Flow Phenomena, Mri Physics, Chapter 6

Front 1

Types of flow

Back 1

Laminar flow
Turbulent flow
Vortex flow
Stagnant flow

Front 2

Most common type of flow

Back 2

Laminar flow

Front 3

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

Back 3

Laminar flow

Front 4

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

Back 4

Laminar flow

Front 5

is flow at different velocities that fluctuates randomly. The velocity difference across the vessel changes erratically.

Back 5

Turbulent flow

Front 6

the direction of the flow is not parallel to the vessel wall and blood flows in different directions.

Back 6

Turbulent flow

Front 7

This type of flow makes an audible noise which can be heard by a stethoscope.

Back 7

Turbulent flow

Front 8

flow can often cause small whirlpools to form in the blood, much like the ones seen in rivers at points of obstruction.

Back 8

Turbulent flow

Front 9

To determinate if flow is turbulent or not we find what?

Back 9

Reynolds number

Front 10

To determinate if flow is turbulent or not find the Reynolds number
For Re > 2000 = turbulent flow

Back 10

Re = D * V * Td / viscosity
D = density
V = velocity
Td = tube diameter

Front 11

Where direction of the flow is spiral

Back 11

Spiral flow

Front 12

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.

Back 12

Vortex flow

Front 13

Flow mechanisms are often termed as follows:

Back 13

First order motion - laminar flow
Second order motion - acceleration
Third order motion - jerk

Front 14

Only ____ ___ ___ can be compensated for.

Back 14

first order flow

Front 15

Flowing nuclei present in the slice for the excitation may have exited the slice before rephasing. This is called

Back 15

Time of flight phenomenon

Front 16

In order to produce a signal, a nucleus must receive

Back 16

an excitation pulse and a rephasing pulse.

Front 17

If a nucleus receives the excitation pulse only and is not rephased ...

Back 17

it does not produce a signal.

Front 18

If a nucleus is rephased but has not previously been excited

Back 18

it does not produce a signal.

Front 19

Time of flight effects depend on:
1.
2.
3.

Back 19

1- the velocity of flow;
2- the TE;
3- the slice thickness.

Front 20

the velocity of flow decreases =

Back 20

Flow related enhancement increases

Front 21

the TE decreases =

Back 21

Flow related enhancement increases

Front 22

90 followed by 180 rephasing (refocusing) pulse
Stationary nuclei within the slice receive both 90 and 180 RF pulse

Back 22

Time of flight in spin echo PS

Front 23

Velocity of flow
Increased : smaller proportion of flowing nuclei are present in the slice for both 90 and 180
This is called:

Back 23

high velocity signal loss

Front 24

Velocity of flow decreased: the time of flight effects decreases.This is called

Back 24

flow- related enhancement

Front 25

TE
increase: =

Back 25

a higher proportion of flowing nuclei have exited the slice between the excitation pulse and the 180 rephasing pulse.

Front 26

Longer TE ->

Back 26

more nuclei have received only one pulse and signal void increases

Front 27

Slice thickness: as the thickness of the slice decreases, the nuclei are more likely to

Back 27

receive only one pulse and signal void increases

Front 28

are often said to be flow - sensitive -Rephasing – gradient
Flowing nuclei are repahsed, too

Back 28

Time of flight in GE pulse sequences

Front 29

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

Back 29

entry slice phenomenon

Front 30

The entry slice phenomenon is related to

Back 30

the excitation history of the nuclei.

Front 31

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.

Back 31

1 the TR;
2 the slice thickness;
3 the velocity of flow;
4 the direction of flow.

Front 32

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?

Back 32

Reduce the magnitude of entry slice phenomenon

Front 33

Entry slice phenomenon decreases in thick slices compared with thin slices due to what?

Back 33

(traveling through thick slices – more RF pulses may receive)

Front 34

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 ?

Back 34

Entry slice phenomenon is therefore decreased as the velocity of floe decrease

Front 35

The most important factor in determining the magnitude of entry slice phenomenon is?

Back 35

Direction of flow

Front 36

Flow that is in the same direction as the slice selection is called:

Back 36

co – current

Front 37

Flow that is in the opposite direction: is called ?

Back 37

counter – current .

Front 38

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?

Back 38

Co – current flow

Front 39

Nuclei flowing along a gradient rapidly accelerate or decelerate depending on the direction of flow and gradient application describe?

Back 39

Intra voxel dephasing

Front 40

If a flowing nucleus is adjacent to a stationary nucleus in a voxel, there is a

Back 40

phase difference between the two nuclei.

Front 41

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 ?

Back 41

Intra voxel dephasing

Front 42

Nuclei within the same voxel are out of phase with each other, which results in ?

Back 42

a reduction of total signal amplitude from the voxel.

Front 43

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

Back 43

intra-voxel dephasing

Front 44

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.

Back 44

1.Even echo rephasing
2.Gradient moment nulling
3.Spatial pre - saturation

Front 45

adding extra gradient pulses to produce even -echo rephasing on the first echo (eliminating dephasing effects) is called

Back 45

Even echo rephasing

Front 46

TE has equal value for each echo (Even number of echos is used)
Dephasing part = rephasing part

Back 46

Even echo rephasing

Front 47

This may also result from the application of multiple gradient echo pulses following the RF pulse.

Back 47

Even echo rephasing

Front 48

The ____ ___ ______ ______ is one of the flow effects observed in MR imaging.

Back 48

even echo rephasing phenomenon