ScreenFilm Radiography II

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Screen-Film Radiography II

• Characteristics of film
• Screen-film system
• Scattered radiation

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Composition and function

• Unexposed film consists of one or two layers of
  film emulsion coated onto a flexible Mylar sheet
• Tabular grain emulsions are used in modern
  radiographic film laser cameras and older
  radiographic films use cubic grain emulsions
• Grains of silver halide (AgBr and AgI) are bound
  in a gelatin base and together comprise the film
  emulsion

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Scanning electron micrographs of film emulsions
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Optical density

• X-ray film is a negative recorder increased
  light (or x-ray) exposure causes the developed
  film to become darker
• Degree of darkness is quantified by the OD,
  measured with a densitometer
• Transmittance and OD defined as

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OD examples
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Hurter and Driffield curve

• The response of film as a function of x-ray
  exposure is nonlinear
• Curve describing OD versus the logarithm (base
  10) of exposure is called the HD curve
• This curve has a sigmoid shape

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A Hurter and Driffield (HD) curve
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Contrast

• Contrast of a radiographic film is related to the
  slope of the HD curve
• Regions of higher slope have higher contrast
• Regions of reduced slope (e.g., the toe and
  shoulder) have lower contrast
• A single number, which defines the overall
  contrast of a given type of radiographic film, is
  the average gradient

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Average gradient from HD curve
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Average gradient

• Always measured at the same OD points
• OD1 0.25 base fog
• OD2 2.0 base fog
• Average gradients for radiographic film range
  from 2.5 to 3.5

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Contrast as a function of exposure or optical
density
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Speed

• Sensitivity or speed of a screen-film system can
  be seen from HD curves
• As the speed of a screen-film combination
  increases, the amount of x-ray exposure required
  to achieve the same OD decreases
• Absolute speed is the inverse of the exposure (in
  roentgens) required to achieve an OD of 1.0
  base fog
• Commercial system for defining speed is a
  relative measure. Par speed systems have an
  arbitrary speed rating of 100.

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HD curves for two different screen-film detectors
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Latitude

• Contrast is desirable in screen-film radiography
• Latitude is the range of x-ray exposures that
  deliver ODs in the usable range
• High contrast systems have reduced latitude
• It is more difficult to consistently achieve
  proper exposures with low-latitude screen-film
  systems

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HD curves for screen-film systems which differ
in contrast
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Screen-film system

• Film emulsion should be sensitive to the
  wavelengths of light emitted by the screen
• Calcium tungstate emits blue light to which
  silver halide is sensitive
• Rare earth screens emit green light sensitizers
  are added to the film emulsion to increase the
  sensitivity of silver halide to these wavelengths
• Screens and films are usually purchased as a
  combination

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Reciprocity law

• The relationship between exposure and OD should
  remain constant regardless of the exposure rate
• For very long or very short exposures, and
  exposure rate dependency between exposure and OD
  is observed, and this is called reciprocity law
  failure

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Contrast and dose

• Screen-film system governs the overall detector
  contrast
• Total x-ray exposure time (motion artifacts) and
  radiation dose to the patient are considerations
• Lower kVp settings yield higher subject contrast,
  especially for bone imaging
• Appropriate kVp for each specific type of
  examination is dogmatic, and these values have
  been optimized over a century of experience
• Adjustments may be required due to patient size

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Entrance skin exposure (ESE) vs peak kilovoltage
(kVp)
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Contrast and dose as a function of peak
kilovoltage (kVp) 1-mm bone chip within a 23-cm
patient
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Scattered radiation

• For virtually all radiographic procedures except
  mammography, most photon interactions in soft
  tissue produce scattered x-ray photons
• Detection of scattered photons causes film
  darkening but does not add information content to
  the image

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Scattered radiation and the scatter-to-primary
ratio
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Effect of collimation

• As the field of view is reduced, the scatter is
  reduced
• An easy way to reduce the amount of x-ray scatter
  is by collimating the x-ray field to include only
  the anatomy of interest and no more

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Scatter-to-primary ratio vs x-ray field size for
different thickness
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Antiscatter grid

• An antiscatter grid is placed between the patient
  and the screen-film cassette
• The grid uses geometry to reduce the amount of
  scattered reaching the detector

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Geometry of an antiscatter grid used in
radiography
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Antiscatter grid (cont.)

• Antiscatter grid is composed of a series of small
  slits, aligned with the focal spot, that are
  separated by highly attenuating septa
• Primary x-rays have a higher chance of passing
  through the slits unattenuated by the adjacent
  septa
• Septa (grid bars) are usually made of lead
  openings (interspaces) between the bars can be
  made of carbon fiber, aluminum, or even paper

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Details of grid construction
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Grid ratio

• Single most important parameter that influences
  the performance of a grid
• Grid ratio is the ratio of the height to the
  width of the interspaces (not the grid bars) in
  the grid
• Grid ratios of 81, 101, and 121 are common in
  general radiography grid ratio of 51 is common
  in mammography

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Distribution of scattered x-rays vs scatter
incidence angle
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Other grid parameters

• Focal length determines the amount of slant of
  the slits in the grid
• Grid frequency refers to the number of grid bars
  per unit length. Grids with 40 and 60 grid lines
  per centimeter commonly available
• Interspace material influences dose efficiency.
  Air or carbon fiber required for mammography
• The Bucky factor is the ratio of the entrance
  exposure to the patient when the grid is used to
  the entrance exposure without the grid

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Bucky factor for four different grid ratios
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Grid orientation errors
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Artifacts caused by grids

• Most artifacts associated with the use of a grid
  have to do with mispositioning
• Generally occurs only during installation or
  servicing of the system
• Portable systems may attach the grid to the
  cassette, increasing the chance of mispositioning
• Radiologist should be able to identify the
  presence and cause of various grid artifacts when
  present

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Air gap geometry for scatter reduction
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Air gaps

• Clinical utility of this approach is compromised
  by several factors
• Additional object magnification, often causing
  loss of spatial resolution unless a very small
  focal spot is used
• Reduced field of view that is imaged by a
  detector of fixed dimensions
• Grids still routinely used in all of radiography
  other than extremity studies, some pediatric
  studies, and magnification mammography views