Development and validation of a 3D anthropomorphic phantom for dental CBCT imaging research

BACKGROUND

Optimization of dental cone beam computed tomography (CBCT) imaging is still in a preliminary stage and should be addressed using task-based methods. Dedicated models containing relevant clinical tasks for image quality studies have yet to be developed.

PURPOSE

To present a methodology to develop and validate a virtual adult anthropomorphic voxel phantom for use in task-based image quality optimization studies in dental CBCT imaging research, focusing on root fracture (RF) detection tasks in the presence of metal artefacts.

METHODS

The phantom was developed from a CBCT scan with an isotropic voxel size of 0.2 mm, from which the main dental structures, mandible and maxilla were segmented. The missing large anatomical structures, including the spine, skull and remaining soft tissues, were segmented from a lower resolution full skull scan. Anatomical abnormalities were absent in the areas of interest. Fine detailed dental structures, that could not be segmented due to the limited resolution and noise in the clinical data, were modelled using a-priori anatomical knowledge. Model resolution of the teeth was therefore increased to 0.05 mm. Models of RFs as well as dental restorations to create the artefacts, were developed, and could be inserted in the phantom in any desired configuration. Simulated CBCT images of the models were generated using a newly developed multi-resolution simulation framework that incorporated the geometry, beam quality, noise and spatial resolution characteristics of a real dental CBCT scanner. Ray-tracing and Monte Carlo techniques were used to create the projection images, which were reconstructed using the classical FDK algorithm. Validation of the models was assessed by measurements of different tooth lengths, the pulp volume and the mandible, and comparison with reference values. Additionally, the simulated images were used in a reader study in which two oral radiologists had to score the realism level of the model's normal anatomy, as well as the modelled RFs and restorations.

RESULTS

A model of an adult head, as well as models of RFs and different types of dental restorations were created. Anatomical measurements were consistent with ranges reported in literature. For the tooth length measurements, the deviations from the mean reference values were less than 20%. In 77% of all the measurements, the deviations were within 10.1%. The pulp volumes, and mandible measurements were within one standard deviation of the reference values. Regarding the normal anatomy, both readers considered the realism level of the dental structures to be good. Background structures received a lower realism score due to the lack of detailed enough trabecular bone structure, which was expected but not the focus of this study. All modelled RFs were scored at least adequate by at least one of the readers, both in appearance and position. The realism level of the modelled restorations was considered to be good.

CONCLUSIONS

A methodology was proposed to develop and validate an anthropomorphic voxel phantom for image quality optimization studies in dental CBCT imaging, with a main focus on RF detection tasks. The methodology can be extended further to create more models representative of the clinical population.

Filtered back projection vs. iterative reconstruction for CBCT: effects on image noise and processing time

OBJECTIVES

To assess the effect of standard filtered back projection (FBP) and iterative reconstruction (IR) methods on CBCT image noise and processing time (PT), acquired with various acquisition parameters with and without metal artefact reduction (MAR).

METHODS

CBCT scans using the Midmark EIOS unit of a human mandible embedded in soft tissue equivalent material with and without the presence of an implant at mandibular first molar region were acquired at various acquisition settings (milliamperages [4mA-14mA], FOV [5 × 5, 6 × 8, 9 × 10 cm], and resolutions [low, standard, high] and reconstructed using standard FBP and IR, and with and without MAR. The processing time was recorded for each reconstruction. ImageJ was used to analyze specific axial images. Radial transaxial fiducial lines were created relative to the implant site. Standard deviations of the gray density values (image noise) were calculated at fixed distances on the fiducial lines on the buccal and lingual aspects at specific axial levels, and mean values for FBP and IR were compared using paired t-tests. Significance was defined as p < 0.05.

RESULTS

The overall mean for image noise (± SD) for FBP was 198.65 ± 55.58 and 99.84 ± 16.28 for IR. IR significantly decreased image noise compared to FBP at all acquisition parameters (p < 0.05). Noise reduction among different scanning protocols ranged between 29.7% (5 × 5 cm FOV) and 58.1% (5mA). IR increased processing time by an average of 35.1 s.

CONCLUSIONS

IR significantly reduces CBCT image noise compared to standard FBP without substantially increasing processing time.

Influence of CBCT parameters on image quality and the diagnosis of vertical root fractures in teeth with metallic posts: an ex vivo study

Objectives

The aim of this study was to evaluate the influence of peak kilovoltage (kVp) and a metal artifact reduction (MAR) tool on image quality and the diagnosis of vertical root fracture (VRF) in cone-beam computed tomography (CBCT).

Materials and Methods

Twenty single-rooted human teeth filled with an intracanal metal post were divided into 2 groups: control (n = 10) and VRF (n = 10). Each tooth was placed into the socket of a dry mandible, and CBCT scans were acquired using a Picasso Trio varying the kVp (70, 80, 90, or 99), and the use of MAR (with or without). The examinations were assessed by 5 examiners for the diagnosis of VRF using a 5-point scale. A subjective evaluation of the expression of artifacts was done by comparing random axial images of the studied protocols. The results of the diagnoses were analyzed using 2-way analysis of variance and the Tukey post hoc test, the subjective evaluations were compared using the Friedman test, and intra-examiner reproducibility was evaluated using the weighted kappa test (α = 5%).

Results

The kVp and MAR did not influence the diagnosis of VRF (p > 0.05). According to the subjective classification, the 99 kVp protocol with MAR demonstrated the least expression of artifacts, while the 70 kVp protocol without MAR led to the most artifacts.

Conclusions

Protocols with higher kVp combined with MAR improved the image quality of CBCT examinations. However, those factors did not lead to an improvement in the diagnosis of VRF.

Influence of Voxel Size on CBCT Images for Dental Implants Planning

Objective  This study was developed to evaluate the influence of voxel size on bone measurements for implant planning.

Materials and Methods  The research was performed by using edentulous synthetic human mandibles with different levels of bone resorption. For each mandible, height and bone thickness were measured with a digital caliper. The PaX-i3d device was used to acquire the volumes of the five mandibles, with 50kVp, 4 mA, and a voxel size of 0.08 mm. After the acquisition, the images were reconstructed in the software CS three-dimensional Imaging, with four different sizes of voxels: 0.1, 0.2, 0.3, and 0.4 mm. All volumes were analyzed by a single evaluator who performed measurements to obtain bone height and thickness, using the reference points that were considered in obtaining the gold standard. The data were analyzed by ANOVA with a significance level of 5%.

Results  There was no significant difference in the measurements obtained with different voxel sizes, both for bone height measurements and bone thickness. There was no statistically significant difference in measurements in thickness in comparison to the gold standard.

Conclusion  When necessary, to measure height and bone thickness, it is possible to recommend voxel images of larger size (0.40 mm) without compromising the quality of the patient's clinical planning.

Characteristics of radiographic images acquired with CdTe, CCD and CMOS detectors in skull radiography

Purpose

The purpose of this study was to evaluate the image quality, diagnostic efficacy, and radiation dose associated with the use of a cadmium telluride (CdTe) detector, compared to charge-coupled device (CCD) and complementary metal oxide semiconductor (CMOS) detectors.

Materials and Methods

Lateral cephalographs of a phantom (type 1) composed of synthetic polymer filled with water and another phantom (type 2) composed of human skull macerated with polymer coating were obtained with CdTe, CCD, and CMOS detectors. Dosimeters placed on the type 2 phantom were used to measure radiation. Noise levels from each image were also measured. McNamara cephalometric analysis was conducted, the dentoskeletal configurations were assessed, and a subjective evaluation of image quality was conducted. Parametric data were compared via 1-way analysis of variance with the Tukey post-hoc test, with a significance level of 5%. Subjective image quality and dentoskeletal configuration were described qualitatively.

Results

A statistically significant difference was found among the images obtained with the 3 detectors (P<0.05), with the lowest noise level observed among the images obtained with the CdTe detector and a higher subjective preference demonstrated for those images. For the cephalometric analyses, no significant difference (P>0.05) was observed, and perfect agreement was seen with regard to the classifications obtained from the images acquired using the 3 detectors. The radiation dose associated with the CMOS detector was higher than the doses associated with the CCD (P<0.05) and CdTe detectors (P<0.05).

Conclusion

Considering the evaluated parameters, the CdTe detector is recommended for use in clinical practice.

Development of a model of soft tissue simulation using ballistic gelatin for CBCT acquisitions related to dentomaxillofacial radiology research

OBJECTIVES

To present the ballistic gelatin as a new material capable of simulating the soft tissues in cone-beam CT (CBCT) images.

METHODS

CBCT images of three piglet heads were acquired with their soft tissues intact (standard group). Subsequently, the piglet heads were fixed in a container using metallic pins and moulded with acrylic resin; the soft tissues were then removed and replaced by ballistic gelatin, with the same thickness of the original soft tissues. The images were evaluated by two oral radiologists, to check the adaptation on bone surfaces, thickness and density, penetration into large bone cavities and cancellous bone, and the presence of air bubbles using a 5-score scale. Additionally, an objective analysis was carried out by one oral radiologist. For each CBCT scan, three axial reconstructions were selected to represent the mandibular, occlusal, and maxillary levels. The mean and standard deviation (SD) of the grey values were calculated in four regions of interest determined on soft tissue areas and compared by two-way ANOVA.

RESULTS

The ballistic gelatin showed subjective scores ranging from good to excellent for all parameters evaluated. There was no significant difference in the mean and SD values of the grey values between ballistic gelatin and the gold standard groups for all levels (p > 0.05). Higher SD values were observed in the occlusal level for both groups (p < 0.05).

CONCLUSIONS

Ballistic gelatin has visual and objective similarity with the gold standard. Thus, the ballistic gelatin is a promising material capable of simulating soft tissues in CBCT images.

Analysis of the deterioration of photostimulable phosphor plates

Objectives:

To assess, objectively to determine the deterioration of the photostimulable phosphor plates (PSPs) after several acquisitions.

Methods:

Two new PSPs without preliminary use were exposed with an intraoral X-ray unit, and the EXPRESS® (Instrumentarium Imaging, Tuusula, Finland) unit was used for scanning of the PSPs. A 12-stepwedge aluminum scale were used superimposed with the PSPs during the X-rays exposures to perform the objective analyses. Objective analysis was carried out by mean gray values using the Image J software through a region of interest (ROI) of 0.1 × 0.1 mm, and the data were analyzed statistically using ANOVA.

Results:

For each, the Express® PSP a total of 1800 images were acquired during 60 days. The objective analysis showed loss of the mean gray values between the initial and final images. Besides, the percentage of mean gray values’ loss was between 0.7%, for the thickest step, and 8.4%, for the lowest thickness step.

Conclusion:

After many X-ray exposures of the Express® PSP a singular deterioration in the mean gray values could be observed.

Impact of movement and motion-artefact correction on image quality and interpretability in CBCT units with aligned and lateral-offset detectors

OBJECTIVES

To evaluate the impact of movement and motion-artefact correction systems on CBCT image quality and interpretability of simulated diagnostic tasks for aligned and lateral-offset detectors.

METHODS

A human skull simulating three diagnostic tasks (implant planning in the anterior maxilla, implant planning in the left-side-mandible and mandibular molar furcation assessment in the right-side-mandible) was mounted on a robot performing six movement types. Four CBCT units were used: Cranex 3Dx (CRA), Ortophos SL (ORT), Promax 3D Mid (PRO), and X1. Protocols were tested with aligned (CRA, ORT, PRO, and X1) and lateral-offset (CRA and PRO) detectors and two motion-artefact correction systems (PRO and X1). Movements were performed at one moment-in-time (t₁), for units with an aligned detector, and three moments-in-time (t₁-first-half of the acquisition, t₂-second-half, t₃-both) for the units with a lateral-offset detector. 98 volumes were acquired. Images were scored by three observers, blinded to the unit and presence of movement, for motion-related stripe artefacts, overall unsharpness, and interpretability. Fleiss' κ was used to assess interobserver agreement.

RESULTS

Interobserver agreement was substantial for all parameters (0.66-0.68). For aligned detectors, in all diagnostic tasks a motion-artefact correction system influenced image interpretability. For lateral-offset detectors, the interpretability varied according to the unit and moment-in-time, in which the movement was performed. PRO motion-artefact correction system was less effective for the offset detector than its aligned counterpart.

CONCLUSION

Motion-artefact correction systems enhanced image quality and interpretability for units with aligned detectors but were less effective for those with lateral-offset detectors.

Quantitative assessment of variation in CBCT image technical parameters related to CBCT detector lateral-offset position

OBJECTIVES

To assess the effect of CBCT detector position (aligned/lateral-offset) on image technical parameters (mean voxel value - MVV, standard deviation of voxel value (SDVV) distribution), comparing peripheral regions of interest (ROIs) to the central ROI in CBCT volumes.

METHODS

40 CBCT volumes of a wax phantom were acquired in six units with aligned and/or lateral-offset detectors: Cranex 3Dx (CRA), Ortophos SL (ORT), Picasso Trio (PIC), Promax 3D Mid (PRO), Scanora 3D (SCA), and X1. Four image-acquisition protocols used an aligned detector, and four a lateral-offset detector. In each volume, 13 ROIs (12 peripheral and 1 central) were evaluated. MVV and SDVV of the peripheral ROI were compared to those of the central ROI in the volume. MVVD (the difference in percentage, between the MVV of a peripheral and the central ROI) was calculated.

RESULTS

For aligned-detectors, MVV increased (ORT and PRO) or decreased (CRA and X1) in the ROIs farther from the centre. For lateral-offset detectors, ROIs farther from the centre showed increased MVV. SDVV for most aligned detectors was lower, the nearer the ROI was to the centre. For lateral-offset detectors, it was lower for the peripheral ROIs, except with PIC. Range for MVVD was -32.8% to 22.8% for units with aligned detectors, and -20.7% to 69.5% for lateral-offset detectors.

CONCLUSION

Lateral-offset detectors to acquire CBCT images significantly change SDVV distribution within the field-of-view, and lead to MVVD with increased range, compared to aligned detectors. This must be taken in consideration in the clinic, if voxel-value dependent measurements are to be performed.

Difference in the artefacts production and the performance of the metal artefact reduction (MAR) tool between the buccal and lingual cortical plates adjacent to zirconium dental implant

OBJECTIVES

To quantify the artefacts production and the performance of the metal artefact reduction (MAR) tool, enabled before or after the acquisition, in cortical plates (buccal and lingual) and other regions adjacent to zirconium implants.

METHODS

Cone beam CT scans were acquired using the OP300 Maxio unit before (control group) and after (implant group) the insertion of a zirconium implant into the posterior region of a dry mandible. Three conditions of MAR tool were tested: "without MAR", with "MAR activated after acquisition", and with "MAR activated before acquisition". The standard deviation (SD), contrast-to-noise ratio (CNR) and voxel values were calculated in the buccal and lingual cortical plates, medullary bone and water region, close to the implant. The structural bone analysis was performed in the medullary bone close to the implant.

RESULTS

Overall, in control and implant groups, the lingual cortical had higher SD, lower CNR and lower voxel values than the buccal cortical, regardless of the MAR condition (p < 0.05). Implant caused higher SD values and lower voxel values in adjacent regions "without MAR" (p < 0.05). MAR activation decreased SD and changed voxel values when the implant was present, regardless of MAR activation mode (p < 0.05). The activation of MAR increased the trabecular thickness values for the implant group (p < 0.05).

CONCLUSION

The expression of artefacts adjacent to zirconium implants is greater in the lingual than in the buccal cortical. The greater the expression of artefacts in this region, the greater the effectiveness of the MAR tool in homogenizing the grey values, regardless of the time of its activation.

The influence of different scan resolutions on the detection of proximal caries lesions

Purpose

This study was conducted to evaluate the effect of different spatial resolutions of a photostimulable phosphor plate (PSP) radiography system on the detection of proximal caries lesions.

Materials and Methods

Forty-five extracted human permanent teeth were radiographed using a PSP system (VistaScan Perio Plus) and scanned at the 4 resolutions (10 lp/mm, 20 lp/mm, 25 lp/mm, and 40 lp/mm) available in the system. Three independent examiners scored the images for the presence and absence of proximal caries lesions using a 5-point scale. The presence or absence of caries was confirmed by histological sections of the examined teeth (defined as the gold standard). Intra- and inter-observer reproducibility was calculated by the weighted kappa test. One-way analysis of variance with the post hoc Tukey test was used to compare the area under the receiver operating characteristic curve for the classifications made with each resolution.

Results

For the detection of enamel lesions, the spatial resolution of 10 lp/mm was significantly superior to the other resolutions. However, the spatial resolution did not affect the detection of caries lesions in dentin (P>0.05).

Conclusion

Spatial resolution may influence the accuracy of the detection of incipient caries lesions in radiographs with PSP plates. Images with low spatial resolution seem to be more appropriate for this purpose.

Influence of scan mode (partial/full rotations) and FOV size in the formation of artefacts in cone beam CT

OBJECTIVE

To evaluate the influence of rotation degree and field of view (FOV) size on the amount of artefacts produced in cone beam CT (CBCT) images.

METHODS

A cylindrical wax utility phantom, with a metallic sample in its interior, was scanned with two FOV sizes (100 x 100 and 40 x 40 mm) and in full (360°) and partial (~180°) rotations. After the acquisitions, images were objectively assessed in the ImageJ software, obtaining the standard deviation in areas around the metal sample. The influence of artefacts produced by the several FOVs and rotation degrees was compared by two-way analysis of variance (α = 0.05).

RESULTS

The images obtained with a large FOV presented a higher amount of noise compared to a restricted FOV, both for partial (p = 0.0037) and full (p = 0.0023) rotation degrees. For images obtained with a restricted FOV, full rotation resulted in images with less noise (p = 0.0259). For images obtained with large FOV, there was no statistically significant difference (p = 0.1494) in noise for both rotation protocols.

CONCLUSIONS

As there were no significant differences in the amount of artefacts in rotation protocols for large FOVs, the partial rotation can be indicated due to its lower exposure to radiation. For acquiring images with restricted FOVs, the full rotation is recommended.

Evaluation of different soft tissue-simulating materials in pixel intensity values in cone beam computed tomography

OBJECTIVE

This study aimed to evaluate different materials as soft tissue simulators and the influence of soft tissues in cone beam computed tomography.

STUDY DESIGN

Images of 5 piglet heads were acquired with intact soft tissues, with the tissues stripped, and with the use of different soft tissue simulators, following the same acquisition protocol. Four different materials were tested, individually or in combination: acrylic, water, utility wax, and expanded polystyrene (EPS). Pixel intensity values of 8 quadrangular regions, that is, upper and lower teeth and alveolar bone, were obtained. The mean values were used for comparison by analysis of variance (ANOVA; α = 5%).

RESULTS

No differences were observed for the "No Material," "EPS," "Acrylic," and "EPS and Wax" groups for the lower anterior and posterior teeth, the upper posterior tooth, and the anterior and posterior bone, and for the "No Material," "EPS," and "EPS and Wax," groups for the lower posterior bone. All groups showed statistical differences for the lower anterior bone and the upper anterior tooth.

CONCLUSIONS

Expanded 2-cm thick polystyrene, with or without 1-cm thick utility wax, was effective for most regions, followed by acrylic 0.5 cm. Soft tissues were not of great influence in most regions. Water was not an effective material for any of the regions.

Influence of VistaScan image enhancement filters on diagnosis of simulated periapical lesions on intraoral radiographs

OBJECTIVE:

To evaluate whether image enhancement filters of VistaScan system improve the diagnostic accuracy of simulated periapical lesions.

METHODS:

10 sockets were prepared on bovine ribs to fit a bovine tooth. Bone defects were created and successively enlarged providing four groups (n = 10): Group 0, absence of lesions; Group 1, periapical lesions with 1.6  mm in diameter; Group 2, with 1.8  mm in diameter; and Group 3, with 2.1  mm in diameter. Periapical radiographs were taken using a photostimulable storage phosphor plate and DBSWIN software. VistaScan filters were applied and the images were allocated into seven groups: Nonfiltered, Fine, Caries 1, Caries 2, Endodontic, Periodontal and Noise Reduction. All the 280 images were assessed about the presence or absence of periapical lesions. Pixel intensities standard deviation were compared between nonfiltered and filtered images. Two-Way Analysis of Variance and the post hoc Tukey's test were used to compare area under the ROC curve, sensitivity and specificity.

RESULTS:

VistaScan filters showed no significant difference for area under receiver operating characteristic curve (p = 0.124), sensitivity (p = 0.835) and specificity (p = 0.832). Area under receiver operating characteristic curve (p = 0.000) and sensitivity (p = 0.000) in 2.1  mm lesions size were significantly higher than in 1.6  mm and 1.8  mm lesions size. Pixel intensities standard deviation was significantly changed in the filtered images compared to nonfiltered ones (p < 0.01), except for Fine in the bone region (p > 0.05).

CONCLUSION:

VistaScan enhancement filters do not influence the diagnostic accuracy of simulated periapical lesions. On the other hand, larger lesions were more frequently detected. The filters change the pixel intensities reducing or intensifying the differences between similar regions.

Metal artifact production and reduction in CBCT with different numbers of basis images

Purpose

To evaluate the effect of different numbers of basis images and the use of metal artifact reduction (MAR) on the production and reduction of artifacts in cone-beam computed tomography images.

Materials and Methods

An acrylic resin phantom with a metal alloy sample was scanned, with 450 or 720 basis images and with or without MAR. Standard deviation values for the test areas (around the metal object) were obtained as a way of measuring artifact production. Two-way analysis of variance was used with a 5% significance level.

Results

There was no significant difference in artifact production among the images obtained with different numbers of basis images without MAR (P=.985). MAR significantly reduced artifact production in the test areas only in the protocol using 720 basis images (P=.017). The protocol using 450 basis images with MAR showed no significant difference in artifact production when compared to the protocol using 720 basis images with MAR (P=.579).

Conclusion

Protocols with a smaller number of basis images and with MAR activated are preferable for minimizing artifact production in tomographic images without exposing the patient to a greater radiation dose.

Comparison of automatic and visual methods used for image segmentation in Endodontics: a microCT study

To calculate root canal volume and surface area in microCT images, an image segmentation by selecting threshold values is required, which can be determined by visual or automatic methods. Visual determination is influenced by the operator's visual acuity, while the automatic method is done entirely by computer algorithms.