Hyperspectral Imaging
Seminar
2012-3
Retrieving Skin
Properties From In
Vivo Spectral
Reflectance
Measurements
Dmitry Yudovsky and Laurent Pilon
In Vivo Spectral Reflectance
Measurements
• Main Challenge:
Demonstrate the capability of reflectance spectroscopy
to:
Determine the structure and thickness of the skin.
Blood volume and oxygen saturation.
Scattering coefficient from in vivo spectral
reflectance measurements.
Rapidly and non-invasive monitoring.
About Skin
The largest organ of the human body.
Total surface area of approximately 1.8 m^2.
Total weight of approximately 11 kg for adult.
• Skin Structure:
Two main layers: Outer – epidermal, Inner -dermis.
Skin Layers
Epidermal layer is composed of dead cells
embedded in a lipid matrix.
Epidermal layer is pigmented by melanin.
Melanin absorbs strongly in the UV.
Melanin concentration is between 0-100 mg/mL of
the epidermal.
Epidermal thickness may vary with anatomical,
gender, age, health. Ranges between 20-150 micro.
Skin Layers Cont.
Dermis is responsible for the skin pliability,
temperature control.
Dermis is composed of collagen fibers perfused by
nerves, capillaries and blood vessels.
Dermis thickness ranges between 450-1000 micro.
Hemoglobin Properties
Blood consists of hemoglobin.
Hemoglobin responsible for oxygen transfer
(Oxyhemoglobin).
Hemoglobin is known as deoxyhemoglobin once it
release its oxygen transfer.
The ratio of oxyhemoglobin to total hemoglobin in
blood – oxygen saturation.
Hemoglobin Properties
Cont.
Depends on body location and tissue health , the
volume of blood in the dermis ranges 0.2-7%
Hemoglobin absorb in the visible and near-infrared
part of the spectrum.
The spectral absorption coefficient of
oxyhemoglobin and deoxyhemoglobin are differ.
The color of the dermis depends of the local oxygen
saturation of the blood.
Uses
Detect a wide variety of medical conditions –
Acute and long inflammation, Acne laser.
Melanin concentration correlates with the risk of
melanoma skin cancer.
To estimate blood volume during alcohol
consumption.
Determine distribution of oxygen saturation in
human skin.
Naïve Approach
Optical tomography or ultrasound.
Sample of the skin is removed and analyzed.
Study light transfer in skin using Monte Carlo
simulator.
Diffusion approximation to model light transfer
through tissue.
Alternative
An inverse method based on a semi empirical
model for the diffuse reflectance from layer
media subject
Motivation
To Retrieve:
Skin melanin volume, epidermal thickness.
Dermal blood volume and oxygen saturation.
Scattering coefficient from diffuse reflectance
spectra of human skin.
Racial differences, UV exposure and anatomical
location.
From spectral reflectance measures.
Measurements
50 male and 21 female of white Caucasian descent.
21 male and 21 female of black African descent.
Illuminating a circular spot of skin, 1 inch in diameter
and collecting the hemispherical reflectance (4401000 nm) .
3 locations: (i) Right inner forearm.
(ii) Forehead over the left eye.
(iii) Left outer forearm.
Inverse Model
• Main Challenge:
Develop method accounting optical properties to
retrieved physiologically meaningful parameters from
the simulated diffuse reflectance spectra of skin.
Diffuse reflectance spectra between 490-620 nm was
generated using Monte Carlo simulations.
In Vivo normalhemispherical spectral
reflectance concept
Optical Properties Of
Human Skin
• The light scattering properties of the skin have been
study in the visible and near infrared region of the
spectrum (between 490-620nm).
• Study light transfer in skin by using Monte Carlo
simulation, a stochastic method.
Monte Carlo Simulation
• Red, the impact of melanin is strong in range 650700 nm, the deep red wavelength specify the
epidermal melanin.
• Green, Yellow, Orange, the impact of blood is
maximal in range 540-580 nm, the yellow
wavelength can specify the blood content.
Optical Properties Of
Human Skin
The linear spectral absorption
The transport scattering coefficient
All units
Optical Properties Of
Epidermis
Absorption in the epidermis is mainly due to melanin
and flesh.
The volume fraction of melanin.
The background absorption of humen flesh :
Absorption coefficient of melanin:
Optical Properties Of
Epidermis
Melanin absorption of UV light is much stronger than
that of near infrared light.
Optical Properties Of
Dermis
Absorption in the dermis is determined by blood and
flesh.
The volume fraction of the dermis occupied by
blood.
Blood absorption:
The absorption coefficient of oxyhemoglobin:
is the molar extinction coefficient of
oxyhemoglobin in units
Optical Properties Of
Dermis, cont.
The average hemoglobin concentration in blood is
150 g/l,
The oxygen saturation
The absorption coefficient of deoxyhemoglobin:
, the molar extinction coefficient of
deoxyhemoglobin.
Optical Properties Of
Dermis, cont.
Oxyhemoglobin is nearly transparent for wavelength
above 600 nm, giving oxygen rich blood its red color.
Spectral Scattering
Coefficients
The spectral scattering coefficients of the epidermis
and dermis are equal.
The power constant b = 1.5, for all locations.
C range between 10^5- 10^6, defends on the
average size of the microscopic feathers of tissue.
Responsible for light scattering in the skin.
Using Inverse Method
• Optical properties of skin depend on the property
vector:
The measured diffuse reflectance spectrum is
denoted by:
A given input parameters
Where
is the jth wavelength and j is an integer
between 1 and k, wavelength range 490-650 nm.
Hemispherical Diffuse
Reflectance From Skin
Yudovsky and Pilon approximate expression for the
diffuse reflectance of skin.
The transport single scattering albedos of:
Epidermis
.
Dermis
.
, Index of refraction of both layers.
Hemispherical Diffuse
Reflectance From Skin
Cont.
The transport single scattering albedo
is defined as:
Reduced Reflectance
From Human Skin
The reduced reflectance is:
The modified optical thickness of the epidermis:
Semi empirical parameter:
Diffuse Reflectance Of
Homogeneous Layer
The diffuse reflectance of semi infinite homogeneous
layer:
The refraction index of tissue
The normal normal reflectivity of tissue/air interface:
The refraction index of air n0 = 1
Inverse Method cont.
• This can be achieved by finding vector
that minimized the sum:
Where :
is the estimate spectral diffuse
reflectance.
is the weight associated with jth.
(W =2 for wavelength < 600 nm, W = 1 for > 600nm)
K is the number of wavelengths.
Wavelengths between 490-650 nm
Inverse Method cont.
• The values of the estimate vector:
Were constrained between physiologically upper and
lower bounds.
Inverse Method cont.
Six values of oxygen saturation, epidermal thickness,
melanosome volume fraction, blood volume fraction.
Three values of scattering constant (C) and b.
Total 11,664 simulated spectra using Monte Carlo.
For each spectrum the estimate vector was found by
minimizing the Eq.
Explore the effect of each parameter separately, two
scenarios were considered.
Inverse Method cont.
First scenario – Assumed that C and b were known.
Second scenario – All parameters assumed unknown.
The minimization was stopped once successive
iteration of the algorithm no longer reduced by more
than 10^-9.
Multiple random initial guesses for the estimate
vector were attempted to prevent convergence to
local minimum.
Reflectance Results
(i) Right inner forearm
Reflectance Results
(ii) Forehead over left eye
Reflectance Results
(i) Left outer forearm
Results
The reconstructed reflectance
spectra
matches the average measured reflectance within
standard deviation, for all locations and for both
populations .
For wave length lower than 600 nm, the absolute
difference was less than 0.05 for all spectra
considered.
Beyond 600 nm, the accuracy diminishes due to
week absorption by melanin and hemoglobin.
Biological properties
determined from the
reflectance spectra and
Inversed method
Conclusion From
Results
Melanin Concentration
Differences in melanin pigmentation occurred due to
racial and exposure to UV.
The value of
calculated for the left outer
forearm and forehead increased by almost identical
amounts compare to the right inner forearm for black
and white objects.
Changes in melanin concentration caused by race
and tanning can be detected by the inverse method.
Melanin concentration
Conclusion From
Results
Epidermal Thickness
Values of epidermal thickness for skin of lightly
versus strongly pigmented subjects were similar.
The estimates of epidermal thickness for all location
was consists with those reported in related work.
Spectral reflectance combined with an inverse
method can retrieve the epidermal thickness.
Epidermal Thickness
Conclusion From
Results
Blood Volume
Blood volume ranges between 0.2-7%, depends on
bodily location.
Blood volume is larger in the forehead than in the
forearm.
The retrieved values are consists for black and white
objects.
Estimate blood volume and oxygen saturation could
be retrieved from the spectral reflectance of human
skin.
Blood Volume
Conclusion From
Results
Scattering constant
The scattering power constant C ranges between
approximately
,
and power law constant b = 1.5.
Varying b between 1.3 and 1.5 influenced only the
value of constant C.
The reduced scattering coefficient was unaffected by
the choice of b for wavelength between 450 and
660nm.
The shape and magnitude of scattering coefficient is
similar to other studies.
Transport scattering
coefficients
For b =1.5 and C =
Summarize
Inverse method was applied to in vivo spectral
reflectance.
Estimate changes in tissue pigmentation due to racial
differences or exposure to UV.
Epidermal thickness from one anatomical location to
other of white and black objects .
Blood volume and oxygen saturation in the dermis of
different body location and populations.
The scattering coefficient of skin.
Summarize
Assess the capability of inversed method to retrieved
properties of the skin from.
Using in vivo experimental data from human skin and
numerically generated reflectance spectra.
Using large population.
Difference skin complextion.
Featured different locations and sun exposures.
Summarize
The proposed technique can be used to monitor
changes in skin:
Melanin volume fraction, epidermal thickness and
blood volume.
Changes occur in the normal course of aging, UV
exposure, results of diseases such as cancer,
ulceration or photo damage.
The changes affect the reflectance spectra of humen
skin.
References
 H. F. Kuppenheim and R. R. Heer, J. Appl. Physiol.
4(10), 800–806 (1952).
 V. V. Tuchin, Tissue Optics: Light Scattering Methods
and Instruments for Medical Diagnosis, (SPIE Press,
San Diego, CA, 2007).
 R. Flewelling, in: The Biomedical Engineering Handbook,
J Bronzion, Ed., (IEEE Press, Boca Roton, FL,
1981) pp. 1–11.
 L. Wang and S. L. Jacques, “Monte Carlo modeling of
light transport in multi-layered tissues in standard C”,
last accessed 3/31/2009, http://labs.seas.wustl.edu/bme/
Wang/mcr5/Mcman.pdf.
References
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Thank you!