Near-infrared light propagation in an adult head
model. II.Effect of superficial tissue thickness
on the sensitivity of the near-infrared
Eiji Okada and David T. Delpy
It is important for near-infrared spectroscopy ?NIRS? and imaging to estimate the sensitivity of the
detected signal to the change in hemoglobin that results from brain activation and the volume of tissue
interrogated for a specific source–detector fiber spacing.
models is predicted by Monte Carlo simulation to investigate the effect of the superficial tissue thickness
on the partial optical path length in the brain and on the spatial sensitivity profile.
source–detector spacing of 30 mm, the partial optical path length depends mainly on the depth of the
inner skull surface whereas the spatial sensitivity profile is significantly affected by the thickness of the
cerebrospinal fluid layer.The mean optical path length that can be measured by time-resolved exper-
iments increases when the skull thickness increases whereas the partial mean optical path length in the
brain decreases when the skull thickness increases.
use the mean optical path length as an alternative to the partial optical path length to compensate the
NIRS signal for the difference in sensitivity caused by variation of the superficial tissue thickness.
© 2003 Optical Society of America
170.0170, 170.3660, 170.3890.
In this study light propagation in adult head
In the case of
These results indicate that it is not appropriate to
Near-infrared spectroscopy ?NIRS? and imaging has
been applied to measure brain activation noninva-
sively.NIRS1,2is widely used to monitor the change
development of commercial instruments has enabled
routine brain monitoring.
for topographic imaging can also obtain a topograph-
ical distribution of the activated region in the brain
cortex by a simple mapping algorithm.3–5
activation measurements by NIRS and topographic
imaging, the source and detector fibers are attached
to the scalp with incident light penetrating the brain
and being scattered back through the scalp.
A multichannel system
absorption of the activated region varies by changes
in the blood volume and oxygenation, one can mea-
sure brain activation by detecting the intensity
change of near-infrared light that passes through the
The fundamental and serious problem of NIRS and
imaging is ambiguity in light propagation in the head
caused by the scattering of biological tissue.
poses difficulties in the quantification of NIRS and
results in poor spatial resolution and contrast in the
near-infrared images.It is clinically important to
know the sensitivity of the detected signal to the
absorption change in the brain and the volume of
tissue interrogated with a particular source–detector
pair.Although these clinically important parame-
ters cannot be obtained directly by experiments, the
partial optical path length in the brain6and the spa-
tial sensitivity profiles7can be predicted by numeri-
cal analysis of light propagation in the head as
indices of the sensitivity and volume of tissue inter-
rogated. The light that passes into the brain must
pass through superficial tissues such as the scalp and
the skull, and hence the thickness and the structure
of these superficial tissues obviously affect the near-
E. Okada ?email@example.com? is with the Department of Elec-
tronics and Electrical Engineering, Keio University, 3-14-1 Hiyo-
shi, Kohoku-ku, Yokohama 223-8522, Japan.
the Department of Medical Physics and Bioengineering, Univer-
sity College London, 11-20 Capper Street, London WC1E 6JA,
Received 13 September 2002.
© 2003 Optical Society of America
D. T. Delpy is with
1 June 2003 ? Vol. 42, No. 16 ? APPLIED OPTICS 2915
14. J. Plucinski, A. F. Frydrychowski, J. Kaczmarek, and W.
of variations in the width of the subarachnoid space,”
J. Biomed. Opt. 5, 291–299 ?2000?.
15. C. R. Simpson, M. Kohl, M. Essenpreis, and M. Cope, “Near-
infrared optical properties of ex vivo human skin and subcu-
taneous tissue measured using the Monte Carlo inversion
technique,” Phys. Med. Biol. 43, 2465–2478 ?1998?.
16. M. Firbank, M. Hiraoka, M. Essenpreis, and D. T. Delpy,
“Measurement of the optical properties of the skull in the
wavelength range 650–950 nm,” Phys. Med. Biol. 38, 503–510
17. P. van der Zee, M. Essenpreis, and D. T. Delpy, “Optical prop-
erties of brain tissue,” in Photon Migration and Imaging in
Random Media and Tissues, R. R. Alfano and B. Chance, eds.,
Proc. SPIE 1888, 454–465 ?1993?.
18. E. Okada and D. T. Delpy, “Near-infrared light propagation in
an adult head model.I.Modeling of low-level scattering in
the cerebrospinal fluid layer,” Appl. Opt. 42, 2906–2914 ?2003?.
19. D. T. Delpy, M. Cope, P. van der Zee, S. R. Arridge, S. Wray,
and J. Wyatt, “Estimation of optical pathlength through tissue
from direct time of flight measurement,” Phys. Med. Biol. 33,
20. B. C. Wilson, “A Monte Carlo model for the absorption and flux
distribution of light in tissue,” Med. Phys. 10, 824–830 ?1983?.
21. S. R. Arridge and J. C. Hebden, “Optical imaging in medicine.
II.Modelling and reconstruction,” Phys. Med. Biol. 42, 841–
22. D. A. Boas, T. Gaudette, G. Strangman, X. Cheng, J. J. A.
Marota, and J. B. Mandeville, “The accuracy of near infrared
spectroscopy and imaging during focal changes in cerebral
hemogynamics,” Neuroimage 13, 76–90 ?2001?.
2922 APPLIED OPTICS ? Vol. 42, No. 16 ? 1 June 2003