Noninvasive imaging of hemoglobin concentration and oxygenation in the rat brain using high-resolution photoacoustic tomography.

Simultaneous transcranial imaging of two functional parameters, the total concentration of hemoglobin and the hemoglobin oxygen saturation, in the rat brain in vivo is realized noninvasively using laser-based photoacoustic tomography (PAT). As in optical diffusion spectroscopy, PAT can assess the optical absorption of endogenous chromophores, e.g., oxygenated and deoxygenated hemoglobins, at multiple optical wavelengths. However, PAT can provide high spatial resolution because its resolution is diffraction-limited by photoacoustic signals rather than by optical diffusion. Laser pulses at two wavelengths are used sequentially to acquire photoacoustic images of the vasculature in the cerebral cortex of a rat brain through the intact skin and skull. The distributions of blood volume and blood oxygenation in the cerebral cortical venous vessels, altered by systemic physiological modulations including hyperoxia, normoxia, and hypoxia, are visualized successfully with satisfactory spatial resolution. This technique, with its prominent sensitivity to endogenous contrast, can potentially contribute to the understanding of the interrelationship between neural, hemodynamic, and metabolic activities in the brain.