Chapter 6 Flashcards
Stationary nuclei always receive both excitation and rephasing pulses, but flowing nuclei present in the slice for the excitation may have exited the slice before rephasing. This is called ______________.
Time of Flight Phenomenon
TOF phenomenon results in a dark vessel. (T or F)
true
a dark vessel because nuclei that were present during the excitation are usually not present when the rephasing pulse is delivered and vice versa.
Which of the following effects does TOF depend on?
Velocity of flow, TE, Slice Thickness
The frequency difference between fat and water is called ___________ and can be used to specifically null the signal from either fat or water.
Chemical Shift
Which of the following is/are result(s) of time of flight?
High velocity signal void,
Flow-related enhancement
Flow affects image quality. (T or F)
True
Which artifact is produced as a result of motion of flowing nuclei?
Phase ghosting
two main uses of spatial pre-saturation
Chemical Pre-saturation
SPIR
Even echo rephasing reduces intra-voxel dephasing (T or F)
True
Another term for time of flight is inflow effect. (T or F)
False
Another term for entry slice phenomenon is inflow effect, as it is most prominent in the first slice of a ‘stack’ of slices.
Any tissue can be nulled as long as an RF pulse matching its precessional frequency is applied to the imaging volume before excitation. (T or F)
True
Pre-saturation techniques increase the RF deposition to the patient. (T or F)
True
STIR is most useful in post contrast imaging. (T or F)
False
In STIR sequences, gadolinium may be nulled along with fat, as gadolinium shortens the T1 recovery time of tissues taking up contrast to that of fat. Therefore, STIR sequences must never be used after giving gadolinium
Vortex
initially laminar but then passes thru a stenosis
laminar
different but consistent velocities
turbulent
different velocities that changes randomly
Counter current flow occurs when flowing nuclei are traveling in the same direction as slice selection. (T or F)
False
Which type of motion can we compensate for in MRI?
First order laminar flow
Determine which of the following parameters would result in the most entry slice phenomenon (i.e. entry slice phenomenon would increase compared to the other options).
Slice 1 of 20, 3000 TR, .7 mm slice thickness
Intra-voxel dephasing results in an increase in total signal amplitude from the voxel (T or F)
False
If a flowing nucleus is adjacent to a stationary nucleus in a voxel, there is a phase difference between the two nuclei. Therefore the nuclei within the same voxel are out of phase with each other, which results in a REDUCTION in total signal amplitude from the voxel
In a dual echo sequence, which of the following combinations would result in reduced flow artifact in the second echo?
TE 40 and 80 ms
As velocity of flow decreases, the time of flight effect decreases. (T or F)
True- This is called flow-related enhancement. See page 202 in the text for more information
What is the chemical shift at 1.5 T?
220 Hz
What is the name of the artifact that is produced as a result of motion of flowing nuclei?
phase ghosting
TOF
flowing nuclei in the slice may have excited before being rephased
ESP
stationary nuclei become saturated.
what result in a dark vessel
TOF
What does TOF depend on
velocity of flow
TE
slice thickness
chemical shift for 1T
147 Hz
chemical shift for 1.5 T
220 Hz
chemical shift for 3 T
441 Hz
Which of the following will increase entry slice phenomenon
first slice of stack, long TR, thin slices, with counter current flow
check q 6 on study guide
_
TRUE or FALSE: A thinner slice is more likely to produce a signal void.
True
During which would Entry-Slice Phenomenon increase
first slice in stack, thin slices, with fast flow, with long TR
TOF phenomenon results in a dark vessel. (T or F)
True- a dark vessel because nuclei that were present during the excitation are usually not present when the rephasing pulse is delivered and vice versa.
The frequency difference between fat and water is called ___________ and can be used to specifically null the signal from either fat or water.
Chemical Shift
Which factors affect the magnitude of entry slice phenomenon
TR, slice thickness, velocity of flow, direction of flow
To produce a signal, a nucleus must receive an excitation pulse and a rephasing pulse. If the nucleus is flowing, it may receive only the excitation pulse or the rephasing pulse, it will not produce a signal.
What does this most likely describe?
Time of Flight
intravoxel dephasing
reduction from amplitude from the voxel
entry slice phenonenom
aka inflow effect
co current flow
flow in same direction
counter current flow
flow in opposite directions
chemical shift
frequency diff between fat and water
fat sat
pre sat pulse precessional frequency of fat
water sat
pre sat pulse at precessional frequency of water
even echo rephasing
reduces intravoxel dephasing in dual echo when second echo is at multiple of the first
Which of the following are true concerning pre-saturation?
uses additional RF to null signal from flowing nuclei
gives flowing nuclei a signal void
increases the RF deposition of the patient
spatial presaturation
nullifies the signal from flowing nuclei to minimize the effects of TOF and ESP
even echo rephasing
uses balanced echoes, demonstrates less dephasing than old echoes
gradient moment rephasing
uses additional gradients to correct altered phase values
Determine which of the following parameters would result in the most entry slice phenomenon (i.e. entry slice phenomenon would increase compared to the other options).
Slice 1 of 20, 3000 TR, .7 mm slice thickness
Stationary nuclei always receive both excitation and rephasing pulses, but flowing nuclei present in the slice for the excitation may have exited the slice before rephasing. This is called ______________.
Time of Flight Phenomenon
two main uses of spatial pre-saturation
Chemical Pre-saturation
SPIR
STIR
never used after gad
SPIR
may be used to null fat after gad
Which type of motion can we compensate for in MRI
First order laminar flow
TRUE or FALSE: Silicone may be saturated to null its signal in breast imaging.
true
result of velocity increases, TOF increases
high velocity signal loss
result of velocity decreases, TOF decreases
flow related enhancement
Which type of flow can be compensated for?
first-order motion, laminar flow
What does this describe?
Each slice is selectively excited by the RF pulse, but the rephasing gradient is applied to the whole body. A flowing nucleus that receives an excitation pulse is rephased regardless of its slice position and produces a signal.
TOF in gradient echo
