Image Receptors

Digital Image Receptors

Panoramic systems are available that use either linear array charge-coupled device (CCD) detectors or photostimulable phosphor plate (PSP) receptors. CCD and PSP receptors are used for intraoral radiography as well but with altered sizes and receptor design. The digital panoramic imaging method is comparable to conventional panoramic radiography but the receptor, processing, display, storage and transmission capabilities differ from film-based imaging.4 Both types of digital image receptors and film will produce diagnostic results.

Direct Digital Image Receptors

CCD linear array design arranges the CCD detectors in a grid with the same vertical dimension as panoramic film but only a few pixels wide. The vertical sensor is moved around the patient opposite to the x-ray source and captures the image one vertical line at a time. In the subsequent photographs, a CCD sensor system is used to capture the image. To initiate the process, a patient file must be created in the computer software system. The patient is aligned in the same head positions as required in film-based panoramic machines. The computer processes the image as it is acquired and the image is displayed on the computer monitor in real time. The image is archived in the patient database. The acquired imaged can be viewed in different ways through the application of software enhancement tools.

Direct Digital Panoramic Imaging

Figure 3.
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Photostimulable Phosphor Plate (PSP)

PSP sensors consist of a rare earth phosphor, barium europium fluorohalide, coated onto a polyester base. PSP plates are inserted into a cassette just like film but without the intensifying screens. The image is captured on the phosphor plate and the energy is stored on the plate until it released during a laser scanning process. When a helium-neon laser beam scans the PSP plate, the energy is released in the form of blue florescent light. The emitted light is captured, intensified by a photomultiplier tube and converted into digital data. PSP plates are reusable but must be exposed to light to erase remnant energy. Recent developments include faster plate scanning and erasure of the plate during the scanning process. It is important to scan the exposed plate without delay as white light or delayed scanning can degrade the result.5,6 PSP imaging is considered an indirect digital imaging method because the data is captured in an analog format like film and converted into digital data via the scanning process.

The obvious advantage of digital panoramic imaging, particularly direct digital panoramic imaging, is elimination of the darkroom including machine cleaning and maintenance, chemical handling and solution change as well as related processing errors. Other advantages include time-saving, electronic image storage, the ability to enhance the captured image with various tools such as density, contrast, measurement and image reversal manipulations and teleradiography capabilities for the purposes of consultation or referral.

However, the image quality remains dependent upon proper patient preparation, patient positioning and exposure selection to produce optimal results and to avoid retakes.

Film-based Panoramic Imaging

Film used for panoramic radiography is more sensitive and packaged differently than intraoral film. The film is either 5” or 6” x 12” (15 x 30 cm) in size. Since the film does not have protective outer wrappings, it must be housed in a cassette holder during exposure. Cassette holders are rigid or flexible configurations with intensifying screens on the internal walls. Screens have a crystalline phosphor layer that produces visible light when exposed to x-rays. The phosphor layer overlays the reflective layer, which reflects the light back to the film and both are coated onto a firm plastic base. A protective layer covers the phosphor layer to reduce damage and wear.

Figure 6.
Intensifying Screen:
Protective Layer
Active Phosphor Layer
Reflecting Layer
Screen Base
Diagrammatic cross-section
of screen
Open cassette showing screen (white)
and film surfaces

Intensifying screens intensify the effect of a single x ray photon by converting it into many light rays. It is the combination of x-rays and light rays that produce the latent image and reduce exposure to the patient. The color of the light emitted by the screens is related to the type of phosphor. Rare earth elements such as lanthanum or gadolinium fluoresce a lime green light and currently are the preferred screen material. Screen film is packaged in boxes containing 50-100 films and must be handled in a darkroom. A single film is loaded between the screens, the cassette is latched closed prior to leaving the darkroom and the cassette is placed into a cassette holder. The screen film type and intensifying screen fluorescence must be matched to produce a diagnostic image.