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.