Multidimensional Imaging: Immediate and Imminent Issues
Allan G. Farman, BDS, PhD, MBA, DSc; William C. Scarfe, BDS, MS, FRACDS; and Michiel van Genuchten, PhD
While dentistry of the future will likely include interventions such as biologic scaffolding and tissue regeneration,1-6 dentistry of the present and near future will continue to be based on the orthodontic movement of teeth and the prosthetic reconstruction and restoration of damaged and missing teeth.7,8 The digital era for implant- and tooth-supported prosthetics based on computer-aided design and computer-aided manufacturing (CAD/CAM) has matured in the past two decades, mostly due to the market-driven development of various generations of visible light impressions. Prior to these developments, clinicians relied on physicochemical impressions and stone models on which to fabricate indirect restorations. Chairside CAD/CAM restoration fabrication, while now mainstream, is still employed by the minority of clinicians. Most certainly, CAD/CAM will grow rapidly at chairside or will be provided by centralized milling laboratories. Similarly, digital data transfer of impressions directly from the mouth and virtual modeling are increasingly being used for planning and executing orthodontic tooth movements.
In the past decade, cone-beam computed tomography (CBCT) has become available for maxillofacial radiographic imaging,9-11 with numerous systems now in use. These rangefrom full maxillofacial field-of-view (FOV) imaging to focused- or limited-field high-resolution imaging of specific regions of individual teeth and jaw segments. While spatial resolution of CBCT systems is insufficient for CAD/CAM restoration of teeth, image-guidance applications of this technologyare expanding and, in some areas, already exceeding purely diagnostic radiology implementation. CBCT is a valuable adjunct to planning dental implant placement, provides sufficient resolution for making virtual or real study models,12 and facilitates construction of surgical templates and, in some systems, the implant-borne prosthesis itself.
The immediate future of dentistry will undoubtedly see great strides toward the fusion of three-dimensional (3-D) data obtained from two or more portions of the electromagnetic spectrum to create a hybrid dataset. Transmission-based imaging (ie, CBCT) will most likely provide the 3-D framework, with supplemental details being given by reflective-based imaging (eg, optical scanners).
The following discussion represents a precis from a brainstorming session involving the three authors concerning the immediate and impending issues related to multidimensional imaging in dentistry with emphasis on CBCT, CAD/CAM, and the prosthodontic workflow. Our purpose was to define areas in which attention should be focused to best use available technologies and to plan and predict implementation strategies for extended use of existing and emerging technologies. As our approach was conceptual rather than specific, we responded to a series of chosen questions.
What has caused the recent attention to multidimensional imaging in dentistry? Fast and inexpensive computers, high-quality digital sensors capable of rapid image acquisition, im-proved image reconstruction algorithms, better image displays, and affordable rapid prototyping systems make it possible for manufacturers to provide digital 3-D cameras and CBCT systems at a price that fits the budget of most dental professionals.10 Many practitioners remember the computers of the late 1980s and early 1990s that preceded even the Windows® operating system-the total memory on the hard drive was not much more than the file size of one large digital image, the displays had low resolution, and the processing times were very slow. By comparison, modern "smart" phones have greater processing capacity.7These developments in enabling technologies now permit multidimensional image guidance in dentistry, from diagnosis to treatment guidance to restoration.
Will multidimensional imaging ever completely replace 2-Dimaging? As the patient is 3 dimensional, diagnosis and treatment should be directed 3-dimensionally. Nevertheless, while there will be ever-greater use of 3-D imaging and simulations, 4-D imaging including the temporal domain, and additional dimensions from fusion of various images such as CBCT and 3-D surface renditions from visible light and other modalities,13,14 there are some circumstances in which 2-D imaging remains best for diagnosis. For example, the radiographic detection of dental caries by intraoral 2-D radiographic detectors is still more reliable and accurate than through CBCT. A beam-hardening artifact from metallic restorations with CBCT would lead to substantial overdiagnosis of dental caries and subsequent treatment of sound tooth surfaces. Furthermore, though most often lower in dose than multislice fan-beam computed tomography (CT), CBCT does impart a higher dose than 2-D transmission imaging. So selection should be based on need. CBCT and 2-D digital radiographic imaging should be viewed as complementary rather than competing modalities.15
How can the radiation dose associated with CBCT be kept as low as reasonably achievable? As with all radiographic diagnostic imaging, the first rule is to make an appropriate selection based on clinically determined need. The second rule is to make excellent images the first time in order to reduce retakes.16,17 The dose necessary to provide these images should be kept to the minimum required to produce an adequate diagnostic image with appropriate exposure parameter selection and collimation of the exposed patient volume to the smallest FOV consistent with the task at hand. For dental implant planning and most orthodontic and orthognathic surgical planning, voxel resolution can be relatively coarse (eg, 0.4 mm), whereas higher resolution combined with a smaller FOV is often the best choice for imaging in endodontics or evaluation of tooth impactions. As the concept of CBCT imaging being the digital virtual framework develops, the fusion of images from systems with higher textural (eg, surface scans) or spatial (eg, intraoral scans) resolution will become more common. Improving CBCT image quality by increasing the exposure parameters (eg, number of base scans, mA, and kVp) will become redundant. All CBCT systems are not made equal-practitioners planning to purchase a CBCT unit should carefully review each product's specifications to determine which best suits their preferred pattern of practice.
The American Academy of Oral and Maxillofacial Radiology (AAOMR) is collaborating with the American Association of Orthodontics to develop a position paper and guidelineson digital imaging for orthodontics, with special attention to CBCT. The AAOMR is also working with the American Association of Endodontists to develop a similar paper for endodontology. These guidelines are expected soon, and additional guidelines will be available for dental implant planning and temporomandibular imaging. The AAOMR is also collaborating with the Academy of General Dentistry to develop a position paper on dental caries imaging.
These guidelines are expected to provide the practitioner with a rational approach for the reasonable use of ionizing radiation based on individual patient needs. As with 2-D radiographs, all clinical decisions should be made on a case-by-case basis using professional judgment. No imaging should be performed simply as routine or to screen for unsuspected pathologies.
Who is responsible for the information included in a CBCT scan? A more appropriate question is: What should we be doing ethically and morally to provide the best service for our patients? A CBCT scan covers a 3-D volume, and the tissues included depend to a large degree on the FOV size. Full FOV scans include the brain, cervical spine, soft tissues of the neck, and paranasal sinuses.18 A dentist is not expected to treat conditions outside his or her professional expertise; however, he or she is not absolved of the moral responsibility to acknowledge deviations within the whole image dataset. If the dental clinician has concerns, then he or she should refer the patient to a relevant healthcare practitioner. If this interpretive standard of care is beyond the dentist's competency, modestly priced over-reads by an oral and maxillofacial radiologist should be considered a necessity, particularly if such over-reads may help detect serious conditions that can be treated in a timely manner.19 An example of a relatively common incidental finding in patients older than 50 is dystrophic calcifications in the soft tissues of the lateral neck consistent with calcified carotid atherosclerosis, which can predispose to stroke. Whether a dentist could be found negligent for overlooking such a finding is uncertain, particularly given the myriad jurisdictions in the United States and state-dependent definitions of standard of care. However, the dental profession has a moral duty to provide the best care for patients. In addition, detection of a peripheral pathology of potential health significance can add to a dental practice's positive reputation.
What standards should we follow in multidimensional imaging? There is but one basic set of standards for image file format: the international Digital Imaging and Communications in Medicine (DICOM) Standard.20 This is the International Organization for Standardization (ISO) referenced standard for image format and communications. It covers all images including visible light and radiographic procedures including simple transmission images, CT, CBCT, magnetic resonance imaging, and ultrasound. The DICOM Standards Committee has 27 active working groups, including WG 22 (Dentistry) and WG 24 (Surgery).21 Such groups often collaborate to advance the DICOM Standard and have recently developed supplements for intraosseous implants, including those used for dental implantology. As dental workflow from imaging to impressions to fabrication and manufacturing of prostheses becomes ever more digital, expansion of the DICOM Standard will be necessary to include all aspects of digital dental production. This is a challenge that will likely be led by interactive project groups of DICOM WG 22 and WG 24. Work item proposals are in process with leading manufacturers involved. To restrain costs for manufacturers and to achieve interoperability for dental practitioners and their patients, universal standards are greatly needed.
What can the dental industry learn from others? Dentistry is not the first industry "going digital." With the development of software in the 1980s, the computer industry changed dramatically and permanently. Today, software is profoundly altering the mobile phone industry with the rise of open operating systems, such as Winmobile, Symbian, and Android. Such software growth affects healthcare, the automotive industry, and many other sectors. The implications transcend engineering; they impact these industries' value chains and business models.22,23
One of the main lessons that these industries have learned-in some cases painfully-is that the amount of software development required to maintain innovation does not allow it to keep building closed end-to-end systems. This lesson certainly applies to the dental industry as the number of its professionals is relatively limited. The development costs of dental software cannot be distributed over as many users as they are in the car or mobile phone industries. To reduce the software development costs per user, both industries have been using standards for many years to provide a consistent user experience.24,25
What can prosthetics learn from dental surgery? The dental surgery value chain has benefited from the DICOM Standard for many years. For example, third-party pre-operative planning software works with data from any CBCT scanner, as long as that scanner generates DICOM-compatible results. This is very different from the current prosthetics value chain comprising scanning, design, and manufacturing elements. In this situation, most manufacturers provide end-to-end "proprietary only" systems to be able to guarantee the prosthetic end results. Currently, there are no standard transfer interfaces defined between scanning and design phases, nor between design and manufacturing. As a result, dental professionals who wish to avail themselves of the totally digital prosthetic workflow currently face being "locked into" a proprietary closed system or can choose from various manufacturers without the full assurance of connectivity, quality, and comprehensive support services that would be facilitated by compliance to a standard. However, with the recent announcement by a prominent manufacturer in the prosthetic value chain to provide open systems, it is apparent that this situation will soon change. It is clear that standards, such as DICOM, will be required to achieve this.
Digital dentistry is in its adolescence-developing rapidly and exploring opportunities in all aspects of diagnosis, treatment, and manufacturing. Efficiencies in both cost and time are being realized, and opportunities are being provided to patients that were not even considered 10 years previously. It is not a matter of whether the prosthetic chain will be entirely digital but when. Dentistry has no crystal ball. Bill Gates has stated:26 "We always overestimate the change that will occur in the next two years and underestimate the change that will occur in the next ten. Don't let yourself be lulled into inaction."
Although preceded by the photography, television, and telephone industries, there is undeniable evidence that multidimensional imaging in dentistry is in transition to a digital environment. As professionals, we need to be assured that the efficiencies offered by these changes are fully realized by developing appropriate guidelines and standards. Interoperability of digital data is a safeguard for the investment made by the dentist purchasing digital systems and is also a protection for sustaining the usability and transferability of patient digital data. Similar to other industries, the dental industry will soon realize that systems based on open standards provide the most competitive solutions in a digital era.
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About the Authors
Allan G. Farman, BDS, PhD, MBA, DSc
Division of Radiology and Imaging Science
Department of Surgical and Hospital Dentistry
University of Louisville
William C. Scarfe, BDS, MS, FRACDS
Division of Radiology and Imaging Science
Department of Surgical and Hospital Dentistry
University of Louisville
Michiel van Genuchten, PhD
Head of Digital Dentistry
Institut Straumann AG
Eindhoven University of Technology
Eindhoven, The Netherlands