| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York |
| Transcript |
"Today we're going to be talking about venous sinus disease in neuro-ophthalmology but specifically for imaging the venous sinus. The reason you need to know that is because we're looking for two different things in our venous sinus disease. We're either looking for venous sinus thrombosis, which is an occlusive clot, or venous sinus stenosis, which is not a clot. As you remember, the venous sinuses: superior sagittal sinus, interior cerebral vein, great vein of Galen, straight sinus, torcula transverse sinus, sigmoid down the jug. One of the reasons we're interested in this venous sinus thrombosis is if you occlude the sinus the back pressure will increase and that will cause increased intracranial pressure because the cerebralspinal fluid has to drain through the arachnoid granulations and get out the venous tree through this cerebral venous sinus. So if you have a cerebral venous sinus thrombosis it might come to us as papilledema, increased intracranial pressure, a venous infarct hemorrhage in the head. It can mimic idiopathic intracranial hypertension so that's why the study of choice for patients who you are working up for what you think is idiopathic intracranial hypertension is an MRI brain with and without contrast and an MR venogram. That is what the book says. However, CT venogram is actually better for looking for the thing we're looking for. In IIH we might now see a thrombosis, we might see a stenosis. That's one of the radiographic findings of increased intracranial pressure because right here as this venous sinus is making the transition from the transverse to the sigmoid sinus, that distal transverse the sigmoid sinus junction, that is where the transverse sinus stenosis occurs in increased intracranial pressure related to any cause but in IIH. So that's one of our radiographic findings of increasing intracranial pressure - not in thrombosis but in stenosis. These are the two things we're looking for in neuro-op when we're doing a venogram. And you might say, "Well, why not just do a CTV then?" and "Why is a CTV better than an MRB?" The short answer is a CT venogram has contrast so in a CT scan we are looking at iodinated contrast. So in CT it just blocks the x-ray or it doesn't block the x-ray. If it blocks the x-ray, it'll be hyper dense, and dense material like iodine will block the x-ray. In a CTV we have direct visualization of the contrast material. When the contrast material goes down this vein, it just won't go. Maybe all of this won't fill either. So CTV is better because it's direct visualization of the contrast. MRI and MRV use the magnet. So when we have an MR venogram, we're using flow to tell us whether the vein is blocked or not. These are flow determined studies and there's different techniques and different types of MRV. You don't have to know all these as an ophthalmologist. The most common are phase contrast and time of flight. We're using the phase of flow in the venous tree - in phase or out of phase in the magnetic field - to tell us about flow or time of flight which is, like it sounds the time of flight of the flow. Flow dependent structures, we can do MR venograms on this: no dye, no contrast. Now of course if you put the gadolinium contrast in there, a post contrast MR venogram is going to be way better than a non-contrast phase contrast (now you can see where the problem is) non-contrast time of flight MRV. But these are so much easier to do if we don't have to time the bolus of the injection and give gadolinium during the MR venogram. This is going to make a lot of trouble for the radiologist. Normally in MRI plus MRV: either phase contrast or time of flight. However because these are flow dependent studies, the place where we're looking at the transverse sinus stenosis could create turbulent flow. In a flow dependent study, that might make an artifact. Those reports will come back as, "We can't tell if there's a thrombus or it's turbulent flow here." Recommend CT venogram. So we'll do the CTV to adjudicate whether the MR venogram was right or not. Now you might be asking yourself, "Why not just do an MRI with gadolinium and a post-contrast CTV?" That actually would be a good combination - but we don't have this because it's going to require that you have two scans in two different machines. So yes, the answer to the question is, if I had choice, what would I pick to look for these things, MRI with or without contrast and CTV - because it's a direct visualization of the dye - because we don't have that, MRI plus MRV." |
