Digital Radiography for the Allied Dental Professional

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Digital Radiography for the Allied Dental Professional
January/February 2007
Vaishali Singhal, DMD and Howard E. Strassler, DMD
Contemporary Dental Assisting

bstract
Many dental offices are converting to digital radiography, and while the purpose of the radiographs remains unchanged, using a digital radiography system requires dental assistants to learn some new skills. Knowing the basics of digital radiography hardware and software will allow assistants to use these new systems to their potential, including image enhancement functions that can assist with diagnosis. The advantages and disadvantages of digital radiography systems are also discussed, as well as how to achieve a seamless transition from using film-based radiography to digital radiography.

Learning Objectives
After reading this article, the reader should be able to:
Ô discuss the differences between digital and film radiography.
Ô describe the differences between direct and indirect systems.
Ô understand what tools can be used with digital radiography to improve diagnosis.
Ô identify advantages and disadvantages of digital radiography.

X-rays were discovered in 1895 by a German physicist, Wilhelm Conrad Roentgen. Two weeks after this discovery was made public, a German dentist made the first dental x-ray in his own mouth using small glass plates that were exposed for 25 minutes.1 Many advances in dental radiography have occurred over the following decades. There have been advances in the devices used to expose film and the sensitivity of the films, which require significantly less radiation dosage to expose them. In recent years much higher speed films have become the standard for dental x-rays.

The new digital era is making a significant impact on the dental profession. The purpose of a digital radiography unit is similar to that of a conventional unit. The digital system can be used to expose periapical and bitewing films, as well as panoramic and cephalometric films.

Digital radiography has been gaining popularity and many offices either have or are considering converting to digital radiography systems.2 A recent survey of Canadian dentists on their perceptions and attitudes toward digital and electronic technologies revealed that they believe the use of digital radiography will increase office efficiency and production.3 The major obstacles to acceptance and general use of the technology include cost and the practitioner's lack of comfort with technology. Within the survey there were no comments on quality of care issues. Another study showed that practice efficiency improves when the practice uses digital radiography.4

Many different studies have evaluated the diagnostic quality of film vs filmless images (digital radiographic images), and film images vs film images scanned into a computer. In the diagnosis and evaluation of periodontal bone defects, digital images have been shown to be equivalent to film images.5 One benefit of digital radiographs is the ability to use a software program to manipulate images (make digital adjustments to contrast, lightness and darkness, and use other tools) to better see conditions to make a more accurate diagnosis. Also, all digital radiographic software programs offer a magnification tool to enlarge the image for an improved evaluation.6 Upon comparison, digital and film radiographs were found equivalent for diagnosing caries.1,7-9 Diagnosis and use of digital radiography during endodontic treatment has also become well accepted.10,11 Digital radiography through the use of digital panoramic units has improved diagnosis and, with the latest 3-dimensional cone technologies, allows for the use of extraoral imaging with excellent diagnostic potential.12,13 Digital radiography also provides an excellent patient education tool with the ability to use the computer monitor at chairside to better demonstrate conditions that have been diagnosed and possible proposed treatments. Table 1 lists some available digital radiography systems.

Fundamentals of Digital Radiography
The purpose of a digital radiography system is the same as a conventional film system, but differences exist in other ways. For film images, the dental office needs the film, film holders, intra- oral (cone head) and extraoral (panoramic) x-ray units, a film processor, film processing chemicals, and a plan to dispose of the chemicals and lead shielding present in film packets. In many cases digital systems use similar types of equipment with some differences in acquiring the digital image.

With digital radiography, film is replaced by an image capture device, an image receptor. Currently there are 2 types of systems for capturing the x-ray image. They are direct imaging systemsÛcharged-coupled device (CCD) and complementary metal oxide semiconductor (CMOS)Ûand indirect imaging systemsÛphotostimulable phosphor (PSP) plates and the use of digitally scanned conventionally processed films that can then be stored as a digital image in a computer hardware device. Scanning existing conventional films allows an office to convert to a digital system for radiography storage and retrieval purposes. It is important that the method of scanning not degrade the image that has been digitized.8

While both types of systems can capture acceptable diagnostic images, each has advantages and disadvantages that can have a direct impact on the dentist, dental assistant, and dental hygienist. These differences relate to the method of image capturing and the physical dimensions of the image capture system. These advantages and disadvantages will be described later in this article.

CCD and CMOS are basically solid state sensors connected to a wire cable that attaches to a computer (Figure 1). In the coming years, manufacturers will be introducing wireless CCD/CMOS sensors similar to the WiFi technology that computers use to access the Internet. The CCD, first invented in the 1960s, is the most commonly used sensor today. When the x-ray beam hits the CCD sensor, electrons, which are stored within electron wells in the sensor, become trapped in the wells. The number of electrons is proportional to the number of x-ray photons striking that particular well. Each electron well is connected to a pixel on the screen. After exposure, the computer reads the wells and each well is given a brightness value via a fiber-optic cable that connects the sensor and the computer screen. These varying values of brightness are displayed on the corresponding pixel on the computer screen. The pixels merge to form an image of varying opacities. We can think of a pixel as a silver halide crystal in the emulsion of a conventional dental film. The CMOS sensor is similar to the CCD sensor and is considered more durable and less expensive to manufacture. In basic terms, x-ray energy from the x-ray unit exposes the sensor, which then converts that energy to a digital signal that is translated by the computer into an image that can be seen on the computer screen. This operation occurs almost instantaneously, without processing.

One major problem with CCD sensors is their thickness. Although the manufacturers provide for sensor holders similar to those used for holding films, these sensors are more difficult to place in a patient's mouth. Some patients will have difficulty tolerating the size and thickness of the sensor. This is especially true with pediatric patients. Also, care must be taken not to damage the cable.

For infection control, the sensor is not autoclaved or disinfected, but rather barriers (plastic sleeves) must be used over the sensor and cable. If necessary a sensor can be wiped down with a disinfectant or ethyl alcohol. It is important that the dental staff and dentist follow the recommendations of each manufacturer. Also, because sensors are expensive, ranging from $4000 to more than $12,000 per sensor, everyone in the office must be diligent about caring for this equipment.

A PSP system uses thin plates similar in shape and size to that of conventional film (Figure 2). Patients therefore may tolerate their use intraorally at a level similar to conventional film. However, PSP systems differ from CCD and CMOS systems in that the images are produced in 2 steps. A reusable, unwired sensor is used. The sensor is a flexible phosphor-coated plate containing rare earth elements. When the plates are exposed to the x-ray beam, they store the energy in the form of photons. Using a manufacturer's specified laser scanner, the PSP plates are scanned in the laser scanning unit (Figure 3). The unit exposes the plate to light released by the laser and then transmits an electronic signal to the computer that the computer software converts to a visible digital image that can be seen on the monitor. Scanning may take from 30 to 90 seconds per exposure to generate the image as compared to the almost instantaneous generation of the CCD/ CMOS sensor. Each exposure will need to be scanned before the sensor can be used to take additional images. It is advisable to scan the PSP plates in the laser scanner immediately after exposure. There have been reports of degradation of image quality if there is a delay in scanning.14,15

The reusable plates are discharged either by the scanner or by exposing them to a very bright light source. The plates must be stored in darkness to extend their time of usability. Infection control requires that the PSP plate be placed in a plastic pouch that is sealed from the oral fluids. Typically a single PSP sensor costs between $20 and $30.

The Digital Image
Resolution and Pixels
Digital images are typically viewed on a computer monitor. A digital image is composed of pixels. "Pixels" is an abbreviation for "picture elements." A pixel is defined as 1 dot in the digital image. The image is composed of all the pixels that form the image, and the image resolution is determined by the size and number of pixels.

Gray Scale
Digital images captured by a sensor are displayed on the monitor in shades of gray. The gray scale displayed by the monitor is comprised of 256 distinct visible shades of gray even though some systems can contain up to 64,000 values.1 When viewing a gray scale image, the human eye normally can only distinguish up to 32 shades of gray. This fact will be important later when the concept of image enhancement is presented.

Technologically, each pixel in the digital image has a number ranging from 0 to 255. When the radiation is captured by the sensor, it is registered, with maximum radiation penetration assigned the number 0 (pure black) and no radiation penetration (total radiopacity) assigned the number 255 (pure white).1 This radiation intensity is sent to the computer and stored.

As clinicians, we rely on the contrast of a radiograph to be able to detect disease processes, and the longer the scale of contrast in the image the easier it is to make a diagnosis. With the ability to enhance gray scales using the system's software, a clinician may be better able to diagnose even subtle changes, which may have been missed previously with conventional films.16-18

Computer Hardware
In most dental offices, the digital x-ray software will run faster with an upgraded computer system.11 Each manufacturer will recommend the minimum computer power needed to adequately run the imaging software. It is always a wise financial decision to purchase more computer than you need because as the software is updated, it may require a hardware upgrade, especially if you are using a computer with the minimum power requirements. It is always less expensive in the long run to purchase hardware upgrades when initially purchasing the computer.

To run digital imaging software, computers should have at least 1 to 2 gigabytes of random access memory (RAM). RAM is the memory in the computer that holds data and the active programs. The greater the RAM, the faster the computer will process images. Storage is also important. It is advisable to purchase at least an 80-gigabyte hard drive. When deciding on how you plan to store the images, have the manufacturer's representative make a recommendation for storage of images and of back-up images so that they will not be lost if the computer malfunctions. Each system may have different requirements. With an optimal processor, RAM, hard drive, and additional hard drive storage, you will be able to input and retrieve data in a reasonable time while treating patients. The current generation of laptops is almost comparable to desktop computer systems.

Whendeciding on a monitor, consider at least a 17-inch monitor, although your office should have at least one 19-inch screen to view and enhance digital radiographs. Before purchasing, try viewing some of your images at your computer retailer.

At times you will need to print the images for a specialist referral or insurance reimbursement, although most will accept electronic images. A medium-grade inkjet printer will be sufficient. For the highest image definition and reproduction, use the manufacturer's recommended paper.

Computer Software
Most digital radiography systems have their own proprietary software. As part of the purchase of the system, the manufacturer will offer training and usually a toll-free number to assist when problems arise. Using the software, dentists, assistants, and hygienists can enhance images by changing their contrast, brightness, and sharpness, as well as other parameters. For some software, a single button will automatically optimize the image. For other software, the dental professional optimizes the images for the best diagnosis.

Even though the images captured by the sensor will be of a diagnostic quality equal to or better than conventional film,17,18 manipulating the images can enhance diagnosis. Some typical examples of image manipulation include: changes in contrast and image density, changes in brightness, digital measurement with an on-screen ruler, magnification, subtraction radiography, image reversal, and embossing. Figure 4 shows an example of a bitewing captured from a wired sensor. Figure 5 shows the image after it was manipulated with digital radiography software to increase contrast and decrease brightness, creating an image that is better for diagnosis. Most software also allows the operator to rotate, flip, and swap images.

Many systems have templates so that a full-mouth series or bitewings can be projected. Some software programs allow the operator to create custom templates, eg, for endodontic treatment. Several images can be displayed and compared side-by-side on the screen. An image can be enlarged and sharpened. Magnification allows the operator to enlarge a specific area to project the details of subtle changes that otherwise may go undetected. However, be careful not to overenlarge or oversharpen an image, which can create digital noise and distortions that can hinder diagnosis.

Contrast and density modifications correct for over and underexposures without distorting the image. With this feature, clinicians can still use an over or underexposed radiograph. With conventional film, an improper exposure will lead to the need to retake the image, exposing the patient to additional radiation. Reversal of the gray scale converts black areas to white and white areas to black. This reversal can aid in better viewing of certain areas or objects by darkening or lightening them.

The operator can colorize an image, with each gray shade assigned a specific color. Colorization can enhance interpretation and diagnosis, as the human eye is more sensitive to colors than shades of gray.

Another feature, digital subtraction, allows the operator to merge 2 images of the same area taken at different visits. The images are then subtracted from each other, leaving behind only the changes between them. After subtraction, a clinician can determine whether or not change has occurred in a specific area over time. This is very useful when monitoring an implant, periodontal disease progression, or the ad-vancement of decay in a specific location. For digital subtraction to work successfully, the 2 images must be taken with identical sensor placement and beam angulation so that the displayed changes show only the patient's true physical changes.

Some software allows the operator to place notes over a particular location on an image, and use a measurement tool. For example, a digital ruler allows the operator to measure bone loss in a patient diagnosed with periodontal disease or measure the length of a root canal during endodontic treatment. All software will allow operators to save the enhanced image separately without changing the archived original image. Also, specific information is accessible for all images, including patient name, date the image was made, and whether or not the image was manipulated.

Advantages of Digital Imaging Systems
There are many advantages for a dental office when using intraoral digital x-ray systems:
1. 50% to 80% less radiation to the patient.19
2. Increase efficiency because images can be viewed instantly, facilitating a quicker diagnosis and treatment plan.
3. Create records that can be electronically transferred to the patient's insurance company or specialist.
4. Allow image enhancements, such as sharpening the image and changing its contrast, lightness, and darkness.
5. Provide an educational tool to explain dental problems and treatment options to patients.
6. Eliminate the need for a darkroom, film processor, and processing chemicals.
7. Eliminate the need to dispose of environmentally hazardous wastes, such as fixer and lead foil.
8. Eliminate the high cost of chemical disposal and film purchases.
9. Eliminate the need for film duplication because images can be reprinted.
10. Reduce the need for paper.

Disadvantages of Digital Imaging Systems
There are also disadvantages to the intraoral digital x-ray systems:
1. High initial set-up cost, though the cost is recovered over several years.19
2. May compromise patient comfort because the sensors are bulkier than conventional film, except when using PSP plates.
3. Requires the use of barrier protection for infection control because the sensors cannot be sterilized.
4. Time lost to training the entire staff to use the system.
5. May cause legal issues because of the capability of the system to enhance the images; a system must have an archival DICOM 3 compliant storage protocol.19
6. Fragile sensors are expensive to replace ($4000 to $12,000 per CCD or CMOS sensor).19
7. Inferior image quality with a resolution of 10 line pairs per mm compared with conventional film that has a resolution of 12 to 20 line pairs per mm.19

Making Digital Radiographs
PSP sensors are similar to film. If using PSP sensors, the transition from conventional film will be almost seamless. As noted earlier, CCD and CMOS sensors are thicker and larger than conventional film. Because of their thickness, these sensors will require a special holder to properly orient them. Enough sensor holders will need to be ordered to accommodate the office's sterilization and image collecting cycles. Also, barriers for infection control will need to be placed over the sensors when capturing images. When making several images or a full-mouth series for a patient, the same sensor is used for all exposures. One additional benefit is knowing right away if the desired image is captured. If the image is undiagnostic, it can be retaken immediately.

The act of exposing a sensor parallels conventional film. The sensor is placed in the patient's mouth and, when the exposure is completed, the sensor stores the latent digital image similar to conventional film. With CCD/CMOS sensors, the latent image is transmitted and converted into a visible image by the computer. The image is displayed on the monitor in pixels as various shades of gray, black, and white. With PSP plates, the sensor must be discharged and scanned in the laser scanner.

Digital radiography offers the patient the benefit of reduced radiation. According to Haring and Howarten, because the sensors are more sensitive to the x-ray photons than conventional film, approximately 50% to 80% less radiation is used when exposing the sensor when compared with exposing F-speed film.19 The same x-ray unit can be used for digital radiography and conventional film radiography. Depending on the x-ray unit, it may need to be adapted to allow it to produce a quicker exposure setting. The manufacturer can provide guidance on what changes need to be made.

Patient Preparationand Procedure
Because digital sensors will be new to most patients, start by explaining the procedure. Before taking an exposure, enter the patient's information and date of exposure (for some software this is automatic) into the computer and select the type of radiographic examination.

After seating the patient, use the same positioning rules and techniques that are used for conventional film exposures. As with any radiographic examination, inspect the oral cavity for any intraoral conditions that may interfere with image capture, including bony tori, partial dentures, tongue or lip piercing, and orthodontic appliances. Also have the patient remove any extraoral objects, such as jewelry. Adjust the dental chair to the appropriate height for the operator and adjust the patient's head so that his or her maxilla is parallel to the floor and his or her midsagittal plane is perpendicular to the floor. Place a lead apron with a thyroid collar on the patient. Turn on the x-ray unit and adjust the settings according to the sensor system's guidelines. It is highly recommended to have a chart listing the recommended settings. In most cases, the exposure setting with a digital sensor will be at least one half of the setting used with F-speed intraoral film.

The sensor should be covered with a disposable plastic barrier and placed in the appropriate holder so the image can be exposed using the paralleling technique. Place the sensor intraorally. Placement of sensors is similar to placement of conventional film. A poorly placed sensor will still produce a poor radiograph, resulting in the need for a retake. Special cushioning tabs on the periphery of the sensor can help make the patient comfortable. With the sensor positioned, position the x-ray cone/position indicating device (PID) using the holder as a guide (Figure 6). Activate the sensor using the computer, leave the room, and complete the exposure by activating the x-ray head.

After capturing the image leave the sensor and PID in place and evaluate the image. If the image is not acceptable, perform the necessary equipment adjustments (ie, adjust the PID or sensor placement) and retake the exposure. If using a PSP sensor, remove the sensor from the patient's mouth after the exposure has been completed. Discard the barrier, place the sensor in the scanner, and view the image on the screen. If a retake is necessary, erase the PSP plate by exposing it to light (with some scanners the plate is automatically discharged), reposition the sensor, and retake the image.

As stated earlier, the image will be saved and a back-up of the image also should be created. Depending on the office's computer system or network, the back-up can be on the computer's hard drive or on a server elsewhere in the dental office. This use of a back-up protects the digital images against computer failures. All images must be backed-up on a daily basis. The images can be stored on tape, a mirrored hard drive, compact discs, or digital video discs.

The Future of Digital Radiography
Since Dr. Francois Mugnon first introduced digital imaging to the dental profession via the RadioVisioGraphy System, digital radiography systems have im-proved significantly.1 As technology improves, digital dental radiography systems improve as well. In the past, these systems had many disadvantages, poor image quality and cost being only 2. Over the past several years image quality has improved tremendously, with significant software improvements that provide for enhanced diagnosis. Digital systems have become more affordable and system software has become easier to use. As a result, many offices are opting to convert to digital systems and many new practitioners are opting to install digital radiography systems in their offices.

Within a short period of time and with experience, digital radiography will prove a valuable tool. Digital radiography technology will save time, provide for reduced radiation exposure for patients and dental staff, allow for immediate image correction, offer the ability to enhance images for better diagnosis, and provide a better method of organizing records that can be easily retrieved with accurate patient and date information.

Photo Credits: Image on page 26 courtesy of Patterson Dental. Images on pages 28 and 29 courtesy of Kodak Dental Products.

Disclosure
Dr. Strassler is a consultant for Kodak Dental Products.

References
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