The Evolution and Application of Dental Maxillofacial Imaging Modalities

During the last decades, an exciting new array of imaging modalities, such as digital imaging, CT, MRI, positron emission tomography, and cone-beam CT (CBCT), has provided astounding new images that continually contribute to the accuracy of diagnostic tasks of the maxillofacial region. The most recent, cone-beam imaging, is gaining rapid acceptance in dentistry because it provides cross-sectional imaging that is often a valuable supplement to intraoral and panoramic radiographs. The information content in such examinations is high and the dose and costs are low. The increasing trend toward the use of CBCT in dental offices may be expected to result in improved diagnosis, but with increased patient dose and health care costs. Using CBCT as a secondary imaging tool helps optimize health-to-risk ratio.

Dental radiology has long played an exciting and critical diagnostic role in dentistry, never truer than now with the rapidly expanding array of imaging modalities. Intraoral radiography was first used within weeks of the discovery of X rays by Roentgen in 1895. Extraoral imaging, including cephalometric radiography, followed soon thereafter. Panoramic radiography has provided broad coverage of the teeth and surrounding structures since the mid-twentieth century. Each of these modalities has adapted to the digital revolution. Recent decades have seen the development of CT, MRI, nuclear medicine, and ultrasonography, imaging modalities that have revolutionized dental and medical diagnosis. CT can be simply defined as the use of the X ray–based imaging method to produce three-dimensional (3D) images usually displayed in the form of image slices. The original CT technology, which is used extensively in medical diagnosis, is designated as medical CT (CT) and the newer modality used primarily in dentistry is cone-beam CT (CBCT). The recent development and application of cone-beam imaging in dentistry provides most of the benefits of CT imaging for many dental applications at a substantial savings of dose and cost. This introductory article provides an overview of the imaging principles underlying each of these technologies, identifies dental applications, and, in particular, focuses on the emerging role of cone-beam imaging in dentistry. Some areas of CBCT that need further attention are also considered. Conventional tomography has been substantially replaced by CBCT and is not considered.

Periapical radiography

Conventional intraoral periapical and bitewing radiographs are familiar and ubiquitous, and, when well made, they provide excellent images for most dental radiographic needs. Their primary use is to supplement the clinical examination by providing insight into the internal structure of teeth and supporting bone to reveal caries, alveolar bone loss associated with periodontal disease, periapical disease, and a wide range of other dental and osseous conditions.

Intraoral imaging

Intraoral imaging still provides the best spatial resolution of any imaging method. However, as a result of collapsing 3D structural information onto a two-dimensional (2D) image, spatial information is lost in the third dimension. For instance, does the radiolucency seen on the crown of a lower first molar come from a lesion in the buccal pit or on the occlusal surface? What is the relationship of the unerupted mandibular third molar to the mandibular canal? The clinician must attempt to reconstruct mentally this 2D image into a 3D reality with limited information from 2D images.

Film

Film is highly flexible, literally and figuratively. Well-processed film radiographs offer highly detailed images at a low cost. Unfortunately, film processing is often suboptimal, with deleterious consequences to image quality. Furthermore, maintaining a darkroom requires space and time and has environmental costs.

Digital

During the last decade, many dental practices replaced film with digital imaging systems. Common reasons for making this transition included improved patient education, lower exposure, greater speed of obtaining images, and the perception of being up to date in the eyes of patients . Two broad types of digital systems are commercially available for dental offices. Most common are the solid-state sensors, made using either charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) technology. The alternative technology is photostimulable phosphor (PSP) plates, also called storage phosphor plates.

Charge-coupled device–complementary metal oxide semiconductor

These systems use rigid sensors placed intraorally to capture the image. A cable usually connects the sensor to the computer. When a radiograph is made, the remnant x-ray beam exiting the patient is captured in a scintillator coating a silicon chip. The scintillator immediately re-emits visible light photons that cause electrons to be released within the silicon chip, resulting in a voltage differential. The amount of the voltage is proportional to the exposure. The smallest unit from which a voltage can be recorded corresponds to a pixel, the smallest picture element. Although the actual voltages in a series of pixels may comprise a continuous spectrum, an analog signal, they are converted to a more limited set of 256, 1024, or more specific (digital) values. This image information is optimized by proprietary software and the resulting image is displayed on a computer monitor within seconds of the exposure.

Photostimulable phosphor

PSP systems use imaging plates to capture an image. These plates are typically thinner than solid-state sensors, have no wires, and are flexible. When an image is made using a PSP system, x-ray photons strike a phosphor coating on the plate and cause electrons to be stored in a higher energy state. Following the exposure, the plate is removed from the patient’s mouth and placed in a laser reader for scanning. When a laser light is directed onto the exposed plate, the electrons return to their ground state and release visible light. This amount of light, which is proportional to the exposure, is read in a photomultiplier tube. The analog signal is digitized, optimized, and displayed on a monitor, as with the solid-state detectors. This process takes from a few seconds to a few minutes.

Clinical considerations

Because film, the CCD-CMOS, and PSP systems all offer essentially equivalent diagnostic information, a decision to “go digital” is made for other reasons. One of the more important reasons for using digital imaging is to facilitate patient education. Displaying a large image on a computer screen provides dentists an excellent opportunity to educate patients regarding their condition and treatment needs. Other important benefits of digital imaging include a reduced chance of losing films and the fact that images can be transferred easily by e-mail, especially useful for second opinions.

The CCD-CMOS systems provide almost immediate images with the sensor still in the mouth, which allows the operator to examine the resulting image, make any adjustments in orientation of the sensor or x-ray aiming tube that may be required for a retake, or proceed to the next exposure, often without having to remove the sensor from the mouth. This consideration is an important one for endodontists. In contrast, with the PSP systems, the sensor must be removed from the mouth after each exposure, as with film. This system is thus more analogous to film and is fully appropriate for restorative dentistry. It is best to scan the plates soon after exposure because the latent image decays with time.

The rigidity of the CCD-CMOS sensors may cause difficulties in positioning the sensor in the patient’s mouth, in terms of patient discomfort and displaying the desired anatomy. This problem is particularly acute in offices where the patient traditionally holds a flexible film in his/her mouth while an exposure is made. The best means to position the sensor is to use holders that can support the sensor deep in the patient’s mouth, well away from the teeth being imaged. This technique will allow the apical end of the sensor to capture tooth apices and surrounding bone with minimal discomfort to the patient. Furthermore, the sensor holder should have an external guide to allow accurate positioning of the x-ray tube head. On the other hand, the flexibility of some of the PSP plates may lead to image distortion if the patient bends it while holding it in the mouth with his/her finger. Again, it is best to use a holder that supports the plate in the middle of the mouth parallel to the teeth, and which has an external guide ring for aligning the x-ray head.

The choice of a digital system can be influenced by costs. CCD and CMOS systems require an initial cash outlay for sensors of several thousands of dollars each. A PSP laser reader costs approximately $10,000 and the plates cost a few tens of dollars each. Both systems may have additional costs for software and computer equipment. Unlike film, PSP plates can be reused, although they may become scratched and require replacement.

Image processing

An often-touted advantage of digital systems is the ability to use image-processing tools such as brightness, contrast, and sharpening routines to improve image interpretation. Although such tools may indeed improve the subjective appearance of images, evidence is scant of improved disease detection or diagnosis given a well-exposed initial image. Indeed, if these tools are applied improperly, degradation of an image, potentially leading to inaccurate interpretation, is a real possibility.

Dose

Many exaggerated advertising claims have been made about dose reduction with digital imaging. The consensus is that CCD and CMOS systems do require somewhat less exposure than F-speed film. However, difficulties with use typically prompt more images to be made than when film is used, thus obviating their dose advantage. PSP plates present another interesting situation. These plates have broad image latitude and are thus able to accept unnecessarily high radiation exposures and still display high-quality images. Care must be taken to assure that low exposures, essentially the same as those used for F-speed film, are used, to avoid unnecessary overexposure of the patient.

Data management

With all digital systems, it is critical to plan for adequate data storage and backup capabilities for the patient management and digital imaging systems. It is often useful to send images to an insurance carrier or a colleague. To accomplish this, images can typically be exported into a nonproprietary format such as JPEG or TIF and sent as e-mail attachments. Images can also be printed onto thermal film or papers but they often lose image quality in this process.

Panoramic imaging

Panoramic imaging has been evolving continuously since its introduction in the 1950s. The basic imaging principle is that of curved surface tomography. A narrow, vertical x-ray beam is directed through the patient’s head. The exit beam then passes through a slit in a shield on the opposite side, where it is captured on a receptor, either film or digital. The x-ray source and film rotate synchronously around the patient’s head. The receptor also travels behind the shield as it rotates around the patient. The image shows on the receptor side of the patient because the rate and direction that the receptor travels behind the shield is the same as the rate at which the x-ray beam passes through the teeth and other structures on the receptor side of the patient. This process results in a sharp image of the teeth and bone on the film side of the patient, whereas structures on the tube side are blurred beyond recognition. The patient doses from CCD and PSP systems are comparable to those received from film/intensifying screens. The resolution of all systems is comparable.

Film

Film has been used in panoramic machines since its inception and always in combination with matching intensifying screens. These screens contain rare-earth elements and fluoresce green or blue light when struck by x-ray photons. This light exposes the silver bromide crystals in the film and forms the latent image that is subsequently made visible by film processing.

Charge-coupled device

CCD sensors have been used in recent years to capture the image in some panoramic machines. A 6 in × 12 in sensor is too expensive to manufacture; a linear array, approximately 4 pixels wide, is used to capture the image. This linear CCD array is read out continuously as the exposure is being made, thus building up the image from one side to the other. The image data are then stored on a hard disk and may be displayed on a monitor.

Storage phosphor plates

PSP technology is also used to acquire panoramic images. In this case, large plates, approximately 6 in × 12 in, are used. The plates are exposed and then read in a laser reader. The resultant image is displayed on a computer monitor and stored like other images.

Clinical considerations

Panoramic radiography is excellent in providing an overview of oral hard tissues, including presence and location of teeth, foreign bodies, cysts, tumors, and other conditions within the jaws. The resolution of panoramic images is sufficient for many dental tasks but less than that provided by intraoral imaging and thus may be insufficient to reveal early or subtle disease. Panoramic radiographs are most useful when full coverage of the jaws is desired or when a specific region that is too large to be seen on a periapical view is desired. For many patients not having extensive dental disease, a thorough clinical examination accompanied by a panoramic view plus four bitewings serves as a good initial examination. Then, on the basis of these images and clinical findings, additional supplemental periapical views may be indicated .

The major advantages of panoramic images are the broad coverage of oral structures, the low patient dose (about 10% of a full-mouth examination), and the moderately low cost of the equipment (compared with cone-beam imaging [see later discussion]). Some newer panoramic machines incorporate cone-beam technology.

The major limitations of panoramic imaging are the reduced resolution compared with intraoral images and the fact that the focal trough is fairly thick, enough to see the full thickness of the alveolar ridges, and thus it presents essentially a 2D projection image of these regions. Also, image distortion and the presence of phantom images of anatomy outside the focal trough, such as overlapping cervical spine images on the anterior maxilla and mandible, can artificially produce apparent changes or may hide significant findings.

Panoramic imaging

Panoramic imaging has been evolving continuously since its introduction in the 1950s. The basic imaging principle is that of curved surface tomography. A narrow, vertical x-ray beam is directed through the patient’s head. The exit beam then passes through a slit in a shield on the opposite side, where it is captured on a receptor, either film or digital. The x-ray source and film rotate synchronously around the patient’s head. The receptor also travels behind the shield as it rotates around the patient. The image shows on the receptor side of the patient because the rate and direction that the receptor travels behind the shield is the same as the rate at which the x-ray beam passes through the teeth and other structures on the receptor side of the patient. This process results in a sharp image of the teeth and bone on the film side of the patient, whereas structures on the tube side are blurred beyond recognition. The patient doses from CCD and PSP systems are comparable to those received from film/intensifying screens. The resolution of all systems is comparable.

Film

Film has been used in panoramic machines since its inception and always in combination with matching intensifying screens. These screens contain rare-earth elements and fluoresce green or blue light when struck by x-ray photons. This light exposes the silver bromide crystals in the film and forms the latent image that is subsequently made visible by film processing.

Charge-coupled device

CCD sensors have been used in recent years to capture the image in some panoramic machines. A 6 in × 12 in sensor is too expensive to manufacture; a linear array, approximately 4 pixels wide, is used to capture the image. This linear CCD array is read out continuously as the exposure is being made, thus building up the image from one side to the other. The image data are then stored on a hard disk and may be displayed on a monitor.

Storage phosphor plates

PSP technology is also used to acquire panoramic images. In this case, large plates, approximately 6 in × 12 in, are used. The plates are exposed and then read in a laser reader. The resultant image is displayed on a computer monitor and stored like other images.

Clinical considerations

Panoramic radiography is excellent in providing an overview of oral hard tissues, including presence and location of teeth, foreign bodies, cysts, tumors, and other conditions within the jaws. The resolution of panoramic images is sufficient for many dental tasks but less than that provided by intraoral imaging and thus may be insufficient to reveal early or subtle disease. Panoramic radiographs are most useful when full coverage of the jaws is desired or when a specific region that is too large to be seen on a periapical view is desired. For many patients not having extensive dental disease, a thorough clinical examination accompanied by a panoramic view plus four bitewings serves as a good initial examination. Then, on the basis of these images and clinical findings, additional supplemental periapical views may be indicated .

The major advantages of panoramic images are the broad coverage of oral structures, the low patient dose (about 10% of a full-mouth examination), and the moderately low cost of the equipment (compared with cone-beam imaging [see later discussion]). Some newer panoramic machines incorporate cone-beam technology.

The major limitations of panoramic imaging are the reduced resolution compared with intraoral images and the fact that the focal trough is fairly thick, enough to see the full thickness of the alveolar ridges, and thus it presents essentially a 2D projection image of these regions. Also, image distortion and the presence of phantom images of anatomy outside the focal trough, such as overlapping cervical spine images on the anterior maxilla and mandible, can artificially produce apparent changes or may hide significant findings.

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Jun 15, 2016 | Posted by in Oral and Maxillofacial Radiology | Comments Off on The Evolution and Application of Dental Maxillofacial Imaging Modalities
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