Nafeesa M A CB Sc (AHS) INTERNEE

Department of Radiology and Imaging SciencesSri Ramachandra University, Porur, Chennai

TIME OF FLIGHT

• Time of flight is one of the MR Angiography

techniques which produces images of the blood

vessels with out using any contrast agent

INTRODUCTION

TIME OF FLIGHT MR ANGIOGRAPHY

Entry slice phenomenon

• Time of flight produces vascular contrast by

manipulating the longitudinal magnetization of the

stationary spins

• The TR is kept well below T1 time of the stationary

tissues so that T1 recovery is prevented

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TIME OF FLIGHT MR ANGIOGRAPHY

• Time of flight MR angiography uses incoherent

gradient echo sequence in combination with gradient

moment rephasing

• Gradient moment rephasing is used to enhance flow

• It uses additional gradients to correct the altered

phases back to their original values

• The flowing spins travelling along the gradients

accumulate zero phase change

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Gradient moment nulling

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For evaluation of Cerebral arteries

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For evaluation of Cerebral veins

TIME OF FLIGHT MR ANGIOGRAPHY2D TOF In 2D TOF the signals are received sequentially

from a series of image slices.

MR Cerebral

Venogram

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3D TOF In 3D TOF the signals are received from the

entire volume of interest

MR Cerebral

Angiogram

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3D TOF

MR Neck

Angiogram

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Advantages Disadvantages

Large area of coverage Lower resolution

Sensitive to slow flow Venetian blind artifact

2D TOF

3D TOF Advantages Disadvantages

High resolution for smaller vessels

Saturation of in-plane flow

Sensitive to T1 effects Small area of coverage

PARAMETERS 2D TOF 3D TOF

TR 40-50ms 40-50ms

TE Minimum permissible Minimum permissible

Flip angle Approximately 600 Approximately 600

Saturation slab Inferior Superior

slice thickness 1.5mm and above 1mm and above

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TIME OF FLIGHT MR ANGIOGRAPHY

Multiple overlapping thin 3D slab acquisition MOTSA

In MOTSA overlapping thin slabs are

used Advantages:• Reduce flow signal

saturation

• High spatial resolution

• Good SNR

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Magnetization Transfer contrast (MTC)

MTC is used to improve the image contrast so as

to enhance the conspicuity of blood vessels by

effectively suppressing the intervening

background tissue signals

WITH MTCWITHOUT MTC

Pathologies

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ANEURYSM

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Conclusion

THANK YOU

IMAGING MODE

TOF MRA can be either acquired in 2d or 3d modeApplications:2D:• Optimal in areas of slower signal velocity flow• Used where larger area of coverage is required.3D:• Used in high areas of velocity flow• For high resolution to visualize small vessels

TIME OF FLIGHT MR ANGIOGRAPHY

Gradient moment nulling

ADVANTAGES AND DISADVANTAGES

Advantages:• Reasonable imaging time• Sensitive to slow flow• Reduced sensitivity to intra voxel dephasing Disadvantages:• Sensitive to T1 effects • Saturation of in plane flow• Enhancement is limited to either flow entering the

fov or very high velocity flow.

VENETIAN BLIND ARTIFACT

venetian blind artifact occurs due to the respiration and the patient motion that moves tissue in and out of the slice that is being suppressed.

MOTSA(multiple overlapping thin slab acquisition)

Multiple thin slab acquisition reduces the signal saturation of slowly flowing blood compared to one thick single-slab acquisition, and overlapping slab acquisition eliminates the signal void from each slab boundary region often called the venetian blind artifact. The slices located in the overlapped region can be acquired twice with an entry slice from the first slab and an exit slice from the next slab.

MOTSA(multiple overlapping thin slab acquisition)

Increasing the amount of slab can reduce the slab boundary artifact. Increasing the slab by 50% completely reduces the slab boundary artifact.The disadvantage is increasing the overlap increases the scan time per unit coverage in the slice direction

Time-of-flight techniques derive contrast between flowing blood and stationary tissues by manipulating the magnitude of the magnetization, such that the magnitude of the magnetization from the moving spins is large and the magnitude of the magnetization from the stationary spins is small. this leads to a large signal from moving blood spinsand a diminished signal from stationary tissue spins.

INTRODUCTION

TIME OF FLIGHT MR ANGIOGRAPHY

Advantages Disadvantages

Large area of coverage Lower resolution

Sensitive to slow flow Venetian blind artifact

2D TOF

3D TOF Advantages Disadvantages

High resolution for smaller vessels

Saturation of in-plane flow

Sensitive to T1 effects Small area of coverage