The skin
Biomedical Optics
Tissue optical properties
E. Göran Salerud
Department Biomedical Engineering
Skin Model 1
I ( λ) = I0 ( λ ) ⋅ Rd ( λ) ⋅ e−2ε m ( λ) c m lm −2ε h ( λ) c h lh
Skin Model 2
ITOT = I0 ( R1 + T12 ⋅ R2 + T12 ⋅ T22 ⋅ R3 + T12 ⋅ T22 ⋅ T32 ⋅ R4 )
Epidermis
•
Epidermis
The total optical absorption coefficient (µa.epi) of the epidermis
depends on a minor baseline skin absorption and a dominant
melanin absorption due to the melanosomes in the epidermis.
Bloodless rat skin
•
Baseline absorption coefficient of melaninless epidermis
Neonatal skin
•
Absorption coefficient of a single melanosome
•
Volume fraction of melanosomes in epidermis
•
Net epidermal absorption coefficient
Epidermis
Based on vaporization
Net epidermal absorption coefficient of
epidermis
Skin type
Fraction melanin
µa.epi = (f.mel)(µa.mel) + (1 - f.mel)( µa.skinbaseline)
light-skinned adults
f.mel = 1.3-6.3%
f.mel ≈ 10%
moderately pigmented adults
f.mel = 11-16%
wavelength [nm]
µa.skinbaseline [cm-1]
µa.mel [cm-1]
µa.epi [cm-1]
darkly pigmented adults
f.mel = 18-43%
694 nm
0.268
228
23
755 nm
0.254
172
17
1064 nm
0.244
55
5.7
µa.epi = (f.mel)( µa.mel) + (1 - f.mel)( µa.skinbaseline)
Absorpion coefficient of epidermis
Scattering coefficient of the epidermis
• There are some difference between the scattering
coefficient of the epidermis μs_epi and of the dermis, μs_derm
but not large
• μs_epi is set equal to μs_derm
• The second major scattering property of a tissue is its
anisotropy, g, with typical values of g in the range of 0.70.95 for skin tissue, and vary with wavelength.
• µs’_epi = (μs_epi)(1-g)
ECE532 Biomedical Optics
©Steven L. Jacques, Scott A.
Prahl
Oregon Graduate Institute
Dermis
Absorption coefficient of whole blood
• The total optical absorption coefficient (µa.derm) of the
dermis depends on a minor baseline skin absorption
and a dominant hemoglobin absorption due to the
cutaneous blood perfusion.
• Baseline absorption coefficient of bloodless dermis
•
µa.skinbaseline
• Absorption coefficient of whole blood
• µa.blood
courtesy V. Venugopalan, http://www.osa.org/meetings/archives/2004/BIOMED/program/#educ
Absorption coefficient of whole blood
Dermis
•
The total optical absorption coefficient (µa.derm) if the dermis is
perfused with blood
•
Absorption coefficient of dermis perfused with blood
• µa.derm=(fblood)(µa.blood) + (1-fblood)(µa.skinbaseline)
at isosbestic point,
µ a = 0.023 mM × 0.09 mm-1 / mM = 0.002 mm-1
Mean free absorption pathlength = 500 mm (!)
Dermis
• The reduced scattering of dermis, μs’_derm,
• is a combination of the contributions due to Mie scattering
by the large cylindrical dermal collagen fibers and the
• Rayleigh limit scattering by the small-scale structure
associated with the collagen fibers and other cellular
structures
• Using Mie theory, one can calculate the scattering
coefficient, μs_derm, and the anisotropy, gderm, then calculate
the μs’_derm.
• The epidermis with its keratin fibers appears to behave
somewhat like dermis, and μs’_epi is tentatively approximated
by the μs’_derm.
Scattering coefficients of dermis
µs’_Mie.fibers (l) = (2x105)(l-1.5) [cm-1]
µs’_Rayleigh(l) = (2x1012)(l-4) [cm-1]
µs’(l) = µs’_Rayleigh(l) + µs’_Mie.fibers (l)
Absorption coefficients in skin
4
µs’
3
Attenuation coefficients
Reduced scattering coefficient
3
µa
2
2
1
1
0
600
650
700
750
800
850
900
950
1000
1050
Wavelength [nm]
ECE532 Biomedical Optics
©Steven L. Jacques, Scott A.
Prahl
Oregon Graduate Institute
CONCLUSION
• The absorption is described in terms of melanin and
hemoglobin absorption and proportional to the
volume fractions of melanosomes and whole blood,
with a slight baseline skin absorption.
• The dermal scattering is described in terms of the
relative contributions of Mie and Rayleigh scattering
due to collagen fibers.
• The epidermal scattering, which is affected by its
keratin fibers, is sufficiently close to that of dermis
and sufficiently thin to not be critical.
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