Spinning to an Answer: Use of Arterial Spin Labeling for Arterial Occlusion Localization
Yoo R, Yun TJ, Rhim JH, Yoon B, Kang KM, Choi SH, et al. Bright Vessel Appearance on Arterial Spin Labeling MRI for Localizing Arterial Occlusion in Acute Ischemic Stroke. Stroke. 2014
The role of imaging is critical to stroke care as it not only confirms the stroke neurologist’s assessment from the physical exam, but it also can pick up subtle strokes with no clinical correlate. Currently, there is a race to discover and develop the next big series or imaging sequence that will provide key information during stroke evaluations and the group from South Korea has brought us a comparative assessment of the use of arterial spin labeling (ASL) and how this compares to susceptibility weighted imaging when using MR angiography as the reference standard to detect arterial occlusions.
The group gathered radiological data on 117 consecutive patients with stroke symptoms who underwent ASL imaging, which is now part of the standard stroke protocol at their institution, as well as DWI, FLAIR, SWI and 3D TOF MR angiography. The best way to really see how a clot appears on ASL imaging is to read the article mentioned and look at the images, however the best way I can explain the appearance is a “lighting up” of the vessel in question, termed by the authors as a “bright vessel appearance”. This was compared to the presence of a susceptibility vessel sign (dark dot in the vessel on SWI imaging) using an MRA cutoff or stenosis as the standard. The group found that 35 of the 117 patients enrolled with stroke symptoms (30%) had arterial occlusion on MRA. ASL bright vessel appearance, susceptibility vessel sign and FLAIR vascular hyperintensity were all more common in the group with occlusion than in the group without (P<.001). Of the 35 patients with MRA arterial occlusions, 33 of 35 (94%) had bright vessel sign on ASL imaging while 23 of 35 (66%) had susceptibility vessel sign, a statistically significant difference between the groups (P= .002). In the MRA negative cases, ASL imaging demonstrated significantly more additional occlusions than the susceptibility vessel sign (21% vs 10%, respectively, P=.012) showing that ASL may be better for detecting these smaller, peripheral occlusions.
The ASL imaging however did occasionally miss stenoses proximal to occlusion sites as well as not showing a bright vessel sign in every area with stroke, only those with occlusion sites. The modality has very good interobserver agreement with kappa=0.86. There may also be applicable uses of the technique for detecting collateral flow (given the detection of an “arterial transit artifact) but this needs to be better studied. The application reflected by this paper however, shows a very reliable, rather easy to interpret, and very accurate modality that can possibly be another sequence that is routinely checked during imaging review during a stroke evaluation. The images yielded by the ASL technique can be interpreted with confidence and with better correlation to true occlusion and better detection of smaller, peripheral occlusions than SWI imaging.