MRA

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what are the two basic principles of MRA

1. Suppress background signal from stationary tissue

2. Enhances signal from moving blood
what is used to process MRA after a sequence so that the background tissues are even more suppressed or completely black

MIP
The MIP allows for visualization of what

vessels and vascular tree
what are the different types of blood flow
- Plug flow
-  Laminar flow
-  Turbulence
-  Vertex flow
-  Pulsatile flow
-  Stagnant flow
what are the two effects motion can cause during and MRA
-  Magnitude effect
-  Phase shift
what is plug flow
- when all blood in the vessel is flowing at the same speed
- it doesnt matter where it is located in the vessel
what is laminar flow

blood flow in which the speed varies in a parabolic fashion across the lumen of the blood vessel
In laminar flow in the blood vessel where is the flow the slowest and fastest and why
- ◦ Slowest at the walls
- ◦ Fastest near the center
because of wall resistance
what is turbulent flow
where does turbulent flow occur mainly
- blood flow that is random in areas of discontinuity
- flows at different velocities that fluctuate
in vessels that bifurcate or through areas of stenosis
what is vertex flow
- Laminar flow that passes through a area of
- stenosis
what is stenosis

area in the blood vessels where there is a blockage where blood cannot pass through as easy
what is a pulsatile flow

normal arterial blood flow
what is stagnant flow
- very slow flow to the point of
- stagnation.
stagnant flow behaves as what
its intensity is based on what
stationary tissue
- depends upon the tissue characteristics
- (T1, T2, or PD);
what are two was to display flowing blood with MRA imaging
- ◦ Time of Flight
- ◦ Phase Contrast
what is time of flight display
- TOF – during an RF excitation, the
- inflow/outflow (Flight) of the proton spins results in the time of flight effect
what is phase Contrast
- PC – during signal sampling while
- the frequency encoding read gradient is on, spins
- moving in the direction of read gradient experience
- a phase shift, resulting in phase contrast effect
MRI systems can only correct for flow that is at a constant ________ and _________ during data acquisition

velocity; direction
in the time of flight phenomenon how do we create an MR signal in stationary tissue

the protons must receive a 90 deg excitation pulse and 180 deg refocusing pulse
in flowing spins what happens if a spin receives a an excitation pulse but not a refocusing pulse

no signal is produced
If a spin misses the excitation pulse and only receives the refocusing pulse

no signal is produced
all stationary spins need to receive what

both 90 deg and 180 deg pulses to produce a signal
What causes a signal void in the time of flight phenomena

not receiving the 90 deg excitation pulse and not receiving the 180 deg rephasing pulse
As a result of a signal void from the spins in TOF what will the blood vessels look like on are image

very dark not what we want we need vessels to appear bright
what characteristics do TOF effects depend on in spin echo
- ◦ Velocity of flow
- ◦ TE
- ◦ Slice thickness
as flow of blood increases or velocity increases does the TOF increase or decrease
why?
- increase B/C ◦ A smaller proportion of flowing spins are present in
- the slice for both the 90deg and the 180deg RF pulses
as flow of blood decreases or velocity decreases does the TOF increase or decrease
why?
- decrease
- A higher proportion of flowing spins are present in
- the slice for both the 90deg and 180deg RF pulses
Increasing the TE time does what in TOF
- A higher proportion of flowing spins have received only one
- RF pulse
Longer TE results higher signal ______ TOF effect is ___________

void; more
entry slice phenomenon or inflow effect is most prominent where

in the first slice of a stack if slices
Moving or flowing spins that are not exposed to repeated RF pulses are not saturated and considered to be _______

fresh
what is flow that is in the same direction as the slice selection

co-current
flow that is in the opposite direction to slice selection is called

counter current flow
why do the flowing spins of co current flow receive more RF excitations

because everything is traveling in the same direction and they become more saturated quicker
what is the entry slice phenomenon
- Contrast difference of flowing nuclei relative to
- the stationary spins because they are fresh
in which direction do flowing spins travel in counter current flow

in the opposite direction
in counter current the flowing spins stay ____ whenever they enter a slice

fresh
what factors cause the entry slice phenomenon to increase
- 1. At the 1st slice in the stack
- 2. Using long TR
- 3. Thin slices
- 4. Fast flow
- 5. Counter-current flow
what is intra Voxel dephasing
- Gradients alter the magnetic field strength,
- precessional frequency and phase of Spins
In gradient sequence of TOF Flowing spins along a gradient rapidly accelerate or
decelerate depending on the _____ of flow and
___________ application

direction; gradient
how do flowing spins gain phase
lose phase?
- if they are accelerated
- if they have been decelerated
If a flowing spin is adjacent to a stationary spin in a voxel what happens

Then the spins are out of phase with each other which results in?

there is a phase difference between the spins

in a reduction of total signal amplitude from the voxel
the magnitude of intra voxel dephasing depends on the degree of ____________

turbulence
how does the artifact ghoasting occur
- Flowing spins can produce a variety of signal
- intensities that may detract or cause artifact
- on the resultant image
ho

Author

MriSensei

339124

Card Set

MRA

Description

mri

Updated

2018-03-27T12:47:18Z

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