Xray Physics

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X-ray Physics

• In a nutshell
• By Dr. Jill Davis

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What you need to know

• The tube how x-rays are produced
• The body how x-rays interact with the body
• The image how x-rays interact with film
• Film processing

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What are X-rays?

• Made of photons
• Travel at speed of light
• Travels in a straight line
• Has no mass nor charge (cannot be focused by
  magnets)
• X-ray beam has a mix of energies
• Maximum energy in a beam kVp
• Diagnostic X-ray range 20-150 kVp

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What are X-rays?
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The X-ray tube
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The X-ray tube parts

• Cathode (-)
• Filament made of tungsten
• Anode () target
• Tungsten disc that turns on a rotor
• Stator
• motor that turns the rotor
• Port
• Exit for the x-rays

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X-ray production

• Push the rotor or prep button
• Charges the filament causes thermionic emission
  (e- cloud)
• Begins rotating the anode.
• Push the exposure or x-ray button
• e-s move toward anode target to produce x-rays

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Hitting the target

• e-s hitting the target creates x-rays two
  different ways
• Characteristic x-rays are due to the material
  the e-s hit (tungsten). Only occurs above 70
  kVp
• Bremsstrahlung (braking) x-rays due to slowing
  down of e- beam.
• lt 70 kVp 100 of X-rays are of this type
• gt 70 kVp 85 of X-rays are of this type

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Characteristic
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Bremsstralung
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Anode Heel Effect
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Exposure Factors

• kVp kilovoltage peak
• mA miliamps (current)
• s seconds (duration of exposure)
• mAs product of mA and s
• Exposure factors are set by radiographer

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X-ray Quality vs. Quantity

• Quality penetrating power / energy
• Quantity of X-rays in beam
• ?kVp ? speed of e- ? quality
• ? kVp efficiency of x-ray production ?
  quantity
• ? mA more e- hit target ? quantity
• ?s longer exposure time ? quantity

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What you need to know

• The tube how x-rays are produced
• The body how x-rays interact with the body
• The image how x-rays interact with film
• Film processing

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Interactions in the Body

• Three things can happen to x-rays as they hit the
  body
• Absorption (photoelectric effect) x-ray is
  absorbed by tissues does not contribute to
  image.
• Scatter (Compton effect) contributes to fog
• Transmission penetrates through body to hit
  radiographic film.

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Interactions in the Body
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Problem

• Only x-rays of sufficient energy (quality) can
  transmit through body to create an image.
• Low energy x-rays dont contribute to the image,
  but add to patient radiation dose.
• Also, different thicknesses, and composition of
  body parts will determine amount of x-ray
  penetration.
• Therefore we need to reduce low energy (low
  quality) x-rays, but at the same time have the
  right quantity of x-rays hitting the body part.

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Filtration

• How we fix the problem is with filtration
• Three kinds of filtration
• Inherent due to tube housing, insulation, etc.
• Added aluminum shielding that blocks low energy
  x-rays.
• Special used to image body parts that have
  varying thickness or density.
• Filtration is measured in terms of half-value
  layer

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Special Filtration
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What you need to know

• The tube how x-rays are produced
• The body how x-rays interact with the body
• The image how x-rays interact with film
• Film processing

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Image Quality
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Density

• Controlling Factors
• mA and s
• ?mAs ?quantity of photons reaching film
  ?density

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Density

• Influencing factors
• ?kVp ?quality (penetration) ?density
• ?SID (source-image distance) ?density
• Due to inverse square law intensity of x-ray is
  inversely proportional to the square of the
  distance from source.
• ?OID (object-image distance) ?density
• Grids (discussed later) grids ?density
• ?Film/screen speed ?density
• ?body part thickness ?density
• ?filtration ?density

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Density and kVp
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Density and SID
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Image Quality
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Contrast

• ?contrast short scale more black and white
  (less detail)
• ?contrast long scale mores shades of grey
  (more detail)

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Contrast

• Controlling factor ? kVp
• ?kVp ? contrast (more shades of grey)

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Contrast

• Influencing factors
• Grid ?fog (scatter) ?contrast
• Collimation narrow collimation ? scatter
  ?contrast
• Anatomic part variation in tissue density
  visible on film
• What are the 5 tissue densities?
• Air, Fat, Water/Tissue, Bone, Metal

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Image Quality
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Recorded Detail

• The sharpness of structural lines in the image
• Geometric unsharpess
• Image receptor unsharpness
• Motion unsharpnesss

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Geometric Unsharpness

• ?SID ?divergence of rays ? unsharpness
• ?OID ?divergence of rays ? unsharpness
• Penumbra geometric unsharpness along the edges
  of the film.

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Image receptor unsharpness

• ?film/screen speed ?detail ?unsharpness

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Motion Unsharpness

• ? motion of patient, image receptor, or tube
  ?unsharpness
• Prevention of motion unsharpness
• ? exposure time
• Patient instruction (i.e. hold breath)
• immobilization

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Image Quality
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Distortion

• Size Distortion
• ?OID ?size distortion (magnification)
• ?SID ?size distortion
• Shape distortion
• Occurs when anatomical part is not parallel to
  the image receptor (elongation or foreshortening)
• Reduced by proper patient positioning and/or tube
  tilt.

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Collimation

• Is located under the port of the X-ray tube.
• Has a light in it for radiographer to see where
  x-rays would hit the patient
• Purpose- restricts beam
• ? patient dose
• ?scatter (?contrast)
• Collimation should be visible on a minimum of
  three sides of the film

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Grids

• Part of the bucky that hold the film cassette
• Reduces scatter radiation that hits film
• Grid is made of lead strips
• Grid ratio height/width of interspace
• Hitting prep button causes grid to vibrate to
  blur out grid lines (doesnt show up on film)

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What you need to know

• The tube how x-rays are produced
• The body how x-rays interact with the body
• The image how x-rays interact with film
• Film processing

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Film

• Photographic film has several layers
• Supercoat protective covering
• Emulsion is radiation and light sensitive
• Made of silver halide and gelatin
• Base plastic for stability
• Film is available in different speeds just like
  35 mm camera film the faster the speed, the less
  radiation is needed to produce an image.

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Image formation

• Latent image invisible image caused by light or
  radiation exposure
• Manifest image shows up after film is developed

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Intensifying screen

• Is located in the cassette that film is placed
  inside of.
• Screen contains phosphors that fluoresces
  when exposed to x-rays.
• Purpose screens amplify x-rays that hit the
  film so you need a lot less mAs to produce an
  image .
• Drawback lose some recorded detail
• Screens also come in different speeds i.e.
  the degree to which it fluoresces upon exposure.

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Film Processing

• May become obsolete as the industry moves to
  digital
• Steps of processing (automatic)
• Developer converts latent image to manifest
  image (22 sec)
• Fixer acetic acid
• Wash- water removes residual chemicals
• Dry blow dryer in the processor

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Radiation Dosimetry - definitions

• Roentgen unit of radiation that will liberate a
  charge of 2.58 x10(-4) coulombs per kilogram of
  air.
• Coulomb unit of electrical charge
• RAD radiation absorbed dose 1 rad is equal to
  the radiation necessary to deposit 100 ergs (unit
  of energy) in 1 gram of irradiated material
• SI unit 1 gray 100 RAD

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Radiation Dosimetry - definitions

• REM rad equivalent man is the unit of
  absorbed dose equivalent is a measure of the
  biological effect of radiation.
• SI unit 1 sievert 100 REM

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Radiosensitivity

• Radiation damages DNA
• Tissues that are sensitive to radiation are
• High lymphocytes, spermatogonia, erythroblasts,
  intestinal crypt cells
• Intermediate- endothelial cells, osteoblasts,
  spermatids, fibroblasts
• Low muscle cells, nerve cells, chondrocytes.
• Rule of thumb the cells that proliferate more
  are more sensitive

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Positioning Tips

• SID (aka FFD or TFD)
• Is either 40 or 72
• Think 40 for all except FS, lat or oblique C-sp
  (air gap), P-A chest
• Tube Tilt
• For every 5 tube tilt, move tube 1 closer
• When to use tube tilt to reduce shape
  distortion
• Example A-P lower cervical 15 cephalad 37

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Positioning Tips

• Central Ray
• Generally aim at middle of anatomy you want to
  see.
• Film Size small film for small part
• 8 x 10, 10 x 12, 14 x 17, 14 x 36
• Collimation how much do you restrict beam?
• Collimation visible on film at least 3 sides
• Include anatomy you want to see

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Positioning Tips

• Ten day rule
• For females of childbearing age
• X-rays not taken after 10 days of start of
  menstrual period.
• Shielding / filters
• Gonadal shield any A-P view that includes the
  pelvis
• Lead apron over body parts not to be visualized
  (extremity views)
• Filters wedge filters (example, for A-P FS,
  wedge over the superior half of spine.

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Youre done!
Are you nuts yet?