MRI Velocity Encoding (VENC)

Setting Up the Phase-Contrast Sequence

Phase-contrast MRI sequences utilize gradient pulses to create phase shifts in moving protons, which are directly proportional to their velocity along the direction of the gradient. This technique differentiates between stationary and moving spins by encoding movement into the phase of the MRI signal. The difference in phase between stationary and moving spins can then be used to calculate velocity.

• Initial Setup: The radiographer sets up a basic MRI sequence with specific parameters that are suitable for capturing the flow of interest. This includes selecting the appropriate plane (either through-plane or in-plane) that will intersect the flow.
• VENC Preliminary Estimate: An initial estimate of the VENC is made based on expected physiological velocities in the vessel or area of interest. For example, higher VENC values are used for arteries with high flow rates (like the aorta), and lower values are used for veins or areas with slower flow

Performing a VENC Scout Scan

• Scout Scans: Before the final flow measurement, a series of scout scans (also known as VENC test scans or range-finding scans) are performed. These scans cover a range of VENC values, starting from a low estimate to higher velocities.
• Purpose of Scout Scans: These scans help in identifying the VENC value at which there is no aliasing (where the velocity measurements wrap around and appear falsely low) and yet the sensitivity of the scan is maximized for the velocities encountered.

Analyzing Scout Scan Results

• Reviewing Images: The radiographer and possibly a radiologist review the images from the scout scans to determine the optimal VENC value. This value is the highest velocity that can be measured without aliasing and provides the best contrast for the flow dynamics being studied.
• Selection of Optimal VENC: The optimal VENC is selected based on a balance between avoiding aliasing and maximizing image quality. If aliasing is observed in the preliminary scans, the VENC is adjusted upward; if the images appear too noisy or the low velocities are not well visualized, it may be adjusted downward.

Setting the Final Phase-Contrast Sequence

• Final VENC Setting: Once the optimal VENC value is determined, it is set as the parameter for the final phase-contrast flow sequence.
• Execution of Final Scan: The final detailed phase-contrast sequence is then run using this optimized VENC setting. This sequence will provide quantitative data on the flow velocities, which can be used for clinical assessment and diagnosis.

Quantitative Analysis

• Post-Processing: After the scan, the images are post-processed using software that calculates flow volumes, velocities, and patterns. These quantitative measures are essential for diagnosing conditions like stenosis, valve malfunctions, or shunts.