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  • Topic ID: id_40022124 Version: 1.3
  • Date: Aug 21, 2022 6:49:46 PM

PC: VENC scan

Consider this information when modifying a 2D Phase Contrast or 3D Phase Contrast scan parameters. For specific scan parameter values, select a protocol from your GE or Site library.

VENC is a value entered to prescribe the highest velocities to be encoded, without aliasing, in Phase Contrast angiography. VENC is the parameter that defines this ceiling. Set the VENC high enough to include all the velocities likely to be encountered within the vessels of interest. Valid values are 5 to 400 cm/sec, in increments of 0.1 cm/sec.

Table 1. Intra-cranial mean velocities (cm/sec)
ArteryVelocity range
Middle cerebral55 ± 13
Anterior cerebral49 ± 18
Posterior cerebral40 ± 7
Internal carotid47 ± 14
Vertebral33 ± 8
Basilar55 ± 14
Opthalmic14 ± 3
Cerebral aqueduct3 ± 1
Transcranial Doppler measurements of arterial flow velocity. (Adapted from Dewitt and Wechsler, Stroke Vol. 19 No. 7, July 1988.)
Table 2. Peripheral arteries peak velocities (cm/sec)
ArteryVelocity range
External iliac119 ± 21
Common femoral114 ± 24
Superficial femoral (proximal)90 ± 13
Superficial femoral (distal)93 ± 14
Popliteal69 ± 13
Aorta (thoracic)100 - 175
Common carotid artery80 - 120
Internal carotid artery80 - 120
"Peak velocities measured by duplex scanning." (Adapted from Jager, Ricketts, Strandness, Jr., "Duplex scanning for the Evaluation of lower Limb Arterial Disease," in Bernstein EF [eds] Non-Inavasive Diagnostic Techniques in Vascular Disease, Mosby & Co., St. Louis, 1985.)

Velocities higher than the VENC are aliased, that is, incorrectly represented as lower velocities, with lower image intensities. Since higher velocities are normally found at vessel center, Phase Contrast aliasing can result in decreased image intensity in the center of a vessel.

Phase aliasing produces flow that appears to have changed direction, identifiable by a group of adjacent black and white pixels. Aliasing is sometimes acceptable. For this effect, deliberately set the VENC below a vessel’s peak velocities. This technique may be useful for highlighting slower flow along arterial walls, or for emphasizing venous anatomy.