2.4 SWIR Dehydration Measurements
Time-resolved SWIR images were acquired of the 20 teeth using a benchtop
system (Fig. 2A) and the handpiece (Fig. 2A) during drying for 60
seconds at 25 psi. The benchtop SWIR reflectance images were captured
using a Model SU320KTSX (Sensors Unlimited, Princeton, NK) InGaAs camera
with a 320 x 256 pixel format, a 25 μm pixel pitch and a bit depth of 12
bit. A Navitar f=35 mm / F1.4 lens was used in combination with a 1 in
diameter f=75 mm planoconvex lens along with an adjustable aperture. The
same 1470 nm SLD was used as a light source. SWIR polarizers were placed
before the light source and camera for cross polarization imaging.
Samples were stored in a moist environment to preserve internal
hydration and the samples were immersed in a water bath before mounting
and performing measurements. A computer-controlled air nozzle with a 1
mm aperture and an air pressure set to 25 psi was positioned 3 cm away
at a 20° angle above the sample plane as shown in Fig. 2A. For the
benchtop system the tooth was held in a horizontal position and for the
handpiece the tooth was held in a vertical position.
After each sample was removed from the water bath, an image was captured
as an initial reference image and the pressurized air nozzle was
activated to dehydrate the sample. Each measurement consisted of
capturing a sequence of images at 1 frame per second for 60 seconds. For
each measurement, the air nozzle and the light source were centered on
the region of interest (ROI) that encompasses the entire sample. The
dehydration setup was completely automated using LabVIEW software from
National Instruments (Austin, TX).
A custom program written using MATLAB from Mathworks (Natick, MA) was
used to calculate DEL, ΔI, rate and %Ifin from each
curve. The first second of each acquisition was removed from each curve
because large fluctuations in intensity occur due to the removal of
water droplets on the tooth surface by the air. Differences between the
four parameters ΔI, DEL, rate and %Ifin between the
active and arrested groups were compared using unpaired two-tailed
t-tests. Prism statistical software from GraphPad Software, Inc., (La
Jolla, CA) was used for the calculations. Significance level was set at
p < 0.05.