Unit 5 - Spatial Encoding

Question 1

Define voxel

Answer 1

finding something in three dimensions (volume)

Question 2

Define pixel

Answer 2

found in two dimensions

Question 3

Define spatial encoding

Answer 3

process of locating signals in three dimensions

Question 4

Define gradients

Answer 4

copper winding that can add or subtract from the main magnetic field

Question 5

Identify the main purpose of gradient coils

Answer 5

create a slope of magnetic field strength that is linear and predictable

Question 6

Define polarity

Answer 6

direction of the magnetic field

Question 7

Define magnetic isocenter

Answer 7

where the magnet is the most homogenous; where the three axis come together (dead center)

Question 8

Define gradient amplitude

Answer 8

the steepness of the slope created
(the steeper = more current)

Question 9

State the direction each gradient controls

Answer 9

X-gradient: right to left
Y-gradient: anterior/posterior
Z-gradient: long axis (head to foot)

Question 10

{?} Explain how slice selection works

Answer 10

when a slice plane is selected on the MR protocol, it determines which gradient will be activated and performs slice-selection during the RF excitation pulse

Question 11

When does slice selection occur in spin echo?

Answer 11

is turned on at the 90 degree RF excitation pulse and during the 180 degree rephasing pulse

Question 12

When does slice selection occur in gradient echo sequences?

Answer 12

during the variable RF excitation pulse

Question 13

Define transmit bandwidth

Answer 13

band of frequencies

Question 14

Define what two ways a system controls slice thickness

Answer 14

1) the slice-select gradient amplitude (slope of the gradient)

2) bandwidth of the transmit RF excitation pulse

Question 15

Explain how frequency encoding works

Answer 15

locates signal along the long axis of the image

Question 16

{?} identify which axis in the image frequency typically controls

Answer 16

X-gradient: axial
Y-gradient: axial (head)
Z-gradient: coronal & sagittal

Question 17

State another name for the frequency encoding gradient

Answer 17

readout gradient

Question 18

Define sampling time

Answer 18

specific time frequency encoding gradient is switched on for

Question 19

Define receive bandwidth

Answer 19

a range of frequencies

Question 20

{?} Discuss how the frequency gradient controls field-of-view (FOV)

Answer 20

the amplitude of the frequency encoding gradient is one of the factors that determines FOV

Question 21

Identify when the frequency gradient is on during the pulse sequence

Answer 21

during the echo of a pulse sequence

Question 22

Explain how phase encoding works

Answer 22

locates signal along the short axis

Question 23

{?} Identify which axis in the image phase typically controls

Answer 23

X-gradient: coronal, axial
Y-gradient: sagittal, axial
Z-gradient: long axis

Question 24

Identify when the phase encoding gradient is on during a pulse sequence

Answer 24

it’s turned on after the RF excitation pulse

Question 25

Define steep phase gradients

Answer 25

has a large phase shift (thin slice)

Question 26

Define shallow phase gradients

Answer 26

has a small phase shift (thick slice)

Question 27

Identify which gradient changes amplitude (slope) from one TR period to the next

Answer 27

phase encoding gradient

Question 28

Define spatial frequency

Answer 28

the number of line pairs per unit length