Virtual Diagnostics Using Cone Beam CT

Advances in medical imaging, computer-aided design (CAD), and computer- aided manufacturing (CAM) are changing the traditional diagnostic processes used in dentistry. Computed tomography (CT) has been the primary imaging process used in dentistry to gather 3-dimensional data about a patient. This information is saved as an isometric data set that can be reformatted into a 1:1 rendering of the patient. The most common use of this data has been for implant planning and the creation of computer-generated models and surgical drill guides.
Conventional CT has been very useful, but the equipment is expensive and bulky, and the imaging facility is usually located a distance from the dental clinic. New cone beam CT machines use lower radiation,1-2 are less expensive, and have faster image acquisition. In addition, image data from cone beam CT can be joined with scan data of patient models and a virtual library of teeth and dental components. Jaw motion can also be recorded digitally and used to control motion of the virtual image of any patient. This data is as accurate as conventional techniques and can be transmitted via the Internet to any individual involved in the patient's treatment. Further, this information can actually be used to manufacture any number of dental devices used in patient treatment.

CONVENTIONAL AND CONE BEAM CT

Figure 1. Conventional CT. Figure 2. Cone beam CT.
Figure 3. Rendering of soft tissue and bone. Figure 4. View of condyle and fossae.

Most dentists are familiar with conventional medical CT machines. These machines are usually located in hospitals or imaging centers. The patient is placed in the prone position and imaged a slice at a time using a fan-shaped x-ray beam. By having the patient positioned on a movable table, the area of interest can be scanned in incremental slices (Figure 1). These machines are quite large and usually occupy an entire room, with a second smaller room for the computer and imaging technician. They are also expensive, since the equipment is quite complex and coordinated movement of the table must be maintained with the imaging x-ray beam and array of sensors.
Cone beam CT is a new form of computed tomography that is simple, less expensive, and uses much less radiation. Unlike the fan-shaped x-ray source in a conventional CT, the cone beam system uses a cone shaped x-ray beam and only exposes the patient to one circular movement of the x-ray source and sensor (Figure 2). This system is simple and does not require segmental movement of the patient through the x-ray source.3 As a result, a motorized table is not required, and the patient can sit in a chair for the scan as in conventional dental radiography. This also allows the soft tissues of the patient to drape in a natural form, and the image can be made with the patient's head in the natural head position.
It is also possible to view the soft tissue, hard tissue, and planned restorations in the same 3-dimensional computer space (Figure 3). The patient's teeth can be positioned in centric relation, centric occlusion, or any other required diagnostic position. This makes it possible to evaluate the osseous relationship of the condyles to the fossae when the teeth are in a specific relationship (Figure 4). Much more information can be gained about the spatial relationship of teeth, soft tissues, and bone using this system when compared to conventional cephalometric films. Cone beam technology also allows for the aesthetic evaluation of a patient and an assessment of the soft-tissue and hard-tissue proportions of the face. Specific measurements of bi-zygomatic width, lip length, interpupil distance, facial proportions, and the angulation of natural and artificial teeth in relation to supporting bone can easily be made.

SCATTER RADIATION

Figure 5. Scatter due to dental restorations.

Figure 6. A 3-dimensional rendering of scatter.

Figure 7. Composite computer model of teeth and bone.

Computed tomography provides excellent 3-dimensional images of osseous tissue, but dental restorations and fillings made of metal, ceramic, and gutta-percha can cause scatter and a blurring of the image (Figure 5). Cone beam machines are noisier than conventional CT due to the inherent cone beam image reconstruction process.4 Scatter is projected along the occlusal plane, and when the CT data is rendered as a 3-dimensional computer image, it may be impossible to determine the actual form of the restorations and teeth (Figure 6).
By joining the CT image with scan data of the patient's dental casts, it is possible to create an excellent virtual model that has micron-level data regarding the teeth with sub-millimeter data regarding the bone and soft tissues. It is important to plan for this problem and to include radiographic markers during the CT scan so that they can be used to orient the scan data from the dental casts. The casts can be scanned with laser, light, or contact digitizers to produce a computer model. Figure 7 illustrates the composite model of the teeth from a contact digitizer and the mandible from cone beam CT.

VIRTUAL TREATMENT PLANNING

Figure 8. View of restoration, drill guide, bone, and implants.

Cone beam CT is commonly used to plan for implant treatment. Generally, the 3-dimensional rendering of bone is used to determine the proper placement and size of dental implants and to create computer-generated drill guides with stereolithography.5 In addition to planning the implants, it is also possible to plan for the actual restoration prior to implant placement. This provides the benefit of knowing how much space is available for restorative materials and the proper contours that are needed for aesthetics and hygiene.
This 3-dimensional model of the implants, bone, restoration, and drill guide can easily be sent to the referring dentist or surgeon via the Internet to coordinate treatment and provide excellent communication. Any object that can be visualized from the CT data or can be scanned directly can be joined in virtual space to create a composite computer model. The model in Figure 8 joins CT data with virtual implants, artificial teeth, and the shape of the drill guide into one composite 3-dimensional image.

COMPUTER-MANUFACTURED RESTORATIONS

Figure 9a. Virtual design for the restoration.

Figure 9b. Restoration for try-in.

Figure 9c. Clinical view of completed restoration.

Figure 10a. Virtual design.

Figure 10b. Completed prosthesis.

Figure 10c. Clinical view of full-mouth implant restorations.

Virtual treatment planning can determine the ideal position for restorative components, and by using computer-generated drill guides it is possible to create an ideal environment for the manufacture of the final restoration using advanced techniques. The tissue and implants can be scanned to create a virtual model of the planned final restoration (Figure 9a). The actual casting can then be produced using a technique called layered manufacturing.
First, the computer model is cut into thin slices, and then an ink-jet technology is used to reproduce the shape of the model in 0.005-inch slices.6-9 Each layer is joined to the previous one until the complete pattern has been manufactured; it can then be cast in any metal and joined to artificial teeth using conventional laboratory techniques (Figures 9b and 9c). This same technique can be used to fabricate fixed maxillary restorations with excellent aesthetics, speech, and contours (Figures 10a to 10c).

CONCLUSIONS

The present process of diagnosis and treatment in dentistry is about to undergo a dramatic transformation. The highly successful methods of treatment developed in the last century provide the foundation for the use of new tools. Cone beam CT make3-dimensional data affordable to the average patient and will improve the diagnostic and restorative capabilities of all clinicians. As we begin the 21st century, medical and dental use of digital imaging, computer-aided design, additive manufacturing, milling, and the Internet will improve the quality of care, improve communication, and reduce costs.


References

1. Mah JK, Danforth RA, Bumann A, et al. Radiation absorbed in maxillofacial imaging with a new dental computed tomography device. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2003;96:508-513.

2. Ludlow JB, Davies-Ludlow LE, Brooks SL. Dosimetry of two extraoral direct digital imaging devices: NewTom cone beam CT and Orthophos Plus DS panoramic unit. Dentomaxillofac Radiol. 2003;32:229-234.

3. Moore WS. Cone Beam CT: a new tool for esthetic implant planning. Tex Dent J. 2005;122:334-340.

4. Endo M, Tsunoo T, Nakamori N, et al. Effect of scattered radiation on image noise in cone beam CT. Med Phys. 2001;28:469-474.

5. Caldwell CS. Practical incorporation of computed tomography into daily implant treatment planning. Tex Dent J. 2005;122:343-354.

6. Schmitt SM. Output: the final step in digital diagnosis and treatment. Academy News - quarterly publication of the Academy of Osseointegration. 2004;15:1,8-9. Available at: www.osseo.org/downloads/2004/news/AO_News_Vol15.pdf. Accessed March 2006.

7. Schmitt S. Changing peoples lives with rapid prototyping and manufacturing. Presented at: 3rd Annual Eugene C. Gwaltney Manufacturing Symposium; October 1-3, 1996; Georgia Institute of Technology, Atlanta.

8. Schmitt SM. Dental lab technology in the digital age. J Dent Technol. 2001;18:18-21.

9. Schmitt S. Digitally designed denture attachments. Collaborative Techniques. 2003;3:4-6.


Dr. Schmitt received his DDS degree from the University of Minnesota School of Dentistry in 1975. He earned his specialty certificate in prosthodontics in 1982 from the Wilford Hall US Air Force Medical Center, San Antonio, Tex, and his master of science degree in prosthodontics from The University of Texas Health Science Center at Houston, Dental Branch. Prior to his retirement from the Air Force, Dr. Schmitt was chairman of the Department of Prosthodontics and program director for graduate prosthodontics at Wilford Hall USAF Medical Center. He also served as consultant to the USAF Surgeon General for prosthodontics. He is a diplomat of the American Board of Prosthodontics, a fellow of the American and International College of Prosthodontics, and a member of the Dental Implant Clinical Research Group. He can be reached at (210) 587-6857 or This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

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