Cerebral near infrared spectroscopy: emitter-detector separation must be increased

Abstract

We have compared the effect of increasing optode separation (range 0.7-5.5 cm) on the sensitivity of near infrared spectroscopy (NIRS) to discrete reductions in scalp and cerebral oxygenation in 10 healthy men (mean age 32, range 26-39 yr) using multichannel NIRS. During cerebral oligaemia (a mean reduction in middle cerebral artery flow velocity of 47%) induced by a mean reduction in end-tidal PCO2 of 2.4 kPa, the decrease in oxyhaemoglobin detected by NIRS became significantly greater with increasing optode separation (P < 0.0001). In response to scalp hyperaemia induced by inflation and release of a pneumatic scalp tourniquet, increases in oxyhaemoglobin became significantly smaller with increasing optode separation (P < 0.0002). These results are consistent with theoretical models of the behaviour of NIR light in the adult head and support the concept of using multi-detector NIRS to separate intra- and extracranial NIR signal changes. However, the emitter-detector separation used by currently available cerebral oximeters is not large enough to provide optimal spatial resolution.

Investigating the post-stimulus undershoot of the BOLD signal—a simultaneous fMRI and fNIRS study

“…Our data are also consistent with simultaneous fMRI and fNIRS experiments showing a high correlation between the BOLD signal and deoxy-Hb (Kida et al, 1996;Mehagnoul-Schipper et al, 2002;Punwani et al, 1997Punwani et al, , 1998Toronov et al, 2003) and with Pouratian et al (2002) who showed a high temporal and spatial correlation between the BOLD signal and optical intrinsic signals measured at 610 nm (particularly sensitive to deoxy-Hb) in humans. Differences to Strangman et al (2002), who found a lower correlation of the BOLD signal with deoxy-compared with oxy-Hb during finger flexion/ extension motor activation, may be explained by their less (three) subjects, different (block) design, their shorter emitter -detector separation (Germon et al, 1999), and non-linear changes of oxyHb in relation to deoxy-Hb in the motor cortex (Wolf et al, 2002). Because of the known linear relationship between changes of oxyand deoxy-Hb in the visual cortex ( Fig.…”

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

“…Previous work has demonstrated that despite the use of large optode distances, there is still a significant contribution from the extracranial tissues. 24,36 Our approach has been to sequentially clamp the ECA and ICA during carotid endarterectomy, allowing the opportunity to separate the intracranial and extracranial signal contributions and to identify changes in the cutaneous circulation affecting the NIRS measurements. 25,30 We have previously found that once the extracranial component is subtracted, NIRS can accurately identify severe cerebral ischemia.…”

Near-Infrared Spectroscopy

“…An increase of inter-optode distance leads to better determination of intracerebral changes in tissue absorption, i.e. the volume of cerebral tissue which is interrogated by NIRS increases (Germon et al, 1999). Also, when using the multi-distance approach, see Fig 2C, the oxygenation changes in cerebral and extracerebral tissue can be separated.…”

“…Transmission cerebral NIRS, in which a near-infrared light source is placed contralateral to the receiver, is most frequently used in infants. In adults this is not believed to be sensitive enough, primarily because of the reduced intensity of the exiting near-infrared light (Jöbsis, 1977) and poor signal-to-noise ratio (Germon et al, 1999). In reflectance-mode NIRS the near-infrared light source is placed ipsilateral to the receiver, it has been developed to overcome the problems of transmission NIRS.…”

“…If these assumptions are correct, then the mean pathlength of the travelling photons should describe an ellipse whose mean depth is proportional (i.e. 1/3) to the optode spacing (Germon et al, 1999;Strangman et al, 2002).…”