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The Evolution of Digital Imaging Sensors
Radiovisiography (RVG) made its debut in the dental community in the late 1980s. The first digital imaging system used a charge-coupled device (CCD) sensor with a fiber-optically integrated scintillation screen within a hard-wired sensor case that measured 40 mm x 20 mm x 14 mm. When exposed to radiation, the wafer of silicon in the CCD sensor stored the resultant charges in the pixels of the silicon, equivalent to the amount of radiation striking the particular location. After transfer of these charges to the analog-to-digital converter (ADC), the charges are assigned grey-level values that became part of the computer-constructed image.
Then, as is the case today, the digital image is made up of a large numbers of pixels in shades of grey, providing the contrast differences across the image. The active, or radiosensitive, area of this first RVG sensor was only 17 mm x 26 mm, which limited the coverage area of the resultant image.1 This RVG system underwent several modifications during the first few years of manufacturing, as developers strove to improve the exposure latitude of the sensor and to increase the variety of image-enhancement tools in the system.2 Even though the third-generation RVG system increased sensitivity (speed) to more than 46% of the first generation, the sensor properties and dimensions of the sensor remained the same. One of the major limitations of this sensor was the bulk of the sensor and the small size of the active area. This meant that the area of coverage with the image was limited and led to a potential increase in the number of radiographs needed, as well as the risk for more retakes, which could be related to the lack of complete coverage of the periapical areas of the teeth in question.
Gendex Dental Systems and Schick Technologies developed competing CCD sensors in the early 1990s. The Gendex Visualix/VIXA featured a radiation-hardened CCD sensor in 1992 that allowed optimally diagnostic exposures that were up to six times faster than D-speed X-ray film.3 Introduced in 1995, Schick Technologies’ Computed Dental Radiography system offered three sensor sizes (corresponding with common film sizes: 0, 1, and 2).4 In the late 1990s, borrowing from technology that was developed for digital cameras, Schick introduced a sensor with a smaller pixel size that incorporated one or more active complementary metal-oxide semiconductor (CMOS) transistors into the pixels.4 This arrangement allowed each pixel to be isolated and directly connected to the transistor and resulted in a more efficient transfer of pixel voltages, which improved on the CCD sensor for dentistry. Since then, manufacturers of solid-state sensors have opted for either CCD- or CMOS-type technology.
Both offer crisp digitally acquired images with good contrast and spatial resolutions and use exposure factors that provide the highest patient-dose reduction when compared to film. Considering exposure factors that will achieve optimum diagnostic performance as compared to F-speed film, solid-state detectors provide radiation dose savings to the patient that are 70% to 80% compared to analog X-ray film.
Photostimulable phosphor (PSP) plate technology had been used in medical radiology for a few years prior to dentistry adapting this technology in the 1990s. PSP systems use a PSP plate that is composed of a plastic base coated with a layer of europium-activated barium fluorohalide. The phosphor layer absorbs and stores energy from X-rays and releases this energy in the form of visible light when scanned with a laser in the range of 600 nm. A photomultiplier tube in the scanner converts the emitted light into electrical energy that is amplified and transmitted to the ADC, where pixels are assigned grey values in much the same manner as that used for solid-state systems.
Solid-state sensors provide a direct connection to the computer via a hard wire, or wirelessly using a radio frequency transfer between the sensor and computer. After a few computer clicks, image capture is virtually instantaneous. With PSP imaging, a latent image is stored on the plate following exposure to X-rays, reminiscent of analog X-ray film. The plate is then read in the imaging scanner, converted to a digital image, and displayed on the computer. This scanning takes about 20 seconds per plate. Most systems erase the plate by flooding it with bright light so that it is ready for reuse once processed. Both systems offer a range of post-processing tools that are designed to aid the clinician with diagnosis.
Sensor Dimensions and Active Areas
CCD/CMOS—Sensor dimension and thickness are important factors in effective digital image capture because the quality of the resultant image is directly related to sensor positioning. One study evaluated sensor parameters, including dimension, by comparing six CCD/CMOS sensors. Including the cable connectors, the thickness of the tested sensors varied from 11.3 mm to 14 mm.5 For the size 2 CCD/CMOS sensors that were tested, the largest active area was 940 mm2 and the smallest was 802 mm2, and F-speed film was 1,235 mm2.
PSP—PSP sensors are manufactured in the same common film sizes of 0, 1, or 2. A size 2 film is 35 mm x 45 mm, and most size 2 plates are 30 mm x 40 mm. Consequently, the active area of the PSP plate is nearly identical to film. PSP plates are slightly thicker on average (1 mm) as compared to film, which is approximately 0.77 mm. In addition, PSP plates have some flexibility and can be slightly bent to increase patient comfort.
Several studies have evaluated sensor comfort during placement.6,7 A 2009 study8 examined the comfort level for patients when performing a radiographic examination using a CCD sensor, a PSP sensor, and radiographic film. In that study, the most comfortable sensor was found to be a PSP sensor. A review article published in 2010 looked at the evidence of patient comfort/discomfort with digital sensors and concluded that patients perceived solid-state sensors as more unpleasant than PSP plates.9
Sensor Sensitivity and Latitude
CCD/CMOS—Solid-state sensors generally require less radiation to achieve optimum diagnostic densities as compared to either film or PSP receptors. Sensitivity of CCD/CMOS sensors depends on detector efficiency, size of the pixels in the sensor, and signal-to-noise ratio (SNR). The noisier the image, the more difficult it will be for the clinician to detect small changes in the image, such as incipient carious lesions. The exposure factors should be as low as possible, while the noise in the image is kept minimal. One study compared SNRs for four solid-state sensors and two PSP sensors.10 This study found that SNRs increased up to maximum level as exposures increased but, depending on the system, the SNR either eventually decreased or stayed the same with increased exposure factors.9 In terms of SNR, solid-state sensors performed as well as PSP receptors.
Latitude can be defined as the ability of a receptor to adequately record a range of exposures while maintaining optimal radiographic density and contrast. For film, an optical density of 2.5 is considered to be the upper exposure limit that will still maintain adequate diagnostic image quality. Solid-state receptors mirror the latitude of analog x-ray film.
PSP—PSP plates are generally low in contrast. However, with the use of image enhancement tools, even low exposures with PSP receptors can achieve relatively high contrast.11 If an optical density of 2.5 is considered to be the upper level on the density gradient for optimal diagnostic capabilities with film, this would be equivalent to approximately two orders of magnitude relative to exposure. PSP receptors have an exposure latitude comparable to five orders of magnitude. The wide latitude of the PSP sensors allows for low patient-exposure factors while maintaining optimum image quality.
Contrast and Spatial Resolution
CCD/CMOS—The ability to distinguish between densities in a radiographic image describes contrast resolution. Contrast resolution is an important parameter to consider when selecting a CCD/CMOS sensor for routine use in the dental office. Attaelmanan et al4 reported that when comparing two sensors from two manufacturers, one sensor demonstrated high-contrast resolution at low-exposure ranges. By comparison, the sensor from the other manufacturer could also achieve similar contrast resolution, but at a 70% higher dose. All the CCD/CMOS receptors tested in this study had peak contrast resolution performance within a narrow range (1,500 µGy to 2,000 µGy).
Spatial resolution describes the capability to distinguish fine detail in an image. Spatial resolution is reported in line pairs per mm (LP/mm). Many factors affect the spatial resolution in solid-state sensors: pixel size, qualities of the scintillator coating, electronic noise, and characteristics of the image display. Some CCD/CMOS sensors have a pixel size of 20 µm, which would be equivalent to a spatial resolution of about 25 LP/mm. This is strictly a theoretical spatial resolution, and when other factors such as those listed above are considered, the actual resolution is in the range of 13 LP/mm to 18 LP/mm.
PSP—Contrast resolution with PSP plates is maintained over a much broader range as compared to CCD/CMOS sensors. Spatial resolution varies with the sensor and system. However, PSP sensors have an effective resolution between 7 LP/mm and 13 LP/mm. The main factor to consider when evaluating the spatial resolution of the sensor is the human eye, which is the true limiting factor in this determination. Without magnification, the human eye is capable of distinguishing only about 6 LP/mm, obviously well within in the range of the resolving power of the digital sensor.
Diagnostic Efficacy of Digital Imaging Sensors
One of the most significant advantages for the use of digital sensors in dentistry is that the technique allows for optimum reduction in the radiation dose to the patient. The need to reduce the patient radiation exposure in medical imaging by as much as possible is encompassed within the principle of ALARA (as low as reasonably achievable). However, the use of a digital sensor must provide the clinician with a resultant image that has optimum density, contrast, and spatial resolution to enable visualization of the smallest details.
Being able to recognize carious lesions in patient images continues to be one of the key diagnostic functions for the dentist. When comparing the efficacy between image receptors, carious lesions are the most often examined pathology in the literature. Numerous studies over the years have performed carious lesion recognition comparisons between x-ray film and digital sensors. A review article written on the use of digital radiography and caries diagnosis was published in 2006.12 Both in vivo and in vitrostudies were selected. In vivo studies reported the hard-wired sensors gave rise to more positioning errors than PSP plates. However, with so few studies that involved patients and the lack of a gold standard for assessing the presence of caries, the efficacy of the digital sensor in clinical studies cannot be fully determined.12 Many more in vitro studies have been conducted throughout the years, with multiple ones comparing digital sensors to various x-ray film speeds. The relatively large number of studies comparing the accuracy for carious-lesion recognition between digital sensors and x-ray film concluded, with a few exceptions, that sensors performed as well as film in recognition of both simulated approximal and occlusal carious lesions.12
The other factor that must be considered when discussing the efficacy of the digital sensor is image display. Just as light transmitted through the x-ray film and ambient light in the viewing area affect the clinician’s ability to detect pathology on the image, display of the digital image is impacted by the way in which it is displayed. Factors such as monitor bit depth, dot pitch, and monitor luminance have been suggested as meaningful considerations for digital image display. In 1999, two studies compared various monitors, including a laptop display, when assessing the accuracy for the recognition of caries with digital images.13,14 Both reports concluded that monitor characteristics and quality did not impact a clinician’s ability to recognize carious lesions.
The Future of Digital Sensors
In medicine, more than 75% of clinics in the United States have converted to digital imaging since 2000, but only 36% of dentists had switched to digital techniques by 2007. However, according to estimates, the number of dentists using digital sensors will approach the numbers in medicine this year.15 This is a significant jump in the use of digital radiographic techniques for the practicing dentist. The need for and the utilization of both solid-state and PSP intraoral sensors for routine use in the dental diagnostic process will continue to flourish this decade. Sensors may undergo some slight improvements in the next few years, with the thickness of the solid-state sensors being thinned and patient comfort being improved, but techniques for acquiring and displaying the digital image will probably change very little.
What about the use of extraoral techniques to acquire intraoral-type images? Historically, panoramic imaging alone could never approach the diagnostic abilities of intraoral imaging due to the magnifying factors, projection geometry distortions, and marked overlapping of teeth. In the last few years, optional bitewing programs have been added to panoramic units, making them capable of intraoral-type bitewing imaging.16 One study compared panoramic imaging, extraoral bitewings, and intraoral bitewings. The results showed that routine intraoral bitewings outperformed either the extraoral bitewing technique or panoramic imaging alone.16 Accuracy of these optional programs will likely improve, but it is much too early to predict the elimination of intraoral imaging in the future.
The availability of digital imaging sensors has been a huge boon for dentistry, and their use has made the diagnostic process more effective and efficient. The sensor has made great strides over the past 20 years and has quelled the naysayers. Conversion to the digital sensor in dentistry has progressed rapidly. The literature has confirmed that the sensor is equivalent in imaging qualities as compared to radiographic film. Digital techniques are cleaner and a more patient-friendly means of imaging.
About the Author
Dr. Cederberg is the Director of Clinical Education and Quality Improvement and a professor in the Department of General Practice and Dental Public Health at the University of Texas Health Science Center at Houston School of Dentistry.
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