Chapter 6 Flashcards
Flowing nuclei exhibit different ___ from their neighboring stationary
nuclei, and originate primarily from nuclei in __
contrast characteristics; blood and CSF
The motion of flowing nuclei causes __ and results in __
mis‐mapping of signals; phase ghosting artifacts
name 4 types of flow
laminar flow, spiral flow, vortex flow, and turbulent flow
laminar flow (__, __)
consistent direction, varying velocities
spiral flow (__)
trajectory of the flow curls around vessel walls
vortex flow (__)
laminar flow loops around after passing through a structure
turbulent flow (__)
different velocities and directions, with no discernible pattern)
If a nucleus receives an excitation pulse but is
not rephased due to it __,
that nucleus is said to experience a __
traveling out of the spatial
region to which the rephasing pulse is applied; time‐of‐flight (TOF) phenomenon
In SE sequences, some nuclear spins may be
__ but not __ (or vice versa), depending on where __ relative to the area __
excited; rephased; they are located when the pulses are applied; to which the pulse delivers RF energy
TOF phenomena lead to
__ in SE
signal void
3 parameters that relate to TOF phenomena leading to signal void in SE
flow velocity, TE, and slice thickness
how does flow velocity relate to TOF phenomena and signal void in SE
as velocity increases, the proportion of nuclei subjected to both pulses decreases, resulting in high velocity signal loss
how does TE relate to TOF phenomena and signal void in SE
as TE increases, the proportion of nuclei receiving both pulses decreases
how does slice thickness relate to TOF phenomena and signal void in SE
as thickness increases, the proportion of nuclei receiving both pulses increases as well
in GE sequences, the RF pulses is applied to __ but the gradient de/rephasing is applied to __
a single slice; the entire volume
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 __
repeated; short; saturated; fresh
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 __
short; stationary; perpendicular; a different signal than the stationary nuclei; entry slice phenomenon
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 __
extremity of the volume where data starts being acquired; opposite extremity of the volume; flow through the volume
the magnitude of entry slice phenomena depends on: (4)
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
co-/counter-currents
flows in the same/opposite direction as slice selection
nuclei traveling in co-current flow receive __ excitations and become saturated __, such that the entry slice phenomenon __
more; faster; decreases
nuclei traveling in counter-current flow receive __ excitations and thus the entry slice phenomena are __
fewer; stronger
nuclei flowing along a gradient rapidly accelerate or decelerate depending on the __ and __
direction of flow and gradient application
flowing nuclei either __ (if they have been accelerated) or __ (if they have been decelerated)
gain phase; lose phase
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 __
phase difference; lost or gained phase; motion along the gradient
when nuclei within the same voxel are __, it results in a reduction of __. this is referred to as __
out of phase with each other; total signal amplitude from the voxel; intra-voxel dephasing
in turbulent flow, intra-voxel dephasing effects __
are irreversible
in laminar flow, intra-voxel dephasing effects __
can be compensated for as long as the velocity and direction of flow are constant
how can intra-voxel dephasing effects be compensated for in laminar flow?
as long as the velocity and direction of flow are constant
gradient moment rephasing compensates for the __ of the nuclei flowing along a gradient by __
altered phase values; using additional gradients to correct the altered phases back to their original values
gradient moment rephasing is performed by __ and/or __, which alter polarity from __ to __ and then to __
slice select; readout gradients; positive; double negative; positive again
a flowing nucleus traveling along gradients experiences different __, and its __ changes accordingly
B strengths; phase
gradient nulling assumes __ and __, so it is effective on __, hence its name: __
constant velocity and direction across gradients; slow laminar flow; first order motion compensation
pulsatile flow is not __, so gradient moment rephasing (gradient nulling) is more effective on __ rather than __ flow. it is also less effective on __
strictly constant; venous; arterial; turbulent fast flow perpendicular to the slice
as nulling uses extra gradients, it increases the __ and thus __, so fewer __, or else the __ and the __ must be increased
minimum TE; more time elapses before signal recording; slices can be read in TR; TR and the scan time
spatial pre-saturation pulses nullify the signal from __ so that __ are minimized
flowing nuclei; entry slice and TOF phenomena
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 __
90 degree; outside the FOV; slice stack; excitation pulse; saturated; fully saturated to 180 degrees; transverse magnetization; signal void
to be effective, pre-saturation pulses should be placed between __ and __ so that signal from __ is nullified
the flow and the imaging stack; flowing nuclei entering the FOV
in __ imaging, pre-saturation pulses are usually placed __ so that __ flow from above and __ flow from below are __
sagittal and axial; above and below the FOV; arterial; venous; saturated
pre-saturation pulses are effective if the flowing nucleus __. pulses are applied __
receives the 90 degree pre-saturation pulse; around each slice just before the excitation pulse
the __ and the __ govern the interval between the delivery of each pre-saturation pulse.
TR and the number of slices
to optimize pre-saturation, use all the __
slices permitted for a given TR
as pre-saturation produces a __, it is used in __ and __ images where __ (__) is dark anyway
signal void; T1 and PD; fluid (blood and CSF)
H exists in different compounds, mainly __ and __, and the __ of H in each compound differs
fat and water; precessional frequency
the frequency difference between fat and water is called __ and can be used to __
chemical shift; null signals
the chemical shift technique is important to differentiate __ (which is mainly water) and __ (which often contains fat)
pathology; normal tissue
to saturate or null either fat or water, the precessional difference between the two must be __ so that __
sufficiently large; they can be isolated from each other
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
pre-saturation; precessional frequency of fat; excitation; magnetic moments; flipped into saturation
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 __
at the same frequency and evenly; dense in fat; with very little fat; less effective
pre-saturation pulses are delivered to __ before __
the FOV; slice excitation
SAT TR is the __ and is equal to __
the interval between pre-saturation pulses; the scan TR divided by the number of slices
if the SAT TR is longer than the T1 of fat or water, they may not be __ because __
saturated; they have had time to recover before each pre-saturation pulse
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.
the precessional frequency of fat; 180 degrees; fat; totally inverted; TI; null point of fat; fat; no transverse magnetization in fat; fat signal
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 __
chemical saturation; STIR; being much less susceptible to field inhomogeneities; T1; precessional frequency; relaxation times
in STIR sequences, __ may be nulled along with fat, as it shortens the __; therefore STIR sequences must never be used after __
Gd; T1 recovery time of tissues taking up contrast to that of fat; giving gadolinium
in SPIR sequences, fat is __, leaving __ untouched (unlike STIR). therefore SPIR may be used to null the signal from fat in sequences where __
selectively inverted and nulled; Gd; Gd has been given
