Chapter 5 Intraoral Digital Imaging
To understand digital image acquisition
To understand the basics of intraoral digital image processing
BASIC CONCEPTS
1. The computer: A computer with digital imaging software is needed. Its function is to receive and view the digital image, identify the image as to orientation and patient/date data, process the image if needed, store the image, and transmit the image to other offices or third party payers. Safeguards must be in place to protect the patient’s privacy as outlined by the Health Insurance Portability and Accountability Act (HIPAA) law that came into effect April 14, 2003. The computer can be a laptop, a wall-mounted type, a display-only monitor connected to a computer by a wire or wireless networking, or a full computer setup in the treatment room. To get a fast, flawless imaging setup, the computer will need at least 512 megs of RAM (random access memory) and a fast processor with 2.5 gigahertz (GHz) speed, as well as adequate temporary storage in the 80 gigabyte (GB) range. Further high capacity hard drive storage and a server will be needed for later storage, archiving, and retrieval of the images.
2. The sensor: Sensors are either wired or not wired. The wired type can be a CCD (charge coupled device) or a CMOS (complementary metal oxide sensor), which transfers the image instantaneously to the computer at the moment of image acquisition (analogous to exposing the film and developing it). The CMOS type is now available in a wireless format. The film-like wireless type is called the PSP (photostimulable phosphor) plate. The exposed plate or sensor (looks a lot like a piece of film) must then be scanned in a laser scanner, which then transfers the image into the computer.
3. The scanner: A laser scanner is needed only for PSP-type sensors. There are currently three brands: DenOptix, Digora, and Scan-x. The Scan-x scanner has significantly faster scanning times.
4. The imaging software: Once the image is in the computer, the software allows the user to process the image. This means the original image can be altered. For security, most software systems do not allow the user to process the original image so it can be later retrieved if needed by third parties such as insurance carriers or courts of law.
5. Image processing: The original image can be copied then altered by processing to enhance its appearance for diagnostics or to highlight a feature for patient education. Some basic processing options (algorithms) include: rotate or flip the image to get it properly oriented on the computer screen because the sensor cannot distinguish left, right, up, or down; lighten/darken the image (histogram shift); change the contrast (histogram stretch); use filters to sharpen the image, smooth edges, or remove noise; zoom, which magnifies a selected area; reverse the image so that it looks like a negative (everything black becomes white, and the whites become black) (histogram reverse); emboss, which produces a 3-D–like look; colorize all or a portion (e.g., a carious lesion or the inferior alveolar canal); and many other options, including some of those just mentioned that are known by different terms to make the individual brands appear different or unique.
• Spatial resolution is really how clean and sharp the image is. It is a function of having small pixels in the 20- to 40-micron size and lots of them (megapixels). Spatial resolution is expressed in line pairs per millimeter (lp/mm). Digital intraoral x-ray systems are capable of image resolution in excess of 20 lp/mm. The human eye can discern somewhere between 12 and 14 lp/mm. Most intraoral films are in the 11- to 12-lp/mm range. Most high-definition monitors are limited to 8 to 10 lp/mm. Most software systems keep only the best 8 lp/mm. The more lp/mm, the greater is the need for storage room. Resolution is limited to the lowest common denominator in the system; thus a system of 8 to 10 lp/mm represents the most current practical system.
• Gray scale resolution is how many shades of gray are in the image; also known as contrast or bit depth. The imaging system is capable of capturing and separating literally thousands of shades of gray. Contrast is expressed in bits. A 1-bit image has only 2 shades (pure black and white—the darkest and lightest shades of gray in the imaging scale) and is expressed as “1 to the power of 1.” A 2-bit image is expressed as “1 to the power of 2,” or 1 × 2 = 2 × 2 = 4 shades of gray. A 3-bit image has 8 shades of gray, or 1 × 2 = 2 × 2 = 4 × 2 = 8. A 4-bit image has 16 shades of gray, and so on. In an 8-bit image, there are 256 shades of gray and this is the standard. However, systems capable of up to 12 bits or 4098 shades of gray presently exist. The more bits in the image, the greater are the storage needs for the images. The human eye of the person in the street can commonly separate 16 shades of gray, a photographer or radiologist can separate about 25 shades of gray, and under laboratory conditions the maximum for the unaided eye to separate is somewhere around 64 shades of gray. The image itself usually does not occupy the entire gray scale as can be seen by viewing the histogram. The image may be confined to about 30 shades of gray. For best results it is desirable to have a system capable of at least 256 shades of gray. This way there is space on the scale to lighten or darken the image (histogram shift) or spread the shades of gray over a bigger part of the scale (histogram stretch). Remember, 8 bits or 256 shades of gray represents the limit of most monitors.
7. Image viewing: The image can be viewed on a monitor, printed on photo quality paper, or printed on a film-like acetate sheet.
8. Sensor reuse: All sensor types can be reused almost indefinitely. However, the PSP types need to be “erased” by exposing them to a bright light for a few minutes before reuse.
9. X-ray equipment: Because of the shorter exposure times needed in digital imaging, the constant potential DC-type x-ray machine with exposure increments in 1/100 seconds is the most desirable. PSP sensors are the most adaptable to older AC machine designs because they are not very sensitive to exposure variations. CCD and CMOS sensors can also be used; however, noise from too little exposure and blooming from too much exposure will be more prone to occur as timer increments will be in impulses at 60 impulses per second. Sometimes the first 1 to 3 impulses produce varying amounts of radiation, especially in older machines, and older machines cannot have an exposure time in increments of less than 1/60 of a second.
10. The next generation: You saw it here first. There will be more use of wireless CCD or CMOS sensors that are currently on the market. These were first developed in Israel for gastrointestinal imaging with the so-called “pill cam.” You will use a lightweight, hand-held, miniature x-ray machine configured much like a digital camera. This product is now ready to be marketed. The wireless sensor will send the image to the back of the camera, as with current digital cameras, or to a palm-held computer to see if it is okay. The image will then either be s/>
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