NewsletterBy Jay B. Reznick, DMD, MD | Jeffery B. Price, DDS, MS
The first dental radiograph was exposed in 1896, a year after German physicist Wilhelm C. Roentgen first produced and subsequently detected what are now known as X-rays. However, it was not until about 1916 that dental X-ray units were commercially available and used in practice (Figure 1). Using this technology, an X-ray beam was directed through dental hard and soft tissues, and those that penetrated through exposed a piece of film, which was placed in a light-restricted pouch that was positioned intraorally. The next innovation did not come until the 1950s with the development of panoramic imaging. This was indeed a revolution, because it enabled clinicians to visualize the entire maxillary and mandibular dentition on a single film. The temporomandibular joints, as well as the floor of the maxillary sinuses and the entire maxilla and mandible, could be seen and evaluated. And, patients loved the fact that nothing needed to be put in their mouths. The panoramic radiograph became the standard of care in oral and maxillofacial surgery, especially because clinicians could see impacted teeth in their entirety, as well as their relationship to the mandibular canal and maxillary sinuses.
In 1994, digital radiography was introduced to dentistry. This technology obviated the need for a dark room and the use of toxic chemicals to process the images. In addition, less radiation was required to expose the sensor and obtain a good image. What’s more, imaging software enabled clinicians to enhance the on-screen image with brightness and contrast adjustments and colorization. The original intraoral sensors were bulky for patients and required a wire to connect them to the image-processing computer. Many of the sensors currently on the market are wireless and much more comfortable for patients.
Despite these radiographic innovations over time, there was one overlying issue that limited what clinicians could do with the images: radiographic images were all still 2-dimensional (2-D). This may not seem like a big deal to most dentists, who had to prove their 3-dimensional (3-D) aptitude to gain admission to and graduate from dental school. However, 2-D radiographs can be very easy to misinterpret due to overlap of anatomic structures (Figure 2). Pathology can be missed, and normal anatomy can be thought to be pathological because of superimposition and/or masking of adjacent structures.In addition, for periapical pathology to be evident radiographically, there must be destruction of the buccal or lingual bony cortex. Patients can go for months without a diagnosis of their odontogenic pain because radiographic signs of the problem are just not evident (Figure 3 and Figure 4). The problem was that, while patients existed in 3-D, dental imaging could only evaluate them in 2-D.
All this began to change in 1998, when the first cone beam computed tomography (CBCT) scan machine was introduced to the European dental market. Three years later, CBCT appeared in the United States. These radiographic units use a technology derived from conventional panoramic radiography to capture a 3-D image of a patient’s dentition and facial skeleton. This image can be manipulated so that the anatomy can be viewed from an infinite number of different angles and with varying thicknesses of tissue. Most CBCT units allow reconstruction of radiographic data to provide a cross-sectional, tangential, axial, sagittal, coronal, and 3-D view. A panoramic reconstruction can also be generated, but it differs in many ways from a conventional 2-D digital panoramic.
Cone beam CT differs from medical-grade CT in a number of ways. First, it uses a cone-shaped, as opposed to a fan-shaped, beam of X-ray photons to expose the image. Rather than a large detector, under which the patient must lie down, the CBCT unit uses a much small detector, which rotates around the head of the sitting or standing patient, similar to a panoramic machine. CBCT can be operated from a standard office PC computer system, and the cost of the equipment and the amount of radiation exposure to the patient is less about one tenth that of a medical CT scanner (Figure 5). These factors make CBCT a practical imaging system for many dental offices, both general practitioners and specialists. A conventional CT scan, with its high-energy dose and powerful computer, is designed to image both hard and soft tissue of the body. It can easily discern between various tissue densities in whatever is imaged. A CBCT uses much lower-energy photons and exposes the patient to considerably less radiation. Because of this, soft tissue is visualized as homogeneous soft tissue, and there is little variation in beam attenuation. Soft-tissue tumors can rarely be visualized. CBCT is optimal for evaluating only calcified structures, such as teeth and bones, which are what dentists are most interested in anyway. On the up side, because the beam is less powerful, scatter due to dental restorations is usually less of a problem with CBCT than with conventional CT.
Three-dimensional dental imaging is now commonplace due to the development of the CBCT scan, and will soon be the standard of care for the evaluation of jaw and dental pathology, facial trauma, and dental implantology.
There are well over a dozen CBCT units on the market. For clinicians considering this technology for their office, the decision-making process can be a daunting task. A number of factors that need to be considered when choosing a unit for the dental office are discussed below. They include: field of view (FOV); the amount of radiation emitted to expose a useful image; the time required to complete the scan; and the software used to evaluate the scanned volume and make a diagnosis.
Field of View
CBCT scanners can be simply divided into large, medium, and small field of view (FOV) units. The type of unit that is best depends on the clinician and what the clinical use will be. Large FOV units are most useful for orthodontics, orthognathic surgery, jaw pathology, temporomandibular joint evaluation, and maxillofacial trauma assessment. These tend to be the most expensive units, ranging from $150,000 to $200,000. The medium FOV units are the most popular with both general dentists and specialists. These are less expensive—in the $110,000 to $165,000 range—and have an average field size of 13 mm to 16 cm in greatest dimension. These are well suited to most indications, including TMJ evaluation, pathology and trauma of the maxilla and mandible, and evaluation of the maxillary sinuses and for planning dental implants. The small FOV units are popular because they are the lowest-priced machines available, at about $85,000 to $120,000. They first became popular in endodontics because they allowed high-resolution scanning of small areas. Many general practitioners are also buying these units, because of their low price point. However, these machines also can be limited in their usefulness because of their small field size, which requires that multiple scans be taken for evaluation of multiple areas of the jaws in the same patient.
The large FOV units are appropriate for the doctor who “wants it all” as far as image volume is concerned. A 15-cm FOV will generally capture most maxillofacial structures of importance, but will not include the back of the patient’s head, the sella turcica, frontal sinuses, or the neck. This may be an issue for some orthodontists and oral and maxillofacial surgeons. If these structures are part of the examination for every patient, a large FOV unit is appropriate; otherwise, supplementing a medium FOV with a conventional antero-posterior and lateral cephalometric projection will usually fill this information deficit when needed. The small FOV units are sufficient for applications involving only endodontics on a single tooth, but for most clinicians, the small field size will soon become a hindrance to patient care.
Radiation Exposure
While it is desirable to expose patients to the lowest radiation dose possible for the sake of their health, there is a trade-off. The higher-energy the photons are, the less background noise will be present in the image, hence a clearer, more diagnostic image. Optimally, it is desirable to have a CBCT that delivers an outstanding image at less than 80
