Cerebral blood volume in humans by NIRS and PET

Near infrared spectroscopy (NIRS) determined changes in the cerebral blood volume (CBV) were compared to those obtained by positron emission tomography (PET) in five healthy volunteers (2 females). Two NIRS optodes were placed on the left forehead and NIRS-CBV was derived from the sum of oxyhaemoglobin and deoxyhaemoglobin. CBV changes were induced by hyperventilation and inhalation of 6 % CO₂. After 2 rain inhalation of labeled carbon monoxide, data were sampled during 8 rain for both PET- and NIRS-CBV as well as for the arterial carbon dioxide tension (P_aCO₂). The region of interest for PET-CBV was "banana-shaped" with boundaries corresponding to the position of the NIRS optodes on the transmission scan and to a depth of approximately 2 cm. During hyperventilation, P_aCO₂ decreased from 5.2 (4.6-5.8) to 4.6 (4.2-4.9) kPa and equally PET-CBV (from 3.9 (2.5-5.2) to 3.6 (3.0-4.8) ml·100 g^-1) and NIRS-CBV were reduced (by -0.14 [-0.38-0.50] ml·100 g^-1). During hypercapnia P_aCO₂ increased to 6.0 (5.9-7.0) kPa accompanied by parallel changes in PET- (to 4.5 (3.9-4.9) ml·100 g^-1) and NIRS-CBV (by 0.04 [-0.02-0.30] ml·100 g^-1) and the two variables were correlated (r=0.78, p<0.05). In conclusion, with a moderate change in the arterial carbon dioxide tension, the cerebral blood volumes determined by near infrared spectroscopy and by positron emission tomography change in parallel but the change in NIRS-CBV is small compared to that obtained by PET.