INTRODUCTION

Carotid stenosis is responsible for up to 20% of strokes in adults.1 Carotid artery stenting (CAS) and carotid endarterectomies (CEA) can decrease stroke risk, but have also been associated with postoperative cognitive impairment independent of cardiovascular risk factors, age, or perioperative complications.2 Some postoperative MRI-based perfusion studies have suggested that variations in CBF are associated with cognitive impairment,3 but there is a relative paucity of data evaluating longitudinal CBF changes and cognitive impairment using arterial spin labelling (ASL) MRI.4 We hypothesize that ASL patterns of CBF can be used to predict cognitive changes from carotid surgical interventions.

METHODS

Under an IRB-approved and HIPAA-compliant protocol, 53 veterans scheduled to undergo carotid interventions (CAS or CEA) at the Veterans Affairs Palo Alto Health Care System were prospectively enrolled into the study. Indications for surgery included severe asymptomatic carotid artery stenosis (>80%) or moderate-to-severe stenosis (>60%) with focal neurological symptoms. In addition to pre-operative and post-operative imaging, subjects underwent neuropsychological testing (prior to, one month after, and 6 months after surgery) that assessed different cognitive functions [i.e., Rey's Auditory Verbal Learning Test (RAVLT),5 Delis–Kaplan Executive Function System (DKEFS),6 Controlled Oral Word Association Test (COWAT)7]. Neuroimaging followed a clinical protocol: use of a GE Discovery MR750 3.0 T MRI scanner (G.E., Waukesha, WI, USA), volumetric 3D fast spoiled gradient echo (TE Min Full; Flip angle 11 degrees; voxel size 1.2mm isotropic; TI 400; FOV 27 cm; NEX 1; Matrix 256x256), and whole brain 3D PCASL ASL imaging (axial acquisition, Freq FOV 24, Slice thickness 4-5mm, 6-8 arms, scan locs 36, NEX 3-4, BW 62.5 PLD 2525.0). CBF maps were generated from product ASL images and were then normalized using a previously reported method.8 Image subtraction analysis was performed on scans taken before, immediately after, and 6 months after surgery. These difference images were then used to compute general linearized model (GLM) contrasts using Statistical Parametric Mapping 8 software.

RESULTS

All subjects demonstrated an overall increase in cerebral perfusion immediately after surgery (p<0.01). In particular, the non-surgical (i.e., contralateral) side demonstrated a larger increase in perfusion relative to the surgical side (p<0.01). All subjects demonstrated an overall decrease in perfusion from immediately after to 6 months after surgery (p<0.01). The nonsurgical (contralateral) side demonstrated a larger decline in perfusion relative to the surgical side (p<0.01). Notably, there were no significant differences in perfusion 6 months after surgery compared to before surgery (p<0.01). Preoperative elevated systolic blood pressure, presence of chronic renal insufficiency, history of prior stroke were correlated with a reduced gain in perfusion immediately after surgery. Hypercholesterolemia was correlated with a diminished reduction in perfusion from post surgery to 6 months after. Elevated BMI was associated with a greater reduction in CBF from post surgery to 6 months after.

DISCUSSION

Using ASL MRI, we observed a biphasic perfusion pattern in the short-term period after carotid revascularization surgery, with an initial increase in cerebral perfusion followed by a decrease in perfusion, as has been seen in other studies using ASL, PET and SPECT. The transience and magnitude of the perfusion changes likely reflects preservation of cerebrovascular autoregulatory functions.9 Conversely, our findings that elevated systolic blood pressure, prior stroke, and chronic renal insufficiency predicted smaller amplitude perfusion changes suggesting an impairment in cerebrovascular autoregulation and vascular reserve. The fact that individual preoperative risk factors, such as hypertension, diabetes, and smoking did not predict differences in perfusion changes also suggest that more advanced derangements of vascular autoregulation may be required before significant differences in perfusion changes are observed. The fact that elevated cholesterol at baseline was associated with a lesser reduction in perfusion and baseline elevated BMI was associated with a greater reduction in perfusion from immediately post surgery to 6 months, also suggests altered vascular mechanics. More time points are needed to characterize the relation between cognition and cerebral perfusion.

Conclusion

Baseline risk factors suggestive of advanced vascular impairment can be more predictive of differences in brain perfusion response as measured by ASL MRI after revascularization surgery. A better understanding of the relationship between these risk factors and cerebral vasoreactivity will enhance the prognostic utility of perfusion imaging modalities in various neurologic conditions.

Acknowledgements

No acknowledgement found.

References

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