Why is MRI important in the management of brain AVMs i.e. over conventional catheter
angiography? The "gold standard" for evaluation of brain AVMs is catheter angiography.
However, the procedure is invasive, involves ionizing radiation, exposure to contrast media
with potential for nephrotoxicity or allergy and carries a 1% risk of morbidity including
stroke. In contrast, MRI is a non-invasive method to evaluate brain AVMs and has the added
advantage over catheter angiography of depicting the anatomical localization of the AVM
within the brain tissue. However, currently MRI is limited by lack of ability to demonstrate
shunting of blood through the AVM, an important indicator that the brain AVM is still present
after treatment.
Susceptibility-weighted imaging (SWI) is a promising new MRI technology which indirectly
evaluates the amount of oxygen within blood vessels. Small case series exploring the utility
of SWI in brain AVMs has been reported suggesting the venous drainage of brain AVMs is often
abnormally hyperintense because of abnormal shunting of oxygenated blood from AVM arteries to
the draining vein(Bharathi D et al.). Typically in normal tissues, oxygenated blood on SWI
images is hyperintense while deoxygenated blood in normal veins is hypointense. Developmental
venous anomalies demonstrating enlarged draining veins are normal variants that must be
distinguished from true AVMs . However, this capability has not been prospectively evaluated
in a systematic fashion.
Our current standard for contrast-enhanced evaluation of brain AVMs is to perform a
contrast-enhanced MRA (CEMRA) followed by a post-contrast T1 volumetric whole brain sequence.
The CEMRA allows depiction of contrast at its maximal intensity passing through the brain on
its first pass. The post contrast T1 scan only demonstrates static contrast pooling within
the brain AVM. However, neither CEMRA nor the post contrast scan provides information about
the speed at which contrast is moving through a brain AVM ie. shunting. Evaluation of the
temporal passage of contrast brain AVM would require a dynamic time-resolved technique with
adequate temporal resolution to distinguish early vs late vs no shunting within a brain AVM.
What is the current technology for MRI of brain AVMs? Susceptibility-weighted angiography
(SWAN) imaging on the GE 3 T has been attempted but the preliminary evidence suggest that the
images are of low resolution and difficult to interpret. In addition, our literature review
found a paucity of studies evaluating staged treatment of brain AVMs with SWAN imaging. In
our institution, brain AVMs may have staged treatment consistent of endovascular embolization
and/or radiosurgery. After each treatment patients are followed with serial imaging MRI and
Digital Subtraction Angiography (DSA). This provides an important opportunity to investigate
the utility of non-invasive MRI to detect residual AVM after treatment.
Thus, there is a significant opportunity to evaluate the value of SWAN and Time Resolved
Magnetic Resonance Angiography (TRMRA) assessment of progressive obliteration of the AVM
nidus. Specifically, this is attractive for brain AVMs that are treated with radiosurgery as
MRI and DSA are required for clinical grounds for treatment planning purposes.
