What Does an Abnormal MRV Brain Scan Mean?

An abnormal MRV (magnetic resonance venography) of the brain means that the veins or venous sinuses draining blood from the skull are not flowing or shaped as expected. The most common reason for ordering an MRV is suspicion of a blood clot in one of the brain’s large venous channels, but the scan can also reveal narrowed sinuses, abnormal connections between arteries and veins, or structural quirks that may or may not need treatment. What makes interpreting these results tricky is that the brain’s venous anatomy varies widely from person to person, so not every unusual-looking finding on an MRV represents a disease.

What MRV Actually Shows

MRV is a type of MRI focused specifically on the veins and venous sinuses inside and around the brain. The large drainage channels you hear about most often are the superior sagittal sinus (running along the top of the skull), the transverse sinuses (running along the back), the sigmoid sinuses (curving down behind the ears), and the straight sinus (deep in the midline). These sinuses collect blood from the brain’s surface and interior and funnel it into the internal jugular veins in the neck.

There are a few ways to perform the scan. Time-of-flight (TOF) MRV uses the natural movement of blood to generate contrast and requires no injection. Contrast-enhanced MRV involves injecting gadolinium dye and tends to show smaller veins more clearly. A comparative study found that certain structures, including the left transverse sinus and deep veins like the thalamostriate veins, were better visualized with contrast-enhanced techniques than with TOF alone.¹ This matters because a sinus that looks absent on a TOF scan might simply be flowing too slowly for the technique to pick up, rather than being blocked.

Normal Variants That Can Look Alarming

One of the biggest pitfalls in reading an MRV is mistaking a normal anatomical variant for disease. The brain’s venous sinuses are not symmetrical in most people. The right transverse sinus is dominant (larger) in roughly the majority of the population, the left is dominant in a smaller group, and some people have roughly equal sinuses. When one transverse sinus is naturally small or even absent, a TOF MRV can flag that side as having no flow, which looks identical to a clot on the image.

Arachnoid granulations are another common source of false alarms. These small, rounded protrusions poke into the walls of venous sinuses and are a normal part of cerebrospinal fluid reabsorption. On MRV, they appear as filling defects inside a sinus and can mimic a thrombus. An experienced neuroradiologist can usually tell them apart by their shape and location, but the initial read sometimes triggers unnecessary concern. If your report mentions a possible filling defect in the superior sagittal or transverse sinus with a suggestion to “correlate clinically,” an arachnoid granulation is often the explanation.

Blood Clots in the Brain’s Veins

Cerebral venous sinus thrombosis, usually abbreviated CVST, is the condition most clinicians are looking for when they order an MRV. A clot in one of the major sinuses blocks normal drainage, which raises pressure in the veins upstream and can lead to brain swelling, bleeding, or stroke. MRV and conventional MRI together can reveal both direct signs of a clot (a filling defect where flow should be) and indirect signs like tissue swelling or hemorrhage.² In one case series, gadolinium-enhanced MRV clearly demonstrated filling defects confirming CVST when clinical suspicion was high.³

CVST can be surprisingly hard to recognize clinically because its symptoms are nonspecific. The most common complaint is headache, often severe and progressive, sometimes accompanied by seizures, vision changes, or weakness on one side of the body. It remains one of the most frequent causes of stroke in young adults, even though it is relatively rare overall.⁴ Because it can mimic other conditions, including brain tumors, clinicians rely heavily on imaging to confirm the diagnosis. One report described CVST presenting in a cancer patient in a way that initially looked like brain metastases, highlighting how misleading the clinical picture can be.⁵

Who Is at Higher Risk for CVST

Pregnancy and the weeks after delivery stand out as a well-recognized risk period. The body’s clotting system shifts into a more active state during pregnancy, which is useful for preventing hemorrhage during birth but also raises the chance of abnormal clotting. The combination of increased venous pressure and this hypercoagulable state makes CVST a particularly dangerous complication in the postpartum window.⁶

Cancer creates a similar hypercoagulable environment. Malignancy is recognized as a predisposing factor for cerebral venous thrombosis, and the diagnosis should be considered when a cancer patient develops new neurological symptoms.⁷ Autoimmune and inflammatory conditions also play a role. In Behçet’s syndrome, for example, cerebral venous sinus involvement occurs in roughly 20 to 25 percent of patients who develop neurological complications.⁸

Head trauma is another underappreciated trigger. A prospective study of patients with traumatic brain injury and skull fractures found that about 14 percent developed dural venous sinus thrombosis, with the risk climbing sharply when the fracture involved multiple bones. Roughly 80 percent of confirmed cases occurred in patients whose fractures extended across the temporal or parietal bones along with the occipital bone.⁹

CVST in Newborns and Children

Venous sinus thrombosis is not just an adult problem. Neonates can develop it too, often after a complicated or assisted delivery. In one study of 52 newborns with confirmed CVST, the most common presenting symptom was seizures, and the clots were confirmed on MRI and MRV in every case. Multisinus thrombosis, meaning clots in more than one venous channel, was the most common pattern.¹⁰

The risk factors and patterns differ between newborns and older children. In neonates, acute systemic illnesses are the leading trigger, and in over a third of cases no identifiable cause is found. Older children are more likely to have local infections, inherited clotting disorders, or a history of head trauma as the precipitant. The location of the clot also tends to differ: deep venous structures are involved in nearly three quarters of neonates, while older children more often have clots in the superficial sinuses. Venous infarctions and bleeding are more frequent in the newborn group.¹¹

Narrowed Sinuses and Raised Intracranial Pressure

Not all abnormal MRV findings involve a clot. Narrowing, or stenosis, of the transverse sinuses is the hallmark imaging finding in idiopathic intracranial hypertension (IIH), a condition where pressure inside the skull is elevated without an obvious cause like a tumor or infection. In a study comparing MRV scans of IIH patients to controls, bilateral transverse sinus stenosis was present in 94 percent of IIH patients but only 3 percent of controls, making it the most sensitive MRI marker for the disease.¹²

The relationship between the narrowing and the high pressure is somewhat chicken-and-egg. Raised intracranial pressure can itself squeeze the sinuses from the outside, creating or worsening the stenosis, which in turn impairs drainage and further raises pressure. For patients who do not respond adequately to medication or weight loss, venous sinus stenting can break this cycle. A prospective trial of stenting found that headaches improved or resolved in about 85 percent of patients, pulse-synchronous tinnitus resolved in all patients treated, and opening pressure dropped by an average of 20 cm of water.¹³

Distended Sinuses and Low Intracranial Pressure

The opposite pressure problem produces the opposite MRV appearance. In intracranial hypotension, usually caused by a spinal cerebrospinal fluid leak, the brain sags downward and the venous sinuses can become engorged. Researchers have described a characteristic change called the venous distension sign, where the underside of the dominant transverse sinus bulges downward on standard MRI sequences.¹⁴ This engorgement of venous structures is one of five classic imaging features of spontaneous intracranial hypotension, along with subdural fluid collections, enhancement of the brain’s lining, pituitary swelling, and sagging of the brain itself.¹⁵ The venous engorgement alone is rarely the most striking finding, but recognizing it on an MRV can be the clue that points the clinician toward a spinal leak rather than a venous sinus problem.

Dural Arteriovenous Fistulas

Sometimes an MRV reveals not a clot or stenosis but an abnormal connection between an artery and a vein within the dura, the tough membrane surrounding the brain. These dural arteriovenous fistulas (DAVFs) can show up as unusual flow patterns in the sinuses or as dilated cortical veins that should not be visible. The concern becomes more urgent when the fistula drains into cortical veins rather than into the sinuses, because this pattern carries a higher risk of hemorrhage. In one series of 91 DAVF patients, about a third had cortical venous drainage, and among those patients, 62 percent presented with bleeding into the brain or the surrounding spaces.¹⁶ DAVFs can also coexist with sinus thrombosis: one case report described a DAVF complicated by clotting in the torcular, left transverse, and sigmoid sinus, with MRV clearly showing the filling defects.¹⁷

How MRV Compares to CT Venography

If your doctor ordered an MRV, you might wonder why they did not just use a CT scan, or vice versa. CT venography (CTV) involves injecting iodinated contrast dye and taking rapid CT images, and it has been shown to be as accurate as MRV for diagnosing cerebral venous sinus thrombosis.¹⁸ A more recent head-to-head study confirmed high agreement between the two modalities, though CTV had an edge in delineating small vessels and produced fewer flow-related artifacts, while MRV provided complementary information without exposing the patient to radiation.¹⁹ In practice, the choice often comes down to availability, speed, and whether the patient can tolerate contrast or hold still in an MRI scanner. Emergency departments tend to favor CTV because it is faster. MRV is preferred when radiation exposure is a concern, such as in pregnant patients or children, and when the clinical picture warrants simultaneous conventional MRI of the brain tissue.

Pulsatile Tinnitus and Structural Abnormalities

An abnormal MRV sometimes surfaces in the workup for pulsatile tinnitus, a rhythmic whooshing or pounding sound in one ear that follows the heartbeat. The venous sinuses run just behind the mastoid bone, and when there is a structural abnormality at that interface, the pulsing blood flow can become audible. In one reported case, MRI revealed a dominant right sigmoid sinus extending abnormally close to the cortex of the mastoid bone, raising suspicion for a dehiscent (uncovered) sigmoid sinus. A follow-up CT confirmed thinning and a gap in the bone overlying the sinus.²⁰ Jugular bulb abnormalities can create similar symptoms. In patients with an elevated jugular bulb, specialized MRI sequences have demonstrated characteristic spiral flow patterns inside the internal jugular vein that depend on the degree of bulb elevation.²¹ These flow disturbances might explain why some patients hear their own blood flow.

Abnormal jugular venous outflow has attracted broader interest as well. A study of patients with internal jugular vein stenosis confirmed by contrast-enhanced MRV found that the narrowing may alter cerebral blood flow and metabolism and contribute to the formation of white matter lesions in the brain.²² This is still an evolving area of research, but it underscores the point that venous outflow problems can have real neurological consequences beyond just headache.

What Happens After a Blood Clot Is Found

If MRV confirms CVST, the standard first-line treatment is anticoagulation, typically heparin followed by oral blood thinners. Most patients undergo follow-up MRV to track whether the clot is dissolving. The recanalization process, meaning the reopening of the blocked sinus, follows a predictable timeline. In one study, about 71 percent of recanalization improvement occurred within the first three months, and roughly 59 percent of patients who achieved complete recanalization did so by three months. Further gains taper off, with progressively smaller percentages improving at six and twelve months.²³

Complete recanalization does not happen for everyone. In a study following 33 patients at six months, 88 percent showed some degree of reopening, but complete recanalization was achieved in only about a fifth of them. The sigmoid sinus recanalized most frequently.²⁴ A larger study found complete recanalization in about 58 percent and partial in 29 percent, with roughly 13 percent of patients never recanalizing. The median time to partial reopening was about four months, and to full reopening about six months.²⁵ Patients whose sinuses never fully reopen may still do well clinically if the brain has developed collateral drainage pathways, but they generally need closer long-term monitoring.

When Standard Treatment Is Not Enough

A small number of CVST patients deteriorate despite anticoagulation. In these refractory cases, interventional neuroradiologists may attempt mechanical thrombectomy, a procedure that physically removes or breaks up the clot using catheters inserted through a vein in the groin. The decision to intervene is made case by case and is typically reserved for patients with extensive clot burden, rapidly worsening neurological deficits, or coma.²⁶ A systematic review concluded that endovascular mechanical thrombectomy is an effective salvage therapy for refractory CVST with a reasonable safety profile, though the evidence base still consists mostly of case series rather than randomized trials.²⁷

Vaccine-Related Venous Thrombosis

During the rollout of adenoviral-vector COVID-19 vaccines, a rare syndrome called vaccine-induced immune thrombotic thrombocytopenia (VITT) drew global attention. VITT could cause cerebral venous sinus thrombosis along with a paradoxical drop in platelet count, typically presenting as a severe headache five to thirty days after vaccination. The diagnosis was confirmed when thrombosis was visible on CTV or MRV alongside low platelets, a very elevated D-dimer, and positive antibodies against platelet factor 4.²⁸ Interestingly, a similar pattern of thrombocytopenia with CVST has since been described in children following respiratory infections rather than vaccination, suggesting the underlying immune mechanism may not be unique to vaccines.²⁹

Artificial Intelligence and the Future of MRV Interpretation

Reading MRV scans for clots requires pattern recognition, and human readers sometimes miss subtle thromboses, especially when the clot is small or located in a deep vein. Researchers have developed deep learning algorithms trained on routine brain MRI sequences that can flag likely CVST without needing a dedicated MRV study. In testing, the best-performing algorithm achieved 96 percent sensitivity and 88 percent specificity for detecting CVST on a per-patient basis, and its sensitivity exceeded that of all four radiologists it was compared against.³⁰ This kind of tool could function as an automated safety net, catching cases that might otherwise be missed on a standard brain MRI that was ordered for a different reason entirely. The technology is still in development, but the prospect of an algorithm that scans every brain MRI for incidental venous clots could meaningfully reduce delayed diagnoses.