Sunday 20 May 2012

MR Angiography Artifactual flow loss

A 60 yo male for stroke evaluation.
3D TOF MRA, MCA main stem on either side show focal flow loss or narrowing.
Diagnosis ?
1. Stenoses.
2. Artifactual flow loss.

Answer: Artifactual flow loss, common in elderly non co operative patients due to motion.



This case provides a reminder that MRA of the brain using 3D TOF technique is usually acquired as two to five slabs, unlike the 100 or more thin slices acquired during a 2D TOF sequence. This approach is called MOTSA (multiple overlapping thin slab acquisition). To make 3D TOF images of the intracranial vessels, rather than include all the region of interest in a single slab, multiple thin slabs are acquired with an overlap and then knit together to appear as one continuous volume. The image contrast for both 2D and 3D techniques is still the result of entry slice enhancement, i.e. unsaturated spins coming from outside into an imaged volume, and you may recall that the advantage of 3D TOF imaging in the brain is its improved depiction of curving vessels. These multiple thin slabs are necessary because the hydrogen spins become progressively saturated by the repeated 90 degree pulses as they experience as they traverse the slab. As a result, when using a single, thick slab there would be no signal recovered from the most distal portions of a vessel. These slabs are acquired sequentially i.e. one after the other.
The above MR Angiography demonstrates misregistration between slabs due to some head motion that occurred between two adjacent slab acquisitions. Another cause of this artifact is the loss of flow-related enhancement that becomes increasingly evident as the vessel approaches the exit side of each slab. This phenomenon called Venetian blind artifact, common when the slabs are stitched together.
To obviate this artifactual signal loss modify the pulse sequence to “add” vascular signal at the exit side of the slab. This is accomplished by using a variable flip angle on this gradient echo acquisition that changes during each slab acquisition. Since the vascular signal increases with an increase in the flip angle, a modulated or “ramped” flip angle can be used to correct for vascular signal loss. This approach can help to provide a seamless appearance to the vasculature across the multiple slabs.

Reference: Practical MR Physics and case file of MR artifacts and pitfalls, Alexander C. Mamourian, MD

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