Microcomputed Tomography Calibration Using Polymers and Minerals for Enamel Mineral Content Quantitation

Abstract

Objective: The aim of this paper was to develop calibration standards (CSs) that are readily available for clinical researchers for the quantitation of enamel mineral content.

Method: Polyethylene terephthalate (PET), acetal, polyphenylene sulfide (PPS), selenite, Egyptian alabaster, aragonite, and fluorite were fashioned into discs, and their densities were measured and stacked for microcomputed tomography examination. Frame averaging, flat-field correction, pre-filtration, and beam-hardening correction were applied. CSs were checked for homogeneity. The linear relationship between the mean greyscale value (GSV) of each disc and its physically calculated density was explored, and reproducibility was tested. A calibration function was established and then validated using a bovine enamel disc and sound enamel of extracted human premolar teeth.

Results: Measured densities were PET (ρ = 1.38 g/cm3), acetal (ρ = 1.41 g/cm3), PPS (ρ = 1.64 g/cm3), selenite (ρ = 2.24 g/cm3), Egyptian alabaster (ρ = 2.7 g/cm3), aragonite (ρ = 2.72 g/cm3), and fluorite (ρ = 3.11 g/cm3). Examination of the profile sections of CSs confirmed the uniformity of GSVs with minimal beam-hardening effect. A squared Pearson correlation coefficient of R2 = 0.994 was determined between the mean GSV of each CS and its calculated density and was reproduced at different settings with R2 >0.99. A linear regression equation of density (y) versus GSV (x) was established using the least squares regression equation method. The estimated density of the bovine enamel disc (2.48 g/cm3) showed high accuracy when compared to the physically measured value (2.45 g/cm3). The -relative error was 1.2%. Densities of sound enamel in the extracted human premolar teeth were 2.6-3.1 g/cm3.

Conclusions: This is a simple, valid, and reproducible method to quantitate enamel mineral content. This simple, yet accurate system could be used to expand knowledge in the field of enamel caries research.

Keywords: Calibration standards; Early enamel lesions; Microcomputed tomography; Minerals; Polymers.

Fig. 1

CSs setup for micro-CT examination. a The seven tested CSs (from bottom to top): A: PET; B: acetal; C: PPS; D: selenite; E: Egyptian alabaster; F: aragonite; and G: fluorite. b The three chosen CSs (A: PET; D: selenite; and G: fluorite) scanned with an extracted human tooth.

Fig. 2

Defect analysis performed on created ROI containing only enamel. Descriptive statistics of GSVs were calculated for the entire volume of intact enamel. a Top-view workplace 2D window displaying defect analysis performed on extracted enamel (area colored red; color in online version only). Surface determination function enabled us to define intact enamel and thus early enamel lesion present was excluded from analysis. b Same as a, but a front-view workplace 2D window. c Same as a, but a right-view workplace 2D window. d Same as a, but 3D window.

Fig. 3

Influence of pre-filtration and beam-hardening correction during reconstruction. a Profile window of fluorite disc scanned with radiation energy (110 kV, 160 µA) with the application of both a Cu fil­ter and the beam-hardening correction module during the reconstruction process. b Same as a, but without application of a Cu filter nor the beam-hardening correction module. c Same as a, but with Cu pre-filtration and without application of the beam-hardening correction module.

Fig. 4

Test of linearity between the mean GSVs versus physically calculated density. a Scatter plot of mean GSV versus calculated density for the seven CSs (A: PET; B: acetal; C: PPS; D: selenite; E: Egyptian-alabaster; F: aragonite; and G: fluorite). Squared Pearson correlation coefficient and calibration function are displayed. b Same as a, but for the three chosen CSs (A: PET; D: selenite; and G: fluorite).