Equipment Operation & Maintenance Review Equipment Operation & Maintenance

ARRT Section Components 30 Questions Radiographic Equipment (21) Evaluation of Radiographic Equipment & Accessories (9)

X-ray Tube Construction Vacuum diode tube cathode anode Radiographic Equipment

X-ray Tube Construction D F C G E WHAT TYPE OF MOTOR IS ROTOR? The energy of electrons comprising the tube current is measured in ______________ ? Radiographic Equipment

Cathode - Filaments Negative Electrode Tungsten with thorium Filament circuit heats (>2200°C) with 3-5 amps Thermionic Emission Vaporization Space charge Saturation current Focusing cup

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

Saturation Current/Voltage Stabilization at specific mA above ~40 kVp

Focusing Cup

Anodes - Target Rotating Anodes 2” to 5” disk (focal track) Induction motor Speed 3000 to 10000 rpm Cu Molybdenum or Graphite base W Cu W Other target materials Tungsten-rhenium Molybdenum Rhodium

What makes the Anode spin? Electromagnetic Induction Motor How fast does it turn? ______________ rpm

A motor is a device that converts electrical energy into mechanical energy

Line-Focus Principle Effective FS < actual FS Small angle =  FS Beveled anode Improves heat capacity Small angle =  FS Small FS =  resolution AHE  Field coverage  Heat loading on smaller anode surface area

© UW and Brent K. Stewart PhD, DABMP Anode Angle and Focal Spot Size (1) Anode angle < range: 7° - 20° Why are anodes beveled? 1. Line focus principle (foreshortening of focal spot length) Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 108-109. © UW and Brent K. Stewart PhD, DABMP

© UW and Brent K. Stewart PhD, DABMP Anode Angle and Focal Spot Size 1. ↓ <→ ↓ apparent focal spot size (B and C) Smaller the angle – smaller the effective focal spot 2. ↑ <→ ↑ heat loading: 3. ↑ <→ ↑ field coverage (B and C) Actual < used trade-off of these factors Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 108-109. 14 © UW and Brent K. Stewart PhD, DABMP

Anode Angles and Heat Smaller the angle = more heat load Larger angle – less heat load Different anode angles are used for different types of equipment: Diagnostic vs special procedures

Which anode angle has greater heat load?

ANODE HEEL EFFECT MORE PRONOUNCED WITH SMALLER ANODE ANGLE (sm anode angle = larger heel effect) 120% ~75% difference of intensity across beam How does this affect positioning? FAT – CAT HIGHER AT CATHODE See pg 139 Bushong

A. AP Thoracic Spine B. AP. Lumbar Spine C. Lateral Femur D. AP FOOT Which of the following projections would take advantage of the anode heel effect on a hyperstenic patient - if the anode was towards the patient’s head? A. AP Thoracic Spine B. AP. Lumbar Spine C. Lateral Femur D. AP FOOT

© UW and Brent K. Stewart PhD, DABMP X-ray Tube Anode Configuration Tungsten anode disk Mo and Rh for mammography Stator and rotor make up the induction motor Rotation speeds Low: 3,000 – 3,600 rpm High: 9,000 – 10,000 rpm Molybdenum stem (poor heat conductor) connects rotor with anode to reduce heat transfer to rotor bearings Anode cooled through transmission Focal track area (spreads heat out over larger area than stationary anode configuration) Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 107. 19 © UW and Brent K. Stewart PhD, DABMP

© UW and Brent K. Stewart PhD, DABMP Heel Effect Reduction of x-ray beam intensity towards the anode side of the x-ray field Although x-rays generated isotropically Self-filtration by the anode and the anode bevel causes Greater intensity on the cathode side of the x-ray field Can use to advantage, e.g., PA chest exposure Orient chest to anode side Abdomen to cathode side Less pronounced as SID ↑ Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 112. 20 © UW and Brent K. Stewart PhD, DABMP

© UW and Brent K. Stewart PhD, DABMP X-ray Filtration Filtration: x-ray attenuation as beam passes through a layer of material Inherent (glass or metal insert at x-ray tube port) and added filtration (sheets of metal intentionally placed in the beam) Added filters absorb low-energy x-rays and reduce patient dose (↑ beam quality) HVL – half value layer (mm Al) Curry, et al., Christensen’s Physics of Diagnostic Radiology, 4th ed., pp. 89, 91. 21 © UW and Brent K. Stewart PhD, DABMP

Voltage generators (power supply)

Extra-focal Radiation Miscellaneous Terms Off Focus Radiation Protective Housing Extra-focal Radiation rebounding e- Leakage Radiation <100 mR/hr @ 1 m

OFF FOCUS RADIATION

SHADOW OF SOMEONE’S HEAD = OFF FOCUS FROM TUBE

LEAKAGE RADIATION may not EXCEED TUBE HOUSING 100mR / HR @ 1 meter

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

Tube Rating Charts (mA)

Tube Rating Charts (kVp)

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

Anode Cooling Chart

Name 3 types of rectifiers

Rectification  AC to DC Keeps e- flowing from cathode to anode HVT tube  AC to DC Keeps e- flowing from cathode to anode Uses rectifying bridge between HVT & tube AC bridge DC HVT tube

SOLID STATE - DIODES – Semi Conductor Allows current to only flow in one direction Most common type used in rectifiers

AEC Operation Exit vs. Entrance Select sensor(s) Select density level Set back-up time Set kVp film Ion chambers film Fluorescent screen Exposure Switch PMT Exit vs. Entrance

Beam Restrictors Collimator Cone Diaphragm

Important Circuit Elements HV subcircuit Power Source X-ray Tube autotransformer Filament subcircuit

X-ray Generator Transformers Rectification Connection to tube cathode Transformers HV (step-up) Filament (step down) Rectification Connection to tube anode F diodes HV

Transformers Autotransformer HV Filament Step-up or step-down Variable TR Controls kV by varying V sent to HVT HV Step-up Fixed TR > 1 (500 to 1000) VS > VP  V to kV Filament Step-down Fixed TR < 1 IS > IP (VS<VP) I in filament to cause e- emission

Transformer Review Turns Ratio Transformer Law Power Step Up Step Down

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)

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

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.

Rectification  AC to DC Keeps e- flowing from cathode to anode HVT tube  AC to DC Keeps e- flowing from cathode to anode Uses rectifying bridge between HVT & tube AC bridge DC HVT tube

Phase, Pulse & Frequency Type # of rectifiers % ripple Wave form 1(self) 1 60 100% 1(half) 1 or 2 1(full) 2 120 4 36p 6 360 6 or 12 12-15% 312p 12 720 3-5% High Freq. 10kHz <1% + - + - + - + - + -

GENERATOR THAT CREATE AN ALTERNATING CURRENT ARE CALLED: AN ALTERNATOR – What does the generator do? CONVERT MECHANICAL ENERGY INTO ELECTRICITY

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

What does a MOTOR due Converts ___________ energy To

OHMS LAW V = IR V = POTIENTAL A = AMPS (CURRENT) R = RESISTANCE (OHMS)

OHM’S LAW: V = Potential difference in volts I = Current in amperes R = Resistance in ohms () V= IR I =V/R R=V/I

Simple SERIES Circuit

Series Circuit Rules: Current: IT = I1 =I2 =I3 Voltage: VT = V1 + V2 + V3 Resistance: RT = R1 + R2 + R3

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

TYPES OF EQUIPMENT FIXED MOBILE DEDICATED MAMMO , CHEST, HEAD

EXPOSURE SWITCH TIMER

RHEOSTAT VAIRIABLE REISITOR regulate the amount of resistance in a circuit

What makes the Anode spin? Electromagnetic Induction Motor How fast does it turn? ______________ rpm