Estimation of cerebral perfusion reserve by blood oxygenation level-dependent imaging: comparison with single-photon emission computed tomography.
Measurement of cerebrovascular reserve capacity predicts the risk of ischemic insult in patients with major vessel occlusion. Blood oxygenation level-dependent (BOLD) imaging has the potential to estimate reserve capacity of the cerebral circulation noninvasively based on changes in the signal that reflect differences in the magnetic susceptibility of intravascular oxyhemoglobin and deoxyhemoglobin. The authors examined the feasibility of using the BOLD technique to assess cerebrovascular reserve capacity in patients with cerebrovascular occlusive disease by comparing results with an established method of measuring CBF. Ten patients with severe or complete occlusion of the internal carotid artery were compared with 17 healthy subjects to evaluate regional differences and identify variables that indicate a change in the BOLD signal. Dilation of cerebral vessels was induced by breath holding, and the R2* change was examined with gradient-echo, echo-planar imaging. Before measuring the regional change in the BOLD signal, actual timing of "activated" and "rest" periods was corrected by shifting the phase of a sine-wave template to obtain the largest correlation coefficient. Percent signal change was calculated on a pixel-by-pixel basis and was compared with CBF measured by single-photon emission computed tomography (SPECT) before and after acetazolamide challenge. The degree of impairment and the distribution of impaired areas detected by the BOLD study correlated with the results of SPECT. Overall sensitivity and specificity of the BOLD technique by visual inspection were 100% and 98.4%, respectively. A negative response (decreased CBF) frequently was observed in areas of exhausted reserve capacity, suggesting that a "steal" phenomenon exists. The percent change and the (Delta)CBF were well correlated (P < 0.01). The mean percent change in most areas of impaired reserve capacity was more than 2 SD below the mean values in healthy subjects. The present method of semiquantitative BOLD analysis can be used to create a map of the cerebral hemodynamic state. Furthermore, the development of reliable, generally accessible techniques for evaluating cerebral hemodynamics opens the door for clinical studies to monitor and treat patients with compromised reserve. This study is an attempt to develop such analysis.