Digital Imaging Presentation PDF

Analog Versus Digital The term digital in digital imaging refers to the numeric format of the image content and its discreteness. Digital images are numeric and discrete in two terms: The spatial distribution of the picture elements (pixels). The di...

Analog Versus Digital The term digital in digital imaging refers to the numeric format of the image content and its discreteness. Digital images are numeric and discrete in two terms: The spatial distribution of the picture elements (pixels). The different shades of gray of each of the pixels. At each pixel of an electronic detector, the absorption of x-rays generates a small voltage whereas more x-rays generate a higher voltage and vice versa.  Analog Signal Signal Value Digital Image At each pixel, the voltage can fluctuate between a minimum and maximum value and is therefore an Analog-to-digital analog signal. conversion (ADC) Sampling means that a small range of voltage values are grouped as a single value. Narrow sampling better mimics the original signal but 1. Sampling Step leads to larger memory requirements for the resulting digital image.  Analog-to-digital conversion (ADC) 2. Quantization Once sampled, the signal is quantized, which means that every sampled signal is assigned a value (pixel). The location of each pixel is uniquely identified by row and column coordinates within the image matrix. The computer organizes the pixels in their proper locations and displays a shade of gray that corresponds to the number that was assigned in the image matrix. The pixels are so small that the image appears smooth at normal magnification.  Digital Image Receptors Photostimulable Solid-state phosphor (PSP) technology technology Charge-Coupled Complementary Metal Photostimulable Phosphor-coated plate Device Oxide Semiconductors Phosphor Scanners Flat-Panel Detectors  Solid-state technology The key clinical feature of these detectors is the rapid availability of the image after exposure. Many manufacturers produce detectors (solid semiconducting material) with varying active sensor sizes that correspond to the different sizes of intraoral film. The components of intraoral detectors are enclosed within a plastic housing to protect them from the oral environment. The expense of the detector increases with increasing matrix size (total number of pixels); however, the pixel size ranges from less than 20 μm to 70 μm. In dentistry, intraoral solid-state detectors are often called sensors. Because digital receptors are intended to be reusable, they must be handled with greater care than their film counterparts.  The sensor components consume part of its real estate so that the active area is smaller than its Solid-State Detectors total surface area. Sensor bulk, is a potential drawback of intraoral Drawbacks detectors. Most detectors incorporate an electronic cable to transfer data to the computer. The cable makes the positioning of the sensor more challenging and requires some adaptation. It increases the liability of the device to fail due to wear of the cable connections from normal use. Manufacturers resolved these issues in many ways: Changing the location of the cable attachment to the corner of the sensor. Offering reinforced cables to reduce accidental damage to the device. Wireless radiofrequency transmission, however, it necessitates some additional electronic components, thus increasing the overall bulk of the sensor. Photostimulable Phosphor (PSP) technology This technology consists of a phosphor-coated plate in which a latent image is formed after X-ray exposure. The latent image is converted to a digital image by a scanning device through stimulation by laser light. This technology is sometimes referred to as storage phosphor based on the idea that the image information is temporarily stored within the phosphor.  Phosphor-coated plate PSP plates absorb and store energy from x-rays. The PSP is made of barium fluoro-halide combined with a polymer that spreads in a thin polyester base material. They are manufactured in standard intraoral sizes, providing handling characteristics like intraoral film. Plates are placed in sealable polyvinyl envelopes that are resistant to oral fluids and light. PSP plates are susceptible to bending and scratching during handling resulting in defects that induce permanent artifacts in the receptor. Before exposure, PSP plates must be erased to eliminate residual images from prior exposures. After exposure, plates should be processed as soon as possible to avoid image degradation in the form of increased image noise.  Photostimulable Phosphor Scanners Current PSP systems integrate automatic plate erasing within the scanner. Many approaches have been adopted for “reading” the latent images on PSP plates including Soredex, Carestream, and Planmeca. Carestream and Planmeca scanners use radio-frequency identification (RFID) technology to link patient information to the plates. This prevents plates from being scanned into the wrong patient file.  Contrast Resolution Spatial Resolution Digital Detector Characteristics Detector Latitude Detector Sensitivity Contrast Resolution Contrast resolution is distinguishing different densities in the radiographic image. This is a function of the interaction of the following factors: Attenuation of the tissues imaged. Capacity of the imaging system to distinguish differences in numbers of x-ray photons and translate them into gray values. Ability of the computer display to portray differences between gray levels. Ability of the observer to recognize those differences. Contrast Resolution Bit depth controls the number of possible gray levels in the image (0 means black, 255 means white). Current digital detectors capture 8, 10, 12, or 16 bits of data. However, regardless of the number of attenuation that the detector can capture, conventional computer monitors can display a grayscale of only 8 bits. While operating systems such as Windows reserve many gray levels for the display of system information, the actual number of gray levels that can be displayed on a monitor is 242. The most important limitation is the human visual system, which can distinguish only about 60 gray levels under ideal viewing conditions. Considering the typical viewing environment in the dental operatory, the actual number of gray levels that can be distinguished decreases to less than 30.  Spatial resolution is the capacity for Spatial Resolution distinguishing fine detail in an image. Test objects consisting of sets of very fine radiopaque lines separated from each other by spaces equal to the width of a line are constructed with a variety of line widths. At least two columns of pixels are required to resolve a line pair, one for the bright line and one for the dark space. A line and its associated space are called a line pair (lp). Resolution is often measured and reported in units of line pairs per millimeter.  Spatial Resolution Typical observers can distinguish about 6 lp/mm without the benefit of magnification. Intraoral film can provide more than 20 lp/mm of resolution. Unless a film image is magnified, the observer will be unable to appreciate the extent of the detail in the image. Spatial Resolution in Solid-state Technology With solid-state digital systems, the theoretical resolution limit is determined by the pixel size: the smaller the pixel size, the higher the resolution. Alternatively, resolution can be expressed in dots per inch (DPI) or points per inch (PPI). At best, there is one dot or point per pixel. Currently the highest-resolution intraoral solid-state detector for dentistry is approximately 20 lp/mm; however, this level of resolution is not obtained clinically due to a variety of reasons. Clinical spatial resolution depends not only on detector characteristics but also on the size of the focal spot, the source-to-object distance, and the object-to-image distance.  Spatial Resolution in Photostimulable Phosphor Technology Resolution in PSP systems is influenced by the thickness of the phosphor material. Thicker phosphor layers require more diffusion and yield a lower resolution. Current PSP systems can provide more than 10 lp/mm of resolution. A magnified periapical image shows a building-block pattern or pixelated appearance.  Detector Latitude The ability of an image receptor to capture a range of X-ray exposures is termed latitude. A desirable quality in intraoral image receptors is the ability to record a broad range of tissue attenuation differences—from gingiva to enamel. At the same time, subtle differences in attenuation within these tissues should be visually apparent. The latitude of solid-state and PSP detectors is like the latitude of film and can be extended with digital enhancement of contrast and brightness.  Detector Sensitivity The sensitivity, or speed is the ability to respond to small amounts of radiation. Unlike conventional films, there are no classification standards for dental digital x-ray receptors. The useful sensitivity of digital receptors is affected by numerous factors, including detector efficiency, pixel size, and system noise. Current PSP systems for intraoral imaging allow dose reductions of about 50% compared with F-speed film with similar diagnostic performance. However, solid-state detectors require less exposure than PSP systems or film. Digital Image Viewing  Electronic Displays The output of laptop displays is limited in intensity and does not have the dynamic range or contrast found in conventional desktop LCDs. The viewing angle of laptop displays is also limited, and the observer must be positioned squarely in front of the display for optimum viewing quality. Current laptop displays are of sufficient quality to be used for typical dental diagnostic tasks. Desktop versions of TFT LCDs have overcome problems of brightness and viewing angle but consume more power and thus are not suited for laptop configurations.  Display Considerations The display of digital images on electronic devices is dependent on the image display software. Even with the same software, the display of images can vary dramatically depending on how the software handles it. For instance, some displays view a full-mouth series of images on a single screen at normal magnification. Bright background illumination from windows or other sources of ambient light reduces visual contrast sensitivity. Light reflecting off a monitor surface may reduce the visibility of image contrast further. Images are best viewed in an environment in which lighting is quiet and indirect.  Image Processing Any operation that acts to improve, restore, analyze, or in some way change a digital image is a form of image processing. Some of these operations are integrated into the image acquisition in the software which are hidden from the user. (For example: the exposed digital image rarely appears too light or too dark because image processing usually includes automatic gray-scale leveling). Others are controlled by the user to improve the quality of the image or to analyze its contents (such as the data histogram or noise measurements).  Image Restoration Brightness and Contrast Image Image Processing Enhancement Sharpening & Smoothing Digital Subtraction Radiography  Depending on the quality of the Image sensor and the choices made by the manufacturer, various other Restoration operations may be applied to the image before it becomes visible on the display. Several preprocessing steps must be performed to correct the image for known defects and to adjust the image intensities so that they are suitable for viewing. These operations are executed very rapidly by the manufacturer, unnoticed by the user, and cannot be changed.  The term image enhancement implies that the adjusted image is Image Enhancement an improved version of the original one. This can be accomplished by increasing contrast, optimizing brightness, improving sharpness, and reducing noise. Image enhancement operations are often task-specific; what benefits one diagnostic task may reduce the image quality for another task Image enhancement operations are also dependent on viewer preference.  Brightness and Contrast Digital radiographs do not always use the full range of available gray values effectively. They can be relatively dark or light, and they can show too much contrast in certain areas or not enough. The image histogram is a convenient tool to determine which of the available gray values and number of pixels, the image is using. The minimum and maximum values and the shape of the histogram indicate the potential benefit of brightness- and contrast-enhancement operations. Brightness Contrast  Sharpening and Smoothing The purpose of sharpening and smoothing filters is to improve image quality by removing blur or noise. Noise represents random intensity variation and is often categorized as high-frequency noise or low-frequency noise. Filters that smooth an image are sometimes called noise filters because they are designed to remove high-frequency noise. Filters that sharpen an image either remove low-frequency noise or enhance boundaries between regions with different intensities (edge enhancement). Sharpening and smoothing filters may make dental radiographic images subjectively more appealing; however, there is no scientific evidence suggesting an increase in diagnostic value.  Digital Subtraction Radiography When two images of the same object are registered and the image intensities of corresponding pixels are subtracted, a uniform difference image is produced. If there is a change in the radiographic attenuation between the baseline and follow-up examinations, this change shows up as a brighter area when the change represents gain and as a darker area when the change represents loss, such as loss of enamel and dentin owing to caries or loss of alveolar bone height with periodontitis. The strength of digital subtraction radiography (DSR) is that it cancels out the complex anatomic background against which this change occurs and reveals subtle changes. However, for DSR to be diagnostically useful, the baseline projection geometry and image intensities must be closely reproduced—a requirement that is difficult to achieve clinically.  Image Storage and Compression Storage of diagnostic images raises new issues The purpose of image compression is to that must be considered, such as capacity, reduce the size of digital image files for reliability, data integrity, and security. archiving or transmission while preserving The file size of dental digital radiographs varies critical image information. considerably, ranging from 200 kilobytes for intraoral images to 6 megabytes for extraoral Version 3.1 of the Digital Imaging and images. Communications in Medicine (DICOM) standard adopted the Joint Photographic As the use of digital imaging in dentistry Experts Group (JPEG) as the compression continues to expand, the implementation of method, which provides a range of standards for preserving original image data compression levels. becomes urgent. Digital Image Backup Considerations Type of backup media Time and method of backup Backup interval Storage location of backup media Recovery time Recovery reliability Security Future compatibility of backup technology