Digital Radiographic Imaging
Is the Dental Practice Ready?
Edwin T. Parks, DMD, MS, Diplomate, ABOMR
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ABSTRACT
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Background. Digital radiographic imaging is slowly, but surely, replacing film-based imaging. It has many advantages over traditional imaging, but the technology also has some drawbacks. The author presents an overview of the types of digital image receptors available, image enhancement software and the range of costs for the new technology.
Practice Implications. The expenses associated with converting to digital radiographic imaging are considerable. The purpose of this article is to provide the clinician with an overview of digital radiographic imaging technology so that he or she can be an informed consumer when evaluating the numerous digital systems in the marketplace.
Key Words: Digital radiography; dental radiography; image enhancementAbbreviations: CCD: Charge-coupled device. • CMOS: Complementary metal-oxide semiconductor. • JPEG: Joint Photographic Experts Group. • LZW: Lempel-Ziv-Welch. • PNG: Portable Network Graphics. • PSP: Photostimulable phosphor.
Digital radiographic imaging offers the clinician an improved way to collect, evaluate and present radiographic data. With film-based imaging, what one sees is what one gets, whereas digital radiographic imaging provides clinicians with the opportunity to enhance the image. More importantly, it displays an enlarged image for the patient to see. What clinicians see as a large carious lesion on a film-based radiograph looks to the patient like a speck of dust on a black rectangle.
The Dental Products Report Radiography Survey, published in 2005, reported that 25 percent of respondents used some form of digital radiographic systems and another 18 percent were planning to purchase digital radiographic equipment in the following year.1 The cost, status of the image receptor technology and inadequate infrastructure (for example, office wiring requirements) continue to be reasons for not purchasing a digital system. Ninety-five percent of respondents who owned a digital radiographic system reported that they were satisfied with the equipment and believed that productivity had increased.1 Two European studies reported similar results.2,3 These data concur with van der Stelt’s4 conclusion that digital radiographic imaging is ready for use in contemporary dental practice.
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DIGITAL IMAGE RECEPTORS
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Charge-coupled device.
The charge-coupled device (CCD) and the complementary metal-oxide semiconductor (CMOS) device are the most commonly used image receptors in direct digital systems. The CCD consists of an array of electron wells covered by a layer of silicon. The silicon emits electrons when a photon of X-radiation strikes its surface. The emitted electrons are collected in the electron well, and the magnitude of the electric charge is converted to produce a grayscale image. The primary advantage of CCD technology is the high signal-to-noise ratio.
CMOS.
The CMOS-based sensors have low energy requirements and are composed of an array of field effect transistors with a polysilicon gate electrode placed on top of a polysilicon insulator.5 Both CCD and CMOS sensors are capable of capturing a 12-bit image (212 gray levels). Traditionally, only 8-bit (28 gray levels) images have been displayed. The range of spatial resolution for either type of sensor is 10 to 21 line pairs per millimeter.6
Sensors.
Rigid sensors can be wired or wireless. The wired sensor is physically attached to the computer. The wireless sensor sends the image data to the computer electronically. The primary advantage of CCD/CMOS-based imaging systems is the instantaneous display of the captured image. This allows the clinician to determine the acceptability of the image and enables the patient to see a representation of his or her dental status in real time.
Rigid sensors have two principal disadvantages: cost and difficulty in sensor placement. From a short-term perspective, the costs associated with converting to rigid digital imaging are high. However, the increase in efficiency (for example, no film processing, timely retakes) tends to offset the initial cost. Receptor placement is more technically demanding than is film placement. One cannot bend the corner of a rigid sensor to obtain the bitewing radiograph. Sensor placement becomes less of an issue as staff members become more familiar with the equipment. In addition, optimal use of available space in the mouth (for example, posterior palate toward the midline, midline floor of the mouth) eases receptor placement. The cost of the battery in the wireless sensor and the potential to lose the sensor are important considerations when deciding whether to go wired or wireless.
Photostimulable phosphor imaging systems.
Photostimulable phosphor (PSP) imaging systems are considered indirect digital systems because the image capture is analog (continuous) and the image is converted to a digital image during the scanning process. Information can be lost during the analog-to-digital conversion in the same way a color photograph loses data when scanned. The image receptor for all PSP imaging systems consists of a thin polyester base coated with materials that can store photon energy (europium-activated barium fluorohalide compounds). When the phosphor plates are scanned with a helium-neon laser (632.8 nanometers; red light), they emit blue light in proportion to the energy stored in the phosphor. The emitted light is converted to a grayscale image.7 Again, these images provide a wide range of spatial resolution (6 to 20 line pairs/mm) and usually are displayed as 8-bit grayscale images.6
The main advantage of PSP imaging systems is the minimal modification in technique needed to convert from film-based imaging. The phosphor plate has the same dimensions as traditional film (whereas rigid sensors are much thicker than film). Disadvantages associated with PSP imaging systems include the processing time and receptor life span. The time required to process phosphor plates is less than that required for traditional film, but the system is slower than CCD/CMOS-based imaging. The phosphor plates are not built to be bent or twisted. If bending occurs, defects will appear in the processed images. Sometimes these plate defects are not visible until an image is captured and processed. Bedard and colleagues8 reported that 95 percent of phosphor plates evaluated produced nondiagnostic images after 50 uses.
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COSTS
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The cost comparison for digital systems is complex. Depending on the system, pricing can be by component (that is, sensor, software and interface sold separately) or bundled. In addition, clinicians must consider the cost of consumables (such as bite blocks, printer paper and ink), warranties and upgrades in the total cost. In 2005, Clinical Research Associates compared 17 digital systems.9 Cost was one of the evaluated criteria. The cost for a size 2 sensor ranged from $7,000 to $12,500. A complete operational system cost between $9,000 and $17,800. The total cost of phosphor plate systems ranged between $11,500 and $14,000. Size 2 phosphor plates cost about $23. As of April 2007, the costs of several sensors and sensor systems were fairly comparable to those reported in the 2005 study. Size 2 sensors (comparable to size 2 film) ranged from $6,500 to $12,500, and operational systems ranged from $6,500 (no software) to $16,000 (including sensor and software) (Adam Wahl, Patterson Technology Consultants, Indianapolis, written communication, April 19, 2007).
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EXTRAORAL SYSTEMS
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Extraoral systems are available with CCD, CMOS or PSP sensors. When considering the purchase of a digital extraoral system, dentists should consider the time needed to capture and offload the image data, the file size (original and compressed) and the receptor cost. As with the intraoral phosphor plate, the PSP image receptor for extraoral imaging is the same size as that of conventional film. The cost to produce an area array the size of a panoramic film by using CCD or CMOS sensors is prohibitive. Therefore, manufacturers have worked to develop array sizes that can capture an extraoral image and offload the captured signal in a timely fashion. Most contemporary panoramic machines can be retrofitted to accept either PSP or CCD/CMOS sensors.
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IMAGE COMPRESSION
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Extraoral image file sizes can be quite large. For example, a traditionally sized lateral cephalo-gram (8 x 10 inches) with a spatial resolution of 10 line pairs/mm would produce a file size in excess of 20 million bytes. Consequently, computer storage space becomes a concern. Image compression algorithms provide a mechanism to decrease data file size. Image compression can be lossless or lossy. No information is lost during a lossless compression. Unfortunately, lossless compression algorithms (for example, Lempel-Ziv-Welch [LZW], Portable Network Graphics [PNG]) generate a 2:1 to 3:1 compression.
Lossy compression algorithms can produce much higher compression ratios but can result in lost data. Joint Photographic Experts Group (JPEG) and wavelet transform are two of the more common lossy compression algorithms. Although these algorithms can produce a compression ratio of up to 300:1, the currently accepted degree of compression for most medical imaging tasks is between 10:1 and 15:1. The JPEG 2000 compression algorithm incorporates features of both the JPEG and wavelet compression algorithms, and it is the most recent algorithm supported by the Digital Imaging and Communications in Medicine standard.10
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IMAGE ENHANCEMENT
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Images from CMOS- and CCD-based sensors are displayed virtually instantaneously. Images from PSP systems must be scanned before display. Regardless of how the image is captured, the displayed data are digital and, thus, can be enhanced with software. It is important to remember, however, that the concept of "garbage in, garbage out" holds true for digital imaging. The best image is one that requires no enhancement. This can be achieved by using appropriate kilovoltage, milliamperage and exposure time.
Tyndall and colleagues11 found that enhanced digital images exhibited less diagnostic accuracy than either film images or unenhanced digital images. A large number of studies have evaluated the utility of enhancements, and the results are mixed.11–18 It seems that most enhancements are task- and operator-specific. Clinicians commonly use several enhancements such as density and contrast, measurement, image reversal, magnification, flashlight and pseudocolor.
Density.
Density is the overall darkness in a radiographic image, and usually it is altered by changing the exposure time. With a digital image, density can be changed by adding or subtracting grayscale values from each pixel (Figure 1A
). For example, an image that is too dark can be salvaged by subtracting grayscale values from each pixel, provided that the pixel is not saturated (that is, black). This can be done without altering the contrast, because of the linear relationship between exposure and density inherent in digital sensors. Light images are a more difficult problem. A light image does not contain sufficient information to enhance. Adding grayscale values to a light image produces a darker, yet still inadequate, image.
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Figure 1. Density and contrast enhancement. A. Density enhancement of original image. B. Unenhanced image. C. Contrast enhancement of the original image (compare with B). Contrast enhancement accentuates the density differences viewed in the image.
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Contrast.
Contrast is the difference in densities viewed on a radiographic image. Contrast usually is altered by changing the kilovoltage of the X-ray beam. Contrast also can be enhanced in a digital image by altering the slope of the curve that relates exposure time to density (compare Figure 1B
[unenhanced] with Figure 1C [enhanced]). As the curve becomes steeper, the difference in grayscale values becomes more distinct. Again, if the image has insufficient density, it will have insufficient contrast, and no enhancement can correct this deficiency.
Measurement tool.
The measurement tool is a valuable software enhancement. It has applications in endodontic therapy, as well as in implant treatment planning and any other tasks that require precise measurement. Most measurement tools allow the operator to calibrate measurement to maintain accuracy. It is critical that the recorded measurement be based on the data collected. If the projection geometry distorts the image, the measurement will reflect the distortion.
Image reversal.
Image reversal is achieved by inverting the grayscale of the image so structures that appeared white in the original image appear black in the altered image (Figure 2). Haak and Wicht12 reported that grayscale reversal did not help with the detection of inter-proximal caries. This enhancement can be used to evaluate the placement of fine endodontic files or to evaluate bone healing. The benefits of image reversal depend completely on the viewer’s visual perception and require that he or she experiment with this enhancement before using it on images.
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Figure 2. Image reversal flips the grayscale of the image. The utility of this enhancement depends on the diagnostic task and the clinician’s vision.
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Magnification.
The ability to enlarge an image is a huge benefit when evaluating a subtle or small change or when educating the patient. However, with regard to magnification, more is not necessarily better. As magnification increases, the image becomes pixilated, which makes it difficult to discern a subtle disruption of an edge, such as an incipient proximal carious lesion.
Flashlight.
The clinician uses the flashlight tool to accentuate a small area of the image. The tool is a histogram equalization of the region of interest (Figure 3). The histogram equalization redistributes the number of gray levels in an image to highlight subtle changes captured in the image. As with all image enhancements, the clinician cannot improve what is not captured, so adequate image density is a necessity.
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Figure 3. Flashlight enhancement accentuates the contrast in a particular region of interest by performing a histogram equalization on the selected area.
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Pseudocolor enhancement.
Adding color to an image that traditionally is black-and-white is eye-catching to the patient (Figure 4). Unfortunately, if the color scheme has no logical progression, the value of this enhancement is minimal. Shi and colleagues13 reported that their indexed color scale (that is, color range with a logical progression from light to dark) performed better than black-and-white images at short exposure times.
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Figure 4. Pseudocolor enhancement is shown, although little evidence suggests improved diagnostic capability with this enhancement.
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Numerous researchers are developing software to enhance the diagnostic quality of digital radiographic images.14–19 The development of more user-friendly software enhancements is simply a matter of time.
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CONCLUSION
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Many factors enter into a dentist’s decision to switch from film-based imaging to digital imaging. CCD/CMOS systems and PSP imaging systems have advantages and disadvantages. Assessment of current imaging needs, office infrastructure and the technical and computer skills of staff members is critical to the success of converting to digital imaging. Perhaps the most important concept regarding digital imaging is to focus on the system that will work best in the practice’s office environment rather than buying the most expensive or sophisticated system.
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FOOTNOTES
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Dr. Parks is a professor, Diagnostic Sciences, Indiana University School of Dentistry, Room S110B, 1121 W. Michigan St., Indianapolis, Ind. 46202, e-mail "edparks{at}iupui.edu". Address reprint requests to Dr. Parks.
Disclosure: Dr. Parks did not report any disclosures.
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ABSTRACT
DIGITAL IMAGE RECEPTORS
