Dental Imaging Centers

Dedicated dental imaging centers have been providing valuable imaging services to the dental community for many years. The centers feature specialized and sophisticated imaging equipment and highly trained personnel and provide photographic and radiographic services for the community. This article discusses selected sophisticated imaging equipment found in dental imaging centers and discusses current and anticipated future services provided by the centers. These future services include the construction of patient-specific interactive three-dimensional models to be used for diagnosis, treatment planning, treatment simulations, communication, and evaluation of treatment outcomes.

Dentistry has historically relied on imaging technology to assist with diagnosis, communication, treatment planning, treatment simulation, and evaluation of outcomes. Until recently, radiographic images have been acquired in general or specialty practice offices. During the past 10 years, there has been an emergence of sophisticated independent dental imaging centers. This article will discuss the role of imaging centers in the dental community, including the role of radiology personnel and the key services provided by these centers. New developments and future goals for imaging centers will also be discussed.

The recent development of designated imaging centers can be attributed in part to the highly specialized imaging equipment and uniquely trained staff that are currently used for imaging. In general, it is no longer economical for an individual dental office to provide the comprehensive services of an imaging center; and, therefore, these services are commonly being referred to a dedicated imaging center. Such centers are most common in the western half of the United States, and California has the highest density. The high density of referring dentists in large metropolitan areas make them ideal sites for imaging centers. Several new centers are being planned for underserved areas in the western half of the United States and Canada. Technological advances in imaging are occurring at a rate faster than can be reasonably assimilated into a private practice, which creates the opportunity for an imaging center to offer advanced imaging services to the dental community. Imaging centers provide photographic and radiographic images that become an essential part of the patient record and are used to aid in diagnosis, treatment, and assessment of outcomes. Imaging centers do not provide treatment and, therefore, do not compete with the services provided by referring dentists. Imaging centers typically service numerous referring dentists and their patients within a geographic area. They work closely with referring dentists to provide images that are optimal for their individual treatment approaches and maintain archived records for future reference. Imaging centers provide a referral slip that lists available services and facilitates the referring of patients. They also participate in local study groups to present and discuss new technologies or methodologies. The services of an oral and maxillofacial radiologist may also be provided to interpret radiographic images.

Personnel

Dental X-Ray Technologist

Modern dental imaging requires personnel who are highly skilled and well-trained. This is partially necessary because sophisticated new X-ray units require the operator to have excellent computer skills and extensive training. To utilize the equipment effectively, the technologist must have a thorough understanding of new technology such as digital capture, digital scanning, sensor technology, digital cameras, and digital printers. Sophisticated X-ray units are complex and require a high skill level for operation. Dental X-ray technologists have specific training required to perform maxillofacial imaging. Licensing guidelines in California allow the dental X-ray technologist to perform imaging services without the direct supervision of a dentist or oral radiologist, but services can only be performed if accompanied by a written or oral requisition. Dental X-ray technologists receive approximately 2 1/2 years of specific training in radiography. This specialized training provides the technologist with the skills, knowledge, and ability to perform the most sophisticated dental imaging.

Oral and Maxillofacial Radiologist

Oral and maxillofacial radiology was recognized by the American Dental Association’s House of Delegates in October 1999 as a specialty, the first new one in 36 years. An oral and maxillofacial radiologist is educated in providing radiographic interpretation of maxillofacial images, including radiographs, CT scans, magnetic resonance imaging, and tomography. Most imaging centers are associated with radiologists who provide radiographic interpretations and technical advice. Oral and maxillofacial radiologists are specifically educated and trained to apply the most appropriate technology for diagnostic purposes for such conditions as temporomandibular joint disorders, infectious diseases, tumors, dental abnormalities, and trauma.

Radiographic Studies

The decision to acquire images occurs as a result of discoveries at the time of the clinical exam and history. There are many available imaging options that can be used in clinical investigations. These include periapical, bitewing, panoramic, cephalometric, tomography, CT scan, magnetic resonance, and bone scans. The ideal imaging solution produces the desired diagnostic information while minimizing the cost and risk to the patient .¹ The ability to fulfill these imaging goals is currently limited by the ability of imaging modalities to represent the anatomy in three dimensions. Imaging centers perform all imaging studies that would normally be performed in a dental office as well as specialized studies requiring sophisticated equipment. The following is a description of some of the equipment and special studies provided by these centers.

Specialized Equipment

Digital and Robotic Equipment

Computer-controlled motion control systems for tomographic and panoramic/ cephalometric imaging have recently become the cornerstone pieces of equipment for dental imaging clinics. The CommCat tomographic unit (Imaging Sciences International, Hatfield, Pa.) (Figure 1) and the OP-100 panoramic/cephalometric unit (Instrumentarium Imaging, Milwaukee, Wisc.) are examples. These robotic imaging units have been designed to image the structures of maxillofacial regions from multiple points of view. Alignment of the X-ray tube and the targeted anatomy occurs via computer control of stepper motors. The computer is the interface that the radiology technologist uses to perform panoramic or tomographic studies. The technologist uses the computer to provide study-specific movement instructions to the imaging instrument, and in turn the instrument produces images that are optimized for the study. There are options to retrofit all film-based imaging systems, including panoramic, cephalometric, and tomographic systems, with a digital sensor. The imaging sensor types include a charged couple device (CCD) and photo-stimulated phosphor sensor. These sensors eliminate darkroom processing procedures and produce a digital image that can be viewed on a monitor, transmitted via Internet or World Wide Web, archived, and printed.

 

Figure 1. CommCat, a robotic tomographic unit produced by Imaging Sciences International.

Specialized Studies

Tomography

Site-specific imaging refers to imaging techniques selected to optimally show specific anatomic regions, such as maxillary sinuses and TMJs. The ideal images show the area of interest with at least two views at right angles to each other produced with minimum superimposition and maximum detail. Tomography is an exceptionally good site-specific imaging technique because it provides high quality images at the desired projection angulation, at low risk (exposure), and relatively low cost. Tomography is a general term used when an imaging technique provides an image of a layer of tissue.² These layers or planes can be oriented to acquire any desired slice of the anatomy under study. The versatility of this technique makes tomography highly desirable for accurate imaging of a wide variety of maxillofacial structures, including the TMJ and cross-sections of the maxilla and mandible.

TMJ Tomographic Study

Corrected tomography has been one of the most widely used techniques to examine the hard tissue of the TMJ because of its ability to image the TMJ quickly and relatively inexpensively. Axially corrected TMJ tomography refers to the alignment of the tomographic beam with the mediolateral long axis of the condyle to produce image layers that are parallel or perpendicular to the mediolateral long axis of the condyle (Figure 2). The laterosuperior and mediosuperior surfaces of the condyle are more difficult to image than the central two-thirds of the condyle with sagittal tomography. Therefore axially corrected para-coronal plane images are recommended for viewing these surfaces.¹

 

Figure 2. A submental vertex projection used to plan the location of the tomographic sections. The tomographic sections are planed to be perpendicular (green lines) and parallel (yellow lines) to a line extending between the medial (orange marker) and lateral poles (yellow markers) of the condyles.

The initial goals for TMJ tomography are to show the size, morphology, and quality of the osseous components and the condyle/fossa spatial relationships in the open and closed mouth positions. To best meet these goals, the tomographic sections must be acquired in planes parallel (para-coronal) and perpendicular (para-sagittal) to the long axis of the condyle (Figures 3a through d). Each section should be thin, approximately 2 to 3 mm thick, and acquired with a complex motion to reduce blurring. Images should be taken with the patient in an upright position.

 

Figure 3a. Figures 3a through d show axially corrected tomographic sections of the temporomandibular joint. Figure 3a shows a para-sagittal image in the closed mouth.

Figure 3b. Figure 3b shows a para-sagittal image in the open mouth position.

Figure 3c. Figure 3c shows the TMJ with evidence of degenerative joint disease and narrowed joint spaces.

Figure 3d. Figure 3d shows a subcondylar fracture with a medial displacement of the proximal fragment.

Implant Tomographic Study

Corrected tomographic sections provide imaging information that optimizes placement of endosseous implants that enhance the success of all subsequent stages of implant placement, including the long-term success. The specific information provided by tomography that increases the long-term success of the biointegration and function of these implants includes jaw size (height and width), orientation of the vertical long axis of the jaw, jaw boundaries, internal anatomy, soft tissue morphology, bone quality, and pathological processes affecting the implant site.³

To provide optimal information, tomographic sections should be acquired parallel and perpendicular to a tangent point on the jaw curve and perpendicular to the occlusal plane (Figures 4 and 5). Similar to TMJ tomographic studies, each tomographic section should be thin, approximately 2 to 4 mm thick, and acquired with a complex motion to reduce blurring. Images should be taken with the patient in an upright position.

 

Figure 4. An occlusal film used to plan the location of the implant sites. The jaw curve and proposed location of tomographic sections were mapped onto the occlusal film parallel and perpendicular to a tangent point on the jaw curve.

Figure 5. Implant tomographs acquired perpendicular and parallel to a point on the jaw curve identified by the metallic marker located and oriented along the path of a proposed implant. The tomographic sections are oriented perpendicular to each other and can be corrected for magnification to allow for accurate measurements of the anatomy. An acrylic tooth coated with a thin layter of barium sulphate supported the metallic marker. The acrylic tooth and marker are used to test the feasibility of this site for implant placement (courtesy of Dr. Monica Crooks).

Photographic Services

Imaging centers provide intraoral and extraoral photo documentation services. Historically these services have been provided with a 35 mm single-lens reflex camera optimized for dentistry, but more recently 35 mm cameras are being replaced by megapixel digital cameras. The photographs can be previewed for quality while the patient is still present. The image is then downloaded into a computer and formatted into a mount series (Figure 6). The images can be catalogued, stored, and printed on photographic-quality paper. The digital camera provides several benefits over standard photography, including the ability to print additional copies from the original photographic series. Any archived digital photographs can be selected and included in a word processing document, such as an interpretative report, or in a presentation package.

 

Figure 6. This mounted photographic series was acquired with a megapixel digital camera. This digital image series or any portion of this series can be printed on phtographic-quality paper, archived for future use, incorporated into word processiong documents, or used for presentations.

Software Enhancement of Standard Series

X-ray film or photographs can be digitized using a flatbed scanner with a transparency adapter or a digital camera. These digital images can be used with software programs to provide additional information. For example, Surgical Planner Software (Imaging Sciences International) (Figure 7) is an interactive software program used by the treating dentist to aid in the diagnosis, treatment planning, and presentation of implant cases that have been imaged with the CommCat tomographic unit. The tomographic images are digitized and prepared by the imaging center and the image files are transferred to the doctor via the Internet or softcopy to be used for diagnosis and treatment planning.

 

Figure 7. A series of digital images displyed in SurgPlan, an implant planning software program by Imaging Sciences International. This is an interactive software program that corrects the images for magnification and spatially cross-correlates selected anatomic sites. This program allows access to and the simulated placement of a database of implants categorized by manufacturer and size.

Future Goals and Services

The goal of imaging is to display the "anatomic truth" as it exists in nature. Current technology is limited because it represents a three-dimensional object in two dimensions. All two-dimensional images are acquired from a selected point of view (e.g., lateral cephalometric projection). These projections create images with superimpositions and dimensional changes in anatomy because of the projection geometry used to acquire them. Anatomy can be obscured because of the chosen projection geometry. To overcome these limitations, multiple modalities are combined to produce a "patchwork quilt" image of the patient. For example, multiple types of images such as periapicals and photographs are used to view the teeth; orthogonal cephalometric projections are used to view the facial skeleton; photographs are used to view the facial soft tissues; and corrected tomographs are used to view the TMJs. Each of these views has its own limitations due to projection geometry, superimpositions, and obscured anatomy. For the clinician to understand the three-dimensional anatomy, he or she must perform a mental reconstruction of the patients’ anatomy using all of the available images. New technology or software programs that aid the clinician in creating a three-dimensional computer reconstruction of the patients’ anatomy would be beneficial. New imaging input devices and software programs are being developed to provide three-dimensional data that will enable the development of accurate three-dimensional anatomic models.

Future Input Devices

Intraoral and extraoral digital cameras utilizing structured light or lasers will be available within months. These cameras will enable the three-dimensional reconstruction of the surface anatomy of the facial soft tissues and teeth. Structured light patterns when combined with photogrammetry to accurately measure the light pattern result in the generation of an accurate three-dimensional map of the lighted structure.

Digital sensor technology combined with robotic imaging machines will be valuable because multiple images can be acquired quickly using precise projection geometry. Furthermore, once multiple digital images have been captured, synthesized tomographic software algorithms can create an infinite number of tomographic slices from the digital images. Instrumentarium Imaging plans to commercially introduce a variation of synthesized tomography called TACT (Tuned Aperture Computer Tomography).⁴ The Instrumentarium system will use the company’s OP-100 robotic panoramic unit with a CCD sensor to acquire multiple images from various angles. Ortho TACT software algorithms will compute the tomographic image layers to be displayed. This is an important advancement because it provides additional information without additional radiation, has the ability to reveal hidden anatomy, eliminates superimpositions, and provides three-dimensional data.

Anatomic Reconstructions

CT scans and magnetic resonance images are now available for three-dimensional reconstruction of anatomy, but these input devices have not been a practical solution because of their limited value and cost. Currently utilizing standard dental input devices, the clinician must mentally conceptualize the three-dimensional anatomy. Software is in development that will use multiple standard two-dimensional radiographs, photographs, and three-dimensional image sets and combine them into a three-dimensional matrix. To combine multiple image sets into a three-dimensional matrix, the image sets must be geometrically corrected to true size and accurately registered into a common spatial reference system. Once the images have been combined into a common three-dimensional matrix, anatomic structures can be defined, measured, and segmented into individual objects. Anatomic objects, such as the mandible, maxilla or teeth, can be analyzed in detail. These models can be used for diagnosis, treatment planning, treatment simulation, communication, and treatment outcomes evaluation. There are no commercial programs available to combine multiple image sets into a three-dimensional matrix. Acuscape International, Inc., (Glendora, Calif.) has software available for research purposes and is in the process of developing a commercial version (Figures 8, 9, and 10).

 

Figure 8a. This series of three-dimensional digital images shows the stages of model construction using a three-dimensional camera that employs structured light. Figure 8a shows a series of points (vertices) that have been connected with lines to form a polygon mesh. The three-dimensional locations of the vertices are being continuously computed as this object is displayed and rotated. Figure 8b. The polygon mesh has been tiled to provide a surface to this object. Figure 8c. The tiled polygon mesh has been smoothed. Figure 8d. The tiled polygon mesh has been textured with the patient’s photograph. Figure 8e. A rotation of the three-dimensionally rendered face.

Figure 9a. A surface-rendered model of a patient’s skull.	Figure 9b. The skull from Figure 9a and a spatially accurate registration of a polygon mesh of the facial soft tissues.	Figure 9c. The skull from Figure 9a rotated to a frontal view. These patient models were created using Acuscape Sculptor software from a standard series of two-dimensional cephalometric and digital photographic projections. Acuscape’s Sculptor program was used to spatially calibrate, register into a three-dimensional matrix and measure these images.

Figures 10a through d. An Acuscape model of the mandible and mandibular teeth. The mandible and teeth have been segmented into individual objects. The locations of these objects, with six degrees of freedom relative to the global patient reference planes, are being continuously computed as the objects are moved during dynamic modeling, treatment simulations and viewing.

Dynamic Modeling

Three-dimensional dynamic modeling is emerging as a useful way to analyze structural and functional interactions. These models can be used to predict muscle, occlusal, and articular biomechanical events during simulated function and examine deviations in form and function. Dr. Alan Hannam, University of British Columbia, has developed dynamic models that can be used to analyze the interactions between form and function. Commercial models are under development by Acuscape International.

Future Role of Imaging Centers Patient-specific three-dimensional interactive models will be a valuable aid to the general dental and specialty practices. However, model construction within individual practices will not be practical because of the time and expertise required. Model building is an ideal task to be outsourced to an imaging center. The imaging center can build the model and send it via the Internet or soft copy to the referring doctor. The referring doctor can view and interact with the model utilizing a special set of software tools designed to extract the desired information. New input devices such as three-dimensional digital cameras and TACT imaging systems will be ideal additions to an imaging center because it has the expertise to adopt the technology early and make it available to the dental community.

Authors

David C. Hatcher, DDS, MSc, is part owner of Diagnostic Digital Imaging in Sacramento, Calif.

Craig Dial, DRT, is part owner of Diagnostic Digital Imaging.

References

1. Hatcher D, Maxillofacial Imaging, Occlusion: Science and Practice, Ed. Charles McNeill, Quintessence Publishing, 1997, 349-364.

2. Dixon C, Diagnostic imaging of the temporomandibular joints. Dent Clin North Am 35:53-74, 1991.

3. Hatcher D, Diagnostic Imaging, Reconstructive Preprosthetic Oral and Maxillofacial Surgery. Davis H, Fonseca R, eds. WB Saunders, Philadelphia, 1995, pp 86-123.

4. Woods D, Commercial applications of Tuned Aperture Computed Tomography. J Cal Dent Assoc, 27(12):XXXX-XXXX, 1999.

To request a printed copy of this article, please contact/David C. Hatcher, DDS, MS, Diagnostic Digital Imaging, 1 Scripps Drive, Suite 101, Sacramento, CA 95825.