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RAD 354 Chapt . 13 Intensifying Screens. Physical purpose: to convert x-ray photons into light photons (done at the phosphor layer). The RESULT does lower patient dose. Most in use – if not ALL – are “rare earth”. Rare earth crystals include (but are NOT limited to): Gadolinium Lanthanum

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rad 354 chapt 13 intensifying screens
RAD 354 Chapt. 13 Intensifying Screens
  • Physical purpose: to convert x-ray photons into light photons (done at the phosphor layer). The RESULT does lower patient dose.
most in use if not all are rare earth
Most in use – if not ALL – are “rare earth”
  • Rare earth crystals include (but are NOT limited to):
    • Gadolinium
    • Lanthanum
    • Yttrium
other intensifying crystals used
Other Intensifying Crystals Used
  • Barium lead sulfate (very early phosphor used)
  • Calcium Tungstate
desired physical properties of crystals
Desired Physical Properties of Crystals
  • High atomic number = high absorption (DETECTIVE QUANTUM EFFICIENCY {DQE})
  • Phosphor should emit a LARGE # of light photons for EACH x-ray photon – CONVERSION EFFICIENCY (CE)
  • Color of light should match the color light the film is sensitive to – SPECTRAL MATCHING
  • ZERO afterglow (“lag”)
important screen terms
Important Screen Terms
  • Luminescence – process of giving off light when stimulated
  • Fluorescence – giving off light ONLY when stimulated
  • Phosphorescence – continuing to give off light after stimualation
  • Intensification factor – amount of radiation reduction WITH screens vs NO screens
screen speed
Screen Speed
  • Can be judged by intensification factor (IF)
  • Increasing speed INCREASES noise
  • Increasing speed REDUCES spatial rresolution
  • Increasing speed INCREASES quantum mottle (line-pair test pattern device is used to measure this)
tech controlable screen items
Tech CONTROLABLE Screen Items
  • Screen attributes the tech can control:
    • Radiation quality (kVp, grid/no grid, filters, etc.)
    • Image processing and temperature
    • Care of and cleaning of screens
cassette construction
Cassette Construction
  • Rigid, light proof protective housing for the film and screens
  • Felt/rubber/sponge “compression” layer to assure good film-screen contact
  • K-edge of crystals determines light spectrum
screen cleaning
Screen Cleaning
  • Compare/contrast screen cleaning solutions (home made vs commercially produced)
  • Cotton balls vs 4 X 4’s
screen film contact test
Screen – Film Contact Test
  • Wire mesh test for screen-film contact and proper resolution/visibility of detail
rad 254 chapt 14 control of scatter
RAD 254 Chapt. 14 Control of Scatter
  • Break down into: Those that reduce patient dose and those that are geometrical in nature and those not
3 primary factors affecting scatter
3 (primary) factors affecting scatter
  • Increased kVp
  • Increased field size
  • Increased patient thickness
spatial resolution contrast resolution
Spatial Resolution & Contrast Resolution
  • Spatial resolution may be thought of as geometric in nature (F.S. size, emission spectrum, OID, SID – dealing with geometric image formation
  • Contrast resolution – driven by scatter and other sources of “noise”
scatter
Scatter
  • INCREASED filed sizes = MORE scatter – collimation is the MOST readily available and EASIEST thing to lower the amount of scatter
  • Patient thickness also INCREASES scatter – compression may be used to help avoid this (IVP’s and mammos are examples where compression may be used)
beam restricting devices limit the radiation to the patient
Beam restricting devices limit the radiation to the patient
  • Aperaturediaphram (size and resultant field size are a DIRECT proportion – draw the damn picture and figure the problem)
  • Cones and cylinders – GREAT for absorbing scatter, but are circular shaped = great for improving contrast and removing scatter, BUT required MUCH MORE mAs as a result
variable aperature diaphram
Variable AperatureDiaphram
  • Mandated in 1974 by the Food and Drug Administration (mandate later removed)
    • Positive Beam Limitation Device (PBL’s)
      • Automatically collimate to the size of the cassette/receptor in the bucky and CANNOT be a BIGGER size than the cassette/receptor
filtration
Filtration
  • Filtration also will DECREASE the low energy rays and LIMIT patient dose and some scatter
the grid
The Grid
  • Only “FORWARD” scatter is of any benefit to the radiographic image – ALL other scatter degrades the image!
scatter lower contrast
Scatter = LOWER Contrast
  • Using a grid (alternating strips of fine leaded strips with alternating radiolucent interspace material) can effectively reduce the amount of ANGLED scatter from reaching the cassette/receptor
grid terms
Grid Terms
  • Grid ratio = height of the lead lines divided by the interspace width
  • Grid frequency/lines per inch = the MORE lines per inch, the more clean up
  • Grid clean up = scatter w/o a grid vs scatter reaching the film/receptor with a grid AKA “Contrast Improvement Factor”
  • Grid function = improved image contrast
bucky factor
Bucky Factor
  • Refers to the AMOUNT of radiation to the patient with a grid vs W/O a grid
    • The HIGHER the grid ratio, the HIGHER the “bucky factor”
    • The HIGHER the kVp, the HIGHER the “bucky factor”
  • Grid WEIGHT refers to how HEAVY the grid is – duhhhh- the MORE lead the heavier it is
grid types
Grid Types
  • Parallel
  • Crossed (cross hatch)
  • Focused
    • Focused crossed
grid problems
Grid Problems
  • Grid cut-off = short SID’s result in the vertical, parallel strips absorbing the “diverging” beam at the OUTER margins of the grid/film/receptor; MOST pronounced at SHORT SID’s
  • Most grid problems are positioning related
    • Uneven grid/off level grid
    • Off centered (lateral decentering)
    • Off focus grid
    • Upside down, focused grid
focused grid misalignment
Focused Grid Misalignment
  • Off level = grid cutoff across image; underexposed image (light OD)
  • Off Center = ditto
  • Off focus = CR centered to one side of the other of a focused grid
  • Upside down grid = SEVER grid cut-off (NO density/OD) at BOTH sides of the image
grid ratio selection
Grid Ratio Selection
  • 8:1 grid is the MOST widely used
  • 5:1 grid is the most PORTABLE use grid ration
  • Grid ratio is kVp driven
    • Higher kVp’s warrant HIGHER grid ratios
    • Higher grid ratios = HIGHER patient dose (more radiation needed to produce an image)
    • As kVp increases pat MAXIUM OPTIMUM kVp, patient dose INCREASES
mas grid considerations
mAs – Grid Considerations
  • AS grid ratio INCREASES, so must mAs
    • 5:1 = 2 X mAs
    • 8:1 = 4 X mAs
    • 12:1 = 5 X mAs
    • 16:1 = 6 X mAs
air gap technique
Air Gap Technique
  • By allowing the scatter radiation to “diffuse” in the atmosphere AFTER the patient but BEFORE the cassette/receptor, the image has HIGHER contrast, as the scatter diffuses and does NOT reach the receptor
    • C-spine is a good example of this
rad 354 chap 15 radiographic technique
RAD 354 Chap. 15 Radiographic Technique
  • Four PRIMARY exposure factors:
    • kVp
    • mA
    • Time
    • distance
in the next 5 minutes
In the next 5 minutes
  • Write down “bullets” about what happens when on RAISES kVp
memory jerk for grids
Memory “jerk” for grids
  • Write the following:
  • 5 2
  • 8 4
  • 12 5
  • 16 6
now what
Now What???
  • 5:1 = 2X mAs
  • 8:1 = 4 X mAs
  • 12:1 = 5 X mAs
  • 16:1 = 6 X mAs
slide32
kVp
  • Beam Qualtiy
    • Primarily responsible for quality, BUT INCREASES in kVp also make x-ray production SLIGHT more productive
  • Penatration
  • Beam intensity
  • HVL
  • Biggest exposure factor affecting CONTRAST
slide33
mA
  • DIRECTLY responsible for AMOUNT of radiation produced (Quantity). As mAs is doubled, so is the number of photons produced and so is PATIENT DOSE
  • mA stations are responsible for focal spot size selection
slide34
Time
  • Exposure times should be practical and short enough to stop patient motion, but the shortest times also result in the most radiation output per unit of time – thus MORE wear and tear on the x-ray tube
  • mAs = time X mA
    • mAs is only measured by tube current
    • Responsible for Optical Density (OD)
distance sid
Distance (SID)
  • The most “forgotten” exposure factor, but perhaps the most important
    • Inverse Square Law
    • Primarily effects Optical Density (OD)
      • NO effect on quality
  • Other distance related terms:
    • FFD, FOD, OFD, FRD, ORD, SSD
  • Other geometric factors (F.S. size, pt. size, part orientation to CR and receptor
filtration kvp driven
FiltrationkVp driven
  • Inherent (.5 mm al equiv)
  • Added (2.0 which may also include some filtration from localizer light apparatus, etc.) in a 70-80 kVp unit
  • Total filtration : inherent + added (2.5 mm al equivalent)
generators
Generators
  • Half wave (120 cycles/sec = 60 impulses per second) – 100% ripple
    • “self rectified” is also half wave where the X-RAY TUBE is the DIODE
  • Full wave rectification (120 cycles per second = 120 impulses per second) – 1--% ripple
  • 3 phase, 6 pulse = 14% ripple (33% more radiation per exposure over full wave)
  • 3phase, 12 pulse = 4% ripple (40% more per exposure over full wave
  • Hi frequency = <1% ripple