Characterisation of Deep Layers of Tissue and Powders: Spatially Offset Raman and Transmission Raman Spectroscopy - Emerging Raman Applications and Techniques in Biomedical and Pharmaceutical Fields - page 52

SORS SORS Inverse SORS Inverse SORS

Δ s Δ s Δ s Δ s

Raman Raman laser laser laser laser Raman Raman

Fig. 3.5. Schematic diagram of conventional SORS and inverse SORS concepts
showing Raman collection and laser beam delivery geometries (reprinted with per-
mission from [34]. Copyright (2006) The Society for Applied Spectroscopy)

The required beam shape is typically generated using a conical lens (axicon)
[33] where the radius of the ring (and hence the spatial offset) is altered, over a
wide and user-selectable range, either by adjusting the distance of the axicon
from the sample [34] or by varying the magnification of a telescope located
between sample and axicon [35, 36]. The laser radiation is coupled into the
sample through a substantially larger area relative to conventional SORS. This
is particularly beneficial in situations where strict illumination intensity limits
are enforced; for example, in the use of laser radiation in potentially explosive
environments or in illumination of human skin in vivo. Additional benefits
include a reduced sensitivity to imaging imperfections in the spectrograph
and detector and the ability to tailor the radius of the ring, and hence the
spatial offset, to match the size and shape of each sample. This is not possible
with conventional ring SORS probes where the ring radii are fixed in the probe
head. The inverse SORS concept was developed independently by Matousek
[34] and Schulmerich et al. [35, 36].

3.3.4 Subsurface Mapping and Tomography

Subsurface Raman imaging can be accomplished, but with spatial resolution
ultimately limited by photon diffusion. The principles are largely the same as
in fluorescence and absorption imaging [37]. In practice the resolution is often
limited by the number and distribution of excitation sources and is perhaps
more appropriately labeled Raman mapping. Subsurface Raman mapping was
first demonstrated by Schulmerich et al. [31] using global illumination and
fibre-by-fibre display of the collected Raman scatter. It was soon realised that
better contrast and deeper penetration could be obtained by an inverse SORS
illumination scheme, which suffered less from surface weighting than the global
illumination method [20]. Proof of principle was demonstrated with polymer
block test systems, as shown in Fig. 3.6.
Diffuse Raman tomography has been demonstrated, with reconstruction
of low-definition three-dimensional maps of phantoms and of canine bone tis-
sue [38, 39]. Spatial priors (i.e. independent information on the location and