A comparative study of new and current methods for dental micro-CT image denoising

Reconstruction of Connected Digital Lines Based on Constrained Regularization

This paper presents a new approach for reconstruction of disconnected digital lines (DDLs) based on a constrained regularization model which ensures connectivity of the digital lines (DLs) in the discrete image plane. The first step in this approach is to determine the order of given pixels of the DDL. To determine connectivity of pixels, we use the usual 8-neighbor connectivity in discrete images. For any neighboring pixels of the DDL that are not connected, we determine a number of new pixel values that need to be reconstructed between these pixels. Next, the integer-valued x - and y -coordinates of the location of the pixels of the DDLs are segregated into two 1D signal vectors. Then the x - and y -coordinates of the missing pixels of the DDLs are estimated using a new constrained regularization. While the solution of this constrained minimization problem provides real values for the x - and y -coordinates of pixels positions, the imposed constraint ensures connectivity of the resulting DLs in the image plane after transforming the computed values from [Formula: see text] to [Formula: see text]. The proposed regularization approach forces connected lines with small curvature. The experimental results demonstrate that the proposed technique improves DL intersection detection, as well. Moreover, this technique has a high potential to be used as a fast approach in binary image inpainting particularly overcoming the shortcomings of conventional methods which cause destruction of thin objects and blurring in the recovered regions.

Influence of reconstruction parameters of micro-computed tomography on the analysis of bone mineral density

Purpose

This study was conducted evaluate the influence of reconstruction parameters of micro-computed tomography (micro-CT) images on bone mineral density (BMD) analyses.

Materials and Methods

The sample consisted of micro-CT images of the maxillae of 5 Wistar rats, acquired using a SkyScan 1174 unit (Bruker, Kontich, Belgium). Each acquisition was reconstructed following the manufacturer's recommendations (standard protocol; SP) for the application of artifact correction tools (beam hardening correction [BHC], 45%; smoothing filter, degree 2; and ring artifact correction [RAC], level 5). Additionally, images were reconstructed with 36 protocols combining different settings of artifact correction tools (P0 to P35). BMD analysis was performed for each reconstructed image. The BMD values obtained for each protocol were compared to those obtained using the SP through repeated-measures analysis of variance with the Dunnett post hoc test (α=0.05).

Results

The BMD values obtained from all protocols that used a BHC of 45% did not significantly differ from those obtained using the SP (P>0.05). The other protocols all yielded significantly different BMD values from the SP (P<0.05). The smoothing and RAC tools did not affect BMD values.

Conclusion

BMD values measured on micro-CT images were influenced by the BHC level. Higher levels of BHC induced higher values of BMD.

Evaluation of a low-dose protocol for cone beam computed tomography of the temporomandibular joint

OBJECTIVE

Evaluation of cone beam CT (CBCT) examination with a low-dose scanning protocol for assessment of the temporomandibular joint (TMJ).

METHODS

34 adult patients referred for CBCT imaging of the TMJ underwent two examinations with two scanning protocols, a manufacturer-recommended protocol (default) and a low-dose protocol where the tube current was reduced to 20% of the default protocol. Three image stacks were reconstructed: default protocol, low-dose protocol, and processed (using a noise reduction algorithm) low-dose protocol. Four radiologists evaluated the images. The Sign test was used to evaluate visibility of TMJ anatomic structures and image quality. Receiver operating characteristic analyzes were performed to assess the diagnostic accuracy. κ values were used to evaluate intraobserver agreement.

RESULTS

With the low-dose and processed protocols, visibility of the TMJ anatomical structures and overall image quality were comparable to the default protocol. No significant differences in radiographic findings were found for the two low-dose protocols compared to the default protocol. The area under the curves (A_z) averaged for the low-dose and processed protocols, according to all observers, were 0.931 and 0.941, respectively. Intraobserver agreement was good to very good.

CONCLUSION

For the CBCT unit used in this study, the low-dose CBCT protocol for TMJ examination was diagnostically comparable to the manufacturer-recommended protocol, but delivered a five times lower radiation dose. There is an urgent need to evaluate protocols for CBCT examinations of TMJ in order to optimize them for a radiation dose as low as diagnostically acceptable (the as low as diagnostically acceptable principle recommended by NCRP).

A quality improvement method for lung LDCT images

BACKGROUND

Low dose computed tomography (LDCT) reduces radiation damage to patients. However, with the decrease of radiation dose, LDCT images of the lung often appear some serious problems such as poor contrast, noise and streak artifacts.

OBJECTIVE

To improve the quality of lung LDCT images, this study proposed and investigated a new denoising method based on classification training structure combined dictionary for lung LDCT images.

METHODS

First, top-hat transform and anisotropic diffusion with a shock filter (ADSF) algorithm are used to enhance the image contrast and image details. Second, an adaptive dictionary is trained and used for noise reduction. Third, more image details are extracted from the residual image by using the atom activity measurement. The final result is obtained by combining the dictionary denoising result with the extracted detail information. The proposed method is then validated by both simulated and clinical lung LDCT images. Four metrics including Contrast-to-Noise Ratio (CNR), Noise Suppression Index (NSI), Edge Preserving Index (EPI), and Blurring Index (BI) are computed to quantitatively evaluate image quality.

RESULTS

The results showed that the CNR, NSI, EPI, and BI of our method reached 8.953, 0.9500, 0.7230 and 0.0170, respectively. Noise and streak artifacts can be removed from lung LDCT images while keeping and retaining more details.

CONCLUSIONS

Comparing with the results of other methods tested using the same dataset, this study demonstrated that our new method significantly improved quality of the lung LDCT images.

Comparison between micro-computed tomography and transverse microradiography of sound dentine treated with fluorides and demineralized by microcosm biofilm

The study aimed to apply micro-computed tomography (micro-CT) and transverse microradiography (TMR) to measure dentine demineralization and to test the preventive effect of titanium tetrafluoride (TiF₄ ) under microcosm biofilm. Sound dentine specimens from bovine root were treated for 6 h with: (i) 4.0% titanium tetrafluoride (TiF₄ ) varnish [pH 1.0, 2.45% fluoride (F-); (ii) 5.42% sodium fluoride (NaF) varnish (pH 5.0, 2.45% F); (iii) 2% chlorhexidine (CHX) gel (pH 7.0); (iv) placebo varnish (pH 5.0); or (v) no agent (untreated). Dentine specimens were then exposed to human saliva mixed with McBain saliva for 8 h. Thereafter, McBain saliva containing 0.2% sucrose was applied daily, for 5 d, onto dentine specimens to stimulate formation of microcosm biofilm. Although a high correlation was found between the results of both methods regarding integrated mineral loss, the results of the methods did not show good agreement in Bland-Altman plots, with significant biases in calculations of lesion depth. Fluoride varnishes were able to reduce dentine demineralization (P < 0.05), while CHX failed to do so. Fluorides are still the best option to reduce dentine demineralization. Micro-CT may be used to measure dentine mineral loss, but not the lesion depth, for which TMR is superior.

Effect of micro-computed tomography reconstruction protocols on bone fractal dimension analysis

OBJECTIVES

To evaluate the influence of the level of three micro-CT reconstruction tools: beam-hardening correction (BHC), smoothing filter (SF), and ring artefact correction (RAC) on the fractal dimension (FD) analysis of trabecular bone.

METHODS

Five Wistar rats' maxillae were individually scanned in a SkyScan 1174 micro-CT device, under the following settings: 50 kV, 800 µA, 10.2 µm voxel size, 0.5 mm Al filter, rotation step 0.5°, two frames average, 180° rotation and scan time of 35 min. The raw images were reconstructed under the standard protocol (SP) recommended by the manufacturer, a protocol without any artefact correction tools (P0) and 35 additional protocols with different combinations of SF, RAC and BHC levels. The same volume of interest was established in all reconstructions for each maxilla and the FD was calculated using the Kolmogorov (box counting) method. One-way ANOVA with Dunnet's post-hoc test was used to compare the FD of each reconstruction protocol (P0-P35) with the SP (α = 5%). Multiple linear regression verified the dependency of reconstruction tools in FD.

RESULTS

Overall, FD values are not dependent on RAC (p = 0.965), but increased significantly when the level of BHC and SF increased (p < 0.001). FD values from protocols with BHC at 45% combined with SF of 2, and BHC at 30% combined with SF of 4 or 6 had no statistical difference compared to SP.

CONCLUSIONS

BHC and SF tools affect the FD values of micro-CT images of the trabecular bone. Therefore, these reconstruction parameters should be standardized when the FD is analyzed.

Modelling of stress distribution and fracture in dental occlusal fissures

The aim of this study was to investigate the fracture behaviour of fissural dental enamel under simulated occlusal load in relation to various interacting factors including fissure morphology, cuspal angle and the underlying material properties of enamel. Extended finite element method (XFEM) was adopted here to analyse the fracture load and crack length in tooth models with different cusp angles (ranging from 50° to 70° in 2.5° intervals), fissural morphologies (namely U shape, V shape, IK shape, I shape and Inverted-Y shape) and enamel material properties (constant versus graded). The analysis results showed that fissures with larger curved morphology, such as U shape and IK shape, exhibit higher resistance to fracture under simulated occlusal load irrespective of cusp angle and enamel properties. Increased cusp angle (i.e. lower cusp steepness), also significantly enhanced the fracture resistance of fissural enamel, particularly for the IK and Inverted-Y shape fissures. Overall, the outcomes of this study explain how the interplay of compositional and structural features of enamel in the fissural area contribute to the resistance of the human tooth against masticatory forces. These findings may provide significant indicators for clinicians and technicians in designing/fabricating extra-coronal dental restorations and correcting the cuspal inclinations and contacts during clinical occlusal adjustment.

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

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.

Synthesis of stabilized hydroxyapatite nanosuspensions for enamel caries remineralization

BACKGROUND

The aim of this study was to develop and evaluate a method for synthesizing a stable suspension of hydroxyapatite nanoparticles and to test its efficacy for remineralizing carious enamel lesions.

METHODS

Hydroxyapatite (HA) particles were synthesized using wet chemistry. Synthesized particles were introduced into a high-pressure homogenizer (5-10 homogenization passes at 15 000 psi) in the presence of different stabilizers. Size and distribution of the resultant particles were determined using dynamic light scattering (DLS). The morphology and composition of the nanoparticles were determined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Subsequently, artificial lesions were treated with HA nanosuspension plus artificial saliva or a fluoride-containing artificial saliva only. Visual analysis and quantification of the lesion mineral density before and after remineralization were performed using microcomputed tomography.

RESULTS

DLS and SEM results confirmed the formation of nonagglomerated HA nanoparticles (20-40 nm) following high-pressure homogenization treatment. Quantitative evaluation of the lesions showed that remineralization of the lesion with hydroxyapatite nanosuspension led to a significantly higher level of mineral gain compared to the control group (P < 0.05).

CONCLUSION

High-pressure homogenization is an effective method for facile preparation of a stable suspension of hydroxyapatite nanoparticles. Treatment of artificial lesions with nonagglomerated spherical HA nanoparticles improves the remineralization of enamel lesion.

Missing Surface Estimation Based on Modified Tikhonov Regularization: Application for Destructed Dental Tissue

Estimation of missing digital information is mostly addressed by one or two-dimensional signal processing methods; however, this problem can emerge in multi-dimensional data including 3D images. Examples of 3D images dealing with missing edge information are often found using dental micro-CT, where the natural contours of dental enamel and dentine are partially dissolved or lost by caries. In this paper, we present a novel sequential approach to estimate the missing surface of an object. First, an initial correct contour is determined interactively or automatically, for the starting slice. This contour information defines the local search area and provides the overall estimation pattern for the edge candidates in the next slice. The search for edge candidates in the next slice is performed in the perpendicular direction to the obtained initial edge in order to find and label the corrupted edge candidates. Subsequently, the location information of both initial and nominated edge candidates are transformed and segregated into two independent signals (X-coordinates and Y-coordinates) and the problem is changed into error concealment. In the next step, the missing samples of these signals are estimated using a modified Tikhonov regularization model with two new terms. One term contributes in the denoising of the corrupted signal by defining an estimation model for a group of mildly destructed samples, and the other term contributes in the estimation of the missing samples with the highest similarity to the samples of the obtained signals from the previous slice. Finally, the reconstructed signals are transformed inversely to edge pixel representation. The estimated edges in each slice are considered as initial edge information for the next slice and this procedure is repeated slice by slice until the entire contour of the destructed surface is estimated. The visual results as well as quantitative results (using both contour-based and area-based metrics) for seven image datasets of tooth samples with considerable destruction of the dentin-enamel junction (DEJ) demonstrates that the proposed method can accurately interpolate the shape and the position of the missing surfaces in computed tomography images in both two and three dimensions (e.g. 14.87 ±3.87 μ m of mean distance (MD) error for the proposed method versus 7.33 ±0.27 μm of MD error between human experts and 1.25 ±~0 % error rate (ER) of the proposed method versus 0.64 ±~0 % of ER between human experts (~1% difference)).

Efficacy of Fluoride Varnishes with Added Calcium Phosphate in the Protection of the Structural and Mechanical Properties of Enamel

The aim of this study was to investigate the efficacy of various fluoride varnishes in the protection of the structural and nanomechanical properties of dental enamel. Demineralized enamel specimens were imaged using a high-resolution micro-CT system and lesion parameters including mineral density and lesion depth were extracted from mineral density profiles. Nanoindentation elastic modulus and hardness were calculated as a function of penetration depth from the load-displacement curves. The average depth of the lesion in specimens with no prior fluoride varnish treatment was 86 ± 7.19 μm whereas the varnish treated specimens had an average depth of 67 ± 7.03 μm (P < 0.05). The mineral density of enamel lesions with no fluoride varnish treatment had an average of 1.85 gr/cm3 which was 25% lower than the corresponding value in varnish treated enamel and 37% lower than sound enamel. While, in the varnish treated group, elastic modulus and hardness values had decreased by 18% and 23%, respectively, the corresponding values in the non-varnish treated specimens had a reduction of 43% and 54% compared to the sound enamel. The findings from this study highlight the preventive role of fluoride varnishes. Addition of calcium and phosphate does not seem to enhance or inhibit the prevention or remineralization performance of fluoride varnishes.

Quantitative characterization and micro-CT mineral mapping of natural fissural enamel lesions

OBJECTIVES

The aim of this study was to characterize the mineral distribution pattern of natural fissural enamel lesions and to quantify structural parameters and mineral density of these lesions in comparison to proximal white spot enamel lesions.

METHODS

Imaging was undertaken using a high-resolution desktop micro-computed tomography system. A calibration equation was used to transform the grey level values of images into true mineral density values. The value of lesion parameters including the mineral density and the thickness of the surface layer of the enamel lesion were extracted from mineral density profiles.

RESULTS

The thickness of the surface layer showed variation among different lesions and it ranged from 0-90 μm in proximal lesions and 0-137 μm in fissural lesions. The average thickness of surface layer in fissural lesions was significantly higher than smooth surface proximal lesions. Sound fissural enamel showed lower mineral density compared to proximal enamel.

CONCLUSION

Micro-CT and the suggested de-noising and visualization method provide an efficient high-resolution approach for non-destructive evaluation of fissural lesions. Using these methods, the current study revealed the exclusive pattern and structure of fissural enamel lesions which may provide a basis for future studies on prevention and remineralization of these lesions.

CLINICAL SIGNIFICANCE

The common demineralization pattern of fissural lesions, which indicates the extension of the lesion in two directions towards the pulp horns, may explain the early inflammation and symptoms of the pulp in fissural lesions even when the lesion base appears far from the pulp roof in normal radiographs. In addition, the presence of the surface layer, indicates that vigorous probing of the occlusal fissures may lead to breakage and cavitation of the enamel lesions.