Cerebral blood volume measured using near-infrared spectroscopy and radiolabels in the immature lamb brain

Near-infrared spectroscopy (NIRS) is a technique that is increasingly being used for the noninvasive measurement of cerebral blood volume (CBV) in newborn infants, but it has not been fully validated against established methods. These experiments in immature lambs (gestation 92 +/- 1 d, mean +/- SEM) compared CBV measured using NIRS-derived estimates of oxygenated Hb (n = 5) with CBV estimated with radiolabeled indicators (I-125-labeled serum albumin and Cr-51-labeled red blood cells, n = 10). Total brain CBV (mL/100 g tissue) measured using NIRS was 2.5 +/- 0.2 compared with 2.5 +/- 0.2 using radiolabels (NS). Regional tissue plasma, red blood cells, and whole blood volumes from radiolabels varied significantly (p less than or equal to 0.05) throughout the brain. Whole blood volume (mL/100 g tissue) was largest in choroid plexus (16.2 +/- 2.1) and least in white matter (0.7 +/- 0.1) with a significant hierarchy evident among regions: choroid plexus > cerebellum > cortex > brain stem = midbrain > white matter. Regional plasma and red blood cell distributions were similar to whole blood, being highest in choroid plexus (13.0 +/- 1.6 and 3.2 +/- 0.9, respectively), and least in white matter (0.8 +/- 0.1 and 0, respectively). These data from the immature lamb brain indicate that total CBV measured with NIRS is essentially identical with the volumes obtained using intravascular radiolabels. Among cerebral regions, white matter contributes little to the global blood volume measured with NIRS because its red blood cell content is very low.