Flow types
- Laminar flow: slow, organized, parallel to vessel walls, parabolic profile
- Vortex flow: swirling, often countercurrent eddies seen at bifurcations and after stenoses
- Turbulent flow: disorganized, random, with high velocities
Factors
Decreased intravascular signal | Increased intravascular signal |
High velocity | Low velocity |
Turbulent or vortex flow | Laminar flow |
Saturation pulses | Gradient-moment nulling |
Odd-echo dephasing | Even-echo rephasing |
Multi-slice acquisition | Single-slice acquisition |
Flow within plane of imaging | Flow perpendicular to plane of imaging |
Slices deep within imaging volume | Slices at ends of imaging volume |
ㅤ | Cardiac gating |
ㅤ | Gadolinium |
Time-of-flight (TOF) effects
- Definition: signal variations resulting from the motion of protons flowing into or out of an imaging volume during a given pulse sequence.
- Flow-related enhancement: In Spin-echo and gradient-echo imaging, inflow of spins results in increased signal;
- High-velocity signal loss {washout): Outflow of spins may result in decreased signal intensity
Thrombus vs slow flow" problem
- Can usually be solved by
- Non-MRI methods, such as CT venography or Doppler ultrasonography.
- Patient could be scheduled to return to the MR center for phase-contrast or gadolinium-enhanced venography
CSF flow study
- Re-phased image (magnitude of flow compensated signal)
- Flow is of high signal
- Background is visible
- Magnitude image (magnitude of difference signal)
- Flow is of high signal (regardless of direction)
- Background is suppressed
- Phase image (phase of difference signal)
- Signal is dependent on direction: forward flow is of high signal; reverse flow is of low signal
- Background is mid-grey
- Look for direction of flow in SSS
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