Technological Features

There are many similarities between film-based and digital radiology. For example, both techniques require the use of an X-ray source and use the paralleling technique. The major differences are found in the type of image receptors used and how the data is processed to produce the images. This section will explain the mechanics of producing a digital image and the three types of digital systems currently being used.

Image production:
In order to produce a radiographic digital image, a source of ionizing radiation, an image detector, a computer, and monitor to display the image are required. As with conventional dental radiography, the detector is positioned in the mouth with a holding device similar to the types used to hold the film in place. X-rays pass through the dental structures carrying information to the detector, first sensing it and then capturing it. This information is then transformed or converted from an analog form (continuous data) to a digital format (discrete data) that can be read by the computer. The image can then be displayed on the computer monitor using up to 256 shades of gray. The image is made up of individual picture elements or pixels (Figure 1). For example, if a cross-stitch picture is viewed from a distance, the colors appear to be continuous, as if from one paintbrush stroke. Upon closer examination of the picture, one can visualize each individual cross-stitch that composes the picture. Thus, each stitch represents a pixel or discrete data point.3

The analog to digital converter measures the amount of radiation registered in the sensor and converts it to digital form by assigning a number using a binary system where each binary digit (bit) is represented by a numeral zero or one. These bits are combined into an eight-bit word, or byte, that allows a maximum combination of 256 (2 to the exponent 8) gray levels. On average, the human eye is able to discern up to 32 levels of gray, therefore 256 gray levels provide the viewer information through different window levels. Windowing allows the information to be represented in different shades of gray making it possible to show soft tissue and hard tissue structures without having to take additional images.4

Types of Digital Imaging and Sensors
Digital images can be acquired directly or indirectly. Direct image acquisition can be accomplished through the use of three different types of detectors. The first type is the charge-coupled device (CCD) detector (Figure 2.). Approximately the size of dental film, the CCD sensor has a slightly smaller sensitive area; a thicker, rigid case; and an electrical lead that attaches to the computer unit. Aradiation-sensitive circuit inside of the case determines the amount of voltage received from the X-ray beam. The specified amount is then converted into a numerical value, which is assigned a gray level displayed on the computer monitor. The image is displayed on the computer monitor almost immediately using this system.

The second type uses a Complementary-Metal-Oxyde-Semiconductor (CMOS) detector. CMOS detectors have the same characteristics as CCD sensors except they use active pixel technology (patented by Schick Technologies, Inc. for use in dental and medical radiology) and are less expensive to manufacture. The sensors are visually indistinguishable.

The third type of digital detector is the photostimulable phosphor (PSP). Used most often in medical radiology, PSP’s are probably more easily adapted to dental radiology. The sensors are manufactured in a variety of sizes similar to dental film (sizes 1, 2, 3, 4, and extraoral). Although slightly thinner than dental film, the sensor can be adapted to most intraoral film holding devices (Figure 3). Unlike the CCD/CMOS, this detector does not require an electrical lead and has properties similar to intensifying screen phosphors. As the phosphor layer of the detector is irradiated, the electrons become trapped in the phosphor. Thus, the plates hold the latent image until it is “processed.” During processing with a laser, the electrons are released and emit a blue light proportional to the intensity of the X-rays attenuated in the phosphor layer. The light is then converted to a digital form, and the data can be displayed and seen on a computer monitor.4

Indirect digital images are made from radiographs acquired from conventional techniques. It involves digitization of the image using a scanner or a digital picture of the image (Figure. 4). The digital picture can then be imported into the electronic patient chart from the camera disk or viewed by any other imaging software such as Adobe Photoshop (San Jose, California).