Flow void

Flow voids is a term used when describing MRI studies and refers to signal loss occurring within moving fluids (usually blood but also frequently seen in CSF or urine) when the fluid is moving at a sufficient velocity relative to the MRI apparatus. Some MRI sequences are more susceptible to flow-induced signal voids than others as it is a combination of time-of-flight and spin-phase effects and thus usually seen in spin-echo techniques (such as T2-weighted images) ².

Flow voids are a form of MRI artifact. Often they are useful as they allow patency and flow to be inferred. Sometimes, however, they can be mistaken for pathology.

Physics

During spin-echo imaging, protons in flowing fluid move out of the plane of spatially-selective radiofrequency pulses, in the time between the initial 90° excitation pulse and the second 180° refocusing pulse. Because of this time-of-flight effect, these protons miss the refocusing pulse and dephase, thereby contributing no signal to that voxel ^1-3. (A different time-of-flight phenomenon occurs in flow-compensated gradient-echo sequences resulting in flow-related enhancement, harnessed in time-of-flight angiography.) The degree of signal loss due to this time-of-flight effect is related to the velocity of the proton out of the plane, the slice thickness, and the time to echo (TE) ¹. 

Spin-phase effects account for signal loss occurring with motion within the same plane of imaging. The velocity of blood varies at different points across the lumen of a vessel, whether during laminar or turbulent flow. Protons moving at different velocities then gain different phases as they move across the gradient applied for spatial encoding. The phase shift depends on the velocity of the proton within the plane, strength of the applied gradient, and time to echo. As these differences occur on a microscopic level, spin dephasing causes signal loss within a voxel. 

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 not present