Objectives: Based on the pixel gray value measurements, establish a beam-hardening artifacts index of the
cone-beam CT tomographic image, and preliminarily evaluate its applicability. Methods: The 5mm-diameter metal ball and resin ball were fixed on the light-cured resin base plate respectively, while four vitro molars were fixed above and below the ball, on the left and right respectively, which have 10mm
distance with the metal ball. Then, cone beam CT was used to scan the fixed base plate twice. The same layer tomographic images were selected from the two data and imported into the Photoshop software. The circle
boundary was built through the determination of the center and radius of the circle, according to the artifact-free
images section. Grayscale measurement tools were used to measure the internal boundary gray value G0, gray value G1 and G2 of 1mm and 20mm artifacts outside the circular boundary, the length L1 of the arc with artifacts
in the circular boundary, the circumference L2. Hardening artifacts index was set A = (G1 / G0) * 0.5 + (G2 / G1) * 0.4 + (L2 / L1) * 0.1. Then, the A values of metal and resin materials were calculated respectively. Results: The A value of cobalt-chromium alloy material is 1, and resin material is 0. Conclusion: The A value reflects comprehensively the three factors of hardening artifacts influencing normal oral
tissue image sharpness of cone beam CT. The three factors include relative gray value, the decay rate and range of artifacts.
KEYWORDS: 3D scanning, 3D modeling, Laser scanners, 3D metrology, Scanners, Error analysis, Statistical analysis, Reverse modeling, Data modeling, Structured light
Objective: To evaluate the measurement accuracy of three-dimensional (3D) facial scanners for facial deformity patients from oral clinic. Methods: 10 patients in different types of facial deformity from oral clinical were included. Three 3D digital face models for each patient were obtained by three facial scanners separately (line laser scanner from Faro for reference, stereophotography scanner from 3dMD and structured light scanner from FaceScan for test). For each patient, registration based on Iterative Closest Point (ICP) algorithm was executed to align two test models (3dMD data & Facescan data) to the reference models (Faro data in high accuracy) respectively. The same boundaries on each pair models (one test and one reference models) were obtained by projection function in Geomagic Stuido 2012 software for trimming overlapping region, then 3D average measurement errors (3D errors) were calculated for each pair models also by the software. Paired t-test analysis was adopted to compare the 3D errors of two test facial scanners (10 data for each group). 3D profile measurement accuracy (3D accuracy) that is integrated embodied by average value and standard deviation of 10 patients’ 3D errors were obtained by surveying analysis for each test scanner finally. Results: 3D accuracies of 2 test facial scanners in this study for facial deformity were 0.44±0.08 mm and 0.43±0.05 mm. The result of structured light scanner was slightly better than stereophotography scanner. No statistical difference between them. Conclusions: Both test facial scanners could meet the accuracy requirement (0.5mm) of 3D facial data acquisition for oral clinic facial deformity patients in this study. Their practical measurement accuracies were all slightly lower than their nominal accuracies.
Objective: The aim of this study is to assess the accuracy of Procrustes analysis(PA)to compute a mid-sagittal
plane(MSP)of three-dimensional(3D) facial data.
Methods: Facial surface data from 30 subjects were acquired by a Face Scan optical 3D sensor. Using the data, 30
asymmetrical facial images with a true MSP for control purpose were constructed using the original facial image.
The MSPs of the 30 images are then computed using Procrustes analysis. The angle between the true-MSP and the
MSP obtained using the PA, and the asymmetry index serve as measures of assessing the accuracy of the
Procrustes analysis to compute the MSP of 3D facial data.
Results: The mean value and the standard deviation of the angle between the true-MSP and the PA-MSP are
0.62° and 0.49° respectively. The t-test for paired groups is used to assess the differences between the two MSPs in the
facial asymmetry index and P values of smaller than 0.05 are considered significant(t=0.783,p=0.440).
Conclusions: There are no significant differences between the PA-MSP and the true-MSP in terms of the
asymmetry index of the 30 subjects. Thus the Procrustes analysis can be used to compute the MSP of 3D facial
data with a significant degree of accuracy.
Objective To make a quantitative analysis between sampling frequencies and micro-movement distance of
mark points on tooth surfaces, and to provide a reference for sampling frequency settings of intraoral
scanning systems.
Methods Mark points affixed to the incisors of five subjects. In total, 3600 groups of tracking point
coordinates were obtained with frequencies of 60, 150 and 300 Hz using an optical 3D tracking system. The
data was then re-sampled to obtain coordinates at lower frequencies (5, 10, 15 and 20 Hz) at equal intervals of
groups of tracking point coordinates. Change in distance (Δd) was defined as the change in position of a
single v from one sampling time point to another, and was valued by clinical accuracy requirement
(20-100μm). The curve equation was fit quantitatively between Δd median (M) and the sampling frequency
(f). The difference between upper and lower incisor mark points were analyzed by a non-parametric test;
α=0.05.
Result When the frequency (f) was 60 Hz, upper jaw Δd median (M) and interquartile (Q) were 14.4 μm
and 9.2 μm, respectively, while the lower Δd(M) and (Q) were 6.4 μm and 10.2 μm, respectively. Every Δd
value was less than 100 μm, while 74% of Δd vales were less than 20 μm. Δd(M) and f satisfy the power
curve equation: Δd(M)=0.526×f-0.979(f∈[5,300]). Significant differences of incisor feature points were noted
between upper and lower jaws of the same subject (P<0.01).
Conclusion Clinical accuracy can be met when the sampling frequency of the intraoral scanning system is
60 Hz.
KEYWORDS: Distance measurement, Machine vision, 3D vision, Computer vision technology, 3D acquisition, 3D metrology, Digital recording, Computing systems, Dentistry, Medicine
Objectives: To quantitatively evaluate the correctness of a computer binocular vision mandibular 3D trajectory recording device.
Methods: A specialized target shooting paper was neatly pasted on a high-precision three-axis electronic translation stage. A linear one-way movement was set at a speed of 1 mm/s along the X, Y, and Z directions for a distance of 10 mm each. The coordinates of 3 pre-set target points were recorded at the start and end by a computer binocular vision system with a frequency of 10 FPS and stored in TXT format. The TXT files were imported to Imageware 13.0, and the straight-line lengths between the start and end were measured. The mean difference between each length and 10 mm were calculated to evaluate the correctness of the distance measurement. The linear movement and recording procedure was repeated 3 times, but the speed was changed to 5 mm/s to simulate the human mandibular movement speed. The trajectories of the 3 target points were fitted and the vertical dimensions from each track point to the fitted lines were measured. The mean difference was calculated between the vertical dimensions and 0 mm to evaluate the correctness of recording trajectories using this device.
Results: The correctness of distance measurements of the points 1, 2, and 3 were 0.06 mm, 0.16 mm, and 0.08 mm, respectively. The correctness of the trajectories of the points 1, 2, and 3 were 0.11 mm, 0.11 mm, and 0.10 mm, respectively.
Conclusion: Using this computer binocular vision device, the correctness of the recorded linear trajectories in the range of 10 mm was better than 0.20 mm.
The aim of this study was to evaluate the surface roughness and wettability of dentin following ultrashort pulsed laser ablation with different levels of fluence and pulse overlap (PO). Twenty-five extracted human teeth crowns were cut longitudinally into slices of approximately 1.5-mm thick and randomly divided into nine groups of five. Samples in groups 1 to 8 were ablated with an ultrashort pulsed laser through a galvanometric scanning system. Samples in group 9 were prepared using a mechanical rotary instrument. The surface roughness of samples from each group was then measured using a three-dimensional profile measurement laser microscope, and wettability was evaluated by measuring the contact angle of a drop of water on the prepared dentin surface using an optical contact angle measuring device. The results showed that both laser fluence and PO had an effect on dentin surface roughness. Specifically, a higher PO decreased dentin surface roughness and reduced the effect of high-laser fluence on decreasing the surface roughness in some groups. Furthermore, all ablated dentin showed a contact angle of approximately 0 deg, meaning that laser ablation significantly improved wettability. Adjustment of ultrashort pulsed laser parameters can, therefore, significantly alter dentin surface roughness and wettability.
The objective was to study the relationship between laser fluence and ablation efficiency of a femtosecond laser with a Gaussian-shaped pulse used to ablate dentin and enamel for prosthodontic tooth preparation. A diode-pumped thin-disk femtosecond laser with wavelength of 1025 nm and pulse width of 400 fs was used for the ablation of dentin and enamel. The laser spot was guided in a line on the dentin and enamel surfaces to form a groove-shaped ablation zone under a series of laser pulse energies. The width and volume of the ablated line were measured under a three-dimensional confocal microscope to calculate the ablation efficiency. Ablation efficiency for dentin reached a maximum value of 0.020 mm3/J when the laser fluence was set at 6.51 J/cm2. For enamel, the maximum ablation efficiency was 0.009 mm3/J at a fluence of 7.59 J/cm2. Ablation efficiency of the femtosecond laser on dentin and enamel is closely related to the laser fluence and may reach a maximum when the laser fluence is set to an appropriate value.
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