ARRT Board Preparation

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ARRT Board Preparation

1. Equipment Operation Maintenance

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Section Components

• 30 Questions
• Radiographic Equipment (21)
• Evaluation of Radiographic Equipment
  Accessories (9)

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X-ray Tube Construction
4
X-ray Tube Construction
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Cathode - Filaments

• Negative Electrode
• Tungsten with thorium
• Filament circuit heats (gt2200C) with 3-5 amps
• Thermionic Emission
• Vaporization
• Space charge
• Saturation current
• Focusing cup

6
Filament I vs. mA

• Heated filament emits electrodes
• Once emission starts small ? in filament I
  large ? in mA
• As kVp rises less filament I needed to reach mA

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Saturation Current/Voltage

• Stabilization at specific mA above 40 kVp

8
Focusing Cup
9
Anodes - Target

• Rotating Anodes
• 2 to 5 disk (focal track)
• Induction motor
• Speed 3000 to 10000 rpm

Cu
W

• Other target materials
• Tungsten-rhenium
• Molybdenum
• Rhodium

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Line-Focus Principle

• Effective FS lt actual FS
• Beveled anode
• Improves heat capacity
• Small angle ? FS
• Small FS ? resolution
• AHE ?
• Field coverage ?
• Heat loading on smaller anode surface area

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Miscellaneous Terms
12
OFF FOCUSRADIATION
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• SHADOW OF
• SOMEONES HEAD
• OFF FOCUS FROM TUBE

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LEAKAGE RADIATIONmay not EXCEED

• TUBE HOUSING 100mR / HR
• _at_ 1 meter

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Tube Failures Prevention

• Failure Causes
• Anode pitting/cracking after single excessive
  exposure
• Bearing damage from numerous long exposures
• Vaporization of the filament
• Filament break
• Coating of glass envelope with tungsten

• Prevention
• Warm up tube before use
• Avoid boost hold exposures if possible
• Use acceptable levels of exposure (e.g. tube
  rating charts)

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Tube Rating Charts (mA)
20
Tube Rating Charts (kVp)
21
Heat Units

• HU measure of thermal energy applied to the
  x-ray tube from an exposure
• Formula based on generator
• HU1Ø kVp x mAs
• HU3Ø6p kVp x mAs x 1.35
• HU3Ø12p kVp x mAs x 1.41
• If multiple, consecutive exposures made,
• HUT HU x exposures

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Anode Cooling Chart
23
AEC

• Operation
• Select sensor(s)
• Select density level
• Set back-up time
• Set kVp

film
Ion chambers
film
Fluorescent screen
Exposure Switch
PMT
Exit vs. Entrance
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Beam Restrictors
25
Important Circuit Elements
HV subcircuit
Power Source
X-ray Tube
autotransformer
Filament subcircuit
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X-ray Generator
cathode

• Transformers
• HV (step-up)
• Filament (step down)
• Rectification
• Connection to tube

anode
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Transformers

• HV
• Step-up
• Fixed TR gt 1 (500 to 1000)
• VS gt VP
• ? V to kV

• Autotransformer
• Step-up or step-down
• Variable TR
• Controls kV by varying V sent to HVT

• Filament
• Step-down
• Fixed TR lt 1
• IS gt IP (VSltVP)
• ?I in filament to cause e- emission

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Transformer Review

• Turns Ratio
• Transformer Law
• Power

• Step Up
• TRgt1
• ? V ?I
• Step Down
• TRlt1
• ?V ?I

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Autotransformer Self Induction

• There is only one wire but works like when
  there are 2 wires
• The windings are used as the primary and
  secondary coils
• The induced voltage varies on where the outside
  wires are connected (KVP Taps)

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TRANSFORMER FORMULAS(STEP UP OR DOWN)

• V voltage
• N turns
• p primary
• s secondary
• I current

• Vp N p
• Vs Ns
• Vp I s
• Vs Ip
• Np I s
• Ns Ip

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Transformer Efficiency

• By design
• Shell most
• Closed core
• Open core
• Air core

• Ideal no loss
• Reality best 95 induction
• Loss due to
• Cu resistance
• ? wire diameter
• Eddy currents
• Laminate core
• Hysteresis
• ? core mag. perm.

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Rectification

• ? AC to DC
• Keeps e- flowing from cathode to anode
• Uses rectifying bridge between HVT tube

AC
DC
bridge
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Phase, Pulse Frequency
Type of rectifiers ripple Wave form
1?(self) 1 60 0 100
1?(half) 1 60 1 or 2 100
1?(full) 2 120 4 100
3?6p 6 360 6 or 12 12-15
3?12p 12 720 12 3-5
High Freq. 10kHz lt1
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generator
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GENERATOR

• THAT CREATE AN ALTERNATING CURRENT ARE CALLED
• AN ALTERNATOR
• CONVERT MECHANICAL ENERGY INTO ELECTRICITY

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• WHAT MEASURES ELECTRIC POTIENTAL VOLT
• CURRENT AMP
• ELECTRIC CIRCUIT IS THE PATHWAY FOR ELECTRIC
  CURRENT

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OHMS LAW V IR

• V POTIENTAL
• A AMPS (CURRENT)
• R RESISTANCE (OHMS)

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OHMS LAW

• V Potential difference in volts
• I Current in amperes
• R Resistance in ohms (?)
• V IR I V/R RV/I

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Simple SERIES Circuit
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Series Circuit Rules

• Current IT I1 I2 I3
• Voltage VT V1 V2 V3
• Resistance
• RT R1 R2 R3

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Parallel Circuit Rules

• Current
• IT I1 I2 I3
• Voltage
• VT V1 V2 V3
• Resistance
• 1 1 1 1
• RT R1 R2 R3
• (REMEMBER TO FLIP SIDES RT/1

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TYPES OF EQUIPMENT

• FIXED
• MOBILE
• DEDICATED
• MAMMO , CHEST, HEAD

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Fluoroscopic Unit

• Conventional fluoroscopy
• User viewed faint image on screen
• User in direct path of beam
• Very high dose to user and patient
• Excellent resolution
• No longer used

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Image Intensified Fluoroscopy

• Electronic conversion of screen image to light
  image that can be viewed on a monitor
• ? resolution
• ? dose

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Image Intensifier Tube

• Vacuum diode tube
• 1. Input phosphor (CsI)
• X-rays ? light
• 2. Photocathode
• Photoemission
• Light ? electron beam
• 3. Electrostatic lenses
• Maintain minify e-
• 4. Anode
• Attracts e- in beam
• 5. Output phosphor (ZnS-CdS)
• e- ? light

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Image Intensifier Terms

• Brightness Gain
• Ability of II to increase illumination

• Minification Gain

• Flux Gain (usually stated rather than calculated)

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Multi-field II Units

• II that allows selection of input phosphor size
• 2 or 3 size selections
• 25/17
• 25/17/12 or 23/15/10
• Smaller input magnifies output by moving focal
  point away from output
• Requires more x-rays to maintain brightness