Behaviour of near-infrared light in the adult human head: implications for clinical near-infrared spectroscopy

Abstract

To test theoretical assumptions supporting the use of near-infrared spectroscopy (NIRS) in clinical practice, we examined the behaviour of NIR light transmission and attenuation in the human head. Sterile probes for emitting and detecting NIR light at a fixed separation of 40 mm were placed in turn on intact skin, skull, dura and cerebral cortex of 10 patients undergoing elective neurosurgery. In the first five patients, the detecting probe was moved through successive extracerebral layers with the emitter on the skin surface. In the second five patients, the process was reversed, with the emitting probe moved and the detector in the same place on the scalp. NIR intensity was measured at each tissue interface and compared with the intensity measured at the skin surface with all layers intact. Removal of bone and dura from the light path caused a significant reduction in detected intensity. The largest mean reduction in light intensity was a 14-fold decrease with removal of bone (unadjusted P < 0.0001; paired t test). The assumptions that extracerebral tissues contribute little to attenuation of NIR light in the adult head and that most of this attenuation occurs superficially in the scalp are drawn into question by this study. We postulate that the skull and/or its interface with other layers may act as an optical 'channel', distorting the behaviour of NIR light in the human head.

Spatially resolved spectroscopy (NIRO-300) does not agree with jugular bulb oxygen saturation in patients undergoing warm bypass surgery

“…23 These have been supported by clinical experimental data, which indicate that cerebral blood flow measured by NIRS is three times greater when the probe is placed on the dura than when it is measured through the scalp.24 More recently, Young et al 25 found that removal of skull bone and dura from the NIRS light path caused a significant reduction in detected intensity (up to 14-fold decrease). These authors suggested that the skull and/or its interface with other layers might act as an optical channel distorting the behaviour of NIRS light in the human head.…”

Illuminating the Black Box: Investigating Prefrontal Cortical Hemodynamics during Exercise with Near-Infrared Spectroscopy

“…It should be noted, however, that estimates of the portion of the path length attributable to cerebral tissue, as opposed to extracerebral tissue, range widely-from 15% to 55%-and the issue is still the subject of some controversy (Young, Germon, Barnett, Manara, & Nelson, 2000). In particular, the fraction of the path length attributable to the adult skull could be substantial (Hueber et al, 2001;Young et al, 2000). Second, it is assumed that the medium being examined with NIRS (e.g., the head) is homogenous.…”

“…Later, Zhao et al (2002) extended these analyses and found that the DPF varied from 6.18 to 7.69 at the forehead, from 7.18 to 8.87 at the somatosensory motor region, and from 6.98 to 9.35 at the occipital region (at 759 nm, with a less than 10% decrease in these values as the wavelength increased to 834 nm). It should be noted, however, that estimates of the portion of the path length attributable to cerebral tissue, as opposed to extracerebral tissue, range widely-from 15% to 55%-and the issue is still the subject of some controversy (Young, Germon, Barnett, Manara, & Nelson, 2000). In particular, the fraction of the path length attributable to the adult skull could be substantial (Hueber et al, 2001;Young et al, 2000).…”

Evaluation of a Near-Infrared Spectrometer (NIRO 300) for the Detection of Intracranial Oxygenation Changes in the Adult Head

“…19,20 The skull itself and its interface with other layers in the human head may distort the penetration of near-infrared light by generating an optical channel. 21 In the adult head NIRS is applied in reflectance mode because pterional transmission is not possible. 2,3,14,16,17 The NIRO 300 (Hamamatsu Photonics) measures tissue oxygenation with the use of spatially resolved reflectance spectroscopy.…”