Pitfalls of MR Angiography due to slow flow in large Aneurysm

This case illustrates a potential pitfall of Non contrast 3D TOF MRA imaging. Contrast enhanced CT study shows a focal aneurysmal dilatation of basilar, but the non contrast 3 D TOF MR Angiography of Brain of same patient fails the aneurysm is not depicted well on lateral view.

The non visualization of aneurysm may be due to slow flow owing to its large size or thrombus formation in the aneurysm because vascular imaging in Non contrast 3D TOF MRA depends on flow related signal from a rapid blood flow in a vessel. In this study the ICAs and basilar show normal flow related signals but the large aneurysm sac appears nearly isointense with nearby static brain.
CT Angiography or MR Angiography with contrast has considerable value in such cases because it is not dependent on flow velocity to create contrast and signals.

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

Neck MR Angiography Pitfall

A young pt with recurrent episodes of TIA advised MR Angiography of neck along with Brain.

Non contrast 2D TOF MRA performed, a MIP image of neck demonstrating of left carotid bifurcation and a row axial image at the level of the ICA origin.

A focal notching in ICA at its origin appears to be:

1. Plaque.

2. External compression.

3. Artifactual.

Answer: it’s an artifact, due to turbulence of flow at the bifurcation as axial sections of contrast enhanced CT Angiography of same patient at the same level are normal. 

Discussion: 

RE CIRCULATION FLOW ARTIFACT on 2 D TOF Non contrast MRA

Carotid bulb is a normal focal expansion of cervical ICA at its origin is patent and relatively prominent in young patient. Again due to vigorous flow there is formation of an eddy circulation in bulb with reversal of flow. This reversed flow leads to diminished signal on 2D TOF MRA because of the linked saturation band that suppresses all caudal flow, regardless of its nature. 

This problem can be obviated by use of contrast during MR Angiography as in contrast enhanced MRA study, flow related signal by contrast is insensitive to direction of flow and demonstrates the normal contour of the carotid bulb.

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

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.

Discussion:

3D TOF MRA ARTIFACTS DUE TO MOTION AND VENETIAN BLIND ARTIFACT

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

MR Venography Artifactual flow loss

A 30 y o postpartum woman presents with headaches and drowsiness. The maximum intensity projection (MIP) image from coronal two-dimensional time-of-flight MR venogram (2D TOF MRV) the part of superior Sagittal sinus near torcula show poor flow related signals.
Diagnosis?
1. Superior sagittal sinus thrombosis.
2. Stenosis.
3. Normal.

Answer: It’s a common finding in 2 D TOF MR Venogram due to inflow artifact, can be passed off as normal.

Discussion:


2 D TIME-OF-FLIGHT : IN-PLANE FLOW ARTIFACTS.

2 D TOF technique of MR Venogram for demonstration of intra cranial venous system without need for any intravenous contrast. The contrast between the vascular lumen and the background is due to entry slice enhancement of blood flow. Because the image is acquired as a stack of individual slices, every slice represents an entry slice for blood flow. This stack of thin images transformed into MIP reconstructions to demonstrate MR Venogram. The advantages to using a 2D TOF technique over 3D TOF is its sensitivity to relatively slow flow and almost unlimited coverage, there are some artifacts that have to be considered as a result of its single plane of acquisition.
Entry slice enhancement is most effective when the orientation of the acquisition slice is perpendicular
to the direction of flow. For MR Venogram commonly coronal 2D TOF is used because the venous flow in the brain is largely anterior to posterior.
It need to be noticed that the superior sagittal sinus (SSS) is well demonstrated throughout with the exception of the posterior segment at its junction with the straight sinus at the torcula. While this appearance can be readily mistaken for intraluminal thrombus or stenosis, it is important to recognize that this is an artifact of in- plane flow. Since intravascular signal is highest when flow is perpendicular to the slice and lowest when flow is in the same plane as the slice, the signal in the SSS drops off in this vertical segment of the sinus that is in plane on a coronal acquisition.
It is important to know how the source data for the 2D TOF study is acquired coronal or sagittal in order to anticipate these artifacts.
Acquiring two 2D TOF MRV scans in perpendicular directions or using contrast can help resolve some of the problems created by in-plane flow.

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