Susceptibility weighted imaging in cerebral hypoperfusion-can we predict increased oxygen extraction fraction?

Increased concentrations of deoxyhemoglobin within veins can induce susceptibility changes resulting in increased conspicuity in susceptibility weighted imaging (SWI). Compensatory mechanisms following reduced cerebral perfusion due to carotid occlusive disease may not be sufficient to meet demands of ischemic tissue and increased tissue oxygen extraction ratio results in relative increase in deoxyhemoglobin levels in the venous blood draining affected hemisphere. We assessed whether patients with carotid disease display prominence of veins over affected cerebral hemisphere.

Eighteen patients with unilateral carotid occlusion or critical carotid stenosis proven by magnetic resonance angiography (MRA) were selected. The medical records and MRI findings including SWI and MRA were reviewed. The SWI images were studied for the presence of asymmetry of veins over the cerebral hemispheres and were correlated with the site and severity of stenosis or occlusion. The veins were assumed to be conspicuous and asymmetric if there were more numerous veins and/or large veins with greater signal loss observed compared with opposite normal hemisphere.

In about half of patients, prominence of veins was noted in the cerebral hemisphere ipsilateral to side of occlusion. This was not observed in patients with significant extracranial carotid stenosis. The SWI abnormalities were seen extending beyond the boundaries of occluded vascular territory. There was good agreement between two observers in all the patients who showed positive finding. Also there was no interobserver variation in patients with negative findings.

The increased susceptibility arising out of increased deoxyhemoglobin to oxyhemoglobin ratio leads to visualization of prominent veins over the affected cerebral hemisphere on SWI.