Non-invasive optical sensing of the brain

In 1977 a pioneering work by Frans Jöbsis reported optical measurements on the brain in the wavelength range 740-865 nm, and gave birth to the field of non-invasive cerebral near-infrared spectroscopy (NIRS). The 1980s saw further validations and first explorations of clinical potential, and 1993 was the year of the first functional NIRS (fNIRS) studies of brain activation. The following 30+ years saw growing research efforts and numerous significant achievements in instrumentation development, data analysis methods, and applications in a number of research and clinical areas. The basic approach is illustrated in Fig. 1.

Fig. 1. Schematic representation of the optical region of sensitivity (banana-shaped shaded area) in noninvasive optical studies of the human brain for one source location and one detector location. The sensitivity of the optical signal to the probed tissue is not spatially uniform (as indicated by the different gray levels within the region of sensitivity) and is greater for superficial tissue layers (scalp and skull) than for brain tissue.

The main appeal of NIRS and fNIRS rests on the possibility of sensing the human brain non-invasively and continuously, using compact and cost-effective instrumentation that can be portable and wearable. These features allow for the realization of diagnostic monitoring in real-time at the bedside, or functional assessment in every-day environments that are beyond the capabilities of more established neuroimaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). However, non-invasive optical techniques for human brain studies suffer from a number of limitations that must be properly taken into consideration:

• Limited penetration depth (2-3 cm), restricting sensitivity to the outermost cortical layers;
• Limited spatial resolution (~1 cm);
• Strong sensitivity to superficial extracerebral tissue layers (scalp, skull, etc.), which results in signal contributions that may confound the targeted optical signals from the brain;
• Complex dependence of the measured optical signals on the temporal and spatial features of the driving physiological quantities (blood volume, blood flow, metabolic rate of oxygen, tissue oxygen saturation, vascular architecture, etc.), as well as their systemic vs. focal origin;
• Challenging absolute measurements of cerebral concentration and oxygen saturation of hemoglobin;
• Sensitivity to motion artifacts and challenges of reliable measurements in the presence of hair.

It should also be considered that non-invasive optical measurements are mostly sensitive to cerebral hemodynamics and oxygenation, which poses an intrinsic limitation to the temporal and spatial resolution of functional brain studies (because of the large temporal and spatial extent of the hemodynamic response to fast and spatially localized neuronal activation). This is a most exciting field of research in biomedical optics.