Application of Near Infrared Spectroscopy in Biomedicine by Thomas Jue & Kazumi Masuda

Author:Thomas Jue & Kazumi Masuda
Language: eng
Format: epub
Publisher: Springer US, Boston, MA

5.4.2 Effect of Multiple Layers

The light path from a light source to the detector follows a banana-shaped curve in which penetration depth into tissue is approximately equal to half the distance between the light source and detector [12]. If light source–detector separation was set at 3 cm, penetration depth was equal to 1–2 cm and measured volume ~4 cm [12, 37]. Hampson et al. studied how the signal from skin might interfere with muscle NIRS measurement, employing NIRSDCWS, changes in skeletal muscle O2 content kinetics and skin deoxygenation during 10-min ischemia in human forearm [13]. They found that muscle O2 content began to fall at onset of arterial occlusion. Increased pressure caused skin oxygenation to rapidly fall within 2–3 min, while muscle oxygenation declined within 6 min. Muscle O2 content was completely depleted (functional anoxia) about 6 min after onset of arterial occlusion. This indicated that the time course of skin deoxygenation changes during ischemia correlate poorly with changes in skeletal muscle. Heating of the thigh at 37°C and 42°C caused a marked increase in cutaneous vascular conductance (CVC) at rest and with exercise, and an increase in SO2 by several percentage points at rest, but not during exercise. One might thus suppose that skin vasodilatation has a marginal influence on NIRS experiments where skin blood flow can change markedly [38]. A fixed source–detector spacing of 4 or 5 cm ensures more accurate quantitation of the oxygenation changes occurring at the muscular level and minimizes the influence of skin vasculature [39].

Subcutaneous adipose tissue thickness has a substantial influence on signal intensity [40–42]: signal intensity for 5-mm fat thickness mm was reduced by ~0.2 (80% of signal for 0 mm) with a source–detector separation of 30–40 mm, and was further reduced by 0.3–0.6 with a separation of 15–20 mm [41, 43]. Other work documented the influence of adipose tissue thickness ranging from 0 to 15 mm with a source–detector separation of 15–40 mm. The curve was based on the results of Monte Carlo simulation as well as in-vivo experiments [41, 43]:

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