Dealing with Flow

Difference between CT & MRI In X-ray angiography a catheter is introduced and contrast injected under pressure. As it flows it displaces the blood and allows imaging of the vessel lumen, making it possible to image no matter what the natural flow dynamics of the vessel may be. MRI (MRA) is a physiologic record of blood flow. If there is no flow, no blood vessels will be seen. For example a disturbance in the pattern of blood flow at a stenotic site causing turbulance may cause a reduced vascular signal or no signal at all.

In X-ray angiography a catheter is introduced and contrast injected under pressure. As it flows it displaces the blood and allows imaging of the vessel lumen, making it possible to image no matter what the natural flow dynamics of the vessel may be.

MRI (MRA) is a physiologic record of blood flow. If there is no flow, no blood vessels will be seen. For example a disturbance in the pattern of blood flow at a stenotic site causing turbulance may cause a reduced vascular signal or no signal at all.

Flow Nuclei such as that found in blood is moving, presenting a unique problem in MRI The flowing nuclei have different contrast or may not show up at all if they do not produce a signal (mismapping) Methods to reduce flow artefact (aka flow phenomena) exist The major three types of flow phenomena are: Time of flight Entry slice phenomenon Intra-voxel dephasing

Nuclei such as that found in blood is moving, presenting a unique problem in MRI

The flowing nuclei have different contrast or may not show up at all if they do not produce a signal (mismapping)

Methods to reduce flow artefact (aka flow phenomena) exist

The major three types of flow phenomena are:

Time of flight

Entry slice phenomenon

Intra-voxel dephasing

Mechanism of Flow Four types of flow: Laminar flow – flow is different but consistent velocity. Flow at center of vessel is faster than near walls. Spiral flow – flow takes the form of a spiral Vortex flow – flow starts off linear until it comes across a stricture or stenosis Turbulent flow – flow at different velocities and is random in nature

Four types of flow:

Laminar flow – flow is different but consistent velocity. Flow at center of vessel is faster than near walls.

Spiral flow – flow takes the form of a spiral

Vortex flow – flow starts off linear until it comes across a stricture or stenosis

Turbulent flow – flow at different velocities and is random in nature

Flow phenomena – Time of flight Stationary nuclei easily receive both the excitation RF pulse and rephasing pulses however if the nuclei is moving it can miss either one or both. This phenomena is called time of flight If flowing nuclei receive an excitation pulse w/o also receiving a rephasing pulse or vice versa a signal void occurs and the vessel appears dark (dark blood imaging)

Stationary nuclei easily receive both the excitation RF pulse and rephasing pulses however if the nuclei is moving it can miss either one or both.

This phenomena is called time of flight

If flowing nuclei receive an excitation pulse w/o also receiving a rephasing pulse or vice versa a signal void occurs and the vessel appears dark (dark blood imaging)

Flow phenomena – Time of flight

As velocity increases less and less nuclei will have the time to receive both the 90 0 and 180 0 RF pulses velocity TOF High velocity signal loss velocity TOF Flow related enhancement

As velocity increases less and less nuclei will have the time to receive both the 90 0 and 180 0 RF pulses

TE The longer the TE the more chance a nuclei may receive one pulse and a signal void is created

The longer the TE the more chance a nuclei may receive one pulse and a signal void is created

Slice thickness In a thicker slice, moving nuclei have more of a chance of receiving both the 90 0 excitation and 180 0 RF pulses vs. a thinner slice . Excited rephased Not rephased thick thin

In a thicker slice, moving nuclei have more of a chance of receiving both the 90 0 excitation and 180 0 RF pulses vs. a thinner slice .

TOF in gradient echo pulse sequences In this sequence, after excitation RF, the nuclei are rephased by a gradient. The imaging volume is pulsed so rapidly that only a small portion of the tissues longitudinal magnetization can be gained between excitations. This causes an overall signal loss in the static volume of tissue. (this can be seen as a form of background suppression). Blood flow having receives less pulses will give off more signal. The key here is that the gradient is applied to the whole body so it covers more than the thickness of a particular slice. Therefore a flowing nuclei will be rephased and produce a signal regardless of its slice position These sequences are said to be flow sensitive .

In this sequence, after excitation RF, the nuclei are rephased by a gradient. The imaging volume is pulsed so rapidly that only a small portion of the tissues longitudinal magnetization can be gained between excitations. This causes an overall signal loss in the static volume of tissue. (this can be seen as a form of background suppression). Blood flow having receives less pulses will give off more signal.

The key here is that the gradient is applied to the whole body so it covers more than the thickness of a particular slice.

Therefore a flowing nuclei will be rephased and produce a signal regardless of its slice position

These sequences are said to be flow sensitive .

Time of Flight ( TOF ) TOF manipulates the longitudinal magnetization of stationary spins. TOF uses coherent gradient echo sequences to enhance flow. The TR is kept below the T1 relaxation time of the stationary tissue ( T1 recovery is prevented ). Inflow of protons produce siginal. Sat bands are used to in the opposite direction of the vessel you want to visualize.

TOF manipulates the longitudinal magnetization of stationary spins. TOF uses coherent gradient echo sequences to enhance flow. The TR is kept below the T1 relaxation time of the stationary tissue ( T1 recovery is prevented ).

Inflow of protons produce siginal. Sat bands are used to in the opposite direction of the vessel you want to visualize.

Disadvantages of TOF If a vessel is parallel to the FOV according to the velocity it can be saturated out and the vessel can be misdiagnosed. High signal from background tissue (fat has a short T1 relaxation time shows bright)

If a vessel is parallel to the FOV according to the velocity it can be saturated out and the vessel can be misdiagnosed.

High signal from background tissue (fat has a short T1 relaxation time shows bright)

TOF 2D AND 3D 2D is slice by slice Coverage large area Sensitive to slow flow Low resolution is a disadvantage In-plane saturation is a disadvantage Venetian blind and pulsation artifacts 3D is a volume (3-6 cm) divided into 1mm sections High SNR Thin contiguous slices Only can cover a small area is the disadvantage Useful when imaging circle of willis

2D is slice by slice

Coverage large area

Sensitive to slow flow

Low resolution is a disadvantage

In-plane saturation is a disadvantage

Venetian blind and pulsation artifacts

3D is a volume (3-6 cm) divided into 1mm sections

High SNR

Thin contiguous slices

Only can cover a small area is the disadvantage

Useful when imaging circle of willis

TOF summary

Entry slice phenomenon This phenomenon is dependent upon the excitation history of nuclei during a TR period For example nuclei having received several RF pulses within an acquisition having a short TR force the magnetic moments of the nuclei past the transverse plane where they are saturated. Due to the short time longitudinal relaxation does not occur Nuclei not receiving repeated RF as they enter a slice are said to be fresh

This phenomenon is dependent upon the excitation history of nuclei during a TR period

For example nuclei having received several RF pulses within an acquisition having a short TR force the magnetic moments of the nuclei past the transverse plane where they are saturated.

Due to the short time longitudinal relaxation does not occur

Nuclei not receiving repeated RF as they enter a slice are said to be fresh

Entry Slice phenomenon Nuclei not subjected to RF entering the first slice are fresh they produce a different signal compared to saturated nuclei As the nuclei moves toward the middle there is less entry slice phenomenon as the nuclei has received more excitation pulses they are “less fresh”

Entry Slice phenomenon The rate at which the nuclei receive excitation pulses determines the magnitude of the phenomenon. This rate is influenced by the following factors: TR Slice thickness Velocity of flow Direction of flow

The rate at which the nuclei receive excitation pulses determines the magnitude of the phenomenon.

This rate is influenced by the following factors:

TR

Slice thickness

Velocity of flow

Direction of flow

TR As a short TR increases the frequency with which RF is delivered, moving nuclei have a better chance of receiving both the excitation pulse and the rephasing pulses. This in-turn decreases the magnitude of the entry slice phenomenon

As a short TR increases the frequency with which RF is delivered, moving nuclei have a better chance of receiving both the excitation pulse and the rephasing pulses.

This in-turn decreases the magnitude of the entry slice phenomenon

Slice Thickness Nuclei traveling through thick slices will have the time to receive more RF pulses than nuclei moving through a thin slice Entry slice phenomenon therefore increases in thick slices compared with thin slices.

Nuclei traveling through thick slices will have the time to receive more RF pulses than nuclei moving through a thin slice

Entry slice phenomenon therefore increases in thick slices compared with thin slices.

Velocity of flow The faster nuclei move, the less chance they have of receiving both the excitation and rephasing RF pulses. Thus if blood velocity is slow entry slice phenomenon is decreased as velocity lowers

The faster nuclei move, the less chance they have of receiving both the excitation and rephasing RF pulses.

Thus if blood velocity is slow entry slice phenomenon is decreased as velocity lowers

Direction of flow Co-current flow in which nuclei travel in the same direction as slice selection. This scenario is best as nuclei has more chance to receive repeated RF excitations, reducing the entry slice phenomenon. Counter –current in which nuclei travel in the opposite direction to slice selection. Nuclei moving opposite to slice excitation when entering slices are less likely to have received excitation pulses and stay (fresh).

Co-current flow in which nuclei travel in the same direction as slice selection. This scenario is best as nuclei has more chance to receive repeated RF excitations, reducing the entry slice phenomenon.

Counter –current in which nuclei travel in the opposite direction to slice selection. Nuclei moving opposite to slice excitation when entering slices are less likely to have received excitation pulses and stay (fresh).

Intra voxel dephasing Each voxel in a MR image contributes signal. If the contents within any one voxel are out of phase with each other, the overall average signal amplitude from the voxel will decrease. This can occur when a moving nuclei interact with a stationary nucleus. Depending on the nuclei motion and how the gradient is applied, it may either lose (slower) or gain phase (faster)

Each voxel in a MR image contributes signal. If the contents within any one voxel are out of phase with each other, the overall average signal amplitude from the voxel will decrease.

This can occur when a moving nuclei interact with a stationary nucleus.

Depending on the nuclei motion and how the gradient is applied, it may either lose (slower) or gain phase (faster)

Flow phenomena compensation All flow phenomena are detrimental to image quality and when possible should be compensated for. There are several methods to accomplish this: Even echo rephasing Gradient moment nulling Spatial pre-saturation

All flow phenomena are detrimental to image quality and when possible should be compensated for.

There are several methods to accomplish this:

Even echo rephasing

Gradient moment nulling

Spatial pre-saturation

Even echo rephasing

Gradient moment rephasing (nulling) This method of dealing with intra-voxel dephasing from flowing nuclei utilizes the slice select gradient and/or the readout (fq) gradients to bring the nuclei back into phase. When in phase both stationary and moving nuclei contribute to the signal and flowing nuclei give a bright signal. STEPS nuclei traveling along a gradient will be of varying speeds ( phases) The slice select/readout gradient alters its polarity from positive to double negative and then back to positive again For GMR to work well flow must be of constant velocity and direction

This method of dealing with intra-voxel dephasing from flowing nuclei utilizes the slice select gradient and/or the readout (fq) gradients to bring the nuclei back into phase.

When in phase both stationary and moving nuclei contribute to the signal and flowing nuclei give a bright signal.

STEPS

nuclei traveling along a gradient will be of varying speeds ( phases)

The slice select/readout gradient alters its polarity from positive to double negative and then back to positive again

For GMR to work well flow must be of constant velocity and direction

Spatial pre-saturation The prefix pre is the major aspect of this flow compensation method as nuclei in a volume outside the FOV receive 90 0 RF pulses to presaturate it prior to it entering the slices. After entering the stack it is hit again with an excitation pulse. If the two add to 180 0 the nuclei will be fully saturated and will not have any transverse component magnetization and thus no signal.

The prefix pre is the major aspect of this flow compensation method as nuclei in a volume outside the FOV receive 90 0 RF pulses to presaturate it prior to it entering the slices.

After entering the stack it is hit again with an excitation pulse.

If the two add to 180 0 the nuclei will be fully saturated and will not have any transverse component magnetization and thus no signal.

Fat saturation As we have learned to satruate fat a 90 0 pre-saturaton pulse will need to be applied at the precessional frequency of fat. This is followed by the excitation RF pluse (total of 180 0 ) which will push the fat nuclei into saturation producing a signal void. The time interval between the pre-saturation pulses is called SAT TR SAT TR = Scan TR / # of slices

As we have learned to satruate fat a 90 0 pre-saturaton pulse will need to be applied at the precessional frequency of fat.

This is followed by the excitation RF pluse (total of 180 0 ) which will push the fat nuclei into saturation producing a signal void.

The time interval between the pre-saturation pulses is called SAT TR

FatSat

Water saturation

Spatial pre-saturation

Spatial inversion recovery (SPIR) SPIR selectively inverts and thus nulls the signal from fat. STEPS RF pulse at precessional frequency of fat is applied (180 0 ) twice as much as chemical pre-saturation technique. The magnetic moments of the nuclei are now in the -Z direction. After a time interval (TI) corresponding to the null point an excitation pulse is applied. Fat signal is nulled as there is no transverse magnetization.

SPIR selectively inverts and thus nulls the signal from fat.

STEPS

RF pulse at precessional frequency of fat is applied (180 0 ) twice as much as chemical pre-saturation technique.

The magnetic moments of the nuclei are now in the -Z direction.

After a time interval (TI) corresponding to the null point an excitation pulse is applied. Fat signal is nulled as there is no transverse magnetization.

SPIR advantage As mentioned chemical fat suppression relies on the precessional fq. of fat and for optium results the precessional frequency of a volume should be the same but this is unrealistic. SPIR is not dependent (like chemical fat suppression) on magnetic field homogeneity and utilizes null point of fat much like STIR

As mentioned chemical fat suppression relies on the precessional fq. of fat and for optium results the precessional frequency of a volume should be the same but this is unrealistic.

SPIR is not dependent (like chemical fat suppression) on magnetic field homogeneity and utilizes null point of fat much like STIR

STIR better but STIR shows more uniform nulling of fat than SPIR but gadolinium may also be nulled along with fat. Thus SPIR should be used to null signal from fat if gadolinium is to be given

STIR shows more uniform nulling of fat than SPIR but gadolinium may also be nulled along with fat.

Thus SPIR should be used to null signal from fat if gadolinium is to be given

Out of phase imaging (Dixon technique) This technique is utilized in gradient echo sequences. This technique will null any signal arising from voxels which contain both fat and water. By utilizing a TE when fat and water are out of phase with each other they will not yield a signal

This technique is utilized in gradient echo sequences.

This technique will null any signal arising from voxels which contain both fat and water.

By utilizing a TE when fat and water are out of phase with each other they will not yield a signal

summary Fat is nulled with the following techniques: Fat saturation STIR SPIR Out of phase imaging (Dixon technique) Even echo rephasing Balanced echoes where even echoes demononstrate less dephasing than odd echoes Reduces intra voxel dephasing Used in T2 weighted sequences

Fat is nulled with the following techniques:

Fat saturation

STIR

SPIR

Out of phase imaging (Dixon technique)

Even echo rephasing

Balanced echoes where even echoes demononstrate less dephasing than odd echoes

Reduces intra voxel dephasing

Used in T2 weighted sequences

summary Gradient moment rephasing: Additional gradients to correct altered phase values Reduces artefact from intra-voxel dephasing Gives flowing nuclei bright signal Used with T2 or T2* Best with slow, laminar flow within the slice Chemical pre-saturation: Extra RF used to null signal from flowing nuclei Reduces artefact from TOF and entry slice phenomenon Creates a signal void from flow Used with T1 Best on both fast or slow flow Increases RF to pt. Used to null signal from fat or water to reduce aliasing.

Gradient moment rephasing:

Additional gradients to correct altered phase values

Reduces artefact from intra-voxel dephasing

Gives flowing nuclei bright signal

Used with T2 or T2*

Best with slow, laminar flow within the slice

Chemical pre-saturation:

Extra RF used to null signal from flowing nuclei

Reduces artefact from TOF and entry slice phenomenon

Creates a signal void from flow

Used with T1

Best on both fast or slow flow

Increases RF to pt.

Used to null signal from fat or water to reduce aliasing.