Evaluation of the 11CO2 Positron Emission Tomographic Method for Measuring Brain pH. II. Quantitative pH Mapping in Patients with Ischemic Cerebrovascular Diseases

Abstract

A practical method has been developed that, using ¹¹CO₂ and positron emission tomography (PET), computes and maps (a) “effective pH” (pH_t), a weighted average of intra- and extracellular pH, and (b) “clearance” ( K₁), product of blood flow and ¹¹CO₂ extraction. This method, together with measurements of cerebral blood flow (CBF) and oxygen extraction fraction (OEF), was applied to 12 patients with cerebral ischemia or stroke. The regional K₁ was positively correlated with CBF ( n = +0.78). The k₁/CBF ratio, representing the extraction fraction ratio of ¹¹CO₂ to H₂¹⁵O, was negatively correlated with CBF ( r = –0.54), suggesting that ¹¹CO₂ extraction decreases as flow increases. In five acute stroke patients within 2 days of onset, the injured cortex had lower CBF (20.6 ml/min/100 g), higher OEF (78.1%), and lower pH_t (6.96) than the contralateral cortex (CBF = 41.4 ml/min/100 g, OEF = 53.3%, pH_t = 7.00), suggesting intracellular acidosis with intact cell membranes. In three stroke patients 5–8 days after onset, the injured cortex had higher CBF (60.9 ml/min/100 g), lower OEF (32.0%), and higher pH_t (7.12) than the contralateral cortex (CBF = 45.3 ml/min/100 g, OEF = 58.0%, pH_t = 7.06), which suggested an increase in extracellular volume compartment reflecting loss of cell membrane integrity. This method provides information on the regional tissue acid–base status and cell membrane integrity, which may be prognostic of tissue viability.