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
TIME OF FLIGHT MR ANGIOGRAPHY
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
TIME OF FLIGHT MR ANGIOGRAPHY
Gradient moment nulling
TIME OF FLIGHT MR ANGIOGRAPHY
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
TIME OF FLIGHT MR ANGIOGRAPHY
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