

Answer 39

- axial - sagittal - coronal

Question 40

when the slice is selected

Answer 40

gradient for that slice is automatically chosen

Question 41

orientation of the phase and frequency gradients are logical

Answer 41

if choosing one direction, the other is automatically chose

Question 42

Z gradient

Answer 42

alters the Long axis - head to foot of patient

Question 43

Y gradient

Answer 43

alters vertical/short axis - front to back

Question 44

X gradients

Answer 44

alters horizontal axis - left to right

Question 45

slice selection

Answer 45

locates the slice within the scan plane

Question 46

frequency encoding

Answer 46

locates the signal along the long axis of the slice

Question 47

phase encoding

Answer 47

locates the signal along the short axis of the slice

Question 48

Oblique slices

Answer 48

combination of two or more gradients

Question 49

gradient magnetic fields are

Answer 49

superimposed on B0

Question 50

different areas of the body can imaged in cross sectional volumes known as

Answer 50

slice