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Determination of blood oxygenation

in the brain by time-resolved

reflectance spectroscopy: influence

of the skin, skull, and meninges

Hanli Liu, Andreas H. Hielscher, Bertrand Beauvoit,

Lihong V. Wang, Steven L. Jacques, et al.

Hanli Liu, Andreas H. Hielscher, Bertrand Beauvoit, Lihong V. Wang, Steven

L. Jacques, Frank K. Tittel, Britton Chance, "Determination of blood

oxygenation in the brain by time-resolved reflectance spectroscopy: influence

of the skin, skull, and meninges," Proc. SPIE 2136, Biochemical Diagnostic

Instrumentation, (21 July 1994); doi: 10.1117/12.180783

Event: OE/LASE '94, 1994, Los Angeles, CA, United States

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Determination of the blood oxygenation in the brain

by time resolved reflectance spectroscopy (II):

Contribution of vascular absorption and tissue background absorption

Hanli Liu1, Andreas H. Hielscher23, Bertrand Beauvoit1, Lihong Wang2,

Steven L. Jacques2, Frank K. Tittel3, and Britton Chance'

1 University of Pennsylvania

Department of Biochemistry and Biophysics

Philadelphia, PA 19104-6089

2 University of Texas, M.D. Anderson Cancer Center

Laser Biology Research Laboratory

Houston, TX 77030

3 Rice University

Department ofElectrical and Computer Engineering

Houston, TX 7725 1-1892

ABSTRACT

The use of near infrared light for the determination of blood oxygenation in the brain

has been of great interest over the last years. However, little attention has been paid to the fact

that the states of blood oxygenation in arteries, veins, and capillaries differ substantially. An

understanding of light absorption by cerebral blood in various blood vessels embedded in the

brain tissue is essential for the interpretation of measured brain oxygenation.

In this study, Monte Carlo simulations for a heterogeneous system were conducted, and

near infrared time-resolved reflectance measurements were performed on a heterogeneous

tissue phantom model. The model was made of a solid polyester resin, which simulates the

tissue background. A network of tubes was distributed uniformly through the resin to simulate

the blood vessels. The time-resolved reflectance spectra were taken with different absorbing

solutions filled in the network. Based on the simulation and experimental results, we

investigated the dependence of the absorption coefficient obtained from the heterogeneous

system on the absorption of the actual absorbing solution filled in the tubes. We show that

light absorption by the brain should result from the combination of blood and blood-free tissue

background. Using multiple wavelengths, we should make the oxygenation determination

more accurately.

1. INTRODUCTION

Near infrared (NIR) techniques, including continuous, pulsed, and amplitude-modulated

light spectroscopy, have been developed to monitor the oxygenation of the brain non-

invasively."2'3'4 In particular, using NIR time-resolved or frequency-resolved spectroscopy has

a great advantage in obtaining the oxygenation state of cerebral blood and tissue since these

techniques can determine accurate path lengths and absorption coefficients.5 However, the

brain is a very complex biological organ, consisting of various blood vessels and brain tissue,

covered by the skin and skull. The relationship between light absorption and vascular

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hemoglobin concentration can be quite complicated since the biological components of the

head have different optical properties. In order to obtain accurate oxygenation values of the

brain using the MR technique, it is necessary to understand the influence of the skin and skull

and the contribution of the blood vessels to the brain oxygenation measurement.

Much effort has been made in studying the optical properties of the skull and brain

tissues.6'7 In the first part of this study,8 we showed that the absorption properties of the brain

tissue can be determined with timeresolved reflectance spectroscopy through the effect of the

skin and skull. It was demonstrated that in layered tissue structures, the outer layers affect only

the earlier part of the measured time resolved reflectance, while the later part is dominated by

the absorption property of the deeper material. Part I of this study8 concludes that one can

extract the absorption information deep in the brain by fitting the analytical solution found for

the diffusion theory to the tail of the time resolved reflectance.

An accurate value of the absorption coefficient of the brain is prerequisite to obtain a

correct oxygenation status. Based on the BeerLambert law, the absorption coefficient, LLa, due

to hemoglobin at two wavelengths, Xi and X2, can be wriuen as

= e[Hb] + eio2 [Hb02]

(1)

2 e[Hb] + eo2 [Hb02]

(2)

where 4

and

Cp,o2

are

extinction coefficients (cm1mM1) of deoxy- and oxy- hemoglobin,

and [Hb] and [Hb02] are concentrations of deoxy- and oxy hemoglobin. Combining these

two equations yields the hemoglobin saturation S02, as follows:

rimr 1

—1:-EHb—EHb

—

L'-"-'2J

P'a

[Hb02}

+ [Hb]

Xl a X2

1Th02 CHb

°2 CHb

This formula is applicable to arterial, venous, or tissue saturation when [Hb] and [Hb02]

are referred to arterial, venous, or tissue hemoglobin concentrations, respectively. An

understanding of what the N spectroscopy measures is essential toward developing brain

oximetry. It is known that in the wavelength range of 700-900 nm, the absorption coefficient

of hemoglobin is much larger than that of tissue or water.9'10 Is it reasonable to ignore tissue

background absorption? Does the absorption coefficient obtained from the NW spectroscopy

result from blood only or from a combination of blood in the vessels and background tissue?

In searching for the answers to the above questions, this study was conducted on a

heterogeneous model, having a simple vascular structure, with 1) time-resolved Monte Carlo

simulation and 2) time-resolved spectroscopy (TRS) in reflectance geometry. Our intention is

to quantify the contribution of blood in the blood vessels to brain oxygenation and then to

obtain the proper algorithm for determination of the brain hemoglobin saturation from the MR

measurement.

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