T2′ Imaging Within Perfusion-Restricted Tissue in High-Grade Occlusive Carotid Disease

Abstract

Quantitative T2' imaging presumably detects regional changes in the relation of oxygenated and deoxygenated hemoglobin. Regional differences in hemoglobin oxygenation might reflect areas with increased oxygen extraction for compensation of reduced perfusion pressure. We investigated quantitative T2' imaging in patients with high-grade stenoses of brain-supplying arteries and hypothesized that T2' values are lower in perfusion-restricted areas as compared with normally perfused tissue. Eighteen patients (15 men; mean age±SD, 54±12.8 years) with unilateral symptomatic or asymptomatic high-grade extracranial or intracranial internal carotid artery or proximal middle cerebral artery stenosis/occlusion were included. MR examination included perfusion-weighted imaging and quantitative, motion-corrected mapping of T2' time. Time-to-peak and mean transit time maps were thresholded for different degrees of perfusion delays (eg, >0 seconds, ≥2 seconds) compared with the contralateral hemisphere. Mean T2' values in areas of impaired perfusion were compared with T2' values in corresponding contralateral or ipsilateral, normoperfused areas. Mean size of perfusion-impaired areas in time-to-peak maps (time-to-peak delay>0 seconds) was 10.8 mL (±6.3) and 11.5 mL (±6.4) in mean transit time maps (mean transit time delay>0 seconds). T2' values were significantly (P<0.01) lower in all perfusion-restricted compared with corresponding contralateral brain areas (ipsilateral versus contralateral). For time-to-peak delay >0 seconds, T2' values were 115 ms (±9) versus 125 ms (±12). For mean transit time delay>0 seconds, T2' values were 115 ms (±9) versus 128 ms (±10). Differences in T2' values increased with the severity of the perfusion delay. Ipsilateral T2' values outside the perfusion-disturbed areas did not differ from contralateral T2' values. Motion-corrected T2' imaging presumably detects areas with increased oxygen extraction within perfusion-restricted tissue in patients with high-grade occlusive vessel disease.