Reviews on TOF, Bright & Dark Blood, etc. Flashcards
What is the Time-of-Flight (TOF) Phenomena used for?
used to visualize desired vascular structures
When does the TOF happen?
happens when acquiring normal MRI images.
What is the TOF based on the timing of?
pulse sequence
slice thickness
flow’s speed
TOF with Spin Echo (SE)
longer time between RF excitation and RF refocusing pulse
flowing protons are not present within the slice for both pulses
vessels appear DARK
TOF with Gradient Echo (GRE)
shorter time from RF excitation and gradient rephasing
flowing protons are present within the slice for both pulses
vessels appear BRIGHT
What are the 2 parts of a vessel?
vessel wall (endothelium)
lumen (where blood flows)
Factors that can alter speed/patterns of blood flow
size
shape
tortuosity of vessel
beating of heart
disease state
4 basic blood flow patterns groups
laminar flow
vortex flow
spiral flow
turbulent flow
Laminar flow
most common flow, parallel to the vessel wall
Does blood flow faster or slower in the center of the vessel?
fast due to less resistance
Does blood flow faster or slower to the wall of the vessel?
slower due to friction
What happens when vessels are bifurcating?
vessels change in size/direction
Vortex flow
stagnant/slow-moving, complex flow
counter-flow
Spiral flow
turbulent flow pattern changes velocities randomly/rapidly
most difficult type of flow to image
What is Flow-Related Enhancement (FRE)?
a phenomenon in MRI where flowing blood appears bright on an image
Why does Flow-Related Enhancement occur?
occurs because the pulse sequence timing allows flowing blood to avoid saturation, while stationary tissues become saturated
What is Flow-Related Signal Loss (FRSL)?
FRSL occurs when flowing blood appears dark on an image due to the timing of the pulse sequence
What is Time-of-Flight (TOF) MRA?
Produces flow-related enhancement
Can use both 2D and 3D acquisitions
This imaging works best when slices are PERPENDICULAR to the flow of blood
What is CE-MRA?
a technique that uses a contrast agent to improve the visibility of blood vessels
What is Phase Contrast MRA?
a technique that measures the velocity and direction of blood flow
What is Bright Blood MRA?
a type of MRI technique that uses a specific pulse sequence to visualize flowing blood
How does Bright Blood MRA work?
by using short TR and TE values, Bright Blood MRA saturates stationary tissues but allows flowing blood to remain unsaturated, making it appear bright on the image
2D TOF
ideal for large, slow flow, and the anatomy to be imaged covers a large area
2D TOF, used in carotids, iliac arteries, and femoral arteries
lower SNR
3D TOF
ideal for tortuous vessels
fast-moving flow (in the brain)
higher resolution
higher SNR
better for visualizing smaller vessels
Spatial Saturation Bands
to minimize venous signal appearing bright on MRAs
suppress venous signal
MOTSA
multiple Overlapping Thin Slice Acquisition
What does MOTSA reduce?
the saturation effect in 3D TOF acquisitions
TE (Bright Blood Imaging)
shorter TE = quicker signal is recorded
flowing protons receive both RF excitation pulse and refocusing gradient
Good for stationary tissues
TR (Bright Blood Imaging)
shorter TR = more saturation
the flow is low, the flow might be saturated as well
reasons for DARK BLOOD on Bright Blood Imaging
changes in flow patterns
saturation
flow is not perpendicular to imaging plane
Dark Blood Imaging
flow-related signal loss
using SE, uses both RF excitation and at least 1 RF refocusing pulse
3 ways to achieve DARK BLOOD
SE is preferred
spatial saturation band placement
inversion recovery techniques
reasons for BRIGHT BLOOD on Dark Blood Imaging
post-contrast imaging
imaging parameters
flow being parallel to the imaging plane
What happens when the flow is very slow and is exposed to both RF excitation and RF refocusing pulse?
Bright Blood on Dark Blood imaging
Timing bolus
injecting a small amount of contrast and seeing how long the contrast takes to arrive at the area of interest
this timing is used to set the delay from the administration of contrast to start the image
Automatic Bolus Detection
uses a Region of Interest (ROI) placed on the desired part of the anatomy that will track the change in signal intensity
Real-time Bolus Tracking
“Fluoro-triggering”
A technique that images a specific area of interest in real time
Tech can visualize the arrival of contrast to the area of interest
Dynamic CE-MRA Acquisition
rapidly and repeatedly imaging the same area of interest over and over again
image acquisition and administration start at the same time
4D or time-resolved angio
What is the purpose of Flow Encoding Gradients?
to reduce artifacts, image, and even measure the speed of flowing tissues
Gradient Moment Nulling
“Flow compensation”
a technique used to reduce the artifact from flowing tissues
Intra-voxel dephasing
a loss of signal amplitude within a voxel due to flowing blood and stationary tissues being out of phase with one another
What is GMN used for?
to reduce intra-voxel dephasing by rephasing the protons within flowing tissues
3 gradients for basic GMN implementation
Initial flow encoding gradient (+ polarity)
2nd flow encoding gradient ( - polarity at double strength)
3rd flow encoding gradient (+ polarity)
Is flow direction crucial for GMN?
yes
Which flow encoding gradient does Phase Contrast enhanced MRA use?
bi-polar flow encoding gradient
(two lobes, 1 positive and 1 negative)
Where does phase shift happen when the bi-polar gradient is turned on?
1st positive lobe
Where do the stationary protons recover the phase shift and go back to where they started?
the negative lobe
What doesn’t the negative lobe do?
rephrasing the flowing protons
What does Phase Contrast Imaging produce?
a difference in signal and phase info. between the stationary and flowing protons
What does Phase Contrast Imaging measure?
the speed and direction of protons in flowing tissues, great for visualizing and quantify blood and CSF flow
How does PC Imaging correct phase shift from an inhomogeneous field?
a 2nd set of flow-encoding bipolar gradients is applied w/ the same strength but opposite polarity
Velocity Encoding (VENC)
the parameter that adjusts phase contrast angiography/venography to image certain blood vessels flowing at a certain speed
Which parameter controls the flow encoding gradient?
VENC
Low VENC = __________
High VENC = _____________
Low VENC = aliasing
High VENC = poor contrast
