Transcript Quality Control Testing of Screen Speed

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QC testing of screen speed should occur on
acceptance and then yearly.
Evaluate first whether similar cassettes
marked with the same relative speed are the
same using the following procedure
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Make an exposure of a step-wedge or
homogenous phantom onto an image
receptor so that the center of the image has
an optical density of about 1.5
Expose each image receptor to the same
technical factors.
Process, and take optical density readings of
the same center area in each. If all have the
same relative speed, the optical density
should not vary by more than +-0.05
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During manufacturing processes,
inconsistencies may occur in which the
phosphor layer is applied more thickly at one
portion of the screen than at another.
Sometimes during cleaning excessive rubbing
may remove more of the phosphor layer.
Screen uniformity testing should be
performed on acceptance and then yearly.
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Make an exposure of a step-wedge or
homogenous phantom onto an image
receptor so that the center of the image has
an optical density of about 1.5
Process and take optical density readings in
the center and in each of the four quadrants
of the image. It should not be more than +0.05.
Any film/screen image receptors that exceed
this limit should be removed from service.
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The film and intensifying screen should
match each other because the film are
sensitive to a specific color.
A blue-violet emitting screen phosphor
should be used with a monochromatic films.
A green-emitting screen phosphor should use
with an orthochromatic film.
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Is a type of black-and-white photographic
film that is sensitive to all wavelengths of
visible light.
A panchromatic film therefore produces a
realistic image of a scene. Almost all modern
photographic film is panchromatic,
 Orthochromatic photography refers
to a photographic emulsion that is
sensitive to only blue and green light,
and thus can be processed with a
red safelight.
 It is used on all radiographic films.
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Intensifying screens should be able to
demonstrate clear images of patient
anatomy so that the proper diagnosis can be
obtained.
The ability of an imaging system to
accurately display images.
There are two types of resolution
 Contrast resolution
 Spatial resolution
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Is the ability of an imaging system to
distinguish structures with similar x-ray
transmission as separate entities (in short
separate shades of grays)
It is affected by the sensitivity of the image
receptor speed and the amount of
radiographic mottle (noise).
If the radiographic mottle is increased,
contrast resolution is decreased.
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Is that the faster the screen speed screens,
the lower the mAs values are used, which in
turns increases the quantum mottle and
lowers the contrast resolution.
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Is the ability of an imaging system to create
separate images of closely spaced objects.
In other words, do the two object appear
sharp and clear, or do they blur together?
This is determined by the amount of light
diffusion that occurs between the screen and
film.
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It is affected by:
 Screen thickness
 Phosphor crystal size
 Film/screen contact.
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The most common method of measuring
spatial resolution is the spatial frequency.
The unit of spatial frequency is the line pairs
per millimeter (lp/mm).
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a line pair is a space and line each being
0.1mm wide.
The greater the line pairs per millimeter value
the smaller the object that can be imaged
and the better the spatial resolution.
The human eye can read up to 5lp/mm, but
most screen system cannot provide this level
of spatial resolution.
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Point spread function (PSF)
Line spread function (LSF)
Edge spread function (ESF)
Modulation transfer function (MTF)
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Is a graph that is obtained with a pinhole
camera and a microdensitometer.
The pinhole camera creates a black dot in the
center of a film and microdensitometer is
used to take readings.
The values are plotted on a graph versus the
distance from the center of the point.
The narrower the peak on the graph, the
better spatial resolution and image quality.
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Is a graph that is more accurate and easier to
obtain than the PSF graph.
It requires an aperture with a slit that is 10um
wide instead of the pinhole camera.
The density readings are taken of the
centerline and plotted.
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It requires a sheet of lead to be placed on a
cassette and exposed.
Density readings are taken at the border
between the black and white areas and
plotted on a graph.
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Is a numeric value that is used to measure the
spatial resolution and is obtained from the
LSF graph with a mathematical process
known as fourier transformation.
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Intensifying screens must be free of dirt,
stains and defects to properly function.
A regular schedule at least every 6 months of
screen cleaning with an antistatic solution
should be a standard department policy.
A UV light may be used to examine the
surface of the screen if there are any stains.
film/screen contact test
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Frequency of test
 Yearly
 As necessary
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Equipment required
 Cassette to be tested
 Test tools:
▪ Box of paper clips or sheet of perforated zinc or fine wire
mesh, large enough to cover a 35 x 43cm (14 x 17” film),
with a square hole, about 10cm from one edge,
approximately 2 to 2cm square)
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Load the cassette to be tested and place it
face up on the tabletop
Cover the whole of the cassette with the test
tool (if cassette distribute evenly).
Set a FFD (SID) of 150cm[60”] ( the longer
FFD (SID) reduces geometric unsharpness)
Collimate to cover whole of cassette
Make exposure using a 50kVp and 6mAs
Process film
If a densitometer is available measure the
image.
 Inspect the image, looking for areas that look
blurred.
 A noticeable area of unsharpness could be
caused by:
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 Damaged cassette
 Screen packing, deterioration
 An air pocket
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When using a close mesh wire test the poor
film/screen contact areas may also have a higher
density.