Time of flight
Entry slice phenomina
Explores artefacts Produced from nuclei
that move during the acquisition of data
Flowing nuclei exhibit different contrast
characteristics from their neighbouring
Originate primarily from nuclei in blood
The motion causes mismapping of signals
and results in artefacts known as flow
motion artefacts or phase ghosting.
The cuases of flow artefact are known as
The principal phenomina
Time of flight
Entry slice phenomina
Mechanisms of flow
Laminar flow – flow that is at different but
consistent velocities. At the centre of the lumen
of the vessel is faster than at the wall. The
volocity difference across the vessel is constant.
Turbulent flow – flow at different velocities
that fluctuates randomly. Velocity difference
across the vessel changes erratically.
Vortex flow – flow that is initially laminar but
then passes through a stricture or stenosis in
the vessel. Flow at the centre of the lumen has
a high velocity but near the walls, the flow
Stagnant flow – the velocity of flow slows
down to a point of stagnation. It behaves like
Laminar vortex turbulance
Laminar flow (constant velocity)is termed first
Time of flight phenomenon
To produce a signal, a nucleus must receive an
excitation pulse and a rephasing pulse.
If a nucleus receive only an excitation pulse but
not rephased it does not produce a signal.
If a nucleus is rephased but not received an
excitation pulse it does not produce a signal
Stationary nuclei always receive both excitation
pulse and rephasing pulse and therefore produce
Flowing nuclei present in the slice for excitation
may exit the slice before rephasing and therefore
not produce signal.
This is called the time of flight phenomenon
The effect of the phenomenon depends on the
types of pulse sequence used.
Time of flight in spin echo pulse sequences
In spin echo a slice is selected and a 900
excitation pulse and a 1800 rephasing pulse are
Every slice is selectively excited and rephased.
Stationary nuclei wihin the slice receive both
pulses and produce a signal
Nuclei flowing perpendicular to the slice may be
present for the excitation pulse, but may not for
the rephasing pulse.
They may not produce a signal
Similarly new nuclei which have not received
excitation pulse may be precsent for the
They also not produce a signal
This will result in a signal void from the nuclei, so
the vessel appear dark
Time of flight phenomenon in spin
excited Not rephased
Factors affecting the time of flight
Velocity of flow
Velocity of flow
At high velocity only a smaller proportion of
nuclei are present for both excitation and
As the velocity increases the time of flight effect
At slow velocity more nuclei will be present for
both excitation and rephasing pulses.
Therefore, as the velocity decreases the effect of
time of flight effect decreases.
This is called flow related enhancement
Effect of TE on TOF
As the TE increases, a higher
proportion of flowing nuclei have
exited the slice between the
excitation pulse and the rephasing
pulse. Therefore with increasing TE
the signal void increases.
Slice thickness & TOF
For a given constant velocity a nucleus take
longer time to travel through a thicker slice.
Therefore the nuclei are more likely to receive
both excitation and the rephasing pulse.
So the TOF effect is less on thick slices than on
Time of flight in gradient echo
In gradient echo a variable RF excitation pulse is
followed by a gradient rephasing.
Each slice is selectively excited by the RF pulse,
but the rephasing gradient is applied to the whole
So the flowing nucleus which was excited by the
RF pulse is rephased by the gradient even if it
has exited the slice, and produces a signal.
In addition the short TR tend to saturate the
stationary nuclei and the flowing nuclei appear to
have a higher signal.
Therefore in GE pulse sequences the flow signal
enhancement is increased.
So the GE sequences are said to be flow sensitive
Time of flight phenomina produce
• flow related enhancement or
• high velocity signal loss
Flow related enhancement increases as
• Velocity of flow decreases
• TE decreases
• Slice thichness increases
High velocity signal void increases as the:
• Velocity of flow increases
• TE increases
• Slice thickness decreases
Entry slice phenomenon (in-flow
Entry slice phenomenon is related to the
excitation history of the nuclei.
Nuclei that receive repeated RF pulses during the
acquisition are said to be saturated or ‘beaten
The NMV of these nuclei eventually reach an
equilibrium position, and produce a signal
according to the TE,TR, flip angle and contrast
characteristics of the tissue.
Nuclei that have not received repeated RF pulses
are said to be ‘Fresh’ as their NMV has not been
The signal produced by the ‘fresh’ nuclei and the
‘beaten down’ nuclei are different.
Saturated and fresh spins
moments beaten spins
down by repeated
Stationary nuclei within a slice become
saturated after repeated RF pulses.
Nuclei flowing perpendicular to the slice
enter the slice fresh.
They therefore produce a different signal
from the stationary signal.
This is called entry slice phenomenon or
in-flow effect (as it is more prominent in
the first slice of a ‘stack’ of slices.
The slices in the middle of the stack
exhibit less entry slice phenomenon ( as
flowing nuclei have received more
excitation pulses by the time they reach
Factors affecting the magnitude
of entry slice phenomenon (ESP)
TR - short TR reduces the
magnitude of ESP
Slice thickness – ESP increase with
the slice thickness
Velocity of flow – ESP increases as
the velocity of flow increases
Direction of flow – ( is the most
important in determining the
magnetude of ESP ) – next slide
Direction of flow
The direction may be co-current (same
direction as the slice selection) or
counter-current (opposite direction to
the slice selection)
Co-current flow – ESP decreases rapidly
in the direction of slice selection
Counter-current flow – ESP is more
prominent and may still be present deep
within the slice stack
Co-current & counter current flow
1 2 3 4
• At the first slice in the stack
• When using a long TR
• In thin slices
• With fast flow
• In counter-current flow
Gradients alter the magnetic field strength,
precessional frequency and phase of nuclei.
Nuclei flowing along a gradient rapidly accelerate
or decelerate depending on the direction of flow
and gradient application.
Flowing nuclei therefore either gain phase or
If a flowing nucleus is adjacent to a stationary
nucleus in a voxel, there is a phase difference
between the two nuclei.
Therefore nuclei wihin the same voxel are out of
phase with each other
This will result in a reduction of total signal
amplitude from the voxal.
This is called intra-voxel dephasing.
Type of flow and IVD
The magnitude of intra-voxel
dephasing depends on the degree of
In turbulent flow, IVD effects are
In laminar flow, the IVD can be
compensated for as long the velocity
Flow affects image quality
Time of flight effects give signal void
Entry slice phenomenon effects give
a different signal intensity to flowing
The signal intensity of the lumen is
also affected by the mechanism of
Flow phenomena compensation
Flowing nuclei produce a very
confusing range of signal intensities.
Ideally, these should be
compensated for, inorder to minimise
their adverse effects on image
quality and interpretation.
There are several methods to help
minimise flow artefacts.
1. Gradient moment rephasing(nulling)
Gradient moment rephasing compensates for the altered
phase values of nuclei flowing along a gradient.
It uses additional gradients to correct the altered phases
back to their original values.
In this way, flowing nuclei do not gain or loose phase due
to the presence of the main gradient.
This is performed by using slice select gradient and/or
The gradient alters its polarity from positive to
double negative and then back to positive again.
A flowing nucleus traveling along these gradients,
experiences different magnetic field strengths.
In order to compensate altered phase values, the
precessional frequency at the beginning of gradient
moment rephasing must be the same as it is at the end.
The net precessional frequency and phase change must
therefore be zero.
Gradient moment rephasing
+4000 +8000 +12000 0
Frequency of a 0 Hz Hz Hz
along a normal
gradient +12000 Hz
+4000 Hz -16000 Hz 0
Frequency of a
Phase of a flowing
Frequency of a 0 +4000 -12000
flowing nucleus Hz 0 0
Result of gradient movement
Gradient moment rephasing reduces intra-
As flowing nuclei have the same phase as
stationary nuclei in the same voxel, their
signals add constructively and therefore a
brighter signal results.
Gradient momemt rephasing gives flowing
nuclei a bright signal as spins are in
Gradient moment rephasing assumes a constant
velocity across the gradients.
Most effective in slow laminar flow (1st order
More effective in venous rather than arterial flow.
As it uses extra grdients minimum TE is increased
Fewer slices may be obtained for a given TR
As flowing nuclei are bright usually used for T2 or
T2* weighted images where fluid (blood & CSF) is
Without GMR – shows With GMR
mismapping of the flowing
nuclei within the aorta
Pre-satuation pulses are used to nullify the signal
from flowing nuclei so that the effects of entry
slice and TOF phenomena are minimised.
It delivers a 900 RF pulse to a volume of tissue
outside the FOV.
The flowing nuclei within the volume receives this
When they enter the slice stack, they receive the
excitation pulse and become saturated.
If it is fully saturated to 1800, it has no
transverse component of magnetization and
produce a signal void.
To be effective, pre-saturation pulses should be
placed between the flow and the imaging stack.
In sagital and axial imaging it is usually placed
above and below the FOV.
Right and left sturation pulses are some times
used in coronal imaging of chest to saturate flow
from subclavion vessels.
Pre-saturation pulses are only useful if they are
applied to tissue.
If they are applied to air they are not effective.
They increase the amount of RF that is delivered
to the patient, and increase heating effect.
Pre-saturation pulses are applied around
each slice just before the excitation pulse.
The TR, and the number of slices govern
the interval between the delivery of each
To optimise pre-saturation, use all the
slices permitted for a given TR.
As pre- saturation produces a signal void,
it is usually used in T1 and proton density
weighted images where fluid (blood &
CSF), is dark anyway.
Example -pre-saturated images
Artefact from flowing
nuclei within aorta
(without saturation) With pre-saturation
T1 weighted imaes
Other uses of pre-saturation
Pre-saturation nullifies signal and can
therefore be used to specifically
eliminate certain signals.
The main uses of this are:
• fat and water saturation
• to reduce aliasing
1. Fat & water saturation
In fat hydrogen is linked to carbon and in water it
is linked to oxygen.
Therfore the precessional frequency of hydrogen
in water and fat are different.
AT 1.5T field strength this difference is 220 Hz (
less in fat).
To saturate fat signal, a 900 pre-saturation pulse
must be applied at the precessional frequency of
fatt to the whole FOV.
To saturate water signal, the pre-saturation pulse
at the precessional frequency of water is applied
to the whole FOV
Fat & water saturation pulses
Sagital T1 weighted images
Without fat saturation With fat saturation
T1 weighted image T1 weighted image
without water with water
2. pre-saturation to prevent
Aliasing is produced when anatomy
exists outside the FOV. It can be
reduced by pre-saturation to prevent
signals from tissue outside the FOV.
However there are anti-aliasing
methods to prevent aliasing
1. Gradient Moment Rephasing:
• Uses additional gradients to correct
altered phase values
• Reduces artefact from intra-voxel
• Gives flowing nuclei a bright signal
• Is mainly used in T2 or T2* weighted
• Is most effective on slow, laminar flow
within the slice
• Uses additional RF pulses to nullify signal from
• Reduces artefacts due to time of flight and
entry slice phnomenon
• Gives flowing nuclei a signal void
• Is mainly used in T1 weighted images
• Is effective on fast and slow flow
• Increases the RF deposition to the patient
• Can be used to nullify signal from fat or water
and to reduce aliasing.