Working with DICOM craniofacial images
2009; Elsevier BV; Volume: 136; Issue: 3 Linguagem: Inglês
10.1016/j.ajodo.2009.04.016
ISSN1097-6752
AutoresDan Grauer, Lucia S.H. Cevidanes, William R. Proffit,
Tópico(s)dental development and anomalies
ResumoThe increasing use of cone-beam computed tomography (CBCT) requires changes in our diagnosis and treatment planning methods as well as additional training. The standard for digital computed tomography images is called digital imaging and communications in medicine (DICOM). In this article we discuss the following concepts: visualization of CBCT images in orthodontics, measurement in CBCT images, creation of 2-dimensional radiographs from DICOM files, segmentation engines and multimodal images, registration and superimposition of 3-dimensional (3D) images, special applications for quantitative analysis, and 3D surgical prediction. CBCT manufacturers and software companies are continually working to improve their products to help clinicians diagnose and plan treatment using 3D craniofacial images. The increasing use of cone-beam computed tomography (CBCT) requires changes in our diagnosis and treatment planning methods as well as additional training. The standard for digital computed tomography images is called digital imaging and communications in medicine (DICOM). In this article we discuss the following concepts: visualization of CBCT images in orthodontics, measurement in CBCT images, creation of 2-dimensional radiographs from DICOM files, segmentation engines and multimodal images, registration and superimposition of 3-dimensional (3D) images, special applications for quantitative analysis, and 3D surgical prediction. CBCT manufacturers and software companies are continually working to improve their products to help clinicians diagnose and plan treatment using 3D craniofacial images. The numbers of clinicians using 3-dimensional (3D) records during diagnosis and treatment planning stages are increasing steadily. Cone-beam computed tomography (CBCT) scanners are becoming more efficient with reduced acquisition time, and software packages developed to process, manage, and analyze 3D images are also undergoing a rapid growth phase. The management of CBCT images differs from that of conventional 2-dimensional (2D) images. Most orthodontists were trained in the 2D era, and the transition to 3D images requires a learning stage. With today's hardware and software improvements, the learning curve is not as steep, but some basic concepts should be taken into account with this new technology.The purpose of this article is to give the clinician some core concepts for 3D diagnosis and treatment planning. The current commercial software applications for clinical management of craniofacial CBCT images are presented and compared with the current standards. The concepts presented here are applicable regardless of the constantly changing software applications.DICOM filesIn the early 1980s, the American College of Radiology and the National Electrical Manufacturers Association joined forces to standardize the coding of images obtained through computed tomography and magnetic resonance imaging. After successive improvements, in 1993, the term digital imaging and communications in medicine (DICOM) was adopted.1DICOM digital imaging and communications in medicine. National Electrical Manufacturers Association (NEMA), Rosslyn, Va2008Google Scholar A DICOM record consists of (1) a DICOMDIR file, which includes patient information, specific information about image acquisition, and a list of images that correspond to axial slices forming the 3D image; and (2) a number of sequentially coded images that correspond to the axial slices. (When those axial slices are combined in the correct order they form the 3D image) (Fig 1).Once a CBCT scan has been acquired, some basic handling and measurements on the data set can be performed with the software provided by the manufacturers. CBCT manufacturers also offer the option through their software to convert their proprietary formats into an exportable DICOM file; this is a first step in managing 3D CBCT information. When ordering a CBCT acquisition through an imaging laboratory, this is normally performed at the laboratory, and the patient or the clinician is given a compact disk containing the DICOM file. If the clinician owns a CBCT scanner, its software allows for exporting images in DICOM format. Further research is needed to validate the process of converting images from a proprietary format into DICOM format.The tools for visualization, landmarking, measurement, registration, superimposition, and computation of 3D images are different from those used in their counterpart 2D images.2Cevidanes L.H. Styner M.A. Proffit W.R. Image analysis and superimposition of 3-dimensional cone-beam computed tomography models.Am J Orthod Dentofacial Orthop. 2006; 129: 611-618Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar The information obtained through 3D visualization in orthodontics has not been completely linked to a diagnostic or prognostic meaning. For instance, when we observe a differently shaped mandibular condyle, it does not necessarily mean pathology. Further research should establish the links between observed morphology, pathology, pathogenesis, and response to treatment.The legal implications of acquiring a CBCT image are also important. More information than the conventional diagnostic records is obtained through a full 3D image of the head and neck, leading to responsibility and accountability issues regarding the diagnosis of pathology outside the region of interest. Whether the orthodontist or a radiologist should be accountable for any pathology beyond the region of interest is a current controversy beyond the scope of this article.3Jerrold L. Liability regarding computerized axial tomography scans.Am J Orthod Dentofacial Orthop. 2007; 132: 122-124Abstract Full Text Full Text PDF PubMed Scopus (12) Google ScholarVisualization of CBCT imagesAmong the increasing number of software packages dedicated to managing and analyzing DICOM images, we focus on 3 with special emphasis in orthodontics. In alphabetical order they are 3dMDvultus software (3dMD, Atlanta, Ga), Dolphin Imaging (Dolphin Imaging, Chatsworth, Calif), and InVivoDental (Anatomage, San Jose, Calif). There are other software packages and applications (even freeware) available to manage DICOM files.A 3D image is composed of a stack of 2D images or slices. In a similar fashion that a 2D image is composed of pixels, a 3D image is composed of voxels. Each voxel has a gray-level value based on indirect calculation of the amount of radiation absorbed or captured by the charge-coupled device and calculated through a filtered-back projection algorithm. Visualization is based on a threshold filter. This filter assigns a binary value, either transparent or visible, to each voxel based on its gray-level value. The user defines the critical value that splits the voxels into visible and invisible. The result is a rendered image on the screen composed of all visible voxels.The operator can visualize the data set by looking at the stack of slices or the rendered 3D image. Computers can reformat the 3D image, allowing the operator to scroll through these 2D images in any direction (Fig 1, C). The most common ones are sagittal, coronal, and axial. All 3 orthodontic programs allow scrolling through the stack of images. A cursor represented by 2 crossing lines indicates the precise localization in virtual space. The data set can also be rotated, panned, or zoomed to allow visualization of the region of interest; at any angle, scale, or position, a rendered image can be created. Multiple threshold filters can be applied to the same image to distinguish between tissues of different density—eg, soft and hard tissues. Transparency can also be applied to allow visualization of hard tissues through the soft tissues (Fig 2). Clipping tools are also available. These allow for isolation and visualization of specific regions—eg, the mandibular condyles. Dolphin Imaging allows for 2 threshold filters: for hard tissues and soft tissues. Transparency can be applied to visualize soft-tissue thickness at various points. InVivoDental allows the user to modify the threshold values through preloaded filters. Additionally, segmentations can be created. The 3dMDvultus software also has threshold filters, in addition to the ability to create segmentations to isolate and define regions of interest (described later).Fig 2Different visualization modes and interfaces of 3 programs: A, Dolphin Imaging interface, with thresholding filters applied to visualize both hard and soft tissues, and a semitransparency applied to the soft tissue to visualize the hard tissue underneath; B, InVivoDental volume interface, with modified thresholding filters applied by a preset visualization “Soft tissue + Bone 1”; C, 3dMDvultus software interface, with hard- and soft-tissue surface models created (segmentations) and a semitransparency applied to the soft-tissue segmentation.View Large Image Figure ViewerDownload Hi-res image Download (PPT)It is crucial to understand that the rendered image is the result of a user-entered threshold value. The visual perception of the operator defines what is bone and what is soft tissue, and many factors can affect this: contrast of the image, noise in the image, individual visual perception and prior knowledge of anatomy among others. For a qualitative assessment, these rendered images are appropriate, but, for a quantitative assessment, they present many challenges that are discussed in the next section.Measurement in CBCT imagesIn 2D radiographs, distances and angles are measured between landmarks. These landmarks are defined by the superimposition of the projection of different structures. This is a property of transmission radiographs. Landmarks can defined as an inflection point in a curved line, the geometric center of a structure, superimposition of projection of different structures, the tip of a structure, or the crossing point of 2 planes. Most landmarks cannot be visualized or are difficult to locate on a curved surface in a 3D image. There are no clear operational definitions for specific cephalometric landmarks in the 3 planes of space.4Bookstein F.L. Landmark methods for forms without landmarks: morphometrics of group differences in outline shape.Med Image Anal. 1997; 1: 225-243Abstract Full Text PDF PubMed Scopus (1152) Google Scholar A second challenge is that the rendered image depends on many factors, including contrast of the image, movement during acquisition, presence of metal that creates noise, overall signal-to-noise ratio of the image, and the threshold filters applied by the operator. Because of all these factors, it make sense that the landmarks should be located in the stack of slices rather than in the 3D rendered volume.5Oliveira A.E. Cevidanes L.H. Phillips C. Motta A. Burke B. Tyndall D. Observer reliability of three-dimensional cephalometric landmark identification on cone-beam computerized tomography.Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2000; 107: 256-265Google ScholarMany studies have assessed the accuracy and reliability of measurements on CBCT images. Those studies can be classified based on 2 criteria. The first is whether they use radiopaque markers or structures of known geometry. This classification yields 2 groups: when landmark location does not need anatomic operational definitions, and when anatomic definitions are important, and another interexaminer or intraexaminer factor (landmark location) is introduced. The second classification, applicable to both groups, is based on where the landmarks were located. According to this second criterion, 3 groups are established: (1) landmarks located in the stack of slices, (2) landmarks located on a segmented surface (more later), and (3) landmarks located on the rendered image.Studies from group 1 report good accuracy regardless of where the measurements were made. For most measurements, there were no statistically significant differences compared with the gold standard (measurements with a caliper or structures of known geometry). Some measurements had statistically significant differences, but those were small and not clinically significant.6Ballrick J.W. Palomo J.M. Ruch E. Amberman B.D. Hans M.G. Image distortion and spatial resolution of a commercially available cone-beam computed tomography machine.Am J Orthod Dentofacial Orthop. 2008; 134: 573-582Abstract Full Text Full Text PDF PubMed Scopus (125) Google Scholar, 7Lascala C.A. Panella J. Marques M.M. Analysis of the accuracy of linear measurements obtained by cone beam computed tomography (CBCT-NewTom).Dentomaxillofac Radiol. 2004; 33: 291-294Crossref PubMed Scopus (380) Google Scholar, 8Marmulla R. Wortche R. Muhling J. Hassfeld S. Geometric accuracy of the NewTom 9000 cone beam CT.Dentomaxillofac Radiol. 2005; 34: 28-31Crossref PubMed Scopus (162) Google Scholar, 9Mischkowski R.A. Pulsfort R. Ritter L. Neugebauer J. Brochhagen H.G. Keeve E. et al.Geometric accuracy of a newly developed cone-beam device for maxillofacial imaging.Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2007; 104: 551-559Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 10Pinsky H.M. Dyda S. Pinsky R.W. Misch K.A. Sarment D.P. Accuracy of three-dimensional measurements using cone-beam CT.Dentomaxillofac Radiol. 2006; 35: 410-416Crossref PubMed Scopus (228) Google Scholar, 11Stratemann S.A. Huang J.C. Maki K. Miller A.J. Hatcher D.C. Comparison of cone beam computed tomography imaging with physical measures.Dentomaxillofac Radiol. 2008; 37: 80-93Crossref PubMed Scopus (178) Google Scholar Studies from group 2 report subclinical accuracy when landmarks were located on segmentations or in the stack of slices,12Hassan B. van der Stelt P. Sanderink G. Accuracy of three-dimensional measurements obtained from cone beam computed tomography surface-rendered images for cephalometric analysis: influence of patient scanning position.Eur J Orthod. 2009; 31: 129-134Crossref PubMed Scopus (133) Google Scholar, 13Ludlow J.B. Laster W.S. See M. Bailey L.J. Hershey H.G. Accuracy of measurements of mandibular anatomy in cone beam computed tomography images.Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2007; 103: 534-542Abstract Full Text Full Text PDF PubMed Scopus (207) Google Scholar but not when they were located on the rendered image.14Periago D.R. Scarfe W.C. Moshiri M. Scheetz J.P. Silveira A.M. Farman A.G. Linear accuracy and reliability of cone beam CT derived 3-dimensional images constructed using an orthodontic volumetric rendering program.Angle Orthod. 2008; 78: 387-395Crossref PubMed Scopus (220) Google Scholar When all studies are considered regardless of their classification, reliability in measurements and landmark identification in CBCT images was reported to be good to very good.5Oliveira A.E. Cevidanes L.H. Phillips C. Motta A. Burke B. Tyndall D. Observer reliability of three-dimensional cephalometric landmark identification on cone-beam computerized tomography.Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2000; 107: 256-265Google Scholar, 10Pinsky H.M. Dyda S. Pinsky R.W. Misch K.A. Sarment D.P. Accuracy of three-dimensional measurements using cone-beam CT.Dentomaxillofac Radiol. 2006; 35: 410-416Crossref PubMed Scopus (228) Google Scholar, 14Periago D.R. Scarfe W.C. Moshiri M. Scheetz J.P. Silveira A.M. Farman A.G. Linear accuracy and reliability of cone beam CT derived 3-dimensional images constructed using an orthodontic volumetric rendering program.Angle Orthod. 2008; 78: 387-395Crossref PubMed Scopus (220) Google Scholar, 15Moerenhout B.A. Gelaude F. Swennen G.R. Casselman J.W. Van Der Sloten J. Mommaerts M.Y. Accuracy and repeatability of cone-beam computed tomography (CBCT) measurements used in the determination of facial indices in the laboratory setup.J Craniomaxillofac Surg. 2009; 37: 18-23Abstract Full Text Full Text PDF PubMed Scopus (53) Google ScholarBased on the available evidence, we can conclude that it is more accurate to locate landmarks in the stack of slices or on a segmented surface; this is possible in all 3 software packages. Landmarks located in the rendered volume must be carefully evaluated.Creation of 2D radiographs from DICOM filesLongitudinal growth databases are no longer allowed for ethical reasons, and there are no normative data in 3 dimensions. However, available 2D growth databases can be used to compare with current clinical data.16Hunter W.S. Baumrind S. Moyers R.E. An inventory of United States and Canadian growth record sets: preliminary report.Am J Orthod Dentofacial Orthop. 1993; 103: 545-555Abstract Full Text PDF PubMed Scopus (21) Google Scholar To be able to compare the new modalities with our current databases, algorithms have been created to extract information from the CBCT image and simulate a conventional cephalogram, panoramic projection, tomographic image of the temporomandibular joint, and posteroanterior cephalogram. Cephalogram registration and superimposition are the most common and efficient ways to quantitatively assess growth and treatment changes. All 3 software packages allow for the extraction of synthetic radiographic projections. The procedure starts by orienting the patient's head image in virtual space similarly to what the technician does in a cephalostat (Fig 3). The advantage of this virtual orientation is the possibility of using a semitransparent image to match bilateral structures and obtain the correct head rotation.Fig 3Creation of synthetic cephalograms: A, unoriented volume; B, oriented to obtain the correct head rotation (note the difference between the orbits and zygomatic bone); C, once oriented, the cephalogram was generated or has been generated (InVivoDental).View Large Image Figure ViewerDownload Hi-res image Download (PPT)Measurements performed on CBCT synthetic cephalograms have proven to be on average similar to those on conventional cephalograms.17Cattaneo P.M. Bloch C.B. Calmar D. Hjortshoj M. Melsen B. Comparison between conventional and cone-beam computed tomography-generated cephalograms.Am J Orthod Dentofacial Orthop. 2008; 134: 798-802Abstract Full Text Full Text PDF PubMed Scopus (88) Google Scholar, 18Kumar V. Ludlow J.B. Mol A. Cevidanes L. Comparison of conventional and cone beam CT synthesized cephalograms.Dentomaxillofac Radiol. 2007; 36: 263-269Crossref PubMed Scopus (136) Google Scholar, 19Moshiri M. Scarfe W.C. Hilgers M.L. Scheetz J.P. Silveira A.M. Farman A.G. Accuracy of linear measurements from imaging plate and lateral cephalometric images derived from cone-beam computed tomography.Am J Orthod Dentofacial Orthop. 2007; 132: 550-560Abstract Full Text Full Text PDF PubMed Scopus (142) Google Scholar, 20van Vlijmen O.J. Berge S.J. Swennen G.R. Bronkhorst E.M. Katsaros C. Kuijpers-Jagtman A.M. Comparison of cephalometric radiographs obtained from cone-beam computed tomography scans and conventional radiographs.J Oral Maxillofac Surg. 2009; 67: 92-97Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar Some statistically significant differences were found between some measurements, but no clinically significant differences were found. When both modalities—conventional and CBCT synthetic cephalograms—are combined in the same longitudinal study, the researcher must account for an increase in landmark error calculation.21Grauer D. Cevidanes L.H. Styner M.A. Heulfe I. Harmon E.T. Zhu H. Proffit W.R. Accuracy and landmark error calculation using CBCT generated cephalograms.Angle Orthod. 2009; (in press)Google ScholarFor the creation of CBCT synthetic cephalograms, Dolphin Imaging allows the user to choose an orthogonal or a perspective projection type, and, with the latter, the projection center can be repositioned to match the transporionic axis. Once created, many visualization filters can be applied to the synthetic cephalogram. The 3 companies are now working to improve the options offered by the cephalogram-creation module. The creation of CBCT synthetic panoramic radiographs starts by delineating the focal trough, its upper and lower limits, and its thickness.Segmentation engines and multimodal imagesThe segmentation process in medical imaging could be defined as the construction of 3D virtual surface models (called segmentations) to best match the volumetric data. There are many different segmentation processes, and this topic is beyond the scope of this article. For more information, the reader is referred to the study of Yushkevich et al.22Yushkevich P.A. Piven J. Hazlett H.C. Smith R.G. Ho S. Gee J.C. et al.User-guided 3D active contour segmentation of anatomical structures: significantly improved efficiency and reliability.Neuroimage. 2006; 31: 1116-1128Crossref PubMed Scopus (4863) Google Scholar The reader must distinguish between a virtual surface and a rendered image. The importance of having a segmentation engine in the software package is twofold. First, it allows the user to export anatomic models in a nonproprietary format; this information can be used in research and will always be accessible regardless of constantly changing software applications. The second advantage is the option of loading anatomic models—segmentations—in a nonproprietary format into the imaging software interface; that allows combining different modalities with the CBCT images. An example is combining digital models obtained through laser or optical scanners with the CBCT data and soft-tissue meshes obtained through 3D cameras. These multimodal images are the foundation of digital dentistry, rapid prototyping, and computer-aided design and computer-aided manufacturing applications.Currently, InVivoDental offers a segmentation engine that allows the user to export anatomic models. Dolphin Imaging allows importing 3D soft-tissue meshes to be combined with the CBCT data. The 3dMDvultus software has a segmentation engine, which performs segmentations by thresholding and smoothing filters (Fig 2, C). The 3dMDvultus software also allows for both exporting and importing segmentations.Registration and superimposition of 3D imagesTraditionally, the best and almost only way to quantitatively assess changes in orthodontics was cephalogram superimpositions. Stable structures described by Bjork,23Bjork A. Sutural growth of the upper face studied by the implant method.Acta Odontol Scand. 1966; 24: 109-127Crossref PubMed Scopus (146) Google Scholar Bjork and Skieller,24Bjork A. Skieller V. Facial development and tooth eruption. An implant study at the age of puberty.Am J Orthod. 1972; 62: 339-383Abstract Full Text PDF PubMed Scopus (522) Google Scholar, 25Bjork A. Skieller V. Growth of the maxilla in three dimensions as revealed radiographically by the implant method.Br J Orthod. 1977; 4: 53-64Crossref PubMed Scopus (298) Google Scholar, 26Bjork A. Skieller V. 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Today, it is possible to register CBCT records acquired at different time points and analyze changes due to treatment, growth, aging, and relapse in 3 dimensions.The 3 software packages can register and superimpose CBCT images from different time points in the same virtual space. The procedure differs slightly between Dolphin Imaging and InVivoDental and 3dMDvultus software. In the first 2 programs, the process includes 5 steps.1.The user loads the 2 CBCT images from different time points.2.The user inputs homologous landmarks found in both images. Those landmarks will be the registration references and must be anatomically stable between time points.3.Once the landmarks are input, the program computes the best fit between the 2 sets of landmarks in each CBCT image. A transformation matrix is obtained (rotation and translation). The program then relocates 1 CBCT image relative to the other based on this transformation matrix, and the result is that both images share the same coordinate system.4.Because of the difficulty of locating stable landmarks in curved surfaces, especially along the cranial base, both programs allow for manually refining the registration process until most cranial base structures match.5.Once the images are registered, the user can evaluate changes in the rendered volume with semitransparencies or at the stack of slices. Changes can be described relative to the registration landmarks (Fig 4, A, B, C, G, and H).Fig 4Registration and superimposition of sequential CBCT images: A, Dolphin Imaging uses a landmark-based registration process that allows the user to manually refine the relative position of the CBCT images until, B, stable structures are matching. C, Once registered, semitransparency visualization allows the user to measure and assess changes. D, The 3dMDvultus software uses a surface-based registration process in which the first 2 images are manually positioned; E, anatomically stable surfaces are selected, and the program refines the registration by matching those surfaces; once registered, changes can be determined. F, Surgical outcome assessment—in this case, maxillary advancement, autorotation of the mandible and genioplasty—can be measured and visualized in the volumetric rendered image and the stack of slices. G and H, Different InVivoDental visualizations of the registered volumes.View Large Image Figure ViewerDownload Hi-res image Download (PPT)The 3dMDvultus software operates in a slightly different manner; the process also consists of 5 steps.1.The images are loaded into the software interface, and segmentations are created.2.The user unlocks the rotation and translation parameters of 1 segmentation.3.The user performs an initial manual registration to approximate the surfaces as much as possible.4.Anatomically stable surfaces must be selected by the user. In this case, the registration is surface-based, rather than landmark-based. The program performs a surface-to-surface registration to refine the initial manual registration.5.Once the segmentations are registered, the user can visualize them by means of semitransparencies and assess changes in the segmentations, the rendered volume, or the stack of slices. Change can be described relative to the registration surfaces (Fig 4, D through F).We believe the latter registration process offers a more precise registration, because it is based on surfaces composed of thousands of landmarks instead of a few landmarks selected by the user; however, it still depends on the precision of the 3D surface models. Researchers at the University of North Carolina have developed a registration process that does not depend on the precision of the 3D surface models. This process compares voxel by voxel between gray-level CBCT images. The region to be compared is defined by the user. A transformation matrix (translation and rotation) is computed and applied to a CBCT image.2Cevidanes L.H. Styner M.A. Proffit W.R. Image analysis and superimposition of 3-dimensional cone-beam computed tomography models.Am J Orthod Dentofacial Orthop. 2006; 129: 611-618Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar, 29Cevidanes L.H. Bailey L.J. Tucker Jr., G.R. Styner M.A. Mol A. Phillips C.L. et al.Superimposition of 3D cone-beam CT models of orthognathic surgery patients.Dentomaxillofac Radiol. 2005; 34: 369-375Crossref PubMed Scopus (245) Google ScholarSpecial applications of quantitative analysisToday, it is easier to analyze the shape and contours of airway passages in 3 dimensions. All 3 programs have tools to measure airway volume. This will open the door to research on airway volume changes with growth, treatment, and pathology. InVivoDental allows for segmenting the airway passages and measuring their volumes. Dolphin Imaging has a tool for segmenting the airway and allows for careful visual examination of airway contours and shapes. Airway volume can also be calculated (Fig 5). The 3dMDvultus software computes airway volume and allows visualization of the cross-section images along the airway. This software detects the smallest cross-sectional area or airway stenosis. A virtual endoscopy is also a feature of this program.Fig 5Airway analysis module by Dolphin Imaging: at the upper right corner, the airway passages are segmented by initialization spheres. Both area and volume can be calculated. The airway segmentation can be rotated, panned, and zoomed in space.View Large Image Figure ViewerDownload Hi-res image Download (PPT)An implant simulation module is offered by InVivoDental softw
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