Magnetic resonance angiography (MRA) and Flow 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.

Principles

• Saturation of stationary spins
• Maximization of inflow effect and minimization of outflow effect
• GE, TR and TE

Optimization

• Orientation of slices perpendicular to flow
• Slow flow: 2D;
• Fast flow: 3D, MOTSA, TONE
• Magnetization transfer, Fat suppression, Saturation band

Results

• 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)

Principles

• Velocity phase encoding gradient in 3 directions
• Subtraction from a non- encoding gradient acquisition
• GE

Optimization

• Choice of encoding speed +++
• 2D: fast, encoding speed test
• 3D: reduction in the number of phase encoding steps to accelerate the sequence

Results

• Data on flow velocity and direction
• Poor visualization of complex or turbulent flows
• Slow in 3D

Principles

• 3D half Fourier fast spin echo
• Prospective ECG synchronization
• STIR preparation

Optimization

• 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

Results

• Coronal
• Possible for thorax/abdomen
• Fast
• Loss of signal for fast flows (alternative: diastolic acquisition)

Principles

• Fast 3D GE, short TR and TE, T1 weighted with destruction of residual magnetization
• Gadolinium bolus injection

Optimization

• Filling of center of k-space when vasculair contrast is at it's peak +++: bolus test, real-time tracking, 4D angioMR
• Fat suppression

Results

• Increase of vascular signal. Exploration of large volumes, Turbulent flow imaging
• Risks and drawbacks of injection