Abraham Noorbakhsh1 and Divya Bolar1
1Department of Radiology, University of California, San Diego, San Diego, CA, United States
Synopsis
A retrospective review was performed of 50 patients undergoing
MRI with pseudo-continuous arterial spin labeling (pCASL) perfusion imaging and
confirmed unilateral fetal PCA. The aim is to determine the frequency of visually-apparent
unilateral PCA territory hypoperfusion in patients with a contralateral fetal PCA,
but without underlying clinical or imaging pathology to suggest true hypoperfusion. Eight of the fifty cases (16%) had visually-apparent
hypoperfusion in the PCA territory contralateral to the fetal PCA. Given these
findings, we advise caution when interpreting ASL in patients with a fetal PCA/variant
circle of Willis anatomy or suggest using longer post-labeling delay times.
INTRODUCTION
Pseudo-continuous arterial spin labeling (pCASL) allows
non-contrast assessment of cerebral perfusion and has become a clinically important
tool in evaluating neurological disease. However, pCASL is prone to artifacts
that can result in erroneous interpretation. Our group periodically encounters
visually-apparent unilateral PCA territory hypoperfusion in patients with the
normal anatomic variant of a contralateral fetal PCA, but who are without
underlying clinical or imaging pathology to suggest true hypoperfusion. The
objective of this study is to determine the frequency of this phenomenon in
patients scanned with pCASL as part of our institution’s routine brain MRI
protocol. METHODS
A search of MRI/MRA reports at our institution from 1/2017
to 3/2019 was performed with inclusion search term of “fetal PCA”, and other
terms to exclude patients with significant neuropathology. Of the resulting 70
patients with both MRI and MRA studies, 50 patients were included who had
unequivocal unilateral fetal PCAs and diagnostic ASL scans, but without
conventional imaging (DWI, FLAIR, or SWI) abnormalities or clinical symptoms
referable to the PCA territories. One patient was excluded due to a basilar
artery stenosis that resulted in ASL arterial transit artifact. The 50 cases
were evaluated by two independent readers (a senior radiology resident and practicing academic
neuroradiologist) for visually apparent PCA territory pCASL
hypoperfusion relative to other vascular territories. The pCASL-CBF perfusion
imaging protocol consisted of the GE product pCASL sequence (General Electric,
Milwaukee, WI) using a 1.5s labeling duration and 2s post-labeling delay (PLD),
adapted from white-paper recommendations (1).RESULTS
The fifty unilateral fetal PCA patients ranged from 25 to 93
in age, 17 male and 33 female. Both readers identified eight cases out of the
fifty (16%) that had visually apparent hypoperfusion in the PCA territory
contralateral to the side of the fetal PCA. Agreement between readers was
perfect (100%, κ=1) In all 8 cases, the PCA territory perfusion signal
ipsilateral to the fetal PCA qualitatively matched the perfusion signal of the
ipsilateral anterior circulation, while the contralateral PCA territory
appeared relatively decreased to the other major vascular territories. None of
the 8 cases showed PCA territory infarction on DWI/FLAIR, increased oxygen
extraction on SWI, or focal neurological symptoms referable to the PCA
territory that would explain a true perfusion abnormality. DISCUSSION
Visually-apparent PCA territory hypoperfusion on pCASL
perfusion imaging is not uncommon in patients with a contralateral fetal PCA
who have no clinical or conventional imaging findings referable to the
territory to suggest true hypoperfusion. This apparent hypoperfusion is almost
certainly artifactual, secondary to a longer arterial transit time of the ASL
label to the non-fetal PCA territory, given a lower blood velocity in the
supplying vertebral arteries compared to the higher-velocity ICA supplying the
fetal PCA territory. Consequently, at a PLD of 2s (at which time imaging begins),
less label has been delivered to the non-fetal side resulting in apparent
hypoperfusion. Additionally, since the fetal-PCA territory is supplied by the
ICA, it has a similar qualitative appearance to the ipsilateral ACA and MCA
territories, which makes the apparent contralateral PCA territory hypoperfusion
even more conspicuous. CONCLUSION
Variation in circle-of-Willis anatomy can periodically result
in artifactual pCASL perfusion deficits, as highlighted by apparent qualitative
PCA hypoperfusion in patients with a unilateral fetal PCA. While quantitative
differences in pCASL-CBF due to this phenomenon have been previously described (2),
this is the first study to evaluate the frequency of qualitative PCA
hypoperfusion in a clinical population with fetal-type PCA anatomy, but without
clinical/imaging deficits. It is important for radiologists to be aware that hypoperfusion
may arise from variant circle of Willis anatomy, which should not be incorrectly
interpreted as reduced perfusion. The use of longer PLD times and/or
velocity-based ASL approaches (3) may correct this asymmetry and is an area of
future research.Acknowledgements
No acknowledgement found.References
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