According to MediLexicon, flow voids occur when the MRI machine is unable to see the metallic elements within the flowing blood. Since MRIs work through magnetization, flow voids can be used to indicate a lack of blood flow or a completely blocked flow.
Mayfield Clinic explains that flow voids appear on an MRI screen as black holes, since traditional MRI machines are not able to read liquid that is moving, such as blood through arteries. To compensate for this, contrast dye containing magnetic properties is used to help the computer pick up blood flowing through veins and arteries. The flow void itself is telling of the blood flow condition within various parts of the brain.
The National Institute of Health notes that flow voids are sometimes helpful in determining the presence of a serious brain injury or condition. For example, a magnetic resonance angiogram will display a flow void if a large aneurysm is present in the brain. In this case, the flow void appears because the aneurysm has caused the blood to slow down significantly. The intensity of the magnetic signals around areas of flow void in the brain are often used to determine the severity of conditions like aneurysms.