Equipment Operation & Maintenance Review Digital Imaging

1
Equipment Operation Maintenance

• Review

2
Objectives

• Review equipment operation and maintenance.
• We start out reviewing primary terminology.

3
Basic Physics Terms

• Familiarize yourself with
• Matter, Mass, Energy, Potential Energy, Kinetic
  Energy, Chemical Energy, Electrical Energy,
  Thermal Energy, Nuclear Energy, Electromagnetic
  Energy, and Ionizing Radiation

4
Atomic Structure

• Atomic Mass Number of protons and neutrons.
• Electron Shells Contain orbital electrons that
  revolve around the nucleus. (Ex. K, L, M, N,
  etc.)
• Atomic Number The number of protons in the
  nucleus atomic number determines the chemical
  element.
• Isotopes Atoms with the same number of protons
  but with a different number of neutrons.
• Electron-Binding Energy Force that holds
  electrons in orbit.
• Octet Rule The outer shell can contain no more
  than 8 electrons.
• Particulate Radiation Alpha/Beta particles.

5
Electromagnetic Radiation

• Photon The smallest amount of any type of
  electromagnetic radiation. Travels at the speed
  of light.
• Sine Waves Wave height is called amplitude
  distance between peaks is called wavelength
  Photon wavelength is associated with photon
  energy level.
• Frequency Number of wavelengths passing a given
  point per unit time. measured in hertz.
• Speed of Travel 186,000 miles per second.
• Gamma Rays Electromagnetic rays produced in the
  nucleus of radioactive atoms. X-rays and gamma
  rays differ only in origin.
• Wave-particle Duality X-rays exist as waves,
  but exhibit properties of particles.
• Law of Conservation of Matter/Energy Created
  nor destroyed.

6
Electricity Magnetism

• Electrostatics Stationary electric charges.
• Electrostatic Law Unlike charges attract / Like
  charges repel.
• Conductor Allows electron flow.
• Insulator Prohibits electron flow.
• Current The flow of electrons along a
  conductor.
• Electromotive Force (EMF) The force with which
  electrons move in an electric circuit.
• Electrodynamics Electric charges in motion.
• Semiconductor Material that may act as an
  insulator or conductor under different
  conditions.
• Electric Resistance Measured in ohms.
• Ohms Law Voltage Current x Resistance

7
Electricity Magnetism

• Circuit Path along which electrons flow.
• Alternating Current Current oscillates back and
  forth.
• Direct Current Current flows in one direction.
• Magnetic Field Surrounds an electric charge in
  motion. Can magnetize ferromagnetic material.
• Magnetic Law Like charges repel while unlike
  charges attract.
• Solenoid Coil of wire wrapped so magnetic field
  lines overlap.
• Electromagnet A solenoid with an iron core
  which concentrates the magnetic field.
• Electromagnetic Induction The process of
  causing current to flow in a conductor when it is
  placed within the force field of another
  conductor.

8
Electricity Magnetism

• Self Induction Opposing voltage created in a
  conductor by passing alternating current through
  it.
• Mutual Induction Inducing current in a
  secondary coil by varying current in a primary
  coil.
• Electric Generator Converts mechanical energy
  to electrical energy.
• Single-Phase, Two Pulse AC Voltage begins at
  zero, peaks at the full value at the crest of the
  wave, and returns to zero.
• Three-Phase AC Special wiring (wye, star.
  Delta) allows for three voltage waveforms out of
  phase with one another. Can have 6 or 12 pulses
  per cycle.6 pulse has 360 pulses per second,
  while 12 pulse has 720 pulses per second.

9
Electricity Magnetism

• Electric Motor Converts electric to mechanical
  energy.
• Transformer Changes voltage and current into
  higher or lower values.
• Step-Up Transformer Increases current/voltage
  has more turns on the secondary coil than on the
  primary coil.
• Step-Down Transformer Decreases current/voltage
  has more turns on the primary coil than the
  secondary coil.
• Autotransformer A transformer that contains an
  iron core and a single winding of wire Provides
  a small increase in voltage before the step-up
  transformer kVp settings are made at the
  autotransformer.
• Rectification Changing AC to DC.
• Line Voltage Compensation Ensures consistent
  voltage supply.

10
The X-ray Circuit
11
X-ray Production

• Three things occur
• 1. Source of electrons
• 2. Acceleration of electrons
• 3. Sudden stoppage of electrons
• against target material.

12
Autotransformer

• A variable transformer that varies incoming
  voltage to the circuit.
• Single coil of wire with iron core.
• Source for selecting kVp.
• Operates on self induction.
• Steps up/down voltage by only a small amount.
• The high voltage transformer then boosts the
  voltage to the kVp.

13
Voltmeter

• Indicates the voltage that is selected.
• Placed in the circuit between the autotransformer
  and the high voltage transformer.

14
Timer

• Regulates the duration of the exposure.
• Synchronous timer Operates from a motor 60
  rotations per second. Shortest time is 1/60 of a
  second.
• Impulse timer Counts voltage pulses.
• mAs timer Provides the safest tube current in
  the shortest possible time. Measures total tube
  current.
• Electronic timer Microprocessor controlled
  Now in most radiographic equipment Allows
  exposures as low as 1 ms (.001 s).

15
AEC

• Used to provide consistency of exposure.
• Contains ionization chamber which is located
  between the patient and the IR. As radiation
  passes through the ionization chamber it ionizes
  the gas within it.
• The level of ionization is directly proportional
  to the density that will appear on the film. At a
  predetermined level of ionization, the exposure
  is stopped by an electronic switch.
• A backup timer will stop the exposure in the
  event of a failure.
• Minimum response time Shortest exposure time
  possible (1ms).

16
Falling Load Generator

• Modern generator that takes advantage of
  extremely short time capabilities and tube heat
  loading potential.
• Calculates the most efficient way of obtaining
  mAs.
• Tube current starts at highest level possible
  until maximum heat load is reached, then the
  generator drops the mA to the next lower level
  that the tube can handle. This continues until
  the desired mAs has been reached.
• This ultimately results in short exposure times.
• Disadvantage Exams such as breathing technique
  exams that require a long exposure time.

17
Step-Up Transformer

• The High-Voltage Transformer.
• Consists of primary and secondary coils.
• Requires AC to operate.
• Operates on principle of mutual induction.
• The turns ratio determines level of voltage
  increase.

18
The Circuit
High Voltage Transformer
19
Rectifier

• X-ray tube requires direct current to operate
  properly.
• Rectifiers change AC from the step-up transformer
  to DC.
• Rectifiers are solid-state semiconductor diodes.
  They are silicon-based n-type and p-type
  semiconductors.
• Located between the step-up transformer and the
  x-ray tube.

20
4-Diode/full-wave rectified/single phase

• Full-wave rectified pulsating current.
• Resultant waveform contains two pulses per cycle
  (120 pulses/sec).
• Uses both portions of rectified AC.
• Results in 100 ripple with voltage dropping to
  zero 120 times per second.

21
Three phase/6 or 12 diodes/fully rectified

• Voltage never drops to zero with three-phase.
• Ripple for 3-phase 6 pulse is 13, so voltage
  used is about 87 of the actual kVp set.
• Ripple for 3-phase 12 pulse is 4, so voltage
  actually used is about 96 of the set kVp.
• High-frequency ripple is about 1, so 99 of what
  is set is used.

22
MA Meter

• Located between the rectifier and tube.
• Measures tube current in milliamperes.

23
mA Control

• Regulates the number of electrons available at
  the filament to produce x-rays.
• Voltage is supplied by tapping windings of the
  autotransformer and sending varying voltage to
  the step-down transformer.
• The step-down transformer reduces voltage and
  increases current in response to variable
  resistors which results in high current to the
  tube filament.

24
X-ray Tube

• Cathode Negative electrode two filaments
  (focal spot) Made of tungsten (small amount of
  thorium).
• - Filament heats up and boils off
  electrons (thermionic emission).
• - Over time the filament vaporizes and
  coats the tube with tungsten
• which leads to failure.
• - Assembly includes the focusing cup
  (focuses space charge on the anode).

25
X-ray Tube

• Anode Tungsten embedded in molybdenum rotating
  positive target. Must withstand high heat loads.
• - Rotates 3300 to 10,000 rpm Faster
  rotations dissipates heat
• better.
• - Tube rotation is achieved by an induction
  motor.
• - Electrons strike the focal track.
• - Target design allows for a large actual
  focal spot with a smaller
• effective focal spot Line-focus
  principle.

26
X-ray Tube

• Glass Envelope/Window Leaded Pyrex glass.
  Window had a thinner leaded area to absorb low
  energy photons. Tube contains a vacuum.
• Encased in the tube housing which is made of
  aluminum with lead lining.
• - Supports and protects the tube,
  while limiting leakage.
• - Also contains oil which
  provides cooling and insulation.

27
X-ray Production

• Review the overview of the x-ray production
  circuit on page 53.
• Brems Radiation Occurs when electrons are
  slowed or braked when they come under the
  influence of the nucleus of target atoms. This
  loss of kinetic energy results in x-ray creation.
  At diagnostic levels, most x-rays produced are
  from brems interactions.

28
X-ray Production

• Characteristic Radiation A projectile electron
  collides with an inner shell electron and ionizes
  it. As outer shell electrons fall down to
  stabilize the atom, characteristic radiation is
  given off.
• - Characteristic rays are produced at kVp
  levels above 70, but only
• in small numbers.
• - Only characteristic rays produced at the
  K-shell have sufficient energy to be used in
  diagnostic radiography.

29
X-ray Properties

• Part of the electromagnetic spectrum
• Highly penetrating
• Invisible
• Travel at the speed of light
• Travel in straight lines as waves Short
  wavelength high frequency high energy. Long
  wavelength low frequency loe energy.
• Can ionize
• Have characteristics of waves and particles
  (wave-particle duality)
• Can cause biologic damage

30
Heating Units

• Single-phase, fully rectified kVp x mAs
• Three-phase, 6 pulse, fully rectified kVp x mAs
  x 1.35
• Three-phase, 12 pulse, fully rectified kVp x
  mAs x 1.41
• Most modern equipment will not allow overloading
  the tube.

31
Fluoroscopy

• Provides dynamic visualization of internal
  anatomy.
• C-arm design ensures proper and safe distance
  between tube IR.
• Tube is operated at 3-5 mA.
• Automatic brightness control Process that
  automatically adjusts kVp and mA during the
  procedure.
• Image-intensifier converts x-ray energy into
  light and then into an electronic image that can
  be displayed on a monitor.

32
Image-Intensifier

• Input Phosphor Cesium iodide Receives exit
  rays from patient and converts them into light.
• Visible light strikes the photocathode which
  releases electrons directly proportional to the
  light that struck it.
• The electrons are focused toward the other end of
  the tube by electrostatic lenses.
• The electrons strike the output phosphor zinc
  cadmium sulfide.
• The energy of the electrons is converted into
  visible light that is 50 -75 times greater than
  at the photocathode.
• The increase in brightness is called the flux
  gain.
• The output phosphor is smaller than the input
  phosphor resulting in an increase in brightness
  minification gain.

33
Image Intensifier
Total brightness gain ranges from 5,000 to 20,000
and decreases with tube age. Images are sent to
a monitor by a vidicon or plumbicon or a
charge-coupled device (CCD).
34
Digital Imaging

• Computed Radiography ( CR ) Uses an imaging
  plate which contains a photostimulable phosphor.
  Absorbs energy exiting the patient. IP is scanned
  with a laser. Energy released as blue light which
  is detected and converted with an ADC, and
  viewed.
• Direct Digital Radiography (DR) Uses fixed
  detectors which communicate directly with a
  computer. Direct uses amorphous selenium to
  convert x-rays directly into electrons. Indirect
  uses a scintillator to convert x-rays into light,
  then into electrons.
• Review key terms on page 63.

35
QC Facts

• Filtration-beam quality Tested with half-value
  layer measurement.
• Timer Testing Single-phase Spinning top test
  one dot appears every 1/120 second.
• - Three-phase
  Motorized spinning top
• or other digital
  timer.
• Review key terms on page 74.

36
The End

• Perform the chapter tutorial at this point.