Safe, Effective Digital Imaging

Allan G. Farman, BDS, PhD, MBA, DSc (AF): The basic radiation safety issues are germane to both digital and analog radiographic techniques. The dentist should always clinically inspect the patient and obtain his or her health and dental history prior to prescribing radiographs based upon the individual's needs. Radiographs should only be taken when they are determined to be clinically necessary. Eliminating unnecessary radiographic procedures is the most important approach to minimize patients' lifetime exposure and increase safety. Selected radiographic procedures should be collimated to limit the radiation beam to the area of interest and the size of the detector. For intraoral radiography, rectangular collimation is preferred over round collimation. In addition, use of a "long-cone" technique is preferred to narrow the cone of radiation and reduce the exposure of extraneous tissues as well as to reduce image distortion through magnification.

When a radiograph is needed, it must be diagnostically acceptable for the tasks at hand. A goal of radiography should be to minimize the need for retakes. In situations where multiple radiographs are taken (eg, intraoral radiographic studies), all should be inspected to determine whether information missing from any specific image is available in a different image. Rapid feedback should not be a reason for increasing the number of retakes to attain "perfection beyond what is clinically necessary." For both digital and analog radiographs, the patient should always be draped in a lead apron, and when possible, a thyroid shield should also be used.

Young children are considered to be especially sensitive to the untoward effects of ionizing radiation; hence, radiographic imaging should be used sparingly on children and always with a "child-size" dose. The guidelines for clinical necessity follow the Image Gently^® campaign. Among very young children who only possess primary teeth, the spacing between the teeth often makes visual inspection and the use of transillumination to detect dental caries diagnostically suitable without the need to use radiography for this purpose.

Digital radiographic systems that use either complementary metal oxide semiconductor (CMOS) sensors or photostimulable phosphors have wide recording ranges that make incorrect exposure unlikely. This is wonderful for avoiding retakes because under- or overexposure sometimes occurs with film imaging, but it means that the practitioner must be especially careful to restrict the dose used to the lowest possible that will still produce a diagnostically acceptable image. Some CMOS detectors provide feedback to reduce the likelihood that the patient is overexposed for multiple images; this is a desirable feature.

ID: Are there still issues with the digital imaging and communications in medicine (DICOM) standard and the Health Insurance Portability and Accountability Act (HIPAA) when transferring radiographic images between practitioners or laboratories?

AF: DICOM is the International Organization for Standardization (ISO)-referenced standard in medicine and dentistry for the transfer of digital images, and it is particularly important for larger image sets, such as those acquired by cone-beam computed tomography (CBCT). It is best to use this standard for the transfer of all diagnostic images to ensure interoperability between systems, sites, and practitioners. HIPAA privacy and security rules for protected health information (PHI) must be followed. For all images, identifiable metadata (eg, patient name, date of birth, etc) must be kept secure. Regarding CBCT image volumes and photographs, patient identification is possible from these data and both are also considered to be PHI. The transfer of such images and metadata must be accomplished  using secure methods rather than public ones or unsecured email transfers. Practitioners must have HIPAA confidentiality agreements in place with all outside entities with which digital images are shared, including laboratories.

ID: Has CBCT become the standard of care for implant, endodontic, and surgical procedures as well as for identifying airway challenges? Has its value shifted from diagnosis to treatment planning?

AF: All radiographic images should be made primarily for diagnostic purposes. This includes image volumes taken for endodontic procedures, for evaluation of the dentition and jaws in those who require orthodontics, for examination of the jaws when assessing surgical and implant procedures, and for examination of the airway in patients with sleep apnea and hypopnea. The use of planning algorithms and simulations certainly add valuable additional uses for diagnostic image volumes, but radiography should not be employed for these purposes exclusively.

In orthodontics, CBCT can be useful to examine complex cases in three dimensions, especially for those involving gross facial asymmetry and those that require surgical procedures. Limited volume CBCT scans are also useful in inspecting for possible root resorption associated with impacted teeth and various jawbone pathoses.

If diagnostically necessary, any CBCT image volume can certainly also be used for secondary purposes, such as a replacement for taking an impression; however, it is undesirable to subsequently take CBCT images in the absence of additional diagnostic necessity.

CBCT is rarely the primary identifier of airway challenges; these are usually ascertained by signs and symptoms experienced by patients or their close relatives. Although CBCT cannot replace a physical workup using air resistance tests and sleep monitoring, it might occasionally be useful in planning certain interventions after apnea or hypopnea has been identified. The results of studies have been contradictory concerning the value of CBCT in measuring the flexible airway, generally in the upright position.

In endodontics, high-resolution, limited-volume CBCT is very useful for looking at cross sections of tooth roots to establish the number of canals present and to more clearly demonstrate the sources of apical pathoses. It can also be useful in examining for root fractures but is not always foolproof in this respect. For complex endodontic cases, one 3-dimensional CBCT image is certainly better than multiple 2-dimensional intraoral images taken at different angles, and it can be acquired with a comparatively similar or lower radiation dose. CBCT is an adjunct to rather than a replacement for intraoral radiography in endodontics.

The area in which CBCT has achieved the most success is undoubtedly in the 3D evaluation of jaws for the placement of dental implants. In my opinion, it should likely be considered the standard of care for excluding pathoses in this area, determining the need for pre-placement bone surgery and grafting, and selecting optimal implant sizes and positions. Many helpful software programs can interact with CBCT image volumes for these purposes.

In addition, CBCT is useful for planning or-tho--gnathic and jaw surgeries involving a healthy jawbone and simulating possible outcomes as well as providing data for model-making for the prefabrication of surgical repair devices. CBCT is also useful for demonstrating fractures of the jaw following trauma. For the diagnostic radiology of tumors and other pathoses that have a soft-tissue component, CBCT is not a substitute for multislice computed tomography (with or without contrast) or magnetic resonance imaging (MRI). CBCT is excellent for imaging bony tissues and the teeth but not for soft-tissue depiction, except for visualizing the boundaries between soft tissues and air-filled open spaces.

ID: As digital radiography becomes more widespread, can you envision a completely film-free future for dentistry?

AF: The newest generation of dentists is being trained almost exclusively using digital radiographic methods and is highly unlikely to regress to analog film. As older dentists retire, the use of analog film will eventually decrease to the point where the dental film industry is no longer viable.

ID: What advances do you foresee in digital extra- and intraoral technology during the next decade?

AF: Going forward, I believe that the digital revolution in dentistry is likely to replace impression materials with laser light scans and that the precision in this will grow to surpass that of conventional impressions. This change will be combined with chairside CAD/CAM applications in mainstream dental practice, and scans will only be sent to special laboratories for certain advanced procedures. For this to be fully realized, improvements will be necessary in the precision of standards for determining and registering tooth color, hue, transparency, etc.

There will be further attempts to use non-ionizing radiation for diagnostic purposes, particularly in pediatric dental practice. Three decades ago, Chester Wang brought a group of inventors together to develop an ultrasound and light system to replace cephalometric radiography. Its name was "Dolphin," and now the only remaining parts of the invention are excellent software programs that are used with cephalometric imaging and CBCT. Given the tremendous advances that have occurred in computers and also in ultrasound, perhaps it is time to consider trying to reinvent the physical machine?

Allan G. Farman, BDS, PhD, MBA, DSc
Independent Consultant
Maxillofacial Imaging Science
Chicago, Illinois