Diagnostic Radiology II

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Diagnostic Radiology II

• X-ray Tubes
• Tube Housing
• Filtration Collimation

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Anode angle

• Anode angle defined as the angle of the target
  surface with respect to the central ray in the
  x-ray field
• Anode angles in diagnostic x-ray tubes range from
  7 to 20 degrees, with 12- to 15-degree angles
  most common

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Focal spot size

• Actual focal spot size is the area on the anode
  that is struck by electrons
• Primarily determined by length of cathode
  filament and width of focusing cup slot
• Effective focal spot size is the length and width
  of the focal spot projected down the central ray
  in the x-ray field

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Anode angle
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Focal spot size

• Effective focal spot width is equal to the actual
  focal spot width
• Effective focal length actual focal length ?
  sin ?
• Foreshortening of the focal spot length, as
  viewed down the central ray, is called the line
  focus principle

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Example

• Actual anode focal area for a 20-degree anode
  angle is 4 mm (length) by 1.2 mm (width). What
  is the projected focal spot size on the central
  axis?
• Effective length actual length ? sin ? 4 mm ?
  sin 20 degrees 4 mm ? 0.34 1.36 mm
• Projected focal spot size is 1.36 mm (length) by
  1.2 mm width.

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Anode angles

• Optimal anode angle depends on the clinical
  imaging application
• Small anode angle (approximately 7 to 9 degrees)
  desirable for small field-of-view image receptors
  (cineangiographic and neuroangiographic
  equipment, where field coverage is limited by the
  image intensifier diameter)
• Large anode angles (12 to 15 degrees) necessary
  for general radiographic work to achieve large
  field area coverage at short focal spot-to-image
  distances

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Variation with position

• Effective focal spot length varies with the
  position in the image plane, in the anode-cathode
  direction
• In the width dimension, the focal spot size does
  not change appreciably with position in the image
  plane
• Nominal size specified at the central ray of the
  beam
• In most radiographic imaging, the central ray
  bisects the detector field

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Variation of effective focal spot size
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Measuring focal spot size

• Pinhole camera
• Slit camera
• Star pattern
• Resolution bar pattern

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Pinhole camera

• Very small circular aperture (10 to 30 ?m
  diameter) in a disk made of a thin, highly
  attenuating metal such as lead, tungsten, or gold
• Image of focal spot recorded with the pinhole
  camera positioned on the central axis between the
  x-ray source and the detector

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Large focal spot
Small focal spot
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Slit camera

• The slit camera consists of a plate made of a
  highly attenuating metal (often tungsten) with a
  thin slit (typically 10 ?m wide)
• Measuring the width of the distribution on the
  image and correcting for magnification yields one
  dimension of the focal spot
• Second radiograph, taken with the slit
  perpendicular to the first, yields the other
  dimension of the focal spot

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Large focal spot
Small focal spot
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Star pattern

• Star pattern test tool contains a radial pattern
  of lead spokes of diminishing width and spacing
  on a thin plastic disk
• Imaging the star pattern at a known magnification
  and measuring the distance between the outermost
  blur patterns on the image provides an estimate
  of the resolving power of the focal spot in the
  directions perpendicular to and parallel to the
  anode-cathode axis

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Star pattern test
Large focal spot
Small focal spot
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Star pattern

• A large focal spot has a greater blur diameter
  than a small focal spot
• Effective focal spot size can be estimated from
  the blur pattern diameter and the known
  magnification

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Resolution bar pattern

• Similar to the star pattern tool, but containing
  radioopaque bars of varying widths and spacings
• Bar pattern images demonstrate the effective
  resolution parallel and perpendicular to the
  anode-cathode axis for a given magnification
  geometry

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Resolution bar pattern test
Large focal spot
Small focal spot
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Heel effect

• The heel effect refers to a reduction in the
  x-ray beam intensity toward the anode side of the
  x-ray field
• For a given field size, the heel effect is less
  prominent with a longer source-to-image distance
  (SID)
• X-ray tube best positioned with the cathode over
  the thicker parts of the patient to balance the
  transmitted x-ray photons incident on the image
  receptor

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Heel effect
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Off-focus radiation

• Off-focus radiation results from electrons in the
  tube that strike the anode outside the focal spot
  area
• Small fraction of electrons scatter from the
  target and are accelerated back to the anode
  outside the focal spot
• Create a low-intensity x-ray source over the face
  of the anode
• Off-focus radiation increases patient exposure,
  geometric blurring, and background fog

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Reducing off-focus radiation

• Small lead collimator placed near the x-ray tube
  output port can reduce off-focus radiation
• Intercepts x-rays that are produced at a large
  distance from the focal spot
• X-ray tubes that have a metal enclosure with the
  anode at the same electrical potential (i.e.,
  grounded anode) reduce off-focus radiation
• Stray electrons as likely to be attracted to the
  metal envelope as to the anode

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Tube housing

• X-ray tube housing supports, insulates and
  protects the x-ray tube insert
• Oil in the housing provides heat conduction and
  electrical insulation
• Lead shielding inside the housing attenuates the
  x-rays that are emitted in all directions
• Regulations limit the maximum leakage exposure
  rate (0.1 _at_ 1m of the exposure rate at the same
  distance on the central axis of the useful beam)

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Grid-biased tubes

• Focusing cup is electrically insulated from the
  cathode filament
• Approximately 2,000 V applied to focusing cup
  shuts off the tube current turning off the grid
  bias allows tube current to flow and x-rays to be
  produced
• Used in applications such as pulsed fluoroscopy
  and cineangiocardiography

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Filtration

• In general diagnostic radiology, added filters
  attenuate the low-energy x-rays in the spectrum
  that have virtually no chance of penetrating the
  patient and reaching the x-ray detector
• Patient radiation dose is reduced
• Aluminum is the most commonly added filter
  material
• Regulations prescribe minimum filtration (e.g.,
  2.5 mm Al for machines designed to operate at
  potentials above 70 kVp)

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Compensators

• Compensators are added filtration with a shape
  intended to change the spatial pattern of the
  x-ray intensity incident on the patient, so as to
  deliver a more uniform x-ray exposure to the
  detector
• Placed close to the x-ray tube port or just
  external to the collimator assembly

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Example compensators

• Trough filter used for chest radiography has
  centrally located vertical band of reduced
  thickness
• Compensates for the high attenuation of the
  mediastinum and reduces exposure latitude
  incident on the image receptor
• Bow-tie filter used in computed tomography (CT)
  to reduce dose to the periphery of the patient

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Sample bow-tie filter
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Collimators

• Collimators adjust the size and shape of the
  x-ray field emerging from the tube port
• Attached to the tube housing at the tube port
  with a swivel joint
• Adjustable parallel-opposed lead shutters define
  the x-ray field
• Positive beam limitation collimators
  automatically limit the field size to the useful
  area of the detector

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