While most intraoral radiography is still performed with film as the recording medium, the use of digital imaging techniques is rapidly increasing. Although it is possible to produce a digital image by scanning a film radiograph, that technique does not provide any of the advantages of speed and radiation dose reduction that are available when digital images are acquired directly.
There are two major techniques for acquiring digital images: (1) a single-step wired system using a charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) sensor and (2) a two-step wireless system using a photostimulable storage phosphor (PSP) plate (Figure 3-2). The CCD or CMOS sensor is a device that transforms the energy from ionizing radiation into an electrical signal that is displayed as an image on a computer monitor within a few seconds. The sensor is housed in a rigid plastic case that is attached to the computer by a long cord. The sensor is placed in the mouth, the computer is activated, and the exposure is made. Once the image appears on the screen (generally within a few seconds) (Figure 3-3), a number of different software
“enhancements” can be applied although some studies have shown the unenhanced images to have higher diagnostic capabilities than the enhanced ones, most likely due to processing done before the image appears on the screen.
With the PSP technique, the imaging plate (sensor) is thinner and more flexible and is not attached to the computer. After the exposure is made, a plate is inserted into a machine that scans it with a laser, converting the latent image into a visual image on the computer screen. This process takes from 30 s to 2.5 min, depending on the system. The sensor plate must then be discharged before reuse.
Both CCD and PSP digital imaging systems are available for both intraoral and panoramic radiography. They use a standard x-ray machine but replace the film or standard panoramic cassette with a digital sensor. Even though digital systems do not have higher diagnostic capabilities than their film-based counterparts do for the detection of dental caries,
periodontal disease,and periapical lesions,there are a number of other advantages to using digital imaging, including reduced radiation exposure, reduced time of image acquisition, the ability to transmit images electronically, and the ability to be used with a number of image analysis tools.
Once the image is in the computer, a number of procedures can be done, depending on the software available. In addition to the standard contrast and brightness enhancements, there is a measurement tool that can be used to determine the dimensions of a lesion. Specialized software is available for digital subtraction, a method of evaluating changes in radiographs over time. Ideally, “before” and “after” radiographs should be identical other than for the area of interest (although algorithms are being developed that can take similar [although not necessarily identical] images and mathematically “warp” them so that the geometry is the same). The two images are then registered, and the gray levels of the same pixels of both images are compared. Typically, increasing mineral (tooth or bone gain) is portrayed as white, and decreasing mineral is shown as black (Figure 3-4). One commercially available program uses red and green to portray bone loss and gain, respectively. Digital subtraction can be useful for evaluating changes in bone height and/or density in periodontitis and for judging the degree of healing and remineralization of periapical lesions after endodontic therapy. Theoretically, any lesion (including bony cysts or tumors) with the potential of change over time can be studied by the subtraction technique, given a method for standardizing the images.
This need for standardization means that the decision to use digital subtraction must be made at the time of the initial imaging so that follow-up images can be made with the same technique since there is a limit to the amount of geometric image correction that can be done. In addition, a step wedge or other device must be incorporated into the imaging system to allow for correction of differences in density and contrast between radiographs, which would affect the subtraction outcome. Because of the difficulty in standardizing panoramic imaging, digital subtraction is currently more feasible for evaluating changes in lesions small enough to be visualized on intraoral views.
Other software is available for the evaluation of other aspects of the digital images, including software for histogram analysis and for a variety of pattern analyses of the trabecular bone.Although these are currently considered research tools, they may have clinical application in the future.
All of the analyses described above can be done on any type of digital image, whether acquired by CCD or PSP or scanned from a film image.
Some interesting new clinical applications of digital imaging will soon be available. With tuned-aperture computed tomography (TACT®), a series of digital images is acquired at slightly varying angles; the computer then reconstructs the
With the PSP technique, the imaging plate (sensor) is thinner and more flexible and is not attached to the computer. After the exposure is made, a plate is inserted into a machine that scans it with a laser, converting the latent image into a visual image on the computer screen. This process takes from 30 s to 2.5 min, depending on the system. The sensor plate must then be discharged before reuse.
Both CCD and PSP digital imaging systems are available for both intraoral and panoramic radiography. They use a standard x-ray machine but replace the film or standard panoramic cassette with a digital sensor. Even though digital systems do not have higher diagnostic capabilities than their film-based counterparts do for the detection of dental caries,
periodontal disease,and periapical lesions,there are a number of other advantages to using digital imaging, including reduced radiation exposure, reduced time of image acquisition, the ability to transmit images electronically, and the ability to be used with a number of image analysis tools.
Once the image is in the computer, a number of procedures can be done, depending on the software available. In addition to the standard contrast and brightness enhancements, there is a measurement tool that can be used to determine the dimensions of a lesion. Specialized software is available for digital subtraction, a method of evaluating changes in radiographs over time. Ideally, “before” and “after” radiographs should be identical other than for the area of interest (although algorithms are being developed that can take similar [although not necessarily identical] images and mathematically “warp” them so that the geometry is the same). The two images are then registered, and the gray levels of the same pixels of both images are compared. Typically, increasing mineral (tooth or bone gain) is portrayed as white, and decreasing mineral is shown as black (Figure 3-4). One commercially available program uses red and green to portray bone loss and gain, respectively. Digital subtraction can be useful for evaluating changes in bone height and/or density in periodontitis and for judging the degree of healing and remineralization of periapical lesions after endodontic therapy. Theoretically, any lesion (including bony cysts or tumors) with the potential of change over time can be studied by the subtraction technique, given a method for standardizing the images.
This need for standardization means that the decision to use digital subtraction must be made at the time of the initial imaging so that follow-up images can be made with the same technique since there is a limit to the amount of geometric image correction that can be done. In addition, a step wedge or other device must be incorporated into the imaging system to allow for correction of differences in density and contrast between radiographs, which would affect the subtraction outcome. Because of the difficulty in standardizing panoramic imaging, digital subtraction is currently more feasible for evaluating changes in lesions small enough to be visualized on intraoral views.
Other software is available for the evaluation of other aspects of the digital images, including software for histogram analysis and for a variety of pattern analyses of the trabecular bone.Although these are currently considered research tools, they may have clinical application in the future.
All of the analyses described above can be done on any type of digital image, whether acquired by CCD or PSP or scanned from a film image.
Some interesting new clinical applications of digital imaging will soon be available. With tuned-aperture computed tomography (TACT®), a series of digital images is acquired at slightly varying angles; the computer then reconstructs the
resultant data to provide information about depth. The image can be manipulated to bring various layers into focus, thus permitting determination of depths of lesions and relationships between structures(Figure 3-5). Another computer reconstruction technique under development uses a Scanora panoramic x-ray machine (Soredex/ Orion Co., Helsinki, Finland) and a special image intensifier to produce three-dimensional data of a cylindrical volume of tissue. The data can be reconstructed to provide an image at any angle through the volume, providing a high-resolution computed tomography (CT) scan at a radiation dose a fraction of that required by conventional CT.The developers have dubbed this technique “ortho-cubic CT” and expect to make it available soon (Figure 3-6).
