Abstract 2912: Quantitative T2'-Imaging detects Increased Oxygen Extraction in High-Grade Internal Carotid and Middle Cerebral Artery Stenosis

Abstract

Background and Purpose: Quantitative T2'-imaging (qT2') detects regional changes in the relation of oxygenated and deoxygenated hemoglobine (Hb) using their different signal characteristics in gradient echo imaging. In contrast to classical T2* imaging, T2’-imaging is corrected for spin-spin effects. Regional differences in Hb oxygenation might reflect areas with increased oxygen extraction for compensation of reduced perfusion pressure. The aim of the study was to investigate qT2’-imaging in perfusion-restricted areas in patients with high-grade stenoses of brain-supplying arteries. We hypothesized that T2’-values are lower in perfusion-restricted areas as compared to normal perfused tissue. Methods: 18 patients (15 men, 3 women; mean age 54 years, SD±12.8) with sonographic unilateral symptomatic or asymtpomatic high-grade extracranial ICA stenosis/occlusion (>70%, NASCET criteria), or MR-angiographic proven unilateral high-grade (>50%) intracranial ICA or proximal MCA stenosis/occlusion were included. Patients with significant bilateral stenoses were excluded. MRI comprised perfusion-weighted imaging and quantitative, motion corrected T2’-imaging. Time-to-peak (TTP)- and mean-transit-time (MTT)-maps were thresholded for different degrees of perfusion delays (e.g. >0 sec, >2 sec) in relation to the contralateral, healthy hemisphere. Mean T2’-values in perfusion-restricted areas were compared to T2'-values in corresponding contralateral or ipsilateral, normoperfused areas. Data are given as mean (+−SD). Results: Mean size of perfusion-restricted areas in TTP-maps (TTP-delay >0 sec) was 10.8 ml (+−6.3) and 11.5 ml (+−6.2) in MTT-maps (MTT-delay >0 sec) decreasing to 4.5 ml (+−4.7) in areas with a TTP-delay >4 sec and 6.9 ml (+−5.5) in areas with a MTT-delay >4 sec, respectively. T2'-values were significantly (p<0.01) lower in all perfusion-restricted compared to corresponding contralateral brain areas: For TTP-delay >0 sec, T2’-values were (ipsilateral) 115.2 ms (+−9) vs. (contralateral) 125.4 ms (+− 11.5). For MTT-delay >0 sec T2’-values were (ipsilateral) 114.5 ms (+−9.1) vs. (contralateral) 127.7 ms (+− 9.5). Differences in T2’-values increased with the severity of the hypoperfusion. Ipsilateral T2’-values outside the perfusion-disturbed areas did not differ from contralateral T2’-values. Conclusions: Motion-corrected T2'-imaging detects areas with increased oxygen extraction within perfusion-restricted tissue in patients with high-grade occlusive vessel disease. T2’-imaging may add important information on the severity of the perfusion-disturbance with respect to its metabolic consequences. Its general suitability and its value for clinical decision making needs further evaluation as well as a thorough comparison with the current gold standard for the assessment of cerebral oxygen consumption, PET.