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ORIGINAL ARTICLE
Year : 2015  |  Volume : 3  |  Issue : 2  |  Page : 33-38

Accuracy of a customized volumetric rendering program in linear measurement of cone beam and multi-slice computed tomography derived three-dimensional images


Department of Prosthodontics, Faculty of Dentistry, University of Jordan, Amman, Jordan

Date of Web Publication22-May-2015

Correspondence Address:
Prof. Wala Majid Amin
Faculty of Dentistry, University of Jordan, P. O. Box: 13455, Amman 11942
Jordan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2321-3841.157515

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  Abstract 

Background: The study aimed to verify the accuracy of volumetric rendering software on cone beam computed tomography (CBCT) and multislice computed tomography (MSCT)-derived surface models. Materials and Methods: Gutta Purcha cone tips were fixed on 14 points at the facial surface of five dry mandibles. The mandibles were scanned twice with each scanner on three occasions. 26 distances were measured 10 times, using digital calipers. The surface models rendered by the software were linearly measured thrice and their means recorded. The data sets were subjected to vigorous statistical treatment using analysis of variance and Tukey test. Results: The caliper measurements, the CBCT and MSCT images were comparable, indicated by an F = 0.0003 and a Tukey T value (8.7), larger than the absolute difference between any two means. Thus, confirming the linear accuracy of the software. Conclusions: "SolidPlanner" volumetric rendering program yields a near 1-to-1 ratio with reality. The program enjoys linear accuracy, which remained submillimeter accurate, emphasizing the validity of the program as a measuring tool.

Keywords: Cone beam imaging, multislice scanning image, three-dimensional surface model, volumetric rendering


How to cite this article:
Amin WM, Saleh MWM. Accuracy of a customized volumetric rendering program in linear measurement of cone beam and multi-slice computed tomography derived three-dimensional images. J Oral Maxillofac Radiol 2015;3:33-8

How to cite this URL:
Amin WM, Saleh MWM. Accuracy of a customized volumetric rendering program in linear measurement of cone beam and multi-slice computed tomography derived three-dimensional images. J Oral Maxillofac Radiol [serial online] 2015 [cited 2019 Nov 15];3:33-8. Available from: http://www.joomr.org/text.asp?2015/3/2/33/157515


  Introduction Top


The morphological and morphometric features of human craniofacial patterns have been the focus of attention of scientists in many disciplines. These patterns were carefully studied and thoroughly analyzed by anatomists, orthognathic surgeons, forensic scientists, and anthropologists. The literature in these disciplines, particularly in the fields of forensic science and physical anthropology, is replete of reports emphasizing the essential role played by the morphometric characteristics of the human craniofacial patterns in gender dimorphism studies and in investigations into sex identification of human remains. [1],[2],[3],[4],[5] Early reports described measurements based on osteological landmarks (craniometry), but investigations reported later relied on direct measurements made on living subjects by palpating the supra-adjacent tissues. After the invention of the X-ray machine, measurements were carried out on cephalometric radiographs "cephalometry." [4] However, the new method, that is, cephalometry, being a two-dimensional (2D) diagnostic rendering from a three-dimensional (3D) structure, it is bound to yield images subject to projection, landmark identification, and measurement errors. [6] Attempts to develop 3D imaging to improve diagnosis has intensified during the late 1990s and culminated during the last decade by achieving the multislice computed tomography ("MSCT"). [7] Nonetheless, owing to the high radiation dose levels required by this technique (MSCT), alternative protocols for facial bone visualization and modeling have been developed and referred to as cone-beam computed tomography ("CBCT"). [8] This new technique enjoyed important advantages over the MSCT, being of a lower radiation dose, a shorter acquisition time, and reduced costs. [9],[10]

During the scanning procedure, whether cone beam or multislice, many projection images are produced and churned by a computer reconstruction algorithm into a 3D volumetric object in a process called rendering. [11] Volumetric rendering algorithm is usually unique for each computer program. The reconstructed 3D surface model helps make measurements, indicate landmarks, move bone fragments, and perform virtual osteotomies. The accuracy of the derived surface model is therefore of utmost importance for treatment planning and outcome. It has been reported that the accuracy of surface models derived from CT scanners seemed to vary as it is subject to identification errors in the segmentation process of the anatomic landmarks used on the surface models. [12],[13],[14],[15],[16] These possible inaccuracies of the surface models might influence the accuracy of the measurement procedure. Therefore, the accuracy of measurement should be calculated to fully determine whether a significant difference existed between surface models and the anatomic truth. [17] To obviate the problem of landmark identification, various markers, such as titanium beads, [14] spherical glass [17] have been used in providing information about linear distances. It has been claimed that by employing such markers, the volumetric renderings from the CT devices produced a 1-to-1 image-to-reality ratio. [15]

The aim of this study was to verify the accuracy of linear measurements made with customized volumetric rendering software on CBCT and MSCT-derived surface models.


  Materials and Methods Top


The test sample of the present investigation comprised of five dry toothless human mandibles selected from a group of dry skulls at the Department of Anatomy, Faculty of Medicine, University of Jordan. Fourteen areas were selected on the facial surface of the body and the right and left rami of each mandible. Tips of Gutta Purcha (GP) cones 1 mm across, 0.5 mm thick (Size F2 Protaper GP cones, Dentsply, Maillefer, Weybridge, Surrey, U.K.) were fixed in the selected areas with cyanoacrylate glue (ALTECO Chemical PTE Ltd., ALTECO Group of Companies, Japan) [Figure 1]a and b. The GP markers were used for the linear measurement in order to minimize the inaccuracies caused by the inherent differences in landmark identification and to establish fiducial anatomic locations. 26 linear distances, representing all three planes of space, were measured between the landmarks [Figure 1]c and d. The center point of each glued GP cone tip was the reference mark. The distances between the reference marks were determined with an electronic digital calipers (Orteam, Carbonari, Milano, ITALY) [Figure 2] on 10 occasions, at 2 days apart, by two observers (W.A. and MW.S.), a senior Faculty and a postgraduate dental student, who are experts in computer-guided clinical Implantology. Prior to conducting the physical and radiographic measurements on the five mandible specimens, the two observers underwent an intensive training tutored by a specialist Orthodontist of a wide experience in digitizing anatomic landmarks and measuring linear distances among points marked on Cephalometric radiographs. The calibration exercise to which the two observers were subjected involved conducting series of tests in which the observers measured different samples (each formed 40% the size of the entire test sample) at the same time, and each observer measured the same sample (but with different codes) at different times. The data sets of the calibration tests were statistically treated using appropriate statistical tests. The mean of the measurements was designated as the physical value or an anatomic truth.
Figure 1: (a) An example of the human mandible specimens used; (b) Gutta Purcha-marked points and distances; (c) a line diagram of a mandible illustrating the selected points on the body and rami of the mandible and the distances between each two points; (d) a line diagram showing distances measured between selected points across the mandible


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Figure 2: The digital caliper used in the physical measurement


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Prior to imaging in the CBCT scanner or the MSCT scanner the mandible was placed in the center of a transparent acrylic Jar and was adjusted with the occlusal plane parallel to the floor and the sagittal laser reference. The CBCT images were acquired with a WhiteFox scanner (WhiteFox, de Götzen SrL ITALY) coinciding with pogonion [Figure 3]; the jar was then filled with water to provide soft tissue equivalent attenuation. [12],[13] and the MSCT images were acquired with a Somatom Emotion scanner (Siemens GmbH, Germany). The preset parameters of the scanners are shown in [Table 1].
Figure 3: Water-immersed mandible being scanned by a cone beam computed tomography scanner


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Table 1: Preset scanning parameters for the WhiteFox CBCT and the Somatom Emotion MSCT scanners


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There were three scanning trials 5 days apart. Each mandible was scanned twice during each trial: Once with the cone-beam scanner and once with the multislice scanner. The CBCT data were exported from the WhiteFox software (WhiteFox Control 3D-00022, version 2.11.1, master version 1.46, inverter version 1.35, de Götzen, Germany) and the MSCT data were exported from the Somatom Emotion software (Syngo CT 2007E, Siemens, GmbH Germany) in DICOM multi-file format and imported into the customized "SolidPlanner Pro" software version 3.2, (Solid Models Co., Amman Jordan) on a Pavilion dv6 Laptop (HP USA) with a dedicated 1 GB video card (Rad con HD 6750 AMD).

The SolidPlanner software designed for the purpose of this study converted the DICOM images into 3D models using the marching cubes algorithm based on surface rendering. The 3D surface models of all mandibular images were generated by the preset threshold value for bone (250-3071 Hounsfield units) as specified by the rendering software.

The exact center of each selected point on the mandible surface [Figure 4] was precisely digitized by a cursor-driven point [Figure 5].
Figure 4: A three-dimensional scanned image of a mandible specimen


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Figure 5: The cursor-driven marker (black dot) used in pinpointing the exact center of a Gutta Purcha-marked point used in the linear measurements


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Customized templates were designed and into which the 3D coordinates of the selected points were registered. The 26 distances between points were measured to the nearest 0.01 mm [Figure 1]c and d using point-to-point mathematical equations.

The measurement values were then exported and saved in Excel file format (Microsoft, Excel 2010, Link). The CBCT and the MSCT images were rendered and measured on three trial sessions. The mean of the three CBCT and that of the three MSCT measurements were referred to as the CBCT and the MSCT measurement value.

Statistical analysis

The precision of the SolidPlanner Pro. Software employed in the linear measurements and its accuracy as a measuring tool was expressed, first, by the absolute error (AE) and second, the absolute percentage error (APE). The AE was defined as the CBCT or MSCT measurement value (measured on the surface models by the volumetric rendering software) subtracted from the reference value (the physical linear measurement made on the real mandible surface by using the electronic digital caliper). [14] The APE was calculated with the following equation:

APE = (AE/physical measurement value) × 100

Means of both "AE" and "APPE" and standard deviations were calculated.

The interobserver agreement and intrareliability correlation coefficients were calculated using Student's t-test.

The linear accuracy of the measurements among the three procedures (direct physical caliper measurement, the CBCT, and the MSCT measurements) was analyzed using analysis of variance (ANOVA) and Tukey test for comparing means by the following equation:



where, N is the number of scores in each group, q is a constant for the corresponding degrees of freedom indicated by the number of means to be compared and the number of scores within the analyzed groups in the ANOVA test.


  Results Top


The accuracy of the SolidPlanner Pro. Software in linear measurements was analyzed by the AE, and the APE, [Table 2]. The calculated AE values were small: 0.01-0.47 mm (0.11 ± 0.08) for the CBCT measurements group and 0.02-0.41 mm (0.09 ± 0.14) for the MSCT measurements group. The APE values were 0.00-2.15% (0.55 ± 0.13) for the CBCT measurements group and 0.15-0.41% (0.39 ± 0.08) for the MSCT measurements group. The results of the intraobserver variation evaluation using Student's t-test showed excellent agreement as demonstrated by a t = 0.987 indicating no significant difference existed. The linear measurements of the digital caliper, the CBCT, and the MSCT images showed excellent comparability indicated by an F = 0.0003 [Table 3].
Table 2: Means, SD, AE and APE values


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Table 3: Analysis of variance table


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The calculated F value, however, did not indicate which two means were comparable. This entailed conducting a more elaborate confirmatory test: The Tukey test for comparison of means. In this test, the calculated T = 8.7 was larger than the absolute difference between any two means [Table 4]. This result has confirmed that there is no statistically significant difference among all means at 99% level of confidence.
Table 4: Mean differences table


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  Discussion Top


The traditional 2D analog films such as the cephalometric radiographs (Ceph.) and the OrthoPanTompgraphs have been widely used and considered important tools in the diagnosis of patients and often employed in evaluating the progress of treatment. In the fields of Orthodontics, Orthognathic Surgery, ridge restoration prior to Implant Prosthodontics, Forensic science, and Physical Anthropology, many types of measurements or norms have been made and helped analyze oral relationships of jaws, teeth, and cranial base, [18] and also helped identify gender in morphometric studies of human skeletal remains.[1],[2],[3],[5] In all of the above-mentioned studies, the accuracy of the craniofacial measurements, linear and/or angular, and the precision of the measuring tool are crucially important. A serious problem associated with the traditional methods of radiography is projection errors. Projection errors can have a considerable effect on linear and angular measurements caused by magnification and distortion and are compounded by incorrect patient positioning. [19],[20]

The aim of this study was to verify a custom made "SolidPlanner" software specifically designed for this study and to assess its validity in converting the CBCT and MSCT-derived DICOM images into 3D surface models. The study also aimed to demonstrate that the obtained 3D surface images were linearly accurate within tolerable and clinically acceptable limits. [21] The results arrived at in this study showed that compared with direct physical caliper measurements (the gold standard or the anatomic truth), the images obtained with both WhiteFox CBCT and Somatom Emotion MSCT scanners and the customized "SolidPlanner" rendering software have accurate values with a variation as small as: 0.11 ± 0.08 mm in linear measurements; for the CBCT renderings and 0.09 ± 0.14 mm for the MSCT renderings. This very small variation can be considered clinically insignificant when the measured linear distances ranged from 10 to 96 mm. The measurement values obtained in the present study were comparable to those previously reported in the literature for differences between 3D CBCT renderings and direct caliper measurements. Our results supported those of Damstra et al., [17] who also reported small mean differences between calipers measured linear distances and those of CBCT renderings. Those researchers measured distances between selected points on dry mandibles each point was marked by a glass sphere 2.4 mm in diameter. In a study conducted by Stratemann et al.[22] a similar conclusion was arrived at; those researchers too reported small values of mean differences between caliper linear measurements and CBCT renderings, however, the reported values of the standard deviations of the mean differences were significantly large (0.41 and 0.22) compared to those presented in this study. The large standard deviations reported by Stratemann et al.[22] could be attributed to the use of metallic markers (2.4 mm diameter chromium balls), which might have caused significant artifacts when rendering the surface models because of scattering.

Despite reluctance of most researchers to use anatomic landmarks instead of selected and marked points in linear measurements in order to avoid falling into misidentification problems, Hassan et al., [16] however, employed anatomic landmarks, instead of markers and reported slightly larger differences of 0.10-0.39 mm between the 3D renderings and the caliper measurements. Those researchers commented that as long as the mean difference between the rendered 3D surface models and the caliper measurements was <0.5 mm level, which was considered by Marmulla et al., [23] as the relevant error limit, and since the mean difference was less than the size of the volume pixel (voxel) of the image, the resulted mean difference, therefore, cannot be regarded a clinically relevant for craniofacial measurements.

Matteson et al., [24] reported that 3D CT was accurate to 0.28% when compared with manual measurements on skulls. Lascala et al., [25] drew the attention that variations of 2-3 mm for distances at the maxillofacial region and 4-6 mm at the skull base area were found. These are far greater variations than the submillimeter mean differences between the caliper measurements and the CBCT or the MSCT renderings shown in this study. Discrepancies in the present findings with those reported by Lascala et al., [25] could be attributed to the fact that Lascala et al., used cuts (axial, coronal and sagittal) of the 3D image to obtain the linear measurements; whereas in this study 3D reconstruction technique was used to determine distances. Furthermore, the markers used in this study were small (1 mm diameter), clear, and better designed for location of the center point.

The present findings as well as those of previously reported similar studies highlighted the potential of 3D volumetric imaging in providing images that can be compared with reality with a 1-1 ratio.

It is obvious that the verification of this 1-1 ratio to reality has provided greater chances for qualitative analysis of craniofacial structures. The verification of the accuracy of 3D image analysis methods paved the way for introducing an entirely different approach in evaluating clinical cases with utmost accuracy and precision in terms of monitoring the progression of an already instituted treatment regimen or in effective planning of alternative treatment modalities.


  Conclusions Top


The accuracy of linear measurements of 3D surface models of CBCT and MSCT scanners and SolidPlanner volumetric rendering was assessed by comparing radiographic measurements with those obtained by using an electronic digital caliper. The results of a rigorous statistical treatment indicated that there is no significant difference neither between the caliper measurement outcomes and the radiographic linear measurements nor between the measurements on CBCT and MSCT. The results showed the accuracy remained submillimeter accurate, thus, emphasizing the validity of the "SolidPlanner" software and verifying its precision and accuracy as an effective measurement tool.


  Disclosures Top


The authors declare they have neither financial disclosure nor conflict of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
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  [Table 1], [Table 2], [Table 3], [Table 4]



 

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