Volumetric analysis of the mandibular condyle using cone beam computed tomography

Objective

The aim was to determine the accuracy of volumetric analysis of the mandibular condyle using cone-beam computed tomography (CBCT).

Materials and methods

Five dry mandibles containing 9 condyles were used. CBCT scans of the mandibles and an impression of each condylar area were taken. The physical volumes of the condyles were calculated as the gold standard using the water displacement technique. After isolating, the condylar volume was sectioned in the sagittal plane, and 0.3 mm thick sections with 0.9 mm intervals were obtained from 3D reconstructions. Using the Cavalieri principle, the volume of each condyle was estimated from the CBCT images by three observers. The accuracy of the CBCT volume measurements and the relation agreements between the results of the three observers were assessed using the Wilcoxon Signed Rank test and Pearson correlation test. The level of statistical significance was set at 0.05.

Results

The results of the Pearson correlation showed that there were highly significant positive correlations between the observers’ measurements. According to the results of the Wilcoxon Signed Rank test comparing the physical and observers’ measurements, there were no statistically significant differences (p > 0.05).

Conclusion

The Cavalieri principle, used in conjunction with a planimetry method, is a valid and effective method for volume estimation of the mandibular condyle on CBCT images.     

Bifid mandibular canals: cone beam computed tomography evaluation

Objectives

For surgical procedures involving the posterior mandible, it is important to be familiar with the details of the bifid mandibular canal. To our knowledge, there have been no systematic studies evaluating the bifid mandibular canal using sectional imaging. The purpose of this study is to evaluate the incidence and configuration of the bifid mandibular canal using cone beam CT.

Methods

252 patients (301 mandible sides) underwent cone beam CT between October 2004 and September 2005 and were included in this study. The cone beam CT images were evaluated for the presence and configuration of the bifid mandibular canal. The patterns of bifurcation were classified into four types according to the classification of Nortjé et al (Variations in the normal anatomy of the inferior dental (mandibular) canal: a retrospective study of panoramic radiographs from 3612 routine dental patients. Br J Oral Surg 1977; 15: 55–63). The diameter of the accessory canal was classified into two categories: 50% or more and less than 50% of the diameter of the main mandibular canal.

Results

Of the 301 subjects, 47 (15.6%) demonstrated a bifid mandibular canal. They were Type I in 2, Type II in 40, Type III in 0, and Type IV in 5 cases. The diameter of the accessory canal was greater than or equal to 50% of the main canal in 23, and less than 50% in 24 cases.

Conclusions

On the basis of the cone beam CT, a bifid mandibular canal was found in 15.6% of cases, a markedly higher proportion than found in previous reports using panoramic images. Cone beam CT is considered a suitable modality for detailed evaluation of bifid mandibular canals.     

Diagnostic value of cone beam computed tomography (CBCT) in occult scaphoid and wrist fractures

ObjectiveEvaluate the diagnostic value of cone beam computed tomography (CBCT) for scaphoid and wrist fractures that are missed on standard radiographs.Materials and methodsBetween September 2014 and October 2015, we prospectively enrolled 49 patients with a clinically suspected scaphoid fracture following an acute injury but had normal radiographs. Each patients underwent radiographs, CBCT and (magnetic resonance imaging) MRI within 7 days of the initial injury event. Both exam were evaluated independently by two radiologists.ResultsFor scaphoid cortical fractures CBCT sensitivity is 100% (95% CI: 75%–100%), specificity 97% (95% CI: 83%–100%). CBCT diagnosed all 24 corticals wrist fractures, corresponding to a sensitivity of 100% (95% CI: 83%–100%), specificity of 95% (95% CI: 75%–100%). Kappa agreement rate between the two radiologists was K = 0.95 (95% CI: 0.85–1) for scaphoid fractures and K = 0.87 (95% CI: 0.73–1) for wrist fractures.ConclusionsCBCT is superior to radiographs for diagnosing occult cortical fractures. Because of its low radiation dose, we believe that CBCT can be used in current practice as a replacement or supplement to radiographs to detect these fractures and optimize the cost-effectiveness ratio by limiting the number of needless immobilizations.     

Optimization of exposure parameters for cone beam computed tomography sialography

Objectives

The assessment of image quality is a crucial step in the development of a new imaging protocol. Having proposed and reported on a preliminary protocol for sialography using cone beam CT (CBCT), the purpose of this study was to further optimize this protocol by maximizing the image signal difference-to-noise ratio (SDNR) and to relate these new data to previously published dosimetric data for CBCT sialography.

Methods

An imaging phantom was constructed using samples with different concentrations of iodine and a water-immersed mandible. The CB MercuRay (Hitachi Medical Systems, Tokyo, Japan) was used to image the phantom using different peak kilovoltage (kVp) and milliamperage (mA) settings. SDNR was then calculated using the raw images based on mean pixel values (MPV) measured in selected regions of interest (ROI). Finally, a figure of merit (FOM) was calculated to examine the trade-off between image SDNR and effective radiation dose.

Results

The SDNR demonstrated an expected increase as the kVp increased from 60 to 120. Also, images made with the higher mA setting (15) had greater SDNR. The iodine concentration also influenced the image quality such that SDNR increased with increased amounts of iodine. The calculated FOM was greatest for the technique using 80 kVp, with equivalent results for 10 mA and 15 mA.

Conclusion

An optimized protocol for CBCT sialography using CB MercuRay entails a 6 inch field of view with 80 kVp and 10 mA.     

Dimensional stability in composite cone beam computed tomography

An automated increase in the field of view (FOV) for multipurpose cone beam CT (CBCT) by "stitching" (joining) up to three component volumes to yield a larger composite volume must still ensure dimensional stability, especially if the image is to form the basis for a surgical splint. Dimensional stability, image discrepancies and the influence of movement artefacts between exposures were evaluated. The first consumer installation of the Kodak 9000 three-dimensional (3D) extraoral imaging system with stitching software was used for the evaluation of a human mandible with three endodontic instruments as markers. The distances between several reproducible points were measured directly and the results compared with the values measured on screen. Displacements of the mandible along all axes between exposures as well as angular displacements were conducted to test the capability of the system. The standard deviations (SD) of the results for the vertical distances varied between 0.212 mm and 0.409 mm (approximately 1-2 voxels; range, 0.6-1.3 mm) and may be considered the systematic error. The SD of the results for the horizontal and diagonal distances varied between 0.195 mm and 0.571 mm (approximately 1-3 voxels; range, 0.6-1.7 mm) if the group with overall horizontal angulations of 10° and a central rotation of 20° was omitted. In conclusion, the evaluated stitching software is a useful tool to expand the options of combined CBCT with an initial small FOV by allowing a merger of up to three component volumes to yield a larger FOV of about 80 × 80 × 37 mm. The dimensional stability was acceptable when seen in relation to the induced disturbance. Further evaluation of this composite CBCT/digital imaging and communications in medicine system for subsequent splint fabrication may yield promising results.     

The usage of digital radiography and cone beam computed tomography among Turkish dentists

Objectives

The aim of this study was to determine the prevalence of the use of digital radiography and report how it was used by Turkish dentists.

Methods

The survey was based on 383 dentists who were present at the sixteenth International Congress organized by the Turkish Dental Association. A questionnaire which consisted of 19 questions was given to the dentists who participated in the study. Data were assessed according to frequency distribution and the χ² test was used to determine the significance of differences between two independent groups.

Results

376 questionnaires were analysed. The mean age of the dentists who participated in the study was 37.14 ± 9.6 years (range: 20–63 years). The distribution of the dentists according to the 7 regions of Turkey were: Marmara 58%, Aegean 13% and Mediterranean 10%, Central Anatolia 8%, Black Sea 7%, Southeastern Anatolia 3%, Eastern Anatolia 1%. 124 dentists (33%) said they did not work with digital radiography. 95 dentists indicated that they did not use digital radiography owing to cost (60%). 252 dentists (67%) said they used digital radiography. 40% of the participants noted that the repetition of periapical radiographs was due to digital radiography. 166 dentists (55.9%) and 79.1% academicians had knowledge about cone beam CT (CBCT).

Conclusion

Digital radiography users are increasing in Turkey and levels of knowledge of CBCT and awareness of radiation safety has also increased.     

Deriving Hounsfield units using grey levels in cone beam computed tomography

Objectives

An in vitro study was performed to investigate the relationship between grey levels in dental cone beam CT (CBCT) and Hounsfield units (HU) in CBCT scanners.

Methods

A phantom containing 8 different materials of known composition and density was imaged with 11 different dental CBCT scanners and 2 medical CT scanners. The phantom was scanned under three conditions: phantom alone and phantom in a small and large water container. The reconstructed data were exported as Digital Imaging and Communications in Medicine (DICOM) and analysed with On Demand 3D® by Cybermed, Seoul, Korea. The relationship between grey levels and linear attenuation coefficients was investigated.

Results

It was demonstrated that a linear relationship between the grey levels and the attenuation coefficients of each of the materials exists at some “effective” energy. From the linear regression equation of the reference materials, attenuation coefficients were obtained for each of the materials and CT numbers in HU were derived using the standard equation.

Conclusions

HU can be derived from the grey levels in dental CBCT scanners using linear attenuation coefficients as an intermediate step.     

Accuracy assessment of the axial images obtained from cone beam computed tomography

Objective

The aim of this study was to evaluate accuracy of linear measurements assessed from axial tomograms and the influence of the use of different protocols in two cone beam CT (CBCT) units.

Methods

A cylinder object consisting of Nylon® (Day Brazil, Sao Paulo, Brazil) with radiopaque markers was radiographically examined applying different protocols from NewTom 3G^TM (Quantitative Radiology s.r.l, Verona, Veneto, Italy) and i-CAT^TM (Imaging Sciences International, Hatfield, PA) units. Horizontal (A–B) and vertical (C–D) distances were assessed from axial tomograms and measured using a digital calliper that provided the gold standard for actual values.

Results

There were differences when considering acquisition protocols to each CBCT unit. Concerning all analysed protocols from i-CAT^TM and Newtom 3G^TM, both A–B and C–D distances presented underestimated values. Measurements of the axial images obtained from NewTom 3G^TM (6 inch 0.16 mm and 9 inch 0.25 mm) were similar to the ones obtained from i-CAT^TM (13 cm 20 s 0.3 mm, 13 cm 20 s 0.4 mm and 13 cm 40 s 0.25 mm).

Conclusion

The use of different protocols from CBCT machines influences linear measurements assessed from axial images. Linear distances were underestimated in both equipments. Our findings suggest that the best protocol for the i-CAT^TM is 13 cm 20 s 0.3 mm and for the NewTom 3G^TM, the use of 6 inch or 9 inch is recommended.     

Dental students' knowledge and attitudes towards cone beam computed tomography in Turkey

Objective

The aim of this study was to evaluate students'' knowledge and attitudes regarding cone beam CT (CBCT).

Methods

A questionnaire consisting of 11 questions was given to 472 dental students (280 pre-graduate and 192 post-graduate) at two institutions located in Ankara, Turkey (Ankara University, Faculty of Dentistry, and Gazi University, Faculty of Dentistry). Differences in responses by institution, education level and sex were statistically assessed with the χ² test.

Results

Statistical results showed that only 63.3% of students had heard of CBCT. Of these, 59.9% said they had learned about CBCT in their classes, 31.0% in seminars and 20.9% from the internet; 76.8% felt that CBCT was not given adequate coverage in their courses; 69% thought that CBCT should be taught as part of their clinical education; 91% thought it essential for CBCT to be available at dental faculties; 53.5% believed that the use of CBCT would become more widespread in the near future; and 84.9% wished to use CBCT technology in their future careers.

Conclusion

This research suggests that efforts should be made to improve students'' knowledge base regarding CBCT and that the dental school curriculum should devote more curriculum time to this promising new technology.     

Comparison of two methods for quantitative assessment of mandibular asymmetry using cone beam computed tomography image volumes

Objectives

The aim of this study was to compare two methods of measuring mandibular asymmetry. The first method uses mirroring of the mandible in the midsagittal plane; the second uses mirroring of the mandible and registration on the cranial base.

Methods

Surface models were constructed from cone beam CT (CBCT) scans of 50 patients with asymmetry. For the first approach, a midsagittal plane was defined for each patient as the plane passing through nasion, anterior nasal spine and basion. Mirrors for both halves of the mandible were created. The second approach consisted of mirroring the image volume by flipping the left and right sides and then registering the mirrored image onto the cranial base using a mutual information maximization method. Surface distances between hemimandibles and mirrors were calculated for nine regions.

Results

There was no statistically significant difference between the mean surface distance measurements obtained with the two approaches and when comparing both halves in most areas.

Conclusion

Both mirroring techniques provided similar quantification of mandibular asymmetry in this cohort.     

Biomonitoring of DNA damage and cytotoxicity in individuals exposed to cone beam computed tomography

Objectives

The aim of the present study was to evaluate DNA damage (micronucleus) and cellular death (pyknosis, karyolysis and karyorrhexis) in exfoliated buccal mucosa cells from adults following cone beam CT exposure.

Methods

A total of 19 healthy adults (10 men and 9 women) submitted to cone beam CT were included.

Results

No significant statistically differences (P > 0.05) in micronucleus frequency were seen before and after cone beam CT exposure. In contrast, the tomography was able to increase other nuclear alterations closely related to cytotoxicity such as karyorrhexis, pyknosis and karyolysis (P < 0.05).

Conclusion

In summary, these data indicate that cone beam CT may not be a factor that induces chromosomal damage, but it is able to promote cytotoxicity.     

Accuracy of chemically created periapical lesion measurements using limited cone beam computed tomography

Objectives

The aim was to assess the accuracy and reproducibility of measurements of chemically created periapical lesions using limited cone beam CT.

Methods

Periapical lesions were chemically created in 18 mandibular cadaver teeth. Mandibles were dissected buccolingually using a bone-cutting burr. Diameters and depths were measured directly in the cross-sectional slices using a precision digital caliper. The cross-sectional slices were then embedded in wax, and cone beam CT images were acquired using a NewTom® 3G Plus scanner with both 6 inch and 9 inch fields of view (FOVs). Two oral radiologists measured the diameter and depth of periapical lesions on the cross-sectional images using the built-in measurement tools. Measurements were repeated after a 1 week interval. Inter- and intraobserver agreement was calculated by ANOVA. Regression analysis was used to test the correlation between the cone beam CT and digital caliper measurements.

Results

No significant differences were found in diameter or depth measurements between or within observers or between 6 inch and 9 inch FOV images. Regression analysis of diameter and depth measurements made by direct caliper versus 6 inch or 9 inch FOV images revealed a high regression coefficient (for diameter: 6 inch FOV, R² _ 94.6%; 9 inch FOV, R² _ 94.8%; P<0.001; for depth: 6 inch FOV, R² _ 99.3%; 9 inch FOV, R² _ 99.3%; P<0.001) showing a strong linear relationship. For the diameter, the mean deviation from direct caliper measurements was 0.0625 mm and 0.08958 mm, respectively; for the 6 inch FOV and 9 inch FOV images, and for depth, the mean deviation was, respectively, −0.1001 mm and 0.09875 mm.

Conclusions

Cone beam CT yielded highly accurate and reproducible results in the quantitative assessment of periapical lesions.     

The estimation of the volume of sheep mandibular defects using cone-beam computed tomography images and a stereological method

Objective: The Cavalieri principle of stereological methods is widely used to estimate the volume of structures. Recently in clinical practice, it has become common to use this approach for daily routine purposes. The Cavalieri principle provides quantitative and unbiased volume estimates which are independent of the observer. In the present study, the efficacy of using the Cavalieri principle to estimate the volume of sheep mandibular defects on cone beam CT (CBCT) scans was tested.Methods: 24 differently sized defects were created on 4 sheep mandibles. Before the defects were created, the outer boundaries of the defects were determined using plaster casts. CBCT scans of the defects were taken. The scans were reconstructed in the coronal plane and sections of 0.2 mm thickness with 0.2 mm and 0.4 mm intervals were obtained. The volume of each defect was estimated using the Cavalieri principle. The models were created using light-body silicone for the estimation of the actual volume of each defect. They were immersed in water using a pycnometer and the actual volumes were obtained on the basis of the Archimedean principle. The actual and estimated volumes of the defects were compared using the Wilcoxon signed-rank test.Results: The results showed that the volumes from the Cavalieri estimates did not differ from the actual volumes of the defects (P > 0.05).Conclusion: We concluded that the volume of mandibular defects can be accurately estimated using the Cavalieri principle on images from a CBCT scan.     

Novel information theory based method for superimposition of lateral head radiographs and cone beam computed tomography images

Objectives

The aim was to introduce a novel alignment criterion, focus mutual information (FMI), for the superimposition of lateral cephalometric radiographs and three dimensional (3D) cone beam computed images as well as the assessment of the alignment characteristics of the new method and comparison of the novel methodology with the region of interest (ROI) approach.

Methods

Implementation of a FMI criterion-based methodology that only requires the approximate indication of stable structures in one single image. The robustness of the method was first addressed in a phantom experiment comparing the new technique with a ROI approach. Two consecutive cephalometric radiographs were then obtained, one before and one after functional twin block application. These images were then superimposed using alignment by FMI where the following were focused on, in several ways: (1) cranial base and acoustic meatus, (2) palatal plane and (3) mandibular symphysis. The superimposed images were subtracted and coloured. The applicability to cone beam CT (CBCT) is illustrated by the alignment of CBCT images acquired before and after craniofacial surgery.

Results

The phantom experiment clearly shows superior alignment when compared to the ROI approach (Wilcoxon n = 17, Z = −3.290, and P = 0.001), and robustness with respect to the choice of parameters (one-sample t-test n = 50, t = −12.355, and P = 0.000). The treatment effects are revealed clearly in the subtraction image of well-aligned cephalometric radiographs. The colouring scheme of the subtraction image emphasises the areas of change and visualizes the remodelling of the soft tissue.

Conclusions

FMI allows for cephalometry without tracing, it avoids the error inherent to the use of landmarks and the interaction of the practitioner is kept to a minimum. The robustness to focal distribution variations limits the influence of possible examiner inaccuracy.     

Assessment of vertical root fractures using three imaging modalities: cone beam CT, intraoral digital radiography and film

Objectives

The aim of this study was to assess the accuracy of cone beam CT (CBCT) in detecting vertical root fractures and to compare the accuracy with images from an intraoral sensor and from conventional intraoral film.

Methods

60 extracted, single-rooted human teeth were divided equally into two groups: a control group of 30 teeth and an induced fracture group of 30 teeth. All teeth were randomly placed into sockets in six dry mandibles. Each tooth was imaged by three modalities: CBCT, intraoral digital radiography and intraoral F-speed film. Three beam angulations (an orthogonal projection and additional projections with ±20° horizontal shifts of the central ray) were used when radiographs were made using film and a digital sensor. Three oral and maxillofacial radiologists evaluated the presence of root fractures twice in each image modality using a five-point confidence rating scale. Areas under receiver operating characteristic curves (A_z) were computed for each observer and modality and were tested for statistical differences using the Kruskal–Wallis test.

Results

There was no statistical difference in the performance of the three modalities (mean of A_z values: CBCT = 0.811, film = 0.797 and sensor = 0.775; p = 0.771).

Conclusions

There was no significant difference between intraoral film, a high-resolution complementary metal oxide semiconductor digital imaging system and CBCT in detecting vertical root fractures in mandibular single-rooted teeth.     

Comparative study between conventional and cone beam CT-synthesized half and total skull cephalograms

Objectives

The aim of this study was to compare cephalometric measurements obtained from conventional cephalograms with total and half-skull synthesized cone beam CT (CBCT) cephalograms.

Methods

Cephalometric analyses of 30 clinically symmetric patients were conducted by a calibrated examiner on conventional and CBCT-synthesized cephalograms (total, right and left). Reproducibility was investigated using the intraclass correlation coefficient (ICC). The Bland–Altman analysis was used to assess the agreement of the measurements from each factor obtained by conventional, total, right and left CBCT-synthesized cephalograms.

Results

The ICC was above 0.9 for most of the 40 cephalometric factors analysed, revealing similar levels of reproducibility. When the measurements obtained from conventional and CBCT-synthesized cephalograms were compared, the Bland–Altman analysis showed a strong agreement between them.

Conclusions

Half-skull CBCT-synthesized cephalograms offer the same diagnostic performance and equivalent reproducibility in terms of cephalometric analysis as observed in conventional and total CBCT-synthesized cephalograms.     

The influence of exposure parameters on jawbone model accuracy using cone beam CT and multislice CT

Objective

The main purpose of this study was to investigate the influence of exposure parameters on jawbone model accuracy when using cone beam CT (CBCT) and multislice CT (MSCT).

Methods

A lower and an upper edentulous human cadaver jaw were scanned using micro-CT (Skyscan 1173 high energy spiral scan micro-CT; Skyscan NV, Kontich, Belgium) at 35 μm to serve as true reference. The in vitro samples were exposed using six CBCT units and one MSCT system. CBCT exposure protocols were chosen according to clinically available settings. The variables were kilovoltage, milliamperage, voxel size and/or scan time. Image segmentation was based on local thresholds using profile lines. The resulting jawbone segmentations were registered with the reference and image processing was done to internally fill the segmentations. A point-based distance calculation was performed between the three-dimensional objects and reference scans and deviation percentages were calculated for 2 mm, 1 mm and 0.5 mm intervals.

Results

All points of the MSCT surface models lay within a 1 mm deviation range and 98.5% within 0.5 mm compared with micro-CT. For the different CBCT systems, accuracy came close to MSCT with mean percentages of 98.9% within 1 mm deviation and 92.8% within 0.5 mm. A difference of approximately 1% between lower and upper jaws could be perceived. For the specific CBCT exposure protocols, only scan time and voxel size revealed certain significant differences.

Conclusion

Jawbone model accuracy using CBCT was comparable with MSCT. The surface models of the upper jaws deviated slightly more than those for lower jaws. CBCT exposure settings had a limited influence on accuracy with scan time and voxel size as the main factors.     

Fractal dimension evaluation of cone beam computed tomography in patients with bisphosphonate-associated osteonecrosis

Objectives

The aim of this study was to (1) evaluate the fractal dimension (FD) in regions of the mandible on cone beam CT (CBCT) images of patients with bisphosphonate-associated osteonecrosis of the jaws (BP-ONJ) and (2) to select the most suitable region of interest (ROI) for further study on detection of bone alterations associated with bisphosphonates.

Methods

CBCT images of patients with BP-ONJ were included with matched controls. Values of FD were compared between groups. Selected ROIs were: ROI-1 — below the mandibular foramen; ROI-2 — above the mandibular foramen; ROI-3 — anterior to the mental foramen; ROI-4 — above the mandibular canal. The area of bone exposure was included as ROI-5. The results were analysed using generalized estimating equations and conditional logistic regression.

Results

There were 36 patients (67% female) with a mean age of 60.7 years. The mean FDs were: ROI-1 — 1.678 for controls and 1.673 for patients (P = 0.81); ROI-2 — 1.657 for controls and 1.653 for patients (P = 0.78); ROI-3 — 1.661 for controls and 1.684 for patients (P = 0.17); and ROI-4 — 1.670 for controls and 1.698 for patients (P = 0.03). The value of the FD in the area of exposed bone was the highest (1.729). The odds of being a BP-ONJ patient vs being a control was six times as high for individuals with a higher FD score at ROI-4, although the confidence interval was quite wide owing to the small sample size.

Conclusion

In this preliminary study, BP-ONJ patients had higher FD values than controls at regions close to the alveolar process. The results suggest that FD is a promising tool for detection of bone alterations associated with BP-ONJ.     

A suggested technique for the application of the cone beam computed tomography periapical index

Objectives

Cone beam CT (CBCT) produces undistorted three-dimensional (3D) images of the maxillofacial region with a radiation dosage lower than conventional CT. The periapical index score (PAI) is commonly used to follow up the lesions in the bone using periapical radiographs. Recently, a new PAI based on CBCT was introduced (CBCT-PAI). The aim of this technical report is to present a modified reproducible method to assess the CBCT-PAI.

Methods

CBCT was used to evaluate a periapical bone lesion observed in the area of tooth number 13 before treatment and 2 years after treatment. The modified CBCT-PAI was applied to both the examinations to measure the lesion. The dimensional analysis of the lesion was performed in each plane, assessing three fixed and reproducible dimensions: mesiodistal (M-D), buccolingual (B-L) and coronoapical (C-A). The images were evaluated by three mutually independent examiners. Data were collected and reported in a chart. The results were compared with each other and with the PAI score from the periapical radiographs.

Results

The three observers reported the same measurements of the lesion for each plane. The CBCT-PAI follow-up showed a reduction of the size of the lesion (5D vs 4D) but also an increase in the erosion of the buccal cortical plate. The comparison of CBCT-PAI with classic PAI showed the first method to be more precise.

Conclusions

This technical report shows how the CBCT-PAI can be applied to the CBCT exam of a periapical lesion in a reproducible way.