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48 Cards in this Set

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Types of flow
Laminar flow
Turbulent flow
Vortex flow
Stagnant flow
Most common type of flow
Laminar flow
The flow at the centre of the lumen of the vessel is faster than at the vessel wall, where resistance slows down the flow. However, the velocity difference across the vessel is constant
Laminar flow
This type of flow is silent: it does not produce an audible noise of any kind, the direction of the flow is parallel to the vessel wall and is basically straight
Laminar flow
is flow at different velocities that fluctuates randomly. The velocity difference across the vessel changes erratically.
Turbulent flow
the direction of the flow is not parallel to the vessel wall and blood flows in different directions.
Turbulent flow
This type of flow makes an audible noise which can be heard by a stethoscope.
Turbulent flow
flow can often cause small whirlpools to form in the blood, much like the ones seen in rivers at points of obstruction.
Turbulent flow
To determinate if flow is turbulent or not we find what?
Reynolds number
To determinate if flow is turbulent or not find the Reynolds number
For Re > 2000 = turbulent flow
Re = D * V * Td / viscosity
D = density
V = velocity
Td = tube diameter
Where direction of the flow is spiral
Spiral flow
flow that is initially laminar but then passes through a stricture or stenosis in the vessel. Flow in the centre of the lumen has a high velocity but, near the walls, the flow spirals.
Vortex flow
Flow mechanisms are often termed as follows:
First order motion - laminar flow
Second order motion - acceleration
Third order motion - jerk
Only ____ ___ ___ can be compensated for.
first order flow
Flowing nuclei present in the slice for the excitation may have exited the slice before rephasing. This is called
Time of flight phenomenon
In order to produce a signal, a nucleus must receive
an excitation pulse and a rephasing pulse.
If a nucleus receives the excitation pulse only and is not rephased ...
it does not produce a signal.
If a nucleus is rephased but has not previously been excited
it does not produce a signal.
Time of flight effects depend on:
1.
2.
3.
1- the velocity of flow;
2- the TE;
3- the slice thickness.
the velocity of flow decreases =
Flow related enhancement increases
the TE decreases =
Flow related enhancement increases
90 followed by 180 rephasing (refocusing) pulse
Stationary nuclei within the slice receive both 90 and 180 RF pulse
Time of flight in spin echo PS
Velocity of flow
Increased : smaller proportion of flowing nuclei are present in the slice for both 90 and 180
This is called:
high velocity signal loss
Velocity of flow decreased: the time of flight effects decreases.This is called
flow- related enhancement
TE
increase: =
a higher proportion of flowing nuclei have exited the slice between the excitation pulse and the 180 rephasing pulse.
Longer TE ->
more nuclei have received only one pulse and signal void increases
Slice thickness: as the thickness of the slice decreases, the nuclei are more likely to
receive only one pulse and signal void increases
are often said to be flow - sensitive -Rephasing – gradient
Flowing nuclei are repahsed, too
Time of flight in GE pulse sequences
Stationary nuclei within a slice become saturated after repeated RF pulses.
Nuclei flowing perpendicular to the slice enter the slice fresh, as they were not present during repeated excitations.
They therefore produce a different signal from the stationary nuclei. This is called the
entry slice phenomenon
The entry slice phenomenon is related to
the excitation history of the nuclei.
Any factor that affects the rate at which a nucleus receives repeated excitations affects the magnitude of the phenomenon. The magnitude of entry slice phenomenon therefore depends on: 1.
2.
3.
4.
1 the TR;
2 the slice thickness;
3 the velocity of flow;
4 the direction of flow.
TR is time between each excitation pulse
Short TR: results in an increase in the rate at which the RF is delivered (decreases the time between successive RF pulses),how does this affect entry slice phenomenon?
Reduce the magnitude of entry slice phenomenon
Entry slice phenomenon decreases in thick slices compared with thin slices due to what?
(traveling through thick slices – more RF pulses may receive)
Velocity of flow
Fast – flowing nuclei are more likely to have traveled to the next slice when RF is delivered than slow nuclei, Entry slice phenomenon is therefore ?
Entry slice phenomenon is therefore decreased as the velocity of floe decrease
The most important factor in determining the magnitude of entry slice phenomenon is?
Direction of flow
Flow that is in the same direction as the slice selection is called:
co – current
Flow that is in the opposite direction: is called ?
counter – current .
The flowing nuclei are more likely to receive repeated RF excitations as they move from one slice to the next.
They therefore become saturated relatively quickly, and so entry slice phenomenon decreases rapidly ; describes what?
Co – current flow
Nuclei flowing along a gradient rapidly accelerate or decelerate depending on the direction of flow and gradient application describe?
Intra voxel dephasing
If a flowing nucleus is adjacent to a stationary nucleus in a voxel, there is a
phase difference between the two nuclei.
If a flowing nucleus is adjacent to a stationary nucleus in a voxel, the flowing nucleus has either lost or gained phase relative to the stationary nucleus due to its motion along the gradient it is know as ?
Intra voxel dephasing
Nuclei within the same voxel are out of phase with each other, which results in ?
a reduction of total signal amplitude from the voxel.
Nuclei within the same voxel are out of phase with each other, which results in a reduction of total signal amplitude from the voxel.
This is called
intra-voxel dephasing
Flowing nuclei produce a very confusing range of signal intensities
To get on image quality, flow artifacts should be reduced
The methods for reducing flow phenomenon are:
1.
2.
3.
1.Even echo rephasing
2.Gradient moment nulling
3.Spatial pre - saturation
adding extra gradient pulses to produce even -echo rephasing on the first echo (eliminating dephasing effects) is called
Even echo rephasing
TE has equal value for each echo (Even number of echos is used)
Dephasing part = rephasing part
Even echo rephasing
This may also result from the application of multiple gradient echo pulses following the RF pulse.
Even echo rephasing
The ____ ___ ______ ______ is one of the flow effects observed in MR imaging.
even echo rephasing phenomenon