Chapter 3 Encoding And Image Formation

Question 1

Def of gradients

Answer 1

Electromagnets or winding coils creating a linear slope in MF strength across the imaging volume in a particular direction. There are 3 sets of gradient coils x y z

Question 2

Shape and function of gradients

Answer 2

Shape- are in pairs.: the pairs of gradients are of equal and opposite polarity

Function- each either increase or decrease the MF by a specific amt

Question 3

What makes the loud sounds in MRI

Answer 3

The gradients

Question 4

What 7 things does the gradients CONTROL

Answer 4

Matrix size
FOV
Min TE
Slice thickness
Slices TR
Ability to obtain oblique images
ETL echo spacing

Question 5

What are the 4 characteristics of gradients

Answer 5

Amplitude
Duty cycle
Rise time
Slew rate

Question 6

What does a gradient do and what are the 3 planes

Answer 6

Creates a plane or environment and isolates the Hs in a specific area

AP-y
SI- z
LR- x

Question 7

Is B1 always parallel or perpendicular to B0

Answer 7

Perpendicular

Question 8

What is the units to measure gradient strength?

Answer 8

B0/distance G/cm or T/m

1G/cm = 10mT/m

Question 9

Why is it important to have higher amplitude in gradients? What’s a good range?

Answer 9

Allows us to do thinner slices, use a smaller FOV and provides a higher resolution

Good range is 10-60 mT/m

15 is good but 33 is best
in magnet the difference and strength allows for faster acquisition for the thinner slices as well as wider range of capabilities

Question 10

What is rise time and how is it measured

Answer 10

It’s how long for gradients to go from zero to maximum amplitude. It is measured in microseconds. The smallest number means less time and is fastest

Question 11

What is the duty time? How was it expressed?

Answer 11

It’s expressed as a percentage. It is the percent of time the gradient works at maximum amplitude. It allows the gradients to combined a small fov with a high matrix, thin slice, flow comp.

The higher a percentage usage the less slices can be done

Question 12

What is slew rate

Answer 12

It is a marketing term. It is the speed and strength of gradients and measured in mT/m/s

The rate the gradients may be turned on or off

The faster the slew rate the shorter the TEs

Question 13

What happens if there is inhomogeneity induced by the gradient in slice selection?

Answer 13

Causes spins to resonate at different frequencies. Only proton spinning at the same frequency as an applied RF pulse will respond

Question 14

What must be in first…. B1 or SS and why?

Answer 14

SS just a nanosecond before to B1

BC SS will select the plane/environment, select slice thickness and adjust the bandwidth… then do the RF pulse

Question 15

When in equilibrium and out of phase where is M? Where is M if protons are in phase?

Answer 15

Longitudinal if out of phase

Transverse if protons are in phase.

Question 16

What is the function of slice thickness (2)

Answer 16

Slope and bandwidth

Question 17

Steep versus shallow slopes and narrow versus wide bandwidth

Answer 17

Steep will provide thin slices, smaller range btwn two pts….however requires more current and gradient strength

Shallow gives thicker slices and large range

Narrow=thin slices
Wide= thick slices
Bandwidth is the size of B1

Question 18

What is a voxel?

Answer 18

It is a three dimensional boxes that make up the FOV. DxWxH

Will affect the resolution of scan. The smaller the FOV the better. So need more voxels within FOV

Question 19

The location of spins in a particular row is determined by what?

Answer 19

Phase shift. Phase shift between spins is directly proportional to their location

it helps determine the rows of a matrix

Question 20

What is phase encoding

Answer 20

It’s the magnetic moment of hydrogens along the gradient in ROWS

Question 21

When choosing a PE do you want the longest or shortest to go along with SS

Answer 21

Shortest

Question 22

When do we apply a frequency encoding (FE) gradient and what does it help determine?

Answer 22

Right before the PE gradient to negate the effect of the SS gradient
Also applied again after the rephasing 180 during read out

Helps determine the Columns of the matrix and the FOV

Question 23

What determines the location of hydrogens along the gradient in columns

Answer 23

FE
Frequency encoding

Long axis

Question 24

Is M parallel or perpendicular with the protons are in phase

Answer 24

Perpendicular

Question 25

What is the FE and PE for the following:
High
Standard
Coarse

Answer 25

High- 512 512-256

Standard- 256 246-128
However the best is 192-256 PE

Coarse 128 128-64

Question 26

What should be higher FE or PE

Answer 26

Frequency

Question 27

If you have a higher PE what does it do to scan time and resolution

Answer 27

Increases

Question 28

When is PE gradient switched on

Answer 28

After the excitation pulse

Question 29

When is the FE gradient switched on

Answer 29

During the collection of the signal….. the echo

Question 30

What is acquisition window

Answer 30

Also known as sampling time

The duration of the readout gradient.

Question 31

What is K space and where is it collected at?

Answer 31

It is raw data formed by collections of echoes as gradient amplitudes change for every TR

The data is collected in the array processor

Question 32

What is the unit of K space

Answer 32

Radians per centimeter

Question 33

What is another name for SS

Answer 33

Physical gradient or logical gradients

Question 34

Short axis? Long axis?

Answer 34

Short is PE

Long is FE

Question 35

Other names for SS

Answer 35

Physical and logical

Question 36

What is a transient MF

Answer 36

Gradients, superpulse electromagnetic field

Question 37

What is M?
Bo
B1

Answer 37

NMV
External MF
RF pulse

Question 38

What is Bandwidth

Answer 38

Range of frequency
Narrow or wide
B1 field