  Multispectral photoacoustic (PA) imaging is a prime modality to monitor
hemodynamics and changes in blood oxygenation (sO2). Although sO2 changes can
be an indicator of brain activity both in normal and in pathological
conditions, PA imaging of the brain has mainly focused on small animal models
with lissencephalic brains. Therefore, the purpose of this work was to
investigate the usefulness of multispectral PA imaging in assessing sO2 in a
gyrencephalic brain. To this end, we continuously imaged a porcine brain as
part of an open neurosurgical intervention with a handheld PA and ultrasonic
(US) imaging system in vivo. Throughout the experiment, we varied respiratory
oxygen and continuously measured arterial blood gases. The arterial blood
oxygenation (SaO2) values derived by the blood gas analyzer were used as a
reference to compare the performance of linear spectral unmixing algorithms in
this scenario. According to our experiment, PA imaging can be used to monitor
sO2 in the porcine cerebral cortex. While linear spectral unmixing algorithms
are well-suited for detecting changes in oxygenation, there are limits with
respect to the accurate quantification of sO2, especially in depth. Overall, we
conclude that multispectral PA imaging can potentially be a valuable tool for
change detection of sO2 in the cerebral cortex of a gyrencephalic brain. The
spectral unmixing algorithms investigated in this work will be made publicly
available as part of the open-source software platform Medical Imaging
Interaction Toolkit (MITK).
