Page 142 - Vibrational Spectroscopic Imaging for Biomedical Applications

118 Cha pte r F o u r

MCT focal plane array and the source emanates from a step-scan
interferometer. Tisinger and Sommer built an identical system for the
infrared region using an MCT array, a zinc selenide prism, and a
step-scan interferometer. 44,47 The amide I image of a fingerprint
collected with the device is illustrated in Fig. 4.10, which demonstrates
the instrument’s capabilities. For this system, the smallest spatial
domain sampled is dependent on the magnification from the sample
to detector and the size of the individual pixels on the array. The total
area sampled (field of view, FOV) is dependent on the size of the
array. For example, Tisinger and Sommer employed a 1:1 magnification
from sample to detector and a 64 × 64 element array with a pixel size
of 61 × 61 μm. In this particular case, the pixel resolution of the
instrument was 61 μm with a FOV of approximately 4 × 4 mm. Tisinger
later improved on the pixel resolution by replacing the zinc selenide
44
prism with a hemisphere. Tisinger demonstrated that a magnifica-
tion factor of 2.4 could be achieved with a resultant pixel resolution
of 25.4 μm. However, the FOV was reduced by a similar factor to
~1.6 × 1.6 mm.
At that time, the standard MCT detector was a 64 × 64 array
with a 61 x 61 μm pixel size, but larger arrays with smaller pixel
sizes soon became available thereby improving both the pixel reso-
lution and the total area sampled. Marcott later reported on the use
of Harrick Fast-IR accessory with a zinc selenide prism, but with a
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256 × 256 array possessing a pixel size of 40 × 40 μm. He was able
to increase the FOV to 7.5 × 7.5 mm with a pixel resolution of 30 μm.
Following the concept of Tisinger, the prism could be replaced with
a zinc selenide or a germanium hemisphere which would increase
the pixel resolution to 12.5 and 7.5 μm, respectively. In effect, micro-
scopic measurements over a large area could be conducted with
these instruments without the use of a microscope. Chan and Kazarian
have investigated this potential principally for the study of pharma-
ceuticals using a SPECAC Golden Gate accessory with a diamond
49
ATR element. The choice of diamond is a compromise between
FOV, pixel resolution, intimate contact considerations and IRE
longevity. Diamond has an identical refractive index to that of zinc
selenide, but is more robust. However, due to cost, the size of the
IRE and thus the FOV are limited. The smaller IRE area and the
greater penetration depth are less problematic when it comes to
achieving intimate contact with the sample. An average penetration
depth for diamond and germanium at 6 μm wavelength is 1.4 μm
and 0.4 μm, respectively.
Whether or not these devices could be employed for the diagnosis of
disease states in biopsied samples remains to be seen. Tissue biopsies are
rather small and the spatial resolution required for an analysis should
be very high. Other applications of these devices include the investi-
gation of skin surfaces, materials applied to skin, and transdermal
drug uptake.