Biomedical Engineering Reference
In-Depth Information
This geometry is ideally suited for breast mounts used in conventional mam-
mography where the breast is inserted between two plates which are slightly
compressed against each other diminishing its thickness to several centimetres.
Previous work adopting a transmission geometry, with Raman spectroscopy,
has demonstrated that for non-absorbing or weakly absorbing diffusely scat-
tering samples, the bias of traditional backscattering geometries to the surface
layers of the sample can be practically eliminated. This has enabled detailed
spectroscopic information relating to the interior of the sample to be obtained
[115].
TransRaman is largely insensitive to the depth from which signal is col-
lected for a given thickness of the probed medium. This leads to a relative
suppression of interference from signals emanating from surface layers of the
sample, such as melanin-induced fluorescence from skin and the depth insen-
sitivity permits the monitoring of the presence of lesions irrespective of their
depth, e.g. whether in the middle or at the opposite side of the sample, in
all the cases yielding a similar level of the Raman signal with respect to the
surrounding tissue.
Unlike SORS, the transmission geometry concept in its basic form does
not offer the ability to separate layers into individual components but instead
it yields, in a simple way, average-volume sample information superseding
that obtainable with conventional approaches [115]. Consequently, it cannot
yield the depth of the probed object. Nevertheless this information is readily
available from mammography or can be obtained using ultrasound or the
abovementioned SORS approach.
This experiment utilised the transmission Raman geometry to evaluate
its possible use as a mammographic adjunct. Polycrystalline standards were
used and placed into 2 mm thick vials with 300
m UV grade quartz windows.
Initially chicken breast and skin tissues were used in this experimental model
due to their homogenous nature. This was followed up by a more accurate
representation of the human breast using a pork phantom. Normal fresh tissue
was obtained from the abattoir and cut to the desired thickness. The samples
were stored in a refrigerator and warmed to room temperature before use. The
Raman spectra in this work were obtained using a modified SORS Raman
apparatus developed for non-invasive spectroscopy of bones described earlier
[111]. Figure 3.3, Chap. 3 shows a schematic diagram of the setup used for this
study. The probe beam was generated using a temperature stabilised diode
laser for Raman spectroscopy and operating at 827 nm (Micro Laser Systems,
Inc., L4 830S-115-TE). The laser power at the sample was about 60 mW and
the laser spot diameter before the sample was
μ
4mm.
Initial results enabled a simple discrimination between calcification types
(buried at 16 mm in chicken breast tissue) using a difference spectrum method
of analysis. Furthermore, signal could be obtained from a thin (100-300
∼
m)
powder layer placed with the tissue. This gave a relative volume of calcifica-
tions to tissue of between 0.625 and 1.875%. This compares with an approxi-
mate physiological level of around 0.05-0.14% [116].
μ
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