Transcript Analog Imaging III

Analog Imaging III
By Professor Stelmark
Many types of radiographic intensifying screens are available, and each
manufacturer uses different names to identify them. Collectively, however,
screens usually are identified by their relative speed expressed numerically.
Screen speeds range from 100 (slow, detail) to 1200 (very fast).
Screen speed is a relative number that describes how efficiently x-rays are
converted into light. Par-speed calcium tungstate screens are assigned a value
of 100 and serve as the basis for comparison of all other screens. Calcium
tungstate screens seldom are used anymore. High-speed rare Earth screens
have speeds up to 1200; detail screens have speeds of approximately 50 to 80.
These and other characteristics are summarized in.
TYPE OF SCREEN
Characteristic Type of
Phosphor
Calcium Tungstate
Oxysulfides and
Oxybromides of Y, La,
Gd
Color of emission
Blue
Green or blue
Approximate speed
50-200
80-1200
Intensification factor
20-100
40-400
Resolution (lp/mm)
8-15
8-15
The speed of a radiographic intensifying screen conveys no information
regarding patient dose. This information is related by the intensification factor
(IF). The IF is defined as the ratio of the exposure required to produce the
same optical density with a screen to the exposure required to produce an
optical density without a scree
Radiographers often use the term image detail or visibility of detail when
describing image quality.
Radiographic intensifying screens have the disadvantage of lower spatial
resolution compared with direct-exposure radiographs.
Cassette
The cassette is the rigid holder that contains the film and radiographic
intensifying screens. The front cover, the side facing the x-ray source, is made
of material with a low atomic number such as plastic. It is thin, yet sturdy. The
front cover of the cassette is designed for minimum attenuation of the x-ray
beam.
Attached to the inside of the front cover is the front screen, and attached to the
back cover is the back screen. The radiographic film is sandwiched between the
two screens.
Between each screen and the cassette cover is some sort of compression
device, such as radiolucent plastic foam, which maintains close screen-film
contact when the cassette is closed and latched.
The back cover is usually made of heavy metal to minimize backscatter. The xrays transmitted through the screen-film combination to the back cover more
readily undergo photoelectric effect in a high-Z material than in a low-Z material.
X-rays can be transmitted through the entire cassette, and some might be
scattered back to the film by the cassette-holding device or a nearby wall. This is
called backscatter radiation and results in image fog.
Backscatter
For mammography, the single screen is on the far side of the emulsion to reduce
screen blur.
Carbon Fiber Material
One of the materials developed early in the space exploration program
was carbon fiber. This material was developed for nose cone applications
because of its superior strength and heat resistance. It consists principally of
graphite fibers (ZC =6) in a plastic matrix that can be formed to any shape or
thickness.
In radiology, this material now is used widely in devices designed to reduce
patient exposure. A cassette with a front that consists of carbon fiber material
absorbs only approximately half the number of x-rays that an aluminum or
plastic cassette does.
Carbon fiber also is used as pallet material for fluoroscopic examination tables
and computed tomography beds.
Carbon fiber not only reduces patient exposure, it also may produce longer xray tube life because of the lower-demand radiographic techniques required.
Double screen cassette
Single screen cassette
Curved cassette
Direct exposure film holder
Dental film holder
Grid cassette
Moire artifact
Computed Radiography cassette
NO SCREENS
Characteristics of a Good Cassette
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Lightweight
Durable
Smooth, washable surface
Lightproof ( analog system)
Capable of maintaining good film-screen contact
Opened and closed easily
Smooth, rounded edges
Problems with cassettes
• Light leaks
• Poor film/screen contact – transient and permanent
• Artifact presence
Radiographic intensifying screens also can be cleaned with mild soap and
water. The screens should be carefully rinsed and thoroughly dried. If the
screen is damp, the film emulsion layer may stick to it, possibly causing
permanent damage
An equally important requirement in caring for radiographic intensifying screens is
maintaining good screen-film contact. Screen-film contact can be checked by
radiographing a wire mesh. If darker areas of blurring are seen, as then screen-film
contact is poor and should be corrected, or the cassette should be replaced.
PROCEDURE
1. Choose a clean location to clean screens and cassettes. Wipe the outside of cassettes and
clean the countertop before cleaning screens.
2. Moisten a lint-free wipe with a small amount of commercially available screen cleaner and
antistatic solution. A mild soap and water solution or a 70% solution of isopropyl alcohol may
be used as an alternative (check with your screen manufacturer to see if this is acceptable),
but a screen cleaner and antistatic solution must be used afterwards.
3. Clean and dry the screen. Be sure to avoid excessive pressure or rubbing on the screen
surface.
4. Use a second lint-free wipe to clean the frame and inside cover (for single-screen
cassettes).
5. Stand the cassettes on edge to dry.
6. Once the screen and cassette cover are dry, inspect them for any particles of dust. Use a
UV light if necessary. If they are clean, reload with fresh film. For mammography cassettes, the
screen surface should be carefully brushed with an antistatic brush and then inspected before
loading with fresh film