Vertebral artery thrombosis

On MR Angio, non visualization of a vessel or a part of vessel needs careful interpretation as it can be due to many reasons right from thrombotic occlusion of vessel, absent vessel as a part of normal anatomical variation or a technical error while doing post processing of raw images of 3D TOF sequences by using add vessel technique.
So diagnosis of vessel thrombosis or passing it off as a normal anatomical vessel should not be solely based on MR Angiography findings.
The diagnostic dilemma occur more so when there is no infarct in corresponding vascular territory.
This problem is most common with vertebral where one of the vertebral is not visualised. Opposite vertebral continues as basilar. In such cases we can take help of other routine parenchymal sequences like FLAIR and T2*GRE.

In above case, thrombosed distal most portion of right intra cranial vertebral near formation of basilar show a focal flow loss on MR Angio, the corresponding portion show an abnormal low signal on T2*GRE and high signal on FLAIR implies to thrombus. 

Vertebral Hypoplastic Vs Thrombosed on MR Angio

On MR Angio, if Diffusion and FLAIR does not show any infarct and single sided vertebral show poor flow related signals or not visualised we may pass it off as hypoplastic vertebral.

But what if diffusion show recent infarcts in posterior circulation territory as in this case.

MRI Brain Dw images show multiple foci of recent ischemia with restricted diffusion involving bilateral cerebellar hemispheres.

Right vertebral show poor flow related signals on MR Angio. 

Can be an occluded or a hypoplastic right vertebral...

What next?

You can try axial thin Fat sat T1sections in the region of neck to cover vertebrals.

Abnormal high signals in the region of right side vertebral with loss of normal T2 flow voids compared with opposite left vertebral and ICAs which show normal T2 flow voids. The high signal intensity on Fat T1 can be attributed to blood clot possible with thrombosis / dissection of vertebral artery. The same protocol is applicable to carotids.