MRI vs Dopplar? How can a vessel be missing in one but not the other?

[kd re: longstanding MR]

Hi -  I am confused and hope you can help me.

I have a history of stroke and TIA.  Several years ago, I had a carotid bypass on the left side.  The gortex graft was placed between the aorta and the internal/external bifurcation.  Within months, it became 100% occluded.  Collaterals were formed from the vertebral system which has been feeding the external carotid creating retrograde flow, which then fed the internal carotid (antegrade flow).  While there was reduced flow to the Circle of Willis, the internal carotid remained functioning with a stenosis per Dopplar of about 10-20%.

Last month I had an MRI/MRA of the head and neck to address some left-side TIA symptoms.  Nothing of importance showed up on the right side of the brain.  My strokes showed up on the left side of the brain.  The main, new "point of interest" was that the internal carotid was not functioning.  When you look at the MRI of 1999 - it shows 4 vessels - the right and left vertebrals and the right and left internal carotids.  The current MRI shows 3 vessels - the right and left vertebrals and the right internal.  There is a black/blank space where the internal used to be.  There is no gray indication of a vessel.  There is no faint indication of where the vessel had been.  It is gone.

I was sent to my vascular doctor and a vascular surgeon for a consult.  They performed a carotid dopplar.  There is flow from the external carotid to the internal carotid with only about 10-20% stenosis.

How can this be?
I could understand if more vessels were missing in the MRI, or if it was a bad picture, or if I had moved, or if there was even a faint impression of the "missing" vessel on the MRI - but there wasn't.  It was out-and-out missing.  I have received a number of partial explanations like, "the MRI is only a screening tool.  These things happen frequently." from a radiologist; to "There may have been some sort of flow problem or equipment problem" from my vascular doc; to just the flat statement from the vascular surgeon, "The MRI was erroneous." and nothing more.

So, my questions to you are:
1. What can create situations like this?
2. What kind of flow problem could create this?  Wouldn't that be diagnostically important?
3. Don't MRIs take several pictures over time so that if it was missing in the lateral view, it might show up in some of the other views?  It was not there in the anterior-posterior views and was not there in the pictures addressing the Circle of Willis.  It was not there in any part of the test yet, it was there in a dopplar a month later.

Thanks for your help.  I really appreciate it.
kd

[To CCF Neuro MD From Pokey]

Thanks for following up too!  It sure does sound like the risks definetly outweigh the benefits. Carotid stenting and bypass are fairly new so maybe they are working on something that will help you in the future. I was wondering if they ever thought of angioplasty on the graft? Dialysis grafts will clot off, and they do angioplasty on them, wonder if they can do that to the carotids as well. I would be great if they could develop some type of blood test to see if patients bodys will accept the grafts. I don't even know if they have that developed, but what a concept, sure would help lots of people. For you, the most important part is the internal is open and working. Doppler study will be a great way to make sure the internal stays open. Are they following you about every 6 months?

[kd re: longstanding MR]

Hi Pokey!
I didn't expect anyone to follow-up and am glad that you did.  The gortex graft was put in from the left aorta to the bifurcation.  It became 100% occluded soon after the surgery in 1995.  No-one could figure out why for years until I (with the help of someone on the internet) investigated antiphospholipid antibody syndrome.  That is a possibility for me.  Therefore, the left common carotid is completely occluded and that was why there was concern that the internal became occluded as well.  I knew the flow wasn't all that hot but the internal had always shown up on previous MRAs.
   In terms of follow-up, there isn't going to be any.  The risk factors are considered to outweigh the results.  Most of my docs have given up on me.  My neuro says that the next step is a major stroke.  I don't handle angios well - they give me major TIAs and they wouldn't be able to get past the occlusion to see the internal flow, anyway.  Surgery is a last ditch thing because of other complications and because it would more than likely clog back up anyway.  
   It's all very circular and frustrating.
Thanks again for your interest,
kd

[To CCF Neuro MD From Pokey]

Interesting how the MRA didn't show flow in the bypass. I would wonder if the graft would of somehow made the MRA not penetrate it. Doppler is very good at detecting slower velocity flow, which may be the reason it was detected on the Doppler study and not the MRA. The "golden standard" that Doctor's like to use is the angiogram - that is the most accurate in picking up stenois, occlusions, slow flow, collaterals etc. The body is an amazing thing - when you talk about flow from the external carotid into the internal carotid that is the body's natural way of getting blood to where it needs to go. Kinda like a river making it's own path, or you could think of it as road construction detour. Is your occlusion in the Common Carotid? I have seen bypasses from the subclavian artery (the artery in the arm) to the carotid bifurcation (where the vessel splits into the internal and external) or is yours off the aortic arch? I hope this helps you a little bit. Just curious what your next step is, what does the vascular surgeon what to do?

[kd re: longstanding MR]

Thank you.
kd

[CCF-Neuro-M.D.-RPS]

Dear kd:

Sorry that you have had an emotional roller coaster ride.  The MRA looking a vessels is actually a good test, however it tends to overcall.  What that means is that the sensitivity is less than the Doppler when looking for blood flow.  It depends on flow to see changes and if the flow is too low, then one doesn't see a vessel.  This is likely what happened.  I would believe the Doppler study and also the fact that you do not have more symptoms and indications of more areas of infarct on the MRI.

Sincerely,

CCF Neuro MD