Accuracy assessment of prototypes produced using multi-slice and cone-beam computed tomography

Abstract

The objective of this study was to assess and quantify the dimensional error of prototypes produced using multi-slice and cone-beam computed tomography (MSCT and CBCT). Titanium screws were inserted into a dry skull at different points of the midface. The skull was scanned using MSCT (LightSpeed16 ® ) with pixel size 0.3 mm and CBCT (i-CAT Cone-Beam 3D™) with voxel sizes 0.25 and 0.4 mm. Prototypes were printed (fabricated) using a ZPrinter 310 ® device. Both the dry skull (gold standard) and the prototypes were measured using a Mitutoyo 3D coordinate measuring system with three perpendicular axes ( X , Y , and Z ). The prototype produced from MSCT data presented a mean dimensional error of 0.62%; the two models produced with CBCT images yielded errors of 0.74% with voxel size 0.25 mm and 0.82% with voxel size 0.40 mm. No significant differences in dimensional errors were observed across the prototypes ( p = 0.767; Friedman’s non-parametric test). Prototypes produced from CBCT data using voxel sizes of 0.25 and 0.4 mm, and also the one produced from MSCT data using pixel size 0.3 mm, showed acceptable dimensional errors and can therefore be used in the fabrication of prototypes in dentistry.

Rapid prototyping (RP) is defined as the production of three-dimensional (3D) physical objects (prototypes) using a virtual model and computer-aided technologies (Computer Aided Design, CAD, and Computer-Aided Manufacturing, CAM). In the field of oral and maxillofacial surgery and traumatology, RP can be used in diagnosis, to simulate osteotomies, resection techniques, and the placement of osseointegrated implants, and also in the planning and treatment of facial defects.

Volumetric data rendered with CBCT systems have been shown to provide highly accurate data compared with physical measures directly from skulls, with less than 1% relative error. The reliability of 3D surface models obtained with CBCT is similar to that of models obtained with MSCT.

CBCT is associated with lower radiation doses, lower rates of image distortion caused by metal artefacts, and reduced costs compared with MSCT, advantages that have contributed to widespread use of this technique. Studies are needed to investigate possible errors generated during CBCT image acquisition for RP. No study has reported whether errors in images acquired by CBCT may affect the use of prototypes in dentistry, or whether the dimensional reproducibility of these prototypes is comparable to measures obtained using the gold standard technique (dry skull).

The objective of the present study was to assess and quantify (using percentages) the dimensional error of prototypes of the middle third of a dry skull (gold standard) produced with MSCT and CBCT images.

Materials and methods

The methodology employed in this study consists of sequential procedures aimed at producing and measuring prototypes. It starts with the acquisition of images of a dry skull (gold standard) using MSCT and CBCT and proceeds to the analysis of the resulting replicas (prototypes).

Bone fixation was achieved using 2.0 mm titanium miniscrews (PROMM, Proto Alegre, Brazil), 7 mm long and with head diameters of 3.15 mm. Miniscrews were inserted into a dry skull at different points of the midface, both on the external cortical bone, palatal face, and alveolar ridge. The skull presented (reduced) fractures and lack of co-adaptation at some points, thus allowing a more detailed analysis of the prototypes ( Fig. 1 ).

Fig. 1
Dry skull with titanium screws inserted at different points on the midface.

The skull was placed in a plastic container and immersed in water to simulate the presence of soft tissues without affecting the calibration of the tomography devices.

MSCT images were obtained using a LightSpeed16 Multi-Slice CT Scanner ® (General Electric Medical Systems ® , Milwaukee, USA). The following image acquisition protocol was adopted: axial and helical planes, skull filter, 512.512 matrix size, slice spacing of 0.625, pixel size of 0.332, and a resolution of 3.012 pixels per mm.

CBCT images were acquired using an i-CAT Cone-Beam 3D Dental Imaging System (Imaging Sciences International, Hatfield, USA) running the Xoran reconstruction software version 3.1.62 (Xoran Technologies, Ann Arbor, USA). Two CBCT image acquisition protocols were used: 0.25 voxel size, voxel spacing of 0.25, resolution of 4 pixels per mm, and an X-ray tube current of 36 mA; and 0.4 voxel size, voxel spacing of 0.4, resolution of 2.5 pixels per mm, and an X-ray tube current of 18 mA.

Images captured using MSCT and CBCT were saved in DICOM format (digital imaging and communication in medicine) with the following parameters: 120 kVp, 16 bits per pixel, and photometric interpretations via the MonoChrome2 software.

Images were sent to the Technology Laboratory of the Product Development Department at Centro de Pesquisas Renato Archer (CenPRA, Campinas, Brazil) and converted into STL (standard template library) format using the Magics ® software (Materialise, Leuven, Belgium). Prototypes were printed (fabricated) using a ZPrinter 310 ® device (MIT, Burlington, USA).

Following fabrication, both the dry skull and the prototypes were measured using a Mitutoyo 3D coordinate measuring system model B231 (Mitutoyo Sul Americana Ltda., São Paulo, Brazil), with a measurement uncertainty of ± 0.005 mm.

Dimensions were defined geometrically in the 3D space, characterized by three perpendicular coordinate axes: Z (vertical axis); X (horizontal axis); and Y (anteroposterior axis) ( Fig. 2 ). All calculations were based on analytical geometry (i.e. vectors). The origin of the object was point 1, where the three coordinates corresponded to 0.000; all the remaining points were calculated thereafter.

Fig. 2
Measurement of one of the prototypes.

The dimensional accuracy of the prototypes was determined by comparing coordinates between key landmarks observed on the dry skull and on the prototypes produced with MSCT and CBCT. The relative difference (%) between measurements obtained on the skull and on the prototypes was calculated based on the studies of Choi et al., Silva et al., and Ibrahim et al., considering the mean of 20 repetitions for each measurement, using the formula:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Mean relative difference(%)=prototype measurement−skull measurement×100skull measurement’>Mean relative difference(%)=prototype measurementskull measurement×100skull measurementMean relative difference(%)=prototype measurement−skull measurement×100skull measurement
Mean relative difference ( % ) = prototype measurement − skull measurement × 100 skull measurement

Results

Mean dimensional errors (%) obtained for the three prototypes in relation to the gold standard (dry skull) on different measurement points and axes are described in Table 1 .

Table 1
Mean dimensional errors (%) observed for the three prototypes in relation to dry skull measurements (gold standard).
MSCT CBCT 0.25 voxel size CBCT 0.4 voxel size
X Y Z X Y Z X Y Z
Point 1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Point 2 4.9 3.2 −1.3 2.5 4.6 4.9 3.3 4.3 5.4
Point 3 0.0 1.9 0.3 0.0 1.0 −0.3 0.0 1.9 0.4
Point 4 −1.0 1.7 2.0 −0.3 1.0 −7.0 1.6 0.0 2.6
Point 5 1.0 1.9 1.4 0.5 1.8 −2.1 1.9 0.2 3.8
Point 6 5.9 1.5 0.0 −5.9 3.2 0.0 0.0 0.9 0.0
Point 7 1.5 1.5 −1.2 0.7 1.8 1.8 4.1 1.7 −0.7
Point 8 −1.0 1.5 −3.3 6.9 1.2 2.2 0.2 −0.1 −0.2
Point 9 0.8 2.5 0.1 2.1 0.7 0.3 1.6 0.2 −0.6
Point 10 3.6 2.6 −2.5 6.8 4.2 −1.3 4.4 2.4 −0.5
Point 11 0.6 −2.1 −0.4 0.3 −3.8 2.1 −0.9 −0.6 2.1
Point 12 −0.4 0.8 1.8 0.5 0.6 1.7 0.4 1.0 1.4
Point 13 0.1 −1.5 −1.3 −0.4 1.0 −0.9 −1.1 0.2 −1.3
Point 14 2.5 0.3 1.0 3.2 −0.4 0.0 2.3 −0.5 0.0
Point 15 −2.7 2.4 −1.6 −3.4 0.3 1.1 0.3 −0.7 2.9
Point 16 1.4 2.2 −5.3 3.0 1.8 −6.4 1.5 0.6 −6.6
Point 17 0.7 3.1 3.7 0.4 3.0 4.8 0.9 3.3 0.0
Point 18 1.2 2.6 −5.0 2.1 3.7 −5.3 3.4 1.7 −4.4
Mean 1.1 1.4 −0.6 1.0 1.4 −0.2 1.3 0.9 0.2
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Jan 26, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Accuracy assessment of prototypes produced using multi-slice and cone-beam computed tomography
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