Quantitative evaluation of cerebral blood flow and oxygen metabolism in the normal anesthetized rat: ¹⁵O-labeled gas inhalation PET with MRI fusion

Objectives The purpose of this study was quantitative measurement of cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO₂), oxygen extraction fraction (OEF), and cerebral blood volume (CBV) by steady state inhalation method in the normal anesthetized rat.

Methods Eight normal male Sprague-Dawley rats (body weight=268±14g) under anesthesia were investigated in this study. Tracheotomy was performed and airway tube was placed into the trachea, which connected to the ventilator (tidal volume: 3ml, frequency: 60/min). The CBF and OEF were measured under artificial ventilation of ¹⁵O-CO₂ and ¹⁵O-O₂ gases delivered through the radioactive gas stabilizer (16min for each study), respectively. CBV was measured by ¹⁵O-CO gas inhalation for 12min and used for the correction of intravascular hemoglobin-bound ¹⁵O₂. Arterial blood sampling (0.1ml) was performed during each ¹⁵O-labeled gas study to measure the radioactivity of whole blood and plasma. MRI was performed with the same holder after PET acquisition. Regions of interests were placed on the whole brain of PET images with reference to the semi-automatically co-registered PET/MRI fused images.

Results The total time of the whole PET scan of each rat was 73.3±5.8 (68 to 85) min. In both ¹⁵O-CO₂ and ¹⁵O-O₂ study, radioactivity count of the brain reached steady state approximately 10 min after the continuous ¹⁵O gas inhalation. Quantitative PET data in the whole brain were as follows: CBF, 32.3 ± 4.5 mL/100ml/min; CMRO₂, 3.23 ± 0.42 mL/100ml/min; OEF, 64.6 ± 9.1 %; and CBV, 5.05 ± 0.45 mL/100ml.

Conclusions This study demonstrated quantitative PET measurement of CBF, CMRO₂, and OEF by steady-state method of ¹⁵O-CO₂ and ¹⁵O-O₂ gases; CBV by ¹⁵O-CO gas in the normal anesthetized rat