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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 3, MARCH 2011 777

Point-of-Care Device for Quantification ofBilirubin in Skin Tissue

Suresh K. Alla*, Adam Huddle, Joshua D. Butler, Peggy S. Bowman, Joseph F. Clark, and Fred R. Beyette Jr.

AbstractSteady state diffuse reflectance spectroscopy is a non-destructive method for obtaining biochemical and physiologicalinformation from skin tissue. In medical conditions such as neona-tal jaundice excess bilirubin in the blood stream diffuses into thesurrounding tissue leading to a yellowing of the skin. Diffuse re-flectance measurement of the skin tissue can provide real timeassessment of the progression of a disease or a medical condition.Here we present a noninvasive point-of-care system that utilizesdiffuse reflectance spectroscopy to quantifying bilirubin from skinreflectance spectra. The device consists of an optical system inte-grated with a signal processing algorithm. The device is then usedas a platform to study two different spectral databases. The firstspectral database is a jaundice animal model in which the jaundicereflectance spectra are synthesized from normal skin. The secondspectral database is the spectral measurements collected on humanvolunteers to quantify the different chromophores and other phys-ical properties of the tissue such as Hematocrit, Hemoglobin, etc.The initial trials from each of these spectral databases have laidthe foundation to verify the performance of this bilirubin quantifi-cation device.

Index TermsBilirubin quantification, hyperbilirubinemia,Jaundice, skin reflectance.

I. INTRODUCTION

STEADY state diffuse reflectance spectroscopy is a nonde-structive method for obtaining biochemical and physiolog-

ical information from tissue. When light strikes a substance, it

can be absorbed, transmitted, scattered or reflected. Reflectance

spectroscopy measures the reflected portion of the light, which

can then be utilized to extract information regarding the absorb-

ing and scattering properties of the material.

Neonatal jaundice, clinically known as hyperbilirubinemia,

is associated with elevated serum bilirubin levels in the blood-

stream of infants. Hyperbilirubinemia is termed as severe when

the serum bilirubin concentration is greater than 12.9 mg/dL,

a condition that has been reported to occur in 10% of new-

borns [1]. As the bilirubin levels increase further, the unbound

Manuscript received July 23, 2010; revised October 14, 2010; acceptedNovember 3, 2010. Date of publication November 18, 2010; date of currentversion February 18, 2011. Asterisk indicates corresponding author.

*S. K. Alla, J. D. Butler, P. S. Bowman, and F. R. Beyette, Jr., are with theDepartment of Electrical and Computer Engineering, University of Cincinnati,Cincinnati, OH 45221-0030 USA (e-mail: allas@mail.uc.edu).

A. Huddle is with the Department of Biomedical Engineering, University ofCincinnati, Cincinnati, OH 45221-0048 USA.

J. F. Clark is with the Department of Neurology, University of Cincinnati,Cincinnati, OH 45267-0536 USA.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TBME.2010.2093132

bilirubin has a greater propensity to cross the blood brain bar-

rier, staining the basal ganglia and causing an irreversible brain

damage known as Kernicterus. The chances of developing ker-

nicterus are higher in sick and preterm newborns even at lower

concentrations of serum bilirubin. This is due to the imma-

ture development of the blood brain barrier [2]. Detection and

accurate quantification of the yellowness caused by bilirubin

deposition in skin tissue provides a window of opportunity to

prevent the onset of kernicterus by introducing proper interven-

tion procedures such as phototherapy and exchange transfusion.

In this letter, we discuss spectral analysis methods (forward

and inverse models) and provide the verification results, which

demonstrate the viability of our proposed noninvasive point-of-

care device for the quantification of bilirubin.

Test results from two different spectral databases are shown.

The first is spectra from an animal model of jaundice. Jaundiced

reflectance spectra were generated by developing a porcine skin

flap soaking method that yields synthesize jaundiced skin from

normal porcine skin samples. A signal processing algorithm

was then developed to simulate the reflectance spectrum of the

porcine skin underjaundice conditionsusing an analyticalmodel

of light transport in skin. The signal processing system coupled

with an optimization algorithm is able to track the changes oc-

curring in the different jaundiced skin tissue spectra. The systemis validated through the analysis of several porcine samples pre-

pared to have different bound bilirubin concentrations.

The second spectral database includes diffuse reflectance

spectra collected from the skin of four human subjects. The

device is tested on the human skin diffuse reflectance spectral

database by validating the model estimated results to the assay

results obtained from blood chemistry.

II. BIOMEDICALOPTICALSYSTEM

The principal hardware components of the device are a white-

light source optimized for the VIS-NIR (3601100 nm), two

SMA connectors used for routing light, a reflectance probe that

both directs incident light to the skin surface and collects the re-

flected light fromthe skin surface, a USB4000 spectrometer, and

a laptop computer. Operationally, the light source is coupled to a

SMA connecter which routes the incident light to a reflectance

probe. The reflectance probe, which provides the mechanical

support and integrity, holds six light illumination fibers, which

direct the light onto the skin surface and a detection fiber, which

collects the reflected light from the skin surface. The bifurcated

bundle assembly splits the fibers from the reflectance probe to

the light source and to the spectrometer. The spectrometer ac-

cepts light energy transmitted through optical fiber anddisperses

it via a fixed grating across the linear CCD array detector, which

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Fig. 1. The system built to capture the reflectance spectral data of human skinand the interface between the spectral data.

resolves the intensity of reflected light at every wavelength. The

spectrometer then converts the optical signal into a digital sig-

nal, which can be processed by a proprietary algorithm whichinterprets the reflected spectrum to predict the concentrations of

the different chromophores (ex. bilirubin) present in the skin.

Fig. 1 shows the system built to capture the reflectance spectral

data of human skin and the interface between the spectral data

III. SIGNALPROCESSINGALGORITHM

The signal processing system used to quantify the serum

bilirubin from skin reflectance spectra is comprised of two parts.

The first part of the system generates the reflectance spectra ex-

pected from the skin with varying absorption and scattering co-

efficients associated with different biological materials present

in different layers of skin. The process of building a spectrumfrom the skin optical properties is defined as developing a for-

ward model.

In the second part of the signal processing system, the biliru-

bin concentration is estimated using the forward model along

with a nonlinear iterative method that adjusts the parameters

of the forward model until a best fit is achieved between the

simulated and measured spectra. Once the best-fit parameters

are obtained, they can be correlated with the experimental con-

ditions associated with the sample preparations.

Fig. 2 shows the flow chart of the signal processing algorithm.

The Rsim is the forward analytical model described in this sec-

tion. The input parameters to the analytical model are the optical

properties and concentration values of different chromophores

in the skin. The Rm in the flow chart is the reflectance spectra

measured using the hardware system defined in the previous

section.

The analytical model of skin reflectance (Rsim ) is derived

usingexisting methods, whichsolve the diffusion approximation

problem that has been extended to a two layer skin tissue system

comprised of epidermal and dermal skin layers [3], [4]. The

existing reflectance model [5] assumes that the skin as is a

semi-infinite turbid media

In a jaundiced individual, the bilirubin flows in the blood

stream bound to albumin. As the bilirubin levels rise beyond the

binding capacity of albumin, unbound bilirubin concentration

Fig. 2. A flow chart showing the engineering flow process in which the sim-ulated spectrum (Rsi m ) is compared with the measured spectrum (Rm) and theparameters are updated using the nonlinear least squares optimization solver tofind the best fit parameters.

rises in the blood stream. This unbound bilirubin diffuses out

of the blood stream and is deposited in the skin tissues where

it binds to collagen and fat. Based on the biophysical system,

a three layered analytical reflectance model has been defined

using the reflectance expression

Reflectance =em m 2le p i ece b e b 2le b

ds

s1 + s2 da

.

The first exponential term takes into account the effect of

melanin present in the epidermal layer. As light traverses twice

through the melanin layer a factor of 2 is introduced in this term.The second exponential term takes into account the effect of ex-

tra vascular bilirubin. The third term in the expression is used

to estimate the back-scattered light from the dermal layer of the

skin tissue. The parameters in the first exponential term: m, m ,

lep i are melanin volume fraction (skin tone), absorption coeffi-

cient of single melanosome, and epidermal layer thickness. The

second exponential term parameters: ceb ,eb , leb are the extra

vascular bilirubin concentration, molar extinction coefficient of

bilirubin, and the path length which the light travels (i.e., the

thickness of the extra vascular bilirubin layer). The parameters

ds andda are the scattering and absorption coefficients of der-

mal layer. The parameters s1 and s2 take into account the effects

probe geometry.

From systems analysis perspective, an inverse model of the

system can be used to extract parameter from measured data. In

this work, the parameters of interest are the physiologic factors

that contribute to the optical properties of the tissue. A nonlin-

ear least squares solver has been adopted as an inverse model

for this work. Sequential quadratic program (SQP) is one of the

most popular and robust algorithms for nonlinear continuous

optimization. In this research, we have used the NLSSOL (non-

linear least squares solver) routine available in the optimization

software TOMLAB interfaced with MATLAB [6]. The primary

goal of this solver is to minimize the error function (differ-

ence between measured and simulated spectra) by updating the

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780 IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 58, NO. 3, MARCH 2011

Fig. 4. (a) Reflectance spectral profile of forehead and finger site of lightpigmented. (Caucasian skin). (b) Reflectance spectral profile of forehead andfinger site of lightly dark pigmented individual (Asian skin).

calculated from biochemistry analysis conducted on the blood

drawn from the volunteers is shown in Table II.

The performance of the model on initial sample size has given

a clear understanding of the system behavior, with a high posi-

tive correlation in the prediction of hemoglobin and hematocrit

value.

The spectral data collected on different skin sites of an indi-

vidual and different colored individuals have been analyzed, as

shown in Fig. 4(a) and (b).

The performance of the algorithm is justified by the higher

numbers for the blood-volume fraction and oxygen saturation

on the finger site compared to the forehead. The spectral data

collected on the finger site of a dark skinned individual has

hemoglobin signal bands that are clearly visible, primarily due

to the absence of melanin on the finger tissue site (vascular side).

The spectral data collected on the forehead has the melanin con-

tent from the epidermal layer masking the hemoglobin bands.

As most of the spectral information used to quantify the biliru-

bin is collected from the surface to a depth of around 12 mm,

which is usually the epidermis and dermal layer. With the prin-

ciple of blanching the skin the light can go further deep and

collect the spectral information from the subcutaneous fat layer

too.

Thein vivohuman data results collected off of the different

individuals was used as a starting step to train the algorithm

for varying optical properties. The poor correlation observed in

the bilirubin device measurement and standard assay could be

strongly improved by collecting more spectral data on healthyand jaundice individuals. As more spectral data is fed into the

feedback mode of the signal processing algorithm inputs from

the standard assay can form the basis to correct for the missing

parameters in the model. This preliminary human data and the

pig skin jaundice model studies has established the fact that

the device has passed the concept and development phase and

could be headed into the clinical trial phase. The point of care

device with the preliminary results through the signal processing

algorithm does give us the proof that multiple parameters, such

as hemoglobin, hematocrit, and oxygen saturation in addition to

bilirubin, could be quantified using a single device.

VI. CONCLUSION

In this work, a noninvasive point-of-care device for the quan-

tification of bilirubin is developed and evaluated. The device

works by analyzing diffuse reflectance spectra collected from

optical reflectance from skin tissue. The system is comprised of

hardware components and a signal processing algorithm. The

signal processing algorithm allows the device to quantify the

different chromophores present in the skin tissue. Initial assess-

ment of the device has been completed using a porcine model

for jaundice that involves soaking biopsied pig skin in varying

concentrations of bilirubin solution. Analysis of these skin sam-

ples shows that the device is able to track the changes associatedwith the absorption of unbound bilirubin in skin tissue.

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