ScreenFilm Radiography I

1
Screen-Film Radiography I

• Geometric principles
• Film cassettes
• Screen characteristics

2
Projection radiography

• Projection imaging refers to the acquisition of a
  two-dimensional image of the patients
  three-dimensional anatomy
• Radiography is a transmission imaging procedure
• In screen-film radiography, the optical density
  (OD) at a specific location on the film is
  (ideally) determined by the x-ray attenuation
  characteristics of the patients anatomy along a
  straight line through the patient between the
  x-ray source and the corresponding location on
  the detector

3
(No Transcript)
4
Basic geometric principles

• Two triangles that have the same shape (the three
  angles of one are equal to the three angles of
  the other) but have different sizes are said to
  be similar triangles
• If two triangles are similar, the ratio of the
  corresponding sides and heights are equal
• Similar triangles are encountered when
  determining image magnification and when
  evaluating image unsharpness

5
(No Transcript)
6
Magnification

• Magnification occurs because the beam diverges
  from the focal spot to the image plane
• Magnification given by
• Largest when object close to focal spot

7
(No Transcript)
8
Extended source

• With magnification, geometric blurring of the
  object occurs in the image
• Similar triangles allow calculation of the edge
  gradient blurring, f, in terms of magnification,
  M, and focal spot size, F

9
Extended source (cont.)

• Blur increases with the size of the focal spot
  and with the amount of magnification
• Focal spot blur can be minimized by keeping the
  object close to the image plane

10
(No Transcript)
11
Screens

• Film by itself can be used to detect x-rays
• Relatively insensitive
• A lot of x-ray energy is required to produce a
  properly exposed x-ray film
• X-ray screens are used to reduce the x-ray dose
  to the patient
• Screens are made of a scintillating material,
  called a phosphor
• X-rays interact in the phosphor visible or
  ultraviolet light is emitted

12
Materials

• For much of the 20th century, calcium tungstate
  (CaWO4) most commonly used
• In early 70s, rare earth phosphors introduced
• Most common used today is gadolinium oxysulfide
  (Gd2O2S)
• Others are lanthanum oxybromide (LaOBr) and
  yttrium tantalate (YTaO4)
• Cesium iodide (CsI) is used in fluroscopy and
  digital radiography too moisture sensitive and
  fragile for use in screen-film radiography

13
(No Transcript)
14
Phosphor thickness

• Usually expressed as the mass thickness (product
  of thickness and density) of the phosphor,
  excluding the binder
• For general radiography, two screens are used,
  with each screen having a thickness of about 60
  mg/cm2, for a total thickness of 120 mg/cm2
• For high-resolution requirements and low energies
  used in mammography, a single screen of
  approximately 35 mg/cm2 is used

15
Screen function

• Screen function is two-fold
• Absorbing (detecting) incident x-rays
• Emitting visible (or UV) light, which exposes the
  film
• Conversion efficiency defined as the fraction of
  the absorbed energy that is emitted as light
• CaWO4 about 5
• Gd2O2STb about 15
• Not all emitted photons reach the film emulsion
  after diffusing through the phosphor and being
  reflected at interface layers

16
Quantum detection efficiency

• QDE of a screen is defined as the fraction of
  incident x-ray photons that interact with it
• Easiest way to increase QDE is to make the screen
  thicker

17
(No Transcript)
18
Thickness effects

• After an x-ray absorption event, visible light
  given off in the depth of a screen propagates
  towards the screens surface to reach the
  adjacent film emulsion layer
• Spreads out in all directions with equal
  probability (isotropic diffusion)
• For thicker screens, light photons propagate
  greater lateral distances

19
(No Transcript)
20
Resolution

• Increasing the screens thickness increases
  detection efficiency and thereby improves the
  screens sensitivity
• Increased thickness causes undesirable loss of
  spatial resolution
• Classic compromise between sensitivity and
  resolution seen in many imaging systems
• Modulation transfer function (MTF) is the
  technical description of spatial resolution for
  most imaging systems

21
(No Transcript)
22
Conversion efficiency

• Total conversion efficiency of a screen-film
  combination refers to the ability of the screen
  or screens to convert the energy deposited by
  absorbed x-rays into film darkening
• Depends on intrinsic conversion efficiency of the
  phosphor, efficiency of light propagation through
  screen to emulsion layer, and efficiency of film
  emulsion in absorbing the emitted light
• Light-absorbing dye may be used to reduce light
  propagation distance and preserve spatial
  resolution

23
(No Transcript)
24
Conversion efficiency (cont.)

• For faster (i.e., higher conversion efficiency),
  lower-resolution screen-film system, a reflective
  layer can be placed between the screen and its
  support
• Intensifying screen by itself is a linear device
  at a given x-ray energy
• If number of x-ray photons is doubled, light
  intensity produced by screen also doubles

25
(No Transcript)
26
(No Transcript)
27
Overall efficiency

• Total efficiency of a screen-film system is the
  product of the conversion efficiency and the
  absorption efficiency
• Screen-film system provides vast increase in
  x-ray detection efficiency over film by itself
• At 80 kVp, a 120 mg/cm2 thickness of Gd2O2S
  detects 29.5 of the incident x-ray energy
• Film detects only 0.65 of the incident x-ray
  energy

28
(No Transcript)
29
Other benefits

• Output requirements of the x-ray system are
  reduced, substantially reducing the requirement
  for powerful generators and high heat capacity
  tubes and reducing costs
• Exposure times are shorter and motion artifacts
  are reduced
• Exposure to personnel from scattered x-rays is
  reduced