Mammography II

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Mammography II

• Compression, scattered radiation, magnification
• Screen-film cassettes processing
• Quality assurance

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Compression

• Firm compression reduces overlapping anatomy and
  decreases tissue thickness of the breast. This
  results in
• Fewer scattered x-rays
• Less geometric blurring of anatomic structures
• Lower radiation dose to the breast tissues
• Uniform breast thickness lessens exposure dynamic
  range and allows the use of higher contrast film

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Compression (cont.)

• Compression paddle should match the size of the
  image receptor, be flat and parallel to the
  breast support table, and not deflect more than 1
  cm at any location when compression is applied
• A right-angle edge at the chest wall produces a
  flat, uniform breast thickness when compressed
  with a force of 10 to 20 newtons (22 to 44 pounds)

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Spot compression

• Smaller compression paddle area ( 5 cm diameter)
  reduces even further the breast thickness in a
  specific area and redistributes the tissue for
  improved contrast and anatomic rendition
• Valuable in delineating anatomy and achieving
  minimum thickness in an area of the breast
  presenting with suspicious findings on previous
  images

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Compression of a suspicious are shows no visible
mass
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Contrast

• Maximum contrast with scatter

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Scatter

• The amount of scatter in mammography increases
  with increasing breast thickness and breast area,
  and is relatively constant with kVp
• The fraction of scattered radiation in the image
  can be reduced by use of antiscatter grids or air
  gaps, as well as vigorous compression

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Scatter depends on breast thickness and field area
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Antiscatter grid

• Parallel linear grids with a ratio of 41 or 51
  are commonly used
• Aluminum and carbon fiber are common interspace
  materials
• Grid frequencies range from 30 to 50 lines/cm for
  moving grids, and up to 80 lines/cm for
  stationary grids

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Linear grid (typically 51), cellular grid for 2D
scatter rejection, and the air gap (intrinsic to
magnification procedures)
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2D copper scatter grid with air interspaces
transmits 87 of primary photons (Argonne
National Labs, 2006)
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Magnification

• Magnification is achieved by
• Placing a breast support platform to a fixed
  position above the detector
• Selecting the small focal spot
• Replacing the antiscatter grid with a cassette
  holder, and
• Using an appropriate compression paddle

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Geometric magnification
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Magnification (cont.)

• Advantages of magnification include
• Increased effective resolution of the image
  receptor by the magnification factor
• Reduction of effective image noise
• Reduction of scattered radiation
• Limitations of magnification include
• Geometric blurring caused by the finite focal
  spot size
• Small focal spot limits tube current, and extends
  exposure times. Even slight breast motion will
  cause blurring during the long exposure times

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Illustration of the need for the 0.1 mm focal
spot in magnification studies
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Screen-film cassettes

• Most cassettes are made of low-attenuation carbon
  fiber and have a single high-definition phosphor
  screen used with a single emulsion film
• The screen is positioned in the back of the
  cassette
• Light spread varies with the depth of x-ray
  absorption
• Most x-rays interact in the layers of the screen
  closest to the film, preserving spatial resolution

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Film-screen system for mammography
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Film processor QA

• Must be done daily, prior to first patient use
• Solution levels and temperatures checked
• Base fog must be within 0.03 of baseline
• Mid-density must be within 0.15 of baseline
• Density difference (contrast) must be within
  0.15 of baseline

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Film processor testing
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Radiation dosimetry

• Mean glandular dose preferred index since
  glandular tissue is most always the site of
  carcinogenesis
• Speed of screen-film system and film OD preferred
  by radiologist are major factors
• Should not exceed 3.0 mGy (300 mrad) per film

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Mammography phantom

• Used to determine adequacy of overall imaging
  system (including film processing) in terms of
  detection of subtle radiographic findings, and to
  assess reproducibility of image characteristics
  (e.g., contrast and optical density) over time

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Phantom (cont.)

• Intended to mimic attenuation characteristics of
  a standard breast of 4.2 cm compressed
  thickness composed of 50 adipose tissue and 50
  glandular tissue
• Wax insert contains
• 6 cylindrical nylon fibers of decreasing
  diameter
• 5 simulated calcification groups (Al2O3 specks)
  of decreasing size
• 5 low contrast disks, of decreasing diameter and
  thickness, that simulate masses

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Wax insert composition and undegraded radiograph
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Phantom QA

• Should be done weekly must be done monthly
• Minimum of 4 largest fibers, 3 largest speck
  groups, and 3 largest masses must be visible
• Number of test objects of each group type visible
  in image should not decrease by more than one
  half
• Background OD should be at least 1.4 and not vary
  more than 0.20 from operating level
• Density difference due to a 4.0 mm acrylic disc
  should be at least 0.40 and not vary more than
  0.05 from established operating level

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Mammographic image of composite phantom