Objetivos de aprendizaje
After reading this chapter, you should be able:
- To describe the different flow phenomena in MRI
- Explain the principle of flow compensation gradients
- Present non-contrast magnetic resonance angiography methods (technique, results, advantages and disadvantages:
- Time-of-flight (TOF) MRA
- Phase contrast (PCA) MRA
- 3D MRA with ultrafast spin echo and ECG synchronization (FBI)
- Describe the Contrast-enhanced MRA technique, its constraints related to the injection of a contrast agent, and its advantages
Flow and MRI
Phenomena associated with flow in MRI
- The inflow effect: flow of nonsaturated blood into the explored zone (vascular hypersignal)
- The outflow effect: the excited blood flows out of the explored zone (loss of vascular signal)
- Dephasing of moving spins in a gradient
This last phenomenon, used in phase contrast MRA, produces artifacts in the other imaging methods. The artifacts caused by flows at constant velocity can be overcome by means of flow compensation gradients.
Time of Flight
- Saturation of stationary spins
- Maximization of inflow effect and minimization of outflow effect
- GE, TR and TE
- Orientation of slices perpendicular to flow
- Slow flow: 2D;
- Fast flow: 3D, MOTSA, TONE
- Magnetization transfer, Fat suppression, Saturation band
- Good visualization of fast flow, Less good for in-plane or turbulent slow flow
- Poor suppression of background signal if short T1 (fat, hematoma, thrombus)
- Velocity phase encoding gradient in 3 directions
- Subtraction from a non- encoding gradient acquisition
- Choice of encoding speed +++
- 2D: fast, encoding speed test
- 3D: reduction in the number of phase encoding steps to accelerate the sequence
- Data on flow velocity and direction
- Poor visualization of complex or turbulent flows
- Slow in 3D
- 3D half Fourier fast spin echo
- Prospective ECG synchronization
- STIR preparation
- Short TE, coronal plane, phase encoding in the direction of the vessels
- Calibration of time interval between R wave and acquisition ++
- Double acquisition to subtract the venous signal
- Possible for thorax/abdomen
- Loss of signal for fast flows (alternative: diastolic acquisition)
- Fast 3D GE, short TR and TE, T1 weighted with destruction of residual magnetization
- Gadolinium bolus injection
- Filling of center of k-space when vasculair contrast is at it's peak +++: bolus test, real-time tracking, 4D angioMR
- Fat suppression
- Increase of vascular signal. Exploration of large volumes, Turbulent flow imaging
- Risks and drawbacks of injection
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