THEORY GUIDE Radiography Theory
10DEC05
TG4825-1
Page
14 of 120
The following phases are necessary to capture and process projection radiographs for both
screen/film systems and CR systems:
Phase of Image Capture Description
Phase 1 -
Making the aerial image
In both screen/film and CR systems:
• an X-ray TUBE emits X-rays in the direction of an IMAGE
RECEPTOR
• when the X-rays reach the body of the patient, some are
absorbed by the patient and some are not. The result is an
“aerial” image with varying degrees of X-ray power (varying
numbers of X-ray photons)
Phase 2 -
Capturing the latent image
When the IMAGE RECEPTOR is exposed to the X-rays in the
aerial image, a latent image is captured on the RECEPTOR:
• screen/film systems - image is captured on sensitized
radiographic film
• CR systems - image is captured on a STORAGE
PHOSPHOR SCREEN. The X-ray photons that reach the
SCREEN charge the PHOSPHOR, making a latent image
on the screen.
Phase 3 -
Capturing, changing, and
storing the visible image
The latent image must be changed to a visible image, which
can be read by the Radiologist, moved from one place to
another, and stored for use at another time:
• screen/film systems - radiographic film is processed through
chemicals and the latent image is fixed onto the film
• CR systems - the latent image on the STORAGE
PHOSPHOR SCREEN is scanned by a laser beam, which
stimulates the charged PHOSPHOR on the SCREEN. Blue
light is emitted from the stimulated PHOSPHOR, assembled,
and changed into analog electrical signals. The analog
image is then changed into digital signals and processed.
The digital image is stored and displayed by a computer
system and can be routed to other computers and
PRINTERS through a network.
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