Flow void

Flow voids refer to a signal loss occurring with blood and other fluids, like CSF or urine, moving at sufficient velocity relative to the MRI apparatus. It is a combination of time-of-flight and spin-phase effects usually seen in spin echo techniques (such as T1 and T2-weighted images) 2.

Physics

Protons in flowing fluid, such as arterial blood, move out of the plane of imaging in the time between the absorption of the energy of the radio wave pulse and the detection of its release, producing no signal 1-3

The amount of signal void is related to the velocity of the proton, the slice thickness, the time of echo (TE), and the course of the vessel 1

A different time-of-flight phenomenon occurs in flow-compensated gradient-echo sequences resulting in flow-related enhancement (time-of-flight angiography).

Practical points

  • flow void is synonymous with vascular patency, representing a normal flow-related signal loss in vessels that contain vigorously flowing blood 
  • sequences with long TE (such as T2 and PD) have most prominent flow voids; when vascular thrombosis is identified on a T1-weighted sequence (short TE), it should be confirmed by the corresponding T2 or PD sequences, as these are less sensitive to slow flow voids and more specific to the diagnosis of thrombosis
  • flow voids can also been seen along transverse T2-weighted images of the spine, as the CSF flows perpendicular to slice direction 2.
  • aqueduct stenosis is a pathologic condition in which CSF flow voids are also present 
MRI physics
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Article Information

rID: 33838
Section: Physics
Synonyms or Alternate Spellings:
  • Flow voids

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Cases and Figures

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    Case 1: cerebral AVM
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    Case 2: bladder diverticulum.
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    Case 3: normal pressure hydrocephalus
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    Dural AVF

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    Case 4: dural arterio-venous fistula
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