digital Radiography

Digital Radiography

Modern digital imaging systems include:

a detector to convert the transmitted light of a conventional radiograph or the remnant x-ray beam into an electronic signal.
a converter to convert the electronic signal into a form the computer can understand, and
a computer to receive, store, process, and retrieve the images.

A sensor replaces the film normally used for traditional radiographs. The most common type of Intraoral sensors are solid-state electronic devices called “charged-coupled devices” (CCD). A CCD is composed of millions of light sensitive cells arranged in a rectangular array on the face of the sensor. The x-ray photons falling upon the material in the sensor create an analog (continuous) electric charge. This signal is converted to digital data that can be understood by the computer. In this way, the image is converted to millions of tiny digital picture elements (pixels) which are reassembled by the computer into a coherent image. CCD's used in dental imaging are essentially the same as the CCD's used in newly popular digital cameras. In your home camera, the CCD contains color filter arrays for each pixel so the image can be reassembled in color. Since dental radiographs are monochrome, the dental CCD does not contain these filters.

A radiograph is composed of shades of gray spanning from black to white, and is known as a "continuous tone" image. This means that the shades of gray blend together with no noticeable interruptions. To convert data from the sensor into digital form, the image is converted into individual pieces of information by an analog to digital or “A to D” converter. This information describes the light intensity (brightness) and its location in relation to the picture as a whole. Each small piece of information is called a pixel (short for "picture element"). The computer reassembles the pixels in the correct order and brightness to build a digital image. Manufacturers of current image processor equipment use a standard 12-bit or 4096 levels of gray for the images. The latest image processors use a 16 bit or 65536 levels of gray. Increasing the number of bits representing the brightness expands the gray scale so that the digital image more closely resembles the original image. The higher the number of pixels used to define the image the closer we approach the spatial appearance of the original image. This means that a properly displayed digital image will appear to an observer to be identical to the same image as presented on an x-ray film. The more pixels and bits of information involved in the pictures, the more memory the computer requires for processing and storing the image.

A typical imaging system is composed of a video camera, a framegrabber with A/D and D/A converter, a host computer with optical disk storage and image processor software or hardware and a video monitor. Once the image is in the computer, it can be manipulated, enhanced, enlarged, filtered, and compared to other images. The technique of capturing the image must be reproducible so two images of the same area taken at different times can be accurately compared.[i]

One alternative to capturing the radiograph digitally is to scan a standard radiograph and convert it into a digital image. For example, software from Televere Systems called TigerView will take the images from the scanner and automatically arrange them in proper orientation and order. These images can be manipulated, rotated, and enhanced. Zoom, contrast, brightness, and orientation are also variable.

Digital images can be transmitted via modem in only seconds. Images can be inserted into a word processing document (like treatment plans) and printed on an office printer with good (although not diagnostic) results.

Radiographs in current patient files can be scanned and stored on magnetic media rather than in physical form, taking up much less room than conventional files. Electronic files can be submitted to insurance companies or for specialist referrals. Radiographs can be displayed in a magnified form for patient education.

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