CE Course 6

Dental Digital Radiographic Imaging

Credit: Continuing Education Hours: 2
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Table of Contents

  1. Describe the two types of radiographic digital imaging.
  2. Describe the three types of digital receptors used in digital dental radiographic imaging.
  3. Discuss the advantages and disadvantages of the different receptors.
  4. Describe infection control procedures that should be used with digital receptors.
  5. Discuss legal issues that surround the use of digital imaging.
  6. Describe pitfalls and artifacts associated with the use of digital receptors.
  7. Discuss the application of the DICOM standard to digital imaging in dentistry.
  8. Explain image processing and its purpose.

Assessment Method: Post-test only


Abstract

Radiographs are an important adjunct to providing oral health care for the total patient. Historically, radiographic images have been produced using film-based systems. However, in recent years, with the arrival of new technologies, many practitioners have begun to incorporate digital radiographic imaging into their practices. Since dental hygienists are primarily responsible for exposing and processing radiographs in the provision of dental hygiene care, it is imperative that they become knowledgeable on the use and application of digital imaging in patient care and record keeping. The purpose of this course is to provide a comprehensive overview of digital radiography in dentistry. Specific components addressed are technological features, diagnostic software, advantages and disadvantages, technique procedures, and legal implications.


Introduction

Since the 1970s, advancements in imaging technology have made dental digital radiographic imaging a reality in oral and maxillofacial radiology.1 Dental digital radiographic imaging includes all non-film- based methodologies and is often referred to as computed dental radiography, direct dental radiography, or simply as digital radiography.

In the late 1980s, the Trophy Corporation introduced the first dental digital radiography system to the dental profession.2 The system was a Charged Coupled Device- (CCD) based system called RadioVisiography (RVG). Since the launching of RadioVisiography, various companies have introduced many more CCD-based systems offering a variety of options. More recently Schick Technologies, Inc. introduced Complementary-Metal-Oxyde-Semiconductor (CMOS) detectors. CMOS is the technology used for all modern microprocessors and memory chips. In addition to CCD/CMOS detectors, in the early 1990s, Digora introduced photostimulable phosphor (PSP) technology to dentistry, a technology that has been used in medicine for a number of years. CCD and PSP detectors offer oral health professionals many advantages, primarily because the images produced with these systems can be manipulated using computer software. However, it is important to note that neither system increases the information available for diagnosis. What digital imaging allows is the alteration on how the information is displayed.

Many dental practices are currently using this technology or beginning to make the transition from film-based imaging to digital imaging. Typically in many oral health practices, dental hygienists expose, process, and utilize radiographic images in their patient care and for use by other clinicians in the practice. Not only do educators need to be aware of its use and prepare future graduates to use this new technology, but practicing dental hygienists unfamiliar with this technology need to update their skills through continuing education. Thus, the purpose of this course is to provide a comprehensive overview of dental digital imaging technology through technological background information, diagnostic software, advantages and disadvantages of the imaging system, technique procedures, and legal implications.


Technical Features

There are many similarities between film-based and digital radiology. For example, both techniques require the use of an X-ray source and use the paralleling technique. The major differences are found in the type of image receptors used and how the data is processed to produce the images. This section will explain the mechanics of producing a digital image and the three types of digital systems currently being used.

Image production:
In order to produce a radiographic digital image, a source of ionizing radiation, an image detector, a computer, and monitor to display the image are required. As with conventional dental radiography, the detector is positioned in the mouth with a holding device similar to the types used to hold the film in place. X-rays pass through the dental structures carrying information to the detector, first sensing it and then capturing it. This information is then transformed or converted from an analog form (continuous data) to a digital format (discrete data) that can be read by the computer. The image can then be displayed on the computer monitor using up to 256 shades of gray. The image is made up of individual picture elements or pixels (Figure 1). For example, if a cross-stitch picture is viewed from a distance, the colors appear to be continuous, as if from one paintbrush stroke. Upon closer examination of the picture, one can visualize each individual cross-stitch that composes the picture. Thus, each stitch represents a pixel or discrete data point.3

The analog to digital converter measures the amount of radiation registered in the sensor and converts it to digital form by assigning a number using a binary system where each binary digit (bit) is represented by a numeral zero or one. These bits are combined into an eight-bit word, or byte, that allows a maximum combination of 256 (2 to the exponent 8) gray levels. On average, the human eye is able to discern up to 32 levels of gray, therefore 256 gray levels provide the viewer information through different window levels. Windowing allows the information to be represented in different shades of gray making it possible to show soft tissue and hard tissue structures without having to take additional images.4

Types of Digital Imaging and Sensors
Digital images can be acquired directly or indirectly. Direct image acquisition can be accomplished through the use of three different types of detectors. The first type is the charge-coupled device (CCD) detector (Figure 2). Approximately the size of dental film, the CCD sensor has a slightly smaller sensitive area; a thicker, rigid case; and an electrical lead that attaches to the computer unit. Aradiation-sensitive circuit inside of the case determines the amount of voltage received from the X-ray beam. The specified amount is then converted into a numerical value, which is assigned a gray level displayed on the computer monitor. The image is displayed on the computer monitor almost immediately using this system.

The second type uses a Complementary-Metal-Oxyde-Semiconductor (CMOS) detector. CMOS detectors have the same characteristics as CCD sensors except they use active pixel technology (patented by Schick Technologies, Inc. for use in dental and medical radiology) and are less expensive to manufacture. The sensors are visually indistinguishable.

The third type of digital detector is the photostimulable phosphor (PSP). Used most often in medical radiology, PSP’s are probably more easily adapted to dental radiology. The sensors are manufactured in a variety of sizes similar to dental film (sizes 1, 2, 3, 4, and extraoral). Although slightly thinner than dental film, the sensor can be adapted to most intraoral film holding devices (Figure 3). Unlike the CCD/CMOS, this detector does not require an electrical lead and has properties similar to intensifying screen phosphors. As the phosphor layer of the detector is irradiated, the electrons become trapped in the phosphor. Thus, the plates hold the latent image until it is “processed.” During processing with a laser, the electrons are released and emit a blue light proportional to the intensity of the X-rays attenuated in the phosphor layer. The light is then converted to a digital form, and the data can be displayed and seen on a computer monitor.4

Indirect digital images are made from radiographs acquired from conventional techniques. It involves digitization of the image using a scanner or a digital picture of the image (Figure 4). The digital picture can then be imported into the electronic patient chart from the camera disk or viewed by any other imaging software such as Adobe Photoshop (San Jose, California).


Digital Imaging and Communications in Medicine (DICOM)

Most digital systems that are being sold today are stand-alone systems and do not necessarily interface with each other. This means that an image obtained with a Gendex sensor cannot be viewed with a TREXtrophy system or vice versa, unless one goes through a series of complicated steps. This hampers oral health professionals in their ability to exchange images without going through a complex process. As a result of the inability of systems to communicate with each other, the American Dental Association Informatics Task Group has adopted the Digital Imaging and Communications in Medicine (DICOM) standard. The DICOM standard has long been used by the medical establishment and specifies file formats to allow images to be viewed cross-platform. Once manufacturers of digital dental equipment become compliant with the DICOM standard, oral health professionals will be able to exchange images with more ease, making image viewing a simpler process. For specific information on the DICOM standard, visit http://ddsdx.uthscsa.edu/DICOM.html or http://www.dicomanalyser.co.uk/ html/introduction.htm.


Advantages and Disadvantages of Digital Imaging

One of the biggest advantages of digital imaging is the ability of the operator to post-process the image. Post-processing of the image allows the operator to manipulate the pixel shades to correct image density and contrast, as well as perform other processing functions that could result in improved diagnosis and fewer repeated examinations (Figure 6). With the advent of electronic record systems, images can be stored in the computer memory and easily retrieved on the same computer screen and can be saved indefinitely or be printed on paper or film if necessary. All digital imaging systems can be networked into practice management software programs facilitating integration of data. With networks, the images can be viewed in more than one room and can be used in conjunction with pictures obtained with an optical camera to enhance the patients’ understanding of treatment (Figure 6). Digital imaging allows the electronic transmission of images to third-party providers, referring dentists, consultants, and insurance carriers via a modem. Digital imaging is also environmentally friendly since it does not require chemical processing. It is well known that used film processing chemicals contaminate the water supply system with harmful metals such as the silver found in used fixer solution.5,6 Radiation dose reduction is also a benefit derived from the use of digital systems. Some manufacturers have claimed a 90% decrease in radiation exposure, but the real savings depend on comparisons. For example, the dose savings will be different if Insight film (F speed film) with rectangular collimation is used versus Ultra-Speed film (D speed film) with round collimation. Clearly, a much greater dose reduction will result from the change of Ultra-Speed film with round collimation to Insight film with rectangular collimation.

Disadvantages
There are also disadvantages associated with the use of digital systems. The initial cost can be high depending on the system used, the number of detectors purchased, etc. Competency using the software can take time to master depending on the level of computer literacy of team members. The detectors, as well as the phosphor plates, cannot be sterilized or autoclaved and in some cases CCD/CMOS detectors pose positioning limitations because of their size and rigidity. This is not the case with phosphor plates; however, if a patient has a small mouth, the plates cannot be bent because they will become permanently damaged (Figure 7). Phosphor plates cost an average of $25 to replace, and CCD/CMOS detectors can cost more than $5,000 per unit. Finally, since digital imaging in dentistry is not standardized, professionals are unable to exchange information without going through an intermediary process. Hopefully, this will change within the next few years as manufacturers of digital equipment become DICOM compliant.


Image Processing Techniques

Once a radiograph has been processed, the image is permanent and further adjustments cannot be made. If the image is too dark or too light, the image has to be repeated. However, this is not the case with digital images. All digital systems employ a stable electronic circuit called a bit, or binary digit. A circuit containing a bit can electronically be switched into two states, off or on. Off is represented by a zero and on is represented by a one. If a shade or color is assigned to the zero and the one then only two colors can be used, black or white. Digital devices used in radiographic imaging must be able to represent more than two colors. To image several shades of gray there must be more than one bit, or multibits.

The number of bits corresponds to the number of gray levels dis-played by a particular system and is calculated as follows: L = 2n where L is equal to the number of gray levels and n the number of bits. For example, an 8-bit unit can display 28 or 256 shades of gray in an image. Since a digital image is made up of pixels, each pixel is assigned a numerical value corresponding to a shade of gray, thus the density and contrast of the image is adjusted by varying the numerical values of each pixel. Human vision can differentiate approximately 32 gray levels, which means that the dynamic range of the X-ray detection system and the human eye do not match. As a result, the computer must be manipulated to show the proper density and contrast of the final image. Most manufacturers treat the raw data with a firmware before the image is displayed. This simply means that the software in the system uses certain algorithms or mathematical computations set by the manufacturer to optimize the image. However, once the image is dis-played, it can be further processed by the operator to change parameters as desired.

The factors controlling the dynamic range are the window level and the window width. The window level is the level within the possible shades that is used to create the middle density in the image. This means the window level controls the image density. The window width is the range of gray shades that will be used in creating the image and therefore controls the contrast of the digital image. A computer usually uses 256 shades of gray to display the image. An increase in window width means that more shades of gray are displayed in the image, resulting in a decrease in image contrast. A too narrow or too wide window width can cause information to be missed in the resulting image. When the entire range of densities is displayed, the image will have lower contrast, or more shades of gray. When a smaller range of densities is displayed, the image will have higher contrast, or fewer shades of grays.

Most software programs are equipped with multiple processing tools and filters, but the most widely used are brightness and contrast adjustments. Many of the image processing tools, such as color conversion and three-dimensional filtering, have no known diagnostic value. Since a digital image can be processed, more sophisticated processing methods can be used. Among these methods are digital subtraction, image synthesis, image restoration, and image analysis. The potential of digital imaging in dentistry lies in the development of practical techniques to perform digital subtraction, Tuned Aperture Computed Tomography (TACT), fractal analysis, and the creation of decision support systems.


Efficacy Studies

In order for a new technology to be embraced by the scientific oral health community, it must be shown that using it results in at least the same diagnostic outcome as what is currently being used. The scientific oral health community is actively engaged in research to answer questions related to new technologies, like digital radiography. A review of the literature revealed an abundance of studies demonstrating that digital radiography performs as well as conventional film in the diagnosing of caries and periodontal disease. In addition, many scientific studies have looked at more advanced imaging techniques for specific tasks, such as file length measurement, detection of bone loss around dental implant sites, the use of algorithms to optimize contrast and density, etc. In most cases, the results overwhelmingly have demonstrated that digital radiography could be substituted for film without any loss in diagnostic information.7-21


Infection Control and Care of Sensors

As with other radiographic procedures, the use of digital detectors requires the same high standards of infection control. Unfortunately, the digital detectors create a greater challenge since they are not disposable. Another problem is that there is a higher potential for damaging them since they are reusable. Damage can result in the production of artifacts that may interfere with the diagnosis of disease. Plastic barriers such as finger cots, plastic bags, or plastic film barrier envelopes have been found in most cases to be effective in protecting the receptor from becoming contaminated. The plastic barriers should be removed after use on each patient to prevent crosscontamination.

PSP sensors can be wiped off with an alcohol swab, but alcohol is not a tuberculocidal-disinfecting agent; therefore every effort should be made to keep saliva from contaminating the plates. Prior to reusing the PSP receptor, the image must be cleared or erased (Figure 8). When clearing the image from the PSP sensor, the viewbox can be draped with a plastic cover for infection control. Another alternative is to turn the viewbox upside down under a counter so that the PSP sensor can be laid on a barrier sheet and placed under the light.


Technique

Radiographic Technique
The recommended technique for imaging intraoral radiographs is the use of the paralleling technique. Rectangular collimation is encouraged with the use of a beam alignment device. Two film-holding devices that align the beam to the receptor are the Rinn XCP Instruments (Rinn Corp., Elgin, Illinois) or Precision Instruments (Isaac Masel, Philadelphia, Pennsylvania). Although all of the systems discussed can be used with a paralleling sensor-holding device, the PSP detector most easily adapts to the majority of film holding devices.

The CCD/CMOS sensors have specially designed devices since these detectors are thicker than film. The holding devices are very similar to the Rinn XCP Instruments and should be positioned in that manner. Manufacturer’s instructions should be followed for determining the display of the images and saving of the images in the patient record. Prior to making an exposure with the PSP sensors, images should be cleared from the sensors by placing them with the phosphor side down on an illuminated viewbox for a minimum of two minutes. Each sensor should then be placed in a plastic barrier envelope with the phosphor side facing the non-transparent side of the barrier envelope. The sensors are then ready to be placed in the film-holding device and exposed with ionizing radiation. After exposure, the sensor is removed from the barrier envelope and immediately placed in a specially designed light-tight plastic receptacle. At the completion of the radiographic exam, the sensors then are processed to display the digital image.

Processing
The digital processor for the PSP sensor looks similar to a large bread maker. (Figure 9). A removable drum is loaded with each exposed sensor placed into the appropriate sized slot, and the sensors are positioned with the phosphor side showing or positioned outward. The loaded drum is then placed in the processor so that the laser can read the sensors. Depending on the number and size of the receptors to be read, the processing can take from two to four-and-a-half minutes. Once processing is completed, the digital image will be displayed on the computer monitor or can be printed or saved. At this time, the image can be enhanced with the several features available on the software package (i.e. magnification, brightness, contrast, etc.).

Since the CCD and the CMOS receptors render an image almost immediately, no additional processing steps are required. Enhancements can be made once the image is displayed.

Technique Pitfalls
Although the sensors are similar in size to conventional dental film, the CCD/CMOS sensors present additional challenges when positioning them in the oral cavity due to their thickness, rigidity, and cord. Both Versteeg et al. and Malarkey et al. reported more errors and more retakes with the CCD receptor when it was compared to conventional film. Specific technique errors that were observed included cone cuts, incorrect packet placement, and vertical alignment.22-23

Care of Sensors
Care should be taken to protect the PSP sensors from being scratched. Once scratched, a permanent artifact will be displayed during subsequent receptor use (Figure 7). Scratches can occur from fingernails or from sliding the sensor across the viewbox or counter top. As with conventional film, bending the corners of the sensor will leave a permanent artifact on the image. The CCD and CMOS sensors should be checked routinely for damage of the casing or frayed wires where most of the damage occurs.


Legal Implications

The new technologies and advancements in dental radiology also have brought forth new legal and ethical issues that should be considered. Although not new in medicine, dentistry and dental-related industries have done little to establish standards for the use of digital imaging in telemedicine and third party reimbursement (submission to insurance carriers). In our litigious society, radiographs have become increasingly important as evidence in a court of law. Therefore, dentistry must consider the ramifications of the use of digital radiography.

The enhancement features and manipulations that are available with the digital imaging software are positive attributes of this technology. The misuse of these devices may cause ethical and legal problems. Once the original digitized image is manipulated or enhanced, then the digitized image becomes a demonstration and not evidence in a legal case. If standards for encrypted images were established, then the images would maintain their value as legal evidence.24 Many companies are currently using encryption software to prevent the original image from being altered. With encryption software, if the original image is altered, the resulting image will be saved as a new image preserving the integrity of the original. The importance of this was demonstrated in two studies that manipulated digital images and showed how they could be used fraudulently.25-26

Other problems that are inherent with the use of computers are inadvertently deleting image files, computer viruses, power outages, and illegal image manipulation. A preventive approach should be developed to help protect against these problems. One approach would be the required use of encryption codes. Other protective mechanisms might include backup disks, non-erasable permanent files for patient radiographic images, and establishing standards for keeping original radiographic images a certain length of time after the completion of a patient’s treatment. Specific issues surrounding teleradiology such as licensure, professional lia-bility, and patient privacy, confiden-tiality, and security are unresolved for dentistry and should be explored.27


Summary

This CE course provided an overview of digital dental radiographic technology. Course content included similarities and differences between film-based and digital technology as well as procedures for exposing and processing digital images. In addition, advantages and disadvantages of using a digital system, implications for the application of digital imaging software for diagnostic purposes, and legal considerations were discussed. Dental hygienists have a pivotal role in facilitating the transition between film-based imaging and digital imaging. Thus, it is imperative for dental hygiene professionals to become knowledgeable and informed about digital technology and its application in dental practice.


Key Terms

1. CCD – Charged Coupled Device
2. CMOS – Complementary-Metal-OxydeSemiconductor
3. PSP – Photostimulable Phosphor
4. Analog – continuous data
5. Digital – discrete data
6. Pixel – discrete data point
7. Bit – binary digit
8. Byte – eight-bit word
9. DICOM – Digital Imaging and Communications in Medicine


Resources

For specific information about each digital system, please refer to www.odont.aau.dk/rad/Digi talx.htm. An explanation on CMOS technology can be found at www.photobit.com/tech.htm


About the Author

Sally M. Mauriello, RDH, MEd, is an associate professor, Department of Dental Ecology; Enrique Platin, RT, MS, EdD, is a clinical associate professor, Department of Diagnostic Sciences, both at the University of North Carolina School of Dentistry, Chapel Hill, North Carolina.