X-ray Physics
Transcript of X-ray 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. < 70 kVp – 100% of X-rays are of this type > 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 don’t 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 (doesn’t 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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You’re done!