Introduction
Measurement of cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO2) by PET is widely used for investigation into the pathophysiology of occlusive cerebrovascular disease. Decreased cerebral perfusion pressure below the lower limit of cerebral autoregulation causes a decrease in CBF with an increase in OEF for maintenance of CMRO2, that is called as “misery perfusion”. On the other hand, decrease in CBF with decrease in CMRO2 is often observed in brain regions suffered from occlusive cerebrovascular disease without any organic lesions on MRI (“matched reduction”). However, cerebral hemodynamics in matched reduction has not been elucidated. In the present study, we have investigated cerebral hemodynamics and oxygen metabolism using PET in both groups of misery perfusion and matched reduction.
Methods
CBF, CBV, OEF, CMRO2, and vascular responses to hypercapnia and acetazolamide stress were measured by PET with H215O, C15O, and 15O2 in 22 patients with stenoocclusive lesions in unilateral major cerebral arteries on 2–6 weeks after onset. All patients were divided into two groups, misery perfusion group (n=6) and matched reduction group (n=16), by ipsilateral/contralateral ratio of OEF (misery perfusion group: OEF ratio > 1.05, matched reduction group: OEF ratio < 1.05).
Results
In both groups, CBF in ipsilateral side was significantly decreased as compared with contralateral side. CMRO2 in ipsilateral side was significantly decreased as compared with contralateral side in the matched reduction group, but not in the misery perfusion group. In the misery perfusion group, CBV and vascular mean transit time (MTT, MTT=CBV/CBF) in ipsilateral side were significantly increased, and vascular responses to hypercapnia and acetazolamide stress in ipsilateral side were significantly decreased as compared with contralateral side. In the matched reduction group, no changes in CBV, MTT, and vascular responses to hypercapnia and acetazolamide stress were observed in ipsilateral side as compared with contralateral side.
Conclusion
In the misery perfusion group, increase in CBV and decrease in vascular responses to hypercapnia and acetazolamide stress were observed, indicating decrease in cerebral vascular reserve, as reported previously. On the other hand, in the matched reduction group, no changes in CBV, MTT, and vascular responses to hypercapnia and acetazolamide stress were observed, indicating preserved cerebral vascular reserve. Reduction of CMRO2 with preserved cerebral vascular reserve in matched reduction indicates that hypoperfusion in this lesion might be caused secondary. It can be speculated that lesions showing matched reduction might have microscopic lesions such as neuronal damage that can not be detected by MRI, or remote effects from other ischemic lesions.