Imaging description

Aliasing artifact occurs in phase-contrast MRI when the peak velocity of flowing blood being imaged exceeds the encoding velocity, or Venc. In phase-contrast MRI, specialized flowsensitive gradients are used to encode flow velocity and flow direction information into grayscale pixel values. The Venc is a parameter, expressed in cm/sec, that specifies the maximal velocity that can be measured by a given phase-contrast acquisition. The Venc is adjustable and is set at the MRI scanner before image aquisition. When the vessel being imaged contains flowing blood that is moving faster than the Venc, aliasing will occur. Pixels will become progressively brighter as velocities approach the Venc; however, pixels that represent velocities exceeding the Venc wrap around and are mapped to dark pixels from the opposite end of the grayscale spectrum (Figure 50.1). Depending on the direction of flow, the inverse situation can also occur, where white aliased pixels may be surrounded by dark non-aliased pixels with velocities just below the Venc (Figure 50.1).

Importance

Aliasing will result in inaccurate measurement of peak velocities within a vessel. Peak velocities are used to estimate the pressure gradient across a stenosis which can dictate treatment decisions. Automated flow measurement software can be used to correct for aliasing if peak velocity is less than three times the Venc. However, repeated acquisitions at an increased Venc setting are preferred for maximum measurement reliability.

Typical clinical scenario

Aliasing artifacts are seen whenever the flow velocity is greater than expected when setting the Venc. This is common in the setting of stenotic vessels or valvular stenosis. Elevated velocities may also be seen within the left ventricular outflow tract in the setting of obstructive hypertrophic cardiomyopathy. Typical maximal velocities for ascending aorta, pulmonary artery, and systemic veins are less than 150 cm/sec, less than 100 cm/sec, and less than 50 cm/sec, respectively. In the setting of stenosis, maximal velocities may increase to 500–1000 cm/sec, and repeated acquisitions with incremental increases of Venc may be necessary to find the optimal Venc to prevent aliasing.