Blood oxygen level-dependent MRI of cerebral CO2 reactivity in severe carotid stenosis and occlusion

Background and Purpose - Impaired cerebrovascular reserve capacity (CVC) is a risk factor for ischemic events in patients with high-grade carotid stenosis and occlusion. In this study, the CVC in response to a CO2 challenge was evaluated with blood oxygen level - dependent ( BOLD) MRI and the results compared with those of a transcranial Doppler CO2 tests. Methods - A T-2*-weighted single-shot multigradient echo-planar imaging sequence was used to determine cerebral CO2 reactivity. T-2* values were calculated for each pixel at rest and during a challenge with 7% CO2, and a reference function was fitted to the T2* time courses. Whole-brain color-coded Delta T2* parameter maps were calculated and visually evaluated for regional differences. Additionally, a region-of-interest analysis was undertaken. Average values for Delta T-2* normalized to changes in end-tidal PCO2 were calculated. Results were correlated with a transcranial Doppler CO2 tests in 20 patients with high-grade stenosis or occlusion of the carotid artery. Results - Color parameter maps showed areas of decreased BOLD effect within the internal carotid artery territory in 12 of 13 hemispheres with impaired CVC in transcranial Doppler CO2 test. Regional normalized Delta T2* was highly correlated with changes of middle cerebral artery blood flow velocity in transcranial Doppler CO2 test. Normalized Delta T-2* was significantly reduced in hemispheres with impaired CVC in transcranial Doppler ( P < 0.0001). Conclusions - BOLD MRI can easily be included in routine MRI exams. The technique is robust and yields diagnostic information concerning the cerebrovascular reserve.