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530: Data Acquisition > Chapter 6 > Flashcards

Chapter 6 Flashcards

1
Q

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

A

contrast characteristics; blood and CSF

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2
Q

The motion of flowing nuclei causes __ and results in __

A

mis‐mapping of signals; phase ghosting artifacts

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3
Q

name 4 types of flow

A

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

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4
Q

laminar flow (__, __)

A

consistent direction, varying velocities

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5
Q

spiral flow (__)

A

trajectory of the flow curls around vessel walls

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6
Q

vortex flow (__)

A

laminar flow loops around after passing through a structure

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7
Q

turbulent flow (__)

A

different velocities and directions, with no discernible pattern)

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8
Q

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

A

traveling out of the spatial

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

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9
Q

In SE sequences, some nuclear spins may be

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

A

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

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10
Q

TOF phenomena lead to

__ in SE

A

signal void

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11
Q

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

A

flow velocity, TE, and slice thickness

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12
Q

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

A

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

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13
Q

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

A

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

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14
Q

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

A

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

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15
Q

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

A

a single slice; the entire volume

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16
Q

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 __

A

repeated; short; saturated; fresh

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17
Q

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 __

A

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

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18
Q

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 __

A

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

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19
Q

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

A

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

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20
Q

co-/counter-currents

A

flows in the same/opposite direction as slice selection

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21
Q

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

A

more; faster; decreases

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22
Q

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

A

fewer; stronger

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23
Q

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

A

direction of flow and gradient application

24
Q

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

A

gain phase; lose phase

25
Q

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 __

A

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

26
Q

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

A

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

27
Q

in turbulent flow, intra-voxel dephasing effects __

A

are irreversible

28
Q

in laminar flow, intra-voxel dephasing effects __

A

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

29
Q

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

A

as long as the velocity and direction of flow are constant

30
Q

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

A

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

31
Q

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

A

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

32
Q

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

A

B strengths; phase

33
Q

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

A

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

34
Q

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

A

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

35
Q

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

A

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

36
Q

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

A

flowing nuclei; entry slice and TOF phenomena

37
Q

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 __

A

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

38
Q

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

A

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

39
Q

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

A

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

40
Q

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

A

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

41
Q

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

A

TR and the number of slices

42
Q

to optimize pre-saturation, use all the __

A

slices permitted for a given TR

43
Q

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

A

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

44
Q

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

A

fat and water; precessional frequency

45
Q

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

A

chemical shift; null signals

46
Q

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

A

pathology; normal tissue

47
Q

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

A

sufficiently large; they can be isolated from each other

48
Q

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

A

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

49
Q

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 __

A

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

50
Q

pre-saturation pulses are delivered to __ before __

A

the FOV; slice excitation

51
Q

SAT TR is the __ and is equal to __

A

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

52
Q

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

A

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

53
Q

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.

A

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

54
Q

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 __

A

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

55
Q

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

A

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

56
Q

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

A

selectively inverted and nulled; Gd; Gd has been given

530: Data Acquisition (13 decks)

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