Biomedical Engineering Reference
In-Depth Information
Depth Probing with Kerr-Gated Raman Spectroscopy
Raman signal from deep within tissues tends to be diminished by elastic scat-
tering and therefore the surface signal is usually significantly stronger. A pos-
sible way to overcome this is by temporal-gating techniques such as picosec-
ond Kerr gating. The Kerr-gating technique uses excitation with a picosecond
pulsed laser, in combination with a fast temporal gating of the Raman scat-
tered light. The scattered light is collected at various time delays following
the laser pulse.
In this way Raman photons originating from differing depths within the
tissue will emerge at different times thereby making their separation in the
temporal domain [108]. It is therefore possible to detect underlying structures
through overlying tissue by the rejection of early photons. There have been
studies utilising the Kerr-gating technique to improve upon Raman spectra ob-
tained from bone interiors [103, 109] as well as test experiments demonstrating
the depth resolving power of the Raman-Kerr-gating concept on artificially
prepared samples.
We performed a study using Kerr-gated surface signal suppression for deep
probing of tissue calcifications in the breast [110] The Kerr-gated experimen-
tal setup is described elsewhere [108]. Experiments were performed to obtain
Raman spectra that could distinguish between type I (COD) and type II
(HAP/COHAP) calcifications. The samples were placed into quartz cuvettes
(windows 300
m thick) and measured alone (for 10 s with five acquisitions
and three cycles) and through sections of chicken breast and fatty tissue;
and through normal and cancerous human breast tissue (20 s with 10 acqui-
sitions and three cycles) to demonstrate the ability to obtain Raman spec-
tra from tissue layers. The recovered spectrum was found to be comparable
to the spectrum from pure hydroxyapatite [110]. There was some mixing of
cancerous breast tissue and HAP signals; however, this was not a problem for
discrimination between buried calcium oxalate and HAP.
μ
Spatially Offset Raman Spectroscopy (SORS) for Deep Probing
of Calcifications
SORS follows a much simpler approach than Kerr-gated RS. It uses continuous
wave laser light and relies on moving the area of collection of the scattered
light away from the laser illuminated zone. Apart from being substantially
less complex in terms of instrumentation, it is also compatible with the use of
lower power continuous wave laser beams and is therefore less likely to induce
any laser radiation-induced changes in the tissue. The technique is based on
the collection of Raman spectra from regions spatially offset, by different
amounts, from the point of incidence of the probe laser beam on the surface
of a diffusely scattering sample. Crucially, such an array of spectra contains
different relative Raman contributions from the surface and sub-surface layers.
Chapter 3 contains more explanation of this approach.
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