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Chest Imaging

Mobile radiography

(also called "portable" or "bed-side" imaging), general term covering all X-ray examinations made by "mobile" X-ray units, in combination with screen-film detectors, digital detectors or C-arms. In most institutions mobile radiography is still performed with conventional screen-film systems. However, digital techniques are taking over rapidly.

Three classes of digital mobile radiography devices can be distinguished:

  • C-arms: offer the advantage of fluoroscopy but lack field of view and image quality. Their application is limited to surgery wards and emergency rooms. Many "digital" C-arms are video-based systems digitized through a frame grabber of about 1k 1k 8bit matrix.

  • Digital off-line detectors for digital mobile radiography are exclusively the field of computed radiography (CR), which is based upon "stimulable phosphor" plates, mounted in a modified conventional cassette. This cassette can be placed behind the patient, with or without grid and uses the same exposure techniques and equipment as the conventional screen-film systems.

  • Digital on-line systems with solid state detectors (such as amorphous selenium or silicon) exist but are not yet in use for digital mobile radiography. These detectors are still too bulky and vulnerable to mechanical damage, and need a cable connection to the network or monitor as well as to the X-ray unit for steering.

    Computed radiography (CR) uses a robust inexpensive cassette off-line from the reading unit, which can be used for other X-ray work. The stimulable-memory phosphors do not require films, but keep the image stored until they are digitized in a reader and can be re-used up to 10,000 times. These phosphor plates have an extreme latitude (1:10 000) compared to film (1:150) and behave linearly where film is typically S-curved (white toe and black shoulder). Over- and underexposure is virtually excluded and produces a stable and excellent image quality independent of exposure technique, dose and patient, allowing an easy day-by-day comparison of chest images. Digital mobile radiography adopts all current exposure techniques ranging from low kVp without grid (7090 kVp) to high kVp with grid (100150 kVp). The image processing behind CR is a necessary component in digital mobile radiography, because it enables a better visualization of the typical problems in intensive care chest radiology.

    JV