Digital imaging is increasingly used for evaluation of dental caries, oral pathology, and presurgical and preorthodontic treatment assessment. [1, 2] With this conversion from legacy to digital imaging has emerged the need for a software strategy that allows the communication of patient, diagnostic, and other acquisition data along with the imaging information. DICOM (Digital Imaging and Communication in Medicine) is a standard for handling, storing, printing, and transmitting information in medical imaging.
A DICOM file contains a patient's x-ray image or series of images and other patient-related information (e.g., patient name, identification number, acquisition modality) as selected from a library of standardized terms. The DICOM library is extensive and continually updated to reflect changing identification standards. DICOM files are fully encrypted to allow safe electronic communication over the internet.
DICOM standards were first developed by the American College of Radiology and National Electrical Manufacturers Association. These standards are regularly revised to improve compatibility with electronic records and improve clinical workflow in the medical environment. By using a standardized format, the images and associated data can be viewed regardless of the proprietary acquisition modality that was used to take the imaging study, allowing for cross-vendor interoperability or connectivity. Dentists are also able to communicate with their medical colleagues via HIPAA-compliant internet connections.
Dentistry has been actively involved in formulation of DICOM standards since 1996, when the American Dental Association joined the DICOM Committee. As a result, DICOM standards began to include definitions for image objects, with special categorization for intraoral projections and color photography. Today, most companies producing imaging devices (termed acquisition modalities) include DICOM image identification. Numerous software systems (termed picture archiving and communication systems, or PACS) have been developed to allow for the storage, retrieval, and viewing of digital DICOM images.
Much work still needs to be done before dentistry is fully integrated with respect to DICOM. For example, DICOM images need to be integrated with the many electronic dental record software products currently available. In February 2010, the DICOM group that represents the dentistry specialty met to discuss issues related to the DICOM standards, including the use of imaging in diagnosis, treatment simulation, treatment guidance, and tissue restoration.  Future work is to include the development of guidelines for standardization of digital photographic structured displays for both intraoral and extraoral projections, the creation of templates for reports, the development of guidelines for presentation states including overlays used in dentistry, and surgical workflow issues within DICOM used in dental implantology.
In DICOM terminology, the device used to take an image is called an acquisition modality. Interoperability is important for an acquisition modality; it should be fully functional and operate without glitches. Theoretically, all devices should be able to connect with any other DICOM-compliant product; this is outlined in the conformance statement.
The DICOM conformance statement should contain the information necessary for technical use when modalities are connected.  DICOM performance statements for various acquisition modalities and other connected machines are typically available for download on the manufacturer's website. The conformance statements for acquisition devices are standardized to allow comparison of DICOM devices from different manufacturers.
The DICOM conformance statement is simply a diagram that demonstrates, for example, that a digital dental x-ray taken by a specific intraoral imaging product will be downloaded to another vendor’s server and then displayed properly on another company’s computer monitor. According to the Radiological Society of North America, “The Conformance Statement must describe how an activity handles associations (i.e., whether the activity initiates associations and accepts multiple associations) for each activity in the model. Some devices, such as the archives in a picture archiving and communication system, must support multiple associations if performance is to be acceptable. Otherwise, only a single activity (e.g., DICOM storage) could be handled at any given time.” 
The following terms are used in the DICOM nomenclature.  Although they may not be typically used by dentists in their day-to-day professional communications, it is important to understand their meaning if digital imaging is being considered or has been implemented.
Association: The communication connection that is established between two DICOM applications by which DICOM information is exchanged. One or more associations may be supported simultaneously.
Attributes: Items that describe something within DICOM, typically used to describe information objects.
Composite objects: Objects that are defined in DICOM corresponding to multiple or parts of multiple entities in the entity-relationship model.
Data elements: The contents of data sets or descriptive attributes that provide the characteristics of entities in the entity-relationship model.
Data set: The formal description of entities in the entity-relationship model, such as patients, equipment, and images, as well as how they are related based on an information-organization perspective.
Explicit VR: A method of including a value representation of an attribute in the attribute itself.
Implicit VR: A method of defining the value representation of an attribute in the data dictionary as defined by the American College of Radiology and National Electrical Manufacturers Association.
Information object instance: An information object to which real-world values have been assigned.
Information objects: Images, reports, and patients for which the function is to carry information; the descriptive attributes have been listed and defined.
Layer: A set of software or hardware that performs specific functions that are needed for the communications process.
Library: The generally accepted terms describing attributes such as patient name, identification number, and date of birth.
Normalized objects: Objects defined in DICOM that correspond to a single entity in the entity-relationship model.
Object-oriented analysis: The process of determining the object or entity-relationship model model that describes a particular activity.
Packet: A portion of a larger message being communicated that has header information to instruct on the correct location and correct order if multiple packets are sent.
Protocol: The set of rules that allow two devices to communicate with each other. The layered communication whereby one layer in a device communicates with a corresponding layer is determined by protocols.
Service: A set of functions that are performed to allow communication between layers within a device.
Stack: A set of layers that allows communication to applications.
Tag: A numeric name for an attribute or data element.
Transfer syntax: The manner by which the value representation of the data elements is presented and their encoding (e.g., the byte order and compression type) is performed.
Unique identifier: A numeric construct that is used when an entity is referenced. It is a unique name that allows the finding and retrieval of the desired entity and allows its distinction from other entities.
Value representation (VR): The description of how the attribute value is represented, such as by text, binary data, or patient name.
An acquisition device is any instrument that produces a digitized image, including computed tomography (CT), magnetic resonance imaging, ultrasound, digital projection radiography, and x-ray machines that allow periapical, panoramic, or cephalometric imaging. Dental digital imaging also includes three-dimensional CT, digital photography, and digitally-driven computer-aided design/manufacturing systems (CAD/CAM). 
Generally, newer standard dental digital imaging equipment (e.g., intraoral digital x-ray systems, panoramic imaging, cone beam CT) is DICOM compliant. However, the standards for DICOM compliance for some devices, including three-dimensional computerized tomography and CAD/CAM systems, as well as interoperability with respect to picture archiving and communications systems (PACS), have not yet been established.
Most medical centers and large physician-based health management organizations use PACS to manage digital DICOM files, including acquisition, storage, retrieval, and viewing. In dentistry, the use of PACS is primarily limited to academic centers and dental clinics in large hospital facilities where there is need for transmission of data between departments.  In these settings, the use of DICOM-compliant imaging is critical for a number of reasons, including HIPAA compliance.
With the implementation of national health care, the growing interest in teledentistry, the continuing pressure for dental compliance with national imaging standards, and the convenience that the technology affords, it is likely that more moderate- to large-size dental facilities will begin to move towards PACS implementation using the DICOM format. Smaller group practices likely will follow their lead.
DICOM Uses and Technical Considerations
Digital imaging systems are being increasingly used in the hospital and academic dental settings, [8, 7, 9, 10, 11, 12, 13] particularly in the specialties of orthodontics, oral surgery, oral medicine, and implantology.
A review article by Grauer et al  described DICOM as it relates to cone beam computed tomography (CBCT) images in orthodontics, the measurement of CBCT images, the creation of 2-dimensional (2D) radiographs from DICOM CBCT files, segmentation engines and multimodal images, the registration and superimposition of 3-dimensional (3D) images, and special applications for quantitative analysis and 3D surgical prediction. Abramovitch and Rice emphasize the importance of new software programs in the incorporation of CBCT into dental practices. 
Most sophisticated software applications allow the transformation of images along different planes. They also allow visualization of a specific region of interest at different angles and the scaling of sections as desired. Typically, there are multiple threshold filters for differentiating tissue density, clipping tools, and transparency filters for soft and hard tissues.
Although rendering images for qualitative assessment is appropriate in 2D imaging, quantitative assessment in 3D rendering has limitations. For example, most landmarks visualized in 2D either cannot be visualized or are difficult to locate on a curved 3D surface. In addition, a rendered image is impacted by many factors, including contrast, movement during acquisition, the presence of metal, signal-to-noise ratio, and the threshold filters that have been applied by the operator. Thus, it is recommended that landmarks need to be located within a stack of slices. With respect to accuracy and reliability of measurements on CBCT images, there may be differences depending on the way landmarks are located within the slices.
Grauer et al  also described the creation of 2D radiographs from DICOM files. Measurements performed on these synthetic cephalograms from CBCT sections are, on average, similar to standard cephalograms. However, there may be an increase in landmark error calculation. Another facet of using multimodal images is in the different segmentation processes. A segmentation engine in the software allows the user to distinguish between the virtual surface and a rendered image. Therefore, the user can export anatomic models and has the option of combining different modalities with the DICOM CBCT images (e.g., combining digital models through laser or optical scanners with the CBCT data).
DICOM 3D software systems have not yet been linked to diagnosis classification. In addition, some of the available tools have not yet been validated with respect to accuracy and precision. Therefore, the data included on a DICOM file should be interpreted with caution. Grauer et al  suggested that further research is needed regarding the interpretation of orthodontic information from CBCT data.
Implant planning may also use DICOM-compliant imaging. In one study, data from a CT scan taken with an intraoral template in place was stored on a CD-ROM in DICOM3 format and then uploaded into an implant software program, where it was then used to calculate parameters necessary for site preparation. The authors noted that this approach may improve surgical accuracy, particularly for regions where exactness is important, such as in cases where anatomical sites have little space (e.g., an atrophic maxillae) or where there have been sinus lifts necessitating a zygomatic approach. To establish accuracy during the actual procedure, infrared light-emitting diodes were connected to rotatory instruments and to the patient’s template, then viewed in real time on a monitor. Such computer-assisted navigation systems in development but DICOM connectivity issues have yet to be resolved. 
Two other areas of dental application related to DICOM are oral surgery and the diagnosis of dental caries. With respect to oral surgery, DICOM files offer considerable information that is useful in diagnosis and surgical planning. However, some evidence suggests that the complexity of the DICOM file data may be problematic in research. A study exploring CBCT-based DICOM files and the properties of these images used to evaluate maxillofacial bone grafts suggested that DICOM files can be used in research looking at issues associated with graft viability, but that the parameters associated with these files may hinder comparison of results between studies, thus compromising their scientific impact. [14, 16]
In the realm of caries diagnosis, one concern with DICOM-compliant digital images is whether they can reliably detect proximal caries when images are viewed on different monitors. Research suggests that the monitor type may not be important. For example, the results of one study evaluating the differences in brightness and contrast adjustment in monitors with different technical standards for the diagnosis of carious lesions suggests that the differences in monitor presentation do not alter a clinician's ability to detect carious lesions, regardless of whether the digital image is presented on a DICOM precalibrated color monitor or a monochromatic monitor. 
Another issue relates to the digital compression of images. It is unclear whether file compression associated with DICOM storage alters the subsequently viewed image in such a way as to hinder diagnosis. At least one study involving assessment of root fracture suggests that it does not. The results of this study failed to show significant differences between uncompressed and compressed images in the detection of root fractures. [18, 19] Other studies also indicate that compression generally does not affect landmark identification of, for example, lateral cephalometric digital radiographs  and reproducible cephalometric point. 
Two innovations in DICOM and PACS implementation include use of the internet for image transfer and a mobile device as a viewer. [22, 23] PACS systems allowing DICOM images to be sent via the internet represent a significant step forward in the communication of digital files. Using the internet, a clinician with proper identification can access his or her clinic's PACS server and upload and display a patient imaging file (using the unique identifier attributed to the DICOM file) while at a distant satellite clinic or other remote location.
Recent advances in the digital display capabilities of mobile devices, including tablets, dramatically increases the clinical possibilities associated with internet-based image transfer.  Internet-based electronic dental records delivery is also currently available but these software systems may not, in all cases, be fully integrated with DICOM imaging solutions. 
In the Health Insurance Portability and Accountability Act (HIPAA), the U.S. government has mandates that health care facilities protect patient information and provide it only to professionals. With DICOM images, 128-byte encryption should generally provide security in the transfer of images across public networks.
However, the implementation of PACS varies with respect to infrastructure. Therefore, there may be inherent weaknesses in the deployment of security standards.  To improve upon the security of PACS and DICOM image transfer, the concept of a digital signature (applied like an invisible watermark on top of the encrypted patient information from the DICOM image header) has been implemented in some locations. In this case, a dedicated PACS security server can be used as a gatekeeper, checking and certifying the image origin and integrity of the requesting user.
Another issue related to security is the transfer of DICOM images via CD-ROM. Use of CD-ROMs to transfer imaging information, including DICOM images, is common in dentistry and medicine. It is assumed that such transfer of imaging data is HIPAA compliant because the integrity of the data is supported by DICOM encryption and the fact that it is delivered directly to the requesting party. However, research assessing data vulnerability has demonstrated that data files on CD-ROM are vulnerable to alteration, and these changes are not detectable without detailed analysis of file structure. Alterations can be done without change in the DICOM readers. Some authors have concluded that CD-ROM transfers should not be considered safe,  particularly when there is a potential for financial or other gain to be had from altering the data and when the copy cannot be cross-checked with the original information.
Finally, it should be appreciated that any transfer of non-DICOM images related to patient care via the internet does not conform to HIPAA standards. Such transfer is subject, at least in the United States, to state and federal consequences.