Chapter 6

Question 1

Flowing nuclei exhibit different ___ from their neighboring stationary
nuclei, and originate primarily from nuclei in __

Answer 1

contrast characteristics; blood and CSF

Question 2

The motion of flowing nuclei causes __ and results in __

Answer 2

mis‐mapping of signals; phase ghosting artifacts

Question 3

name 4 types of flow

Answer 3

laminar flow, spiral flow, vortex flow, and turbulent flow

Question 4

laminar flow (__, __)

Answer 4

consistent direction, varying velocities

Question 5

spiral flow (__)

Answer 5

trajectory of the flow curls around vessel walls

Question 6

vortex flow (__)

Answer 6

laminar flow loops around after passing through a structure

Question 7

turbulent flow (__)

Answer 7

different velocities and directions, with no discernible pattern)

Question 8

If a nucleus receives an excitation pulse but is
not rephased due to it __,
that nucleus is said to experience a __

Answer 8

traveling out of the spatial

region to which the rephasing pulse is applied; time‐of‐flight (TOF) phenomenon

Question 9

In SE sequences, some nuclear spins may be

__ but not __ (or vice versa), depending on where __ relative to the area __

Answer 9

excited; rephased; they are located when the pulses are applied; to which the pulse delivers RF energy

Question 10

TOF phenomena lead to

__ in SE

Answer 10

signal void

Question 11

3 parameters that relate to TOF phenomena leading to signal void in SE

Answer 11

flow velocity, TE, and slice thickness

Question 12

how does flow velocity relate to TOF phenomena and signal void in SE

Answer 12

as velocity increases, the proportion of nuclei subjected to both pulses decreases, resulting in high velocity signal loss

Question 13

how does TE relate to TOF phenomena and signal void in SE

Answer 13

as TE increases, the proportion of nuclei receiving both pulses decreases

Question 14

how does slice thickness relate to TOF phenomena and signal void in SE

Answer 14

as thickness increases, the proportion of nuclei receiving both pulses increases as well

Question 15

in GE sequences, the RF pulses is applied to __ but the gradient de/rephasing is applied to __

Answer 15

a single slice; the entire volume

Question 16

nuclei which receive __ RF pulses during an acquisition with a __ TR are more likely to be __. nuclei which do not receive such pulses are said to be __

Answer 16

repeated; short; saturated; fresh

Question 17

if the TR is __, __ nuclei in a slice become saturated, while nuclei flowing __ to the slice enter the slice fresh and produce __. this is called __

Answer 17

short; stationary; perpendicular; a different signal than the stationary nuclei; entry slice phenomenon

Question 18

the entry slice phenomenon is more common at the __, and then less and less common toward the __ because fresh nuclei become more and more saturated as they __

Answer 18

extremity of the volume where data starts being acquired; opposite extremity of the volume; flow through the volume

Question 19

the magnitude of entry slice phenomena depends on: (4)

Answer 19

TR: the shorter the TR, the larger the effect
slice thickness: the thinner the slice, the larger the effect
flow velocity: the faster the flow, the larger the effect
flow direction

Question 20

co-/counter-currents

Answer 20

flows in the same/opposite direction as slice selection

Question 21

nuclei traveling in co-current flow receive __ excitations and become saturated __, such that the entry slice phenomenon __

Answer 21

more; faster; decreases

Question 22

nuclei traveling in counter-current flow receive __ excitations and thus the entry slice phenomena are __

Answer 22

fewer; stronger

Question 23

nuclei flowing along a gradient rapidly accelerate or decelerate depending on the __ and __

Answer 23

direction of flow and gradient application

Question 24

flowing nuclei either __ (if they have been accelerated) or __ (if they have been decelerated)

Answer 24

gain phase; lose phase

Question 25

if a flowing nucleus is adjacent to a stationary nucleus in a voxel, there is a __. this is because the flowing nucleus has either __ or __ relative to the stationary nucleus due to its __

Answer 25

phase difference; lost or gained phase; motion along the gradient

Question 26

when nuclei within the same voxel are __, it results in a reduction of __. this is referred to as __

Answer 26

out of phase with each other; total signal amplitude from the voxel; intra-voxel dephasing

Question 27

in turbulent flow, intra-voxel dephasing effects __

Answer 27

are irreversible

Question 28

in laminar flow, intra-voxel dephasing effects __

Answer 28

can be compensated for as long as the velocity and direction of flow are constant

Question 29

how can intra-voxel dephasing effects be compensated for in laminar flow?

Answer 29

as long as the velocity and direction of flow are constant

Question 30

gradient moment rephasing compensates for the __ of the nuclei flowing along a gradient by __

Answer 30

altered phase values; using additional gradients to correct the altered phases back to their original values

Question 31

gradient moment rephasing is performed by __ and/or __, which alter polarity from __ to __ and then to __

Answer 31

slice select; readout gradients; positive; double negative; positive again

Question 32

a flowing nucleus traveling along gradients experiences different __, and its __ changes accordingly

Answer 32

B strengths; phase

Question 33

gradient nulling assumes __ and __, so it is effective on __, hence its name: __

Answer 33

constant velocity and direction across gradients; slow laminar flow; first order motion compensation

Question 34

pulsatile flow is not __, so gradient moment rephasing (gradient nulling) is more effective on __ rather than __ flow. it is also less effective on __

Answer 34

strictly constant; venous; arterial; turbulent fast flow perpendicular to the slice

Question 35

as nulling uses extra gradients, it increases the __ and thus __, so fewer __, or else the __ and the __ must be increased

Answer 35

minimum TE; more time elapses before signal recording; slices can be read in TR; TR and the scan time

Question 36

spatial pre-saturation pulses nullify the signal from __ so that __ are minimized

Answer 36

flowing nuclei; entry slice and TOF phenomena

Question 37

spatial pre-saturation delivers a __ RF pulse to a volume of tissue __. a flowing nucleus within the volume receives this pulse. when it then enters the __, it receives a __ and is __. if it is __, it has no __and produces a __

Answer 37

90 degree; outside the FOV; slice stack; excitation pulse; saturated; fully saturated to 180 degrees; transverse magnetization; signal void

Question 38

to be effective, pre-saturation pulses should be placed between __ and __ so that signal from __ is nullified

Answer 38

the flow and the imaging stack; flowing nuclei entering the FOV

Question 39

in __ imaging, pre-saturation pulses are usually placed __ so that __ flow from above and __ flow from below are __

Answer 39

sagittal and axial; above and below the FOV; arterial; venous; saturated

Question 40

pre-saturation pulses are effective if the flowing nucleus __. pulses are applied __

Answer 40

receives the 90 degree pre-saturation pulse; around each slice just before the excitation pulse

Question 41

the __ and the __ govern the interval between the delivery of each pre-saturation pulse.

Answer 41

TR and the number of slices

Question 42

to optimize pre-saturation, use all the __

Answer 42

slices permitted for a given TR

Question 43

as pre-saturation produces a __, it is used in __ and __ images where __ (__) is dark anyway

Answer 43

signal void; T1 and PD; fluid (blood and CSF)

Question 44

H exists in different compounds, mainly __ and __, and the __ of H in each compound differs

Answer 44

fat and water; precessional frequency

Question 45

the frequency difference between fat and water is called __ and can be used to __

Answer 45

chemical shift; null signals

Question 46

the chemical shift technique is important to differentiate __ (which is mainly water) and __ (which often contains fat)

Answer 46

pathology; normal tissue

Question 47

to saturate or null either fat or water, the precessional difference between the two must be __ so that __

Answer 47

sufficiently large; they can be isolated from each other

Question 48

to saturate fat, a __ pulse must be applied at the __. the __ pulse is then applied, and the __ of fat nuclei are __ to produce a signal void

Answer 48

pre-saturation; precessional frequency of fat; excitation; magnetic moments; flipped into saturation

Question 49

pre-saturation RF is transmitted __ and __ to the whole FOV, so that an area __ receives the same pre-saturation energy as an area __. under these circumstances, fat saturation is __

Answer 49

at the same frequency and evenly; dense in fat; with very little fat; less effective

Question 50

pre-saturation pulses are delivered to __ before __

Answer 50

the FOV; slice excitation

Question 51

SAT TR is the __ and is equal to __

Answer 51

the interval between pre-saturation pulses; the scan TR divided by the number of slices

Question 52

if the SAT TR is longer than the T1 of fat or water, they may not be __ because __

Answer 52

saturated; they have had time to recover before each pre-saturation pulse

Question 53

in spatial inversion recovery (SPIR), an RF pulse at __ is applied to the imaging volume, but unlike chemical pre-saturation this pulse has a magnitude of __. the magnetic moments of __ are therefore __. after a time (__), which corresponds to the __, the 90 degree excitation pulse is applied. as __ has no longitudinal magnetization at this point, the excitation pulse produces __. therefore __ is nullified.

Answer 53

the precessional frequency of fat; 180 degrees; fat; totally inverted; TI; null point of fat; fat; no transverse magnetization in fat; fat signal

Question 54

SPIR is analogous to __ and to __, but it has the advantage of __ because the null point of fat depends on its __ rather than on its __ and __

Answer 54

chemical saturation; STIR; being much less susceptible to field inhomogeneities; T1; precessional frequency; relaxation times

Question 55

in STIR sequences, __ may be nulled along with fat, as it shortens the __; therefore STIR sequences must never be used after __

Answer 55

Gd; T1 recovery time of tissues taking up contrast to that of fat; giving gadolinium

Question 56

in SPIR sequences, fat is __, leaving __ untouched (unlike STIR). therefore SPIR may be used to null the signal from fat in sequences where __

Answer 56

selectively inverted and nulled; Gd; Gd has been given