Download Exam II Review Fall 08

November 11, 2008
HM Fall 2008
Fundamentals of Diagnostic Imaging
Exam II Review
Chapter 2: The X-ray Beam
Beam Filtration (pg. 37)
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2 Main Types
o Inherent Filtration
 Inherent to the x-ray tube & housing
 Glass Envelope
 Oil surrounding the tube
 Collimator Mirror
 No energy below 15keV allowed to pass
o Added Filtration
 Added to the port of the x-ray tube
 Aluminum (Al) primarily used
 Absorbs low energy but allows high energy to pass
Total Filtration
o Sum of Added + Inherent filtration
o Current U.S. standard
 X-ray tubes operating over 70kvp must have a minimum total filtration of 2.5
mm of Aluminum (Al) equivalent
Selective / Special / Compensating Filtration – between x-ray beam/collimator & patient
o Added to the primary beam to alter its intensity
o Examples: Gonadal shielding, Eye shielding, Trough (double wedge filter)
o Most common type: Simple Wedge Filter
 Thick Side & Thin Side
 Made of aluminum or lead
o Also used to “even out” an image
Heat Units (HU)
 Amount of heat produced by any given exposure
 mA x Time (S) x kVp x Generator factor = HU
o High Frequency Generator Factor = 1.45
Tube Rating Charts
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Describes exposure limits of the x-ray tube
Most modern machines will not allow for tube-damaging exposures
o Theoretically, you can’t burn anode
November 11, 2008
HM Fall 2008
Chapter 3: Radiographic Image Formation (p. 45)
Interactions with Matter
5 Main types of interactions with matter
 3 Within Diagnostic Range
o Compton = outer shell – creates scatter
o Photoelectric = interacts with k shell (inner shell) – most desired for film
o Coherent (Classic) Scattering – Non–ionizing - never ejects an actual electron from
shell
 2 Outside Diagnostic Range (Nuclear interactions)
o Pair production
o Photodisentigration
Scatter increases film density – Creates fog
Differential Absorption
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Process where some of the x-ray beam is absorbed in the tissue & some pass through a
given object
Different tissues absorb differently (listed from radiolucent [BLACK] to radiopaque
[WHITE])
5 Radiographic Intensities
o Inherent / Endogenous – Normally found in Human body
1. Air
2. Fat
3. Water (H2O)
4. Bone
o Exogenous – NOT normally found in Human body
5. Metal
Beam Attenuation
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The energy reduction of a primary x-ray beam as it passes though a given object
3 processes occur
1. Absorption – Good
2. Scattering – NOT good – only degrades image quality
3. Transmission – doesn’t interact with anything
November 11, 2008
HM Fall 2008
Chapter 6: Scatter Control (p.133)
Field Size
Types of Beam-Restricting Devices
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Cones
Cylinders
Aperture Diaphragms
Collimators – most commonly used in diagnostic radiology
Primary Ways to Control (Prevent) Production of Scatter
1. Field size (example: collimator)
2. kVp (lower kVP = less scatter)
3. Patient Thickness (Anatomical Part thickness / Size) – Dr. has least amount of control over
a. Methods to reduce patient size
i. Can lay patient down
ii. Use compression bands
Focal Film Distance (FFD) (p. 164)
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Air-Gap Technique - Increase Object-Image Distance (OID)
o Move image receptor away from the patient (create gap), less scatter reaches the
image receptor
Differential Radiation
Field Size
Grids
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Grid Ratio
o Height of lead strips / Distance between the lead strips
o Ex: 6:1, 8:1, 10:1, 12: 1, 16:1
o 10:1 or 12:1 ideal ratios to be used for diagnostic imaging
Types of Grids
o Linear Grid
o Crossed Grids
o Focused Grids - Focused grids follow beam divergence
 Linear Focused Grids – Most commonly used in diagnostic radiology
 Crossed focused Grids
Grid Performance
o Purpose is to increase radiographic contrast, reduce scatter, but reduces density so
additional mAs is required
o Grid conversion factor (aka Bucky factor) used to determine adjustment of mAs
needed
o Mathematically expressed
November 11, 2008
HM Fall 2008
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 Bucky factor = mAs with the grid/mAs without a grid
Frequency (# of lead lines / unit length)
o Typical: 40 lines/cm or 103 lines/inch for diagnostic radiology
Grid Cut-off (most only occur with focused grids )
o Off-Center Grid
 Lateral decentering
o Off-Focus Grid
o Upside-Down (Focused) Grid
 A focused grid placed upside-down
 Appears as significant loss of density along the edges
 Dark Middle but Nothing on the SIDES of image
o Off-Level Grid – MOST COMMON
 X-ray beam is angled across the lead strips
 Occurs with both focused & parallel
If a part measures 10 cm or Greater thickness – Use a grid
Increase patient dose – because MaS must be increased to maintain density
Advantages vs Disadvantages
Chapter 7: Image Receptors (p. 169)
Cassettes
Radiographic Film
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Construction (Layers of Film)
o Supercoat
 Protective outer layer
 Made of gelatin
o Emulsion
 Radiation/light-sensitive layer
 Active ingredient: Silver halides
 Dominant Silver halide: Silver Bromide (AgBr) 90-99%
 Silver iodide (AgI) only 10-1% of emulsion layer
o Adhesive Layer
o Base
 Blue dye or tint added - to reduce eye strain
Latent Image – Image that exists on the film after exposure, but prior to processing (aka prior to
development)
 Formed by: Gurney-Mott Theory
 Can’t be seen with eyes
 After developing film Latent Image becomes Manifest image
Manifest Image – image that exists on film after exposure & developing
 The Radiographic Image
November 11, 2008
HM Fall 2008
Crossover
o A problem unique to having 2 layers of emulsion (Double emulsion or Duplitized
film)
o Light produced by the intensifying screen exposes the back emulsion, crosses over
the base layer & exposes the front emulsion
o Blurs the Image
o Modern film has built in t-grain technology & anti-crossover layers that virtually
eliminate crossover
Types of Film
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Direct Exposure (Non-screen)
o Intended for cardboard holder without an intensifying screen
Screen Film
o Most widely used
o Used with one-two intensifying screens
o More light sensitive
o Single Emulsion
o Double Emulsion (aka Duplitized)
Duplicating (aka Copy film)– designed to allow a copy of an original film
o Solarization – process by which copy film acts (opposite/ the reverse) as it should to
light
Exposure Latitude – aka Forgiveness of the film
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Range of exposure techniques (kVp & mAs) that will produce an acceptable image
 Contrast & Latitude = Inverse Relationship
o High contrast film = Narrow (low) latitude
o Low contrast film = Wide (high) latitude
 Wide Latitude = more favorable for diagnostic imaging
 It’s more forgiving
 Don’t have re-x-ray patient as much because there are a wider range
of exposures that fit
 Characteristic curve (aka H&D curve or Sensitometric curve)
o Graphically demonstrates (plots):
 Film speed
 Contrast
 Latitude
o Describes the relationship between Optical Density & Radiation Exposure
Object-Image Distance – between object being x-rayed and image receptor
 Increase in OID = increase contrast (ie: air-gap technique)
o Scatter doesn’t have enough Energy to get to the film
November 11, 2008
HM Fall 2008
Chapter 7: Image Receptors (Continued) (p. 59)
Intensifying Screens
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Primary function: Reduce patient dose
Active layer: Phosphor layer
o Emits light during stimulation by x-rays
o Converts energy of x-ray beam into visible light
o Layer of crystals
Modern intensifying screens: Rare Earth phosphor Intensifying Screens
o Rare Earth phosphors (atomic number between 57 to 71)
 Gadolinium – produces green light when stimulated
 Lanthanum – Blue light when stimulated
 Yttrium – Ultraviolet/Blue light when stimulated
Factors that effect screen speed
o Size of crystals
o Thickness of phosphor layer (crystal layer)
o Phosphor used
 Rare Earth phosphor Intensifying Screens
 Faster, More Efficient, Faster Conversion Ratio than Calcium Tungstate
 Conversion ratio – taking photons in and converting it to white light
o Intensification factor
 Exposure required without screens/Exposure required with screens
Luminescence
o Florescence
 Type of Luminescence do intensifying screens produce
o SCREENS Don’t use Phosphorescence
 Produces screen after-glow or screen lag
 NOT favorable
 Would overexpose radiograph
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Quantum Mottle
o Image noise, “Noise in the film”
o Statistical fluctuation in the quantity of x-ray photons that contribute to image
formation per square millimeter
o If you use too low of mAs the image may appear “salt & pepper” like
o Decreases recorded detail
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Efficiency
Conversion Ratios
Spectral Matching
o Film should be sensitive to the same color as intensifying screen
 Spectral emission of the screen must match sensitivity of the x-ray film
 Example: Green screen, should use green sensitive film
November 11, 2008
HM Fall 2008
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Calcium tungstate screen speed - Standard screen speed or Par-Speed
o Speed at which other screens are measured against/compared to
o Speed: 100
Faster screen  increases Density
Faster screen  decreases (lowers) Detail (doesn’t affect contrast)
Chapter 4: Radiographic Image Quality: Photographic Properties (p. 59)
Know whole Chapter
Density
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Controlling Factors
o Miliamperage (Ma)
o Exposure Time (S)
Contrast
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Controlling Factor:
o Kilovoltage(kVp)
 Inverse relationship to Contrast
 Example: High kVp = low Contrast
 Direct relationship to Gray Scale
 Example: High kVp = Long Gray Scale (Long-scale)
Gray Scale Inverse relationship to Contrast
o Example: Long Gray Scale (long-scale) = low Contrast
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KNOW CHART (p. 102)
Inverse-Square Law (p. 70)
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Relationship between FFD (aka SID – Source-Image Distance) & Density
Density (intensity) of x-ray beam is inversely related to Square of Distance (Distance2)
o 2x FFD = MaS should be increased by 4x (factor of 4) to maintain same densitty
o Example #1: If distance is increased from 40in to 80in (double [2x] distance)
 Density of film will be ¼ of density that it originally was (at 40in)
 Must increase MaS by 4X to maintain original density
o Example #2: If distance is decreased from 60in to 30in
 MaS must be decreased by ¼ to maintain same Density
o Short FFD = increase Density
o Long FFD = decrease density
o Closer you are to SOURCE the darker the image should be
Grids
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Decrease density
When using a grid: Must increase Time (S) to maintain Density (to prevent density from being
lowered)
November 11, 2008
HM Fall 2008
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Grid Ratio
o 10:1 or 12:1 ideal ratios to be used diagnostically
Increase Contrast (limits/prevents/reduces Scatter hitting film)
Collimation (field size)
 Limits field size – limits area being radiated
 Increases Contrast (limits/reduces Scatter)
Air Gap Technique
 Increases Contrast (limits/reduces Scatter)
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EXAM II (Details)
50 to 60 Multiple Choice questions
No MACA
Fauber book
o Parts of Chapters: 2, 3, 4, 6, 7