Flow voids are dark areas that appear on magnetic resonance imaging (MRI) scans, representing regions where fluids, primarily blood, are moving rapidly. Common during brain imaging, they relate to how MRI detects signals. Understanding their nature often requires further medical evaluation to determine their significance, as they can represent both normal physiological processes and various medical conditions.
How Flow Voids Appear on MRI
MRI detects signals from hydrogen protons in water molecules. A strong magnetic field aligns protons. Radiofrequency pulses momentarily knock them out of alignment, and as they realign, they emit a signal the scanner detects.
When blood flows rapidly through a vessel, the hydrogen protons within that blood move out of the MRI scanner’s imaging area before they can fully contribute a signal. This phenomenon, known as a “time-of-flight” effect, results in a “void” or dark area on the image. Faster blood flow, or longer echo times, lead to more pronounced signal loss and a darker appearance, particularly in spin-echo sequences like T2-weighted images.
Normal Physiological Causes
Flow voids are frequently a normal and expected finding in brain MRI scans, indicating healthy and unobstructed blood circulation. This appearance is due to the rapid flow of blood within the brain’s major arteries and venous structures. The presence of these expected arterial flow voids suggests that the central arteries supplying the brain are open and functioning as they should. Examples of such normal flow voids include those seen in the large intracranial arteries, such as the internal carotid and basilar arteries, and within the major dural venous sinuses like the superior sagittal sinus and transverse sinuses. These dark areas confirm the patency of these vessels, meaning they are open and allowing brisk blood flow.
Medical Conditions Associated with Flow Voids
While often normal, abnormal flow voids can indicate underlying medical conditions due to alterations in blood flow dynamics. These variations can range from turbulent flow to unusually high blood volume in specific areas.
Aneurysms
Aneurysms, which are balloon-like bulges in blood vessel walls, can present with flow voids. The rapid and often turbulent blood flow within an aneurysm creates signal loss on MRI, appearing as a dark area. This turbulent flow causes the protons to dephase, leading to a loss of signal that can sometimes mimic the appearance of an aneurysm.
Arteriovenous malformations (AVMs)
Arteriovenous malformations (AVMs) are abnormal tangles of blood vessels where arteries connect directly to veins, bypassing the normal capillary network. The high-velocity shunting of blood through these abnormal connections results in prominent flow voids on MRI, often described as a “bag of worms” appearance.
Dural arteriovenous fistulas (DAVFs)
Dural arteriovenous fistulas (DAVFs) are abnormal connections between arteries and veins located in the dura mater, the tough outer covering of the brain and spinal cord. High-flow DAVFs can manifest as multiple, tortuous flow voids on T2-weighted MRI, particularly at the base of the brain or around the tentorial incisura. Enlarged deep and superficial draining veins, also appearing as prominent signal voids, may accompany these findings.
Highly vascular tumors
Highly vascular tumors, such as some meningiomas or juvenile nasopharyngeal angiofibromas, can also exhibit flow voids due to their rich blood supply and rapid internal blood flow. These intratumoral signal voids correspond to dilated vessels within the tumor, indicating significant vascularity. Less common high-flow vascular lesions, like carotid-cavernous fistulas, can also present with similar flow void characteristics.
Interpreting Flow Voids and Next Steps
Flow voids on a brain MRI require careful interpretation by a qualified medical professional, such as a radiologist or neurologist. While many are normal, their location, size, and appearance provide clues about their significance. The interpreting physician considers these imaging characteristics alongside the patient’s symptoms and other clinical information.
If an abnormal flow void is suspected, further specialized imaging is often recommended to confirm the diagnosis and provide more detailed information about the vascular anatomy. This may include magnetic resonance angiography (MRA), computed tomography angiography (CTA), or conventional catheter angiography. These advanced imaging techniques offer a clearer picture of blood vessel structure and flow, guiding management and treatment decisions.