Non-invasive cerebral blood flow monitoring by a dye bolus method: separation of extra- and intracerebral absorption changes by frequency-domain spectroscopy

Tracking a bolus of contrast agent travelling through the cerebral vasculature provides a measure of the blood supply and the blood flow velocity in the respective cerebral tissue. This principle has been the basis for the first approaches in functional MR imaging and is of great value when investigating stroke and other vascularly compromised patients. While the bolus measurement is a standard procedure in clinical MR imaging, optical bolus tracking has not yet become a reliable protocol. Here optical absorption changes induced by bolus signals of the dye indocyanine-green (ICG) are studied by near infrared spectroscopy (NIRS) on volunteers. The aim is to assess the latency and shape of the absorption change. For an application on the human brain the differentiation of extra- and intracerebral vascular compartments is essential. For this purpose frequency domain measurements are analysed by a Monte Carlo based model for the photon transport in tissue. We show that based on measurements of the photon-mean time of flight (phase) in addition to intensity the bolus signal of the deeper tissue layers (cortical signals) has a peak of about 10 s width, while the absorption change in the upper layers, i.e. extracortical tissue, has a much longer recovery time. This is in qualitative agreement with Gd-MR bolus measurements. The potential clinical interest of this study is to monitor cerebral perfusion changes induced by pathological conditions i.e. stroke or cerebral occlusion.