Page 240 - Vibrational Spectroscopic Imaging for Biomedical Applications

216 Cha pte r Se v e n

Laser light

Optical
window

Stratum
corneum

Stratum
granulosum
EPIDERMIS

Stratum
spinosum
Stratum
bosale

DERMIS

FIGURE 7.14 (a) Layer structure of human skin as seen in a microscope after
staining, showing the morphology of dermis, basal layer, stratum spinosum,
stratum granulosum, and stratum corneum. Cells of the stratum corneum have
no nucleus (missing dark stain spots), and form a relatively homogeneous
optical medium well suited for Raman measurements. For visible wavelengths,
the excitation light has a penetration depth of about 400 μm, and stays within
the 0.7 to 2 mm-thick stratum corneum, as indicated.

diameter spots show that skin carotenoid concentrations vary signifi-
cantly only on a microscopic scale, but that they are rather uniform
over mm-scale spot sizes. For Raman detection of average skin carot-
enoid levels, imaging is therefore not needed. Instead, a mm-scale
beam spot size is adequate to integrate over these microscopic spatial
concentration changes, and thus to obtain a reproducible noninvasive
Raman measure of skin carotenoid content in selected tissue sites.
A recent field-usable instrument configuration that evolved out
of the development of RRS for in vivo skin carotenoid measure-
30
ments is shown in Fig. 7.16. It is based on a miniaturized, fiber-
based, computer-interfaced spectrograph with high light throughput,
31
and hand held probe module, as shown in Fig. 7.16a. For a RRS skin
carotenoid measurement, the palm of the hand is held against the
window of the probe head module and the tissue is exposed for about
10 seconds with 488-nm laser light at laser intensities of ~10 mW in a
2-mm-diameter spot. Carotenoid RRS responses are detected with a
CCD array integrated into the spectrograph. Typical skin carotenoid
RRS spectra measured in vivo, are shown in Fig. 7.16b. The raw
spectrum shown at the top of the panel (trace 1) was obtained
directly after laser exposure, and reveals a broad, featureless, strong