Flow & Quantitative MRI

LEVEL 6 (BOARD INTENSIVE)

Phase contrast physics, VENC optimization, and hemodynamic calculations.

Need-to-know for clinical practice

Phase Contrast / VENC Lab

Exhaustive Hemodynamics Simulation

Magnitude (Anatomy)

Velocity Encoding (VENC)150 cm/s

Peak Blood Velocity100 cm/s

Status

Diagnostic

Dynamic Range

67%

The VENC Rule

VENC must be > Peak Velocity. If blood moves faster than VENC, the phase shift exceeds 180° (π radians) and is incorrectly recorded as flow in the opposite direction.

Quick Challenges

1. Physics of Phase Contrast

Phase-contrast MRI (PC-MRI) takes advantage of the fact that protons moving through a magnetic gradient accumulate a phase shift that is proportional to their velocity.

The Math

For a fellow, the key relationship is: Δφ = γ · G · Δt · v. The change in phase (Δφ) depends on the gradient strength (G) and the velocity (v). By measuring this phase shift, we can calculate the velocity of every pixel.

2. Mastering VENC

Velocity Encoding (VENC) is the maximum velocity that can be measured before the phase wraps around.

If VENC is too low: You get aliasing (bright flow turns black). You cannot calculate peak velocity.
If VENC is too high: The "dynamic range" is wasted. You won't be able to see slow flow against the noise.

Board Rule of Thumb

Always set your VENC ~10-25% higher than the expected peak velocity. If you see aliasing, re-scan with a higher VENC.

3. Hemodynamics: Qp/Qs & 4D Flow

One of the most exhaustive board topics is the calculation of shunts (ASD, VSD, PDA). Modern MRI goes beyond simple 2D flow to 4D Flow, which visualizes the volume of blood over time.

4D Flow Vector Lab

Visualize streamlines and turbulence

Aortic Stenosis Severity0%

Board Insight: Stenosis causes flow acceleration (Vmax increases) and post-stenotic turbulence. 4D flow allows us to visualize 'Wall Shear Stress' and 'Turbulent Kinetic Energy' (TKE).

The Shunt Calculation (Qp/Qs)

1. Measure net stroke volume in the Main Pulmonary Artery (Qp).
2. Measure net stroke volume in the Ascending Aorta (Qs).
3. Qp/Qs = Qp / Qs.

Note:A ratio > 1.5 usually indicates a significant left-to-right shunt requiring intervention.

Knowledge Check1 / 3

You are measuring flow in the ascending aorta. If the VENC is set to 150 cm/s and the actual peak velocity is 200 cm/s, what will you see on the phase image?

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