Chapter 5: Spatial Encoding

Question 1

Spatial encoding is a process in which signal is located and pinpointed, and it is performed by what?

Answer 1

Gradients

Question 2

Graded or sloped magnetic fields are generated by cylindrical electromagnets situated in the warm bore of the cryostat. These coils are called?

Answer 2

Gradient coils

Question 3

If current flows clockwise through the gradient coil, then the magnetic field induced around the coil _____ to Bo.

Answer 3

adds; This increased the magnetic field strength relative to Bo.

Question 4

If current flows anticlockwise through the gradient coil, then the magnetic field induced around the coil _____ from Bo.

Answer 4

subtracts; The magnetic field is decreased relative to Bo.

Question 5

The middle of the axis of the gradient remains at the field strength of the main magnetic field even when the gradient is switched on. This is called the what?

Answer 5

Magnetic isocenter

Question 6

The changes in field strength and therefore frequency imposed by gradients are small. They usually vary the magnetic field and frequency of magnetic moments of spins located along them by less than ____ percent

Answer 6

one

Question 7

What determines the rate of change of the magnetic field strength along the gradient axis?

Answer 7

Gradient amplitude

Question 8

Which type of gradient slope alter the magnetic field strength between two points more, steep or shallow?

Answer 8

Steep

Question 9

The _____ of current passing through the coil determines the amplitude, strength, or slope of the gradient.

Answer 9

amount

Question 10

The _____ of the current passing through the coil determines its polarity.

Answer 10

direction

Question 11

What spatially locates or encodes signal depending on where it is located along these three gradients.?

Answer 11

Gradients

Question 12

Gradients spatially locate (encode) signal along the long axis of the slice, this is called ______ encoding.

Answer 12

frequency

Question 13

Gradients spatially locate (encode) signal along the short axis of the slice, this is called _____ encoding.

Answer 13

phase

Question 14

In SE pulse sequences, when is the slice-select gradient switched on?

Answer 14

During the application of the 90 degree RF excitation pulse and during the 180 degree RF rephasing pulse.

Question 15

In gradient-echo sequences, when is the slice-select gradient switched on?

Answer 15

Only during the RF excitation pulse.

Question 16

The amplitude of any gradient is determined by how much ______ passes through it when it is switched on.

Answer 16

current

Question 17

To achieve ______ slices, a steep slice-select slope and/or ______ transmit bandwidth is applied.

Answer 17

thin; narrow

Question 18

To achieve ______ slices, a shallow slice-select slope and/or ______ transmit bandwidth is applied.

Answer 18

thick; broad

Question 19

What 2 things determine the range of nuclei that resonate on either side of the center frequency?

Answer 19

Transmit bandwidth and gradient slope

Question 20

The center frequency is called the _____ frequency

Answer 20

carrier

Question 21

The transmit bandwidth is a small range of frequencies on either side of the carrier frequency that typically has a range of ___ to ____ KHz.

Answer 21

1 to 2

Question 22

What 2 things determines the gap between slices?

Answer 22

gradient slope and the thickness of the slice

Question 23

Thin slices are produced by passing a ______ current through the slice-select gradient.

Answer 23

large

Question 24

Thick slices are produced by passing a ______ current through the slice-select gradient

Answer 24

smaller

Question 25

A large current passing through the slice-select gradient coil produces a ______ or _____ gradient.

Answer 25

high-amplitude or steep

Question 26

Once a slice is selected, signal coming from it is located along both axes of the image. Signal is usually located along the long axis of the anatomy by a process known as _______.

Answer 26

frequency encoding

Question 27

The frequency-encoding gradient is switched on when signal is received (i.e. when the echo occurs) and is often called the _____ or ______ gradient.

Answer 27

readout or measurement

Question 28

The frequency-encoding gradient is switched on for a very specific time. This is called the ______ time/window.

Answer 28

sampling

Question 29

A steep frequency-encoding gradient produces a ____ FOV dimension in the frequency axes of the image.

Answer 29

small

Question 30

A shallow frequency-encoding gradient produces a _____ FOV dimension in the frequency axes of the image.

Answer 30

large

Question 31

Signal is located along the remaining short axis of the anatomy, and this localization of signal is called ______.

Answer 31

phase encoding

Question 32

The number of steps in the ladder or how many times the phase-encoding gradient is switched on to a different amplitude or polarity during the sequence is determined by the ________ we select in the scan protocol.

Answer 32

phase matrix

Question 33

The steepest slope of phase-encoding gradient, either positive or negative, determines ________ in the phase direction of the image.

Answer 33

pixel size

Question 34

To achieve a small pixel, the highest or lowest step in the ladder is selected using a ______ phase-encoding gradient.

Answer 34

steep

Question 35

To achieve a large pixel, the highest or lowest step in the ladder is selected using a ______ phase-encoding gradient.

Answer 35

shallow

Question 36

______ is the math that gets us from frequencies in the echo to the image.

Answer 36

FFT (Fast Fourier Transform)