The first mNIRS

Near infrared light (700–1,000 nm) can penetrate tissue because of the relatively low absorbance of biological chromophores in this wavelength range. However, those chromophores that do absorb NIR light can change colour in response to changes in oxygen supply or metabolism. In particular, oxyhemoglobin, oxymyoglobin, deoxyhemoglobin, deoxymyoglobin and mitochondrial cytochrome c oxidase have significant oxygen-sensitive signals.

In general, myoglobin and hemoglobin cannot be distinguished spectrally because of their structural similarity (although see a later method). Therefore, there are essentially three NIR-detectable chromophores: oxy[heme], deoxy[heme] and mitochondrial cytochrome c oxidase.

These were first reported in the seminal 1977 paper by Frans Jöbsis:

Jöbsis FF. “Noninvasive, infrared monitoring of cerebral and myocardial oxygen sufficiency and circulatory parameters.” Science 198 (1977): 1264–1267.

This paper showed the anoxic–normoxic NIR spectra of a transilluminated dog heart. So, this is the first published mNIRS spectrum. Although Jöbsis was actually most interested in the mitochondrial cytochrome signal, this has proved tricky to study because of its lower in vivo concentration (see a later method). Consequently, nearly all subsequent mNIRS studies have focused on changes in the oxy[heme] and deoxy[heme] signals.

Actually, the first ever in vivo muscle NIR spectrum may have occurred when Jöbsis had his “Eureka” moment of shining NIR light through his food. Joseph LaManna, who worked with Jöbsis, recalled this in an email to me:

“I remember the experiments of measuring NIR light through supermarket raw beef steak samples as a proof of concept; Frans originally thought that heart was to be a good target. We grilled and ate the steaks afterwards.”

For this story and others about the discovery and early development of NIRS, see:

Jöbsis-vanderVliet FF. “Discovery of the near-infrared window into the body and the early development of near-infrared spectroscopy.” Journal of Biomedical Optics 4(4) (1999): 392–396.