week 2 c chapter 9 the x-ray tube and tube rating charts 71
TRANSCRIPT
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Chapter 9 The X-ray Machine
• The x-ray machine is divided into four major components.
• The Tube
• The Operating Console
• The High Voltage Section
• The Film Holder , Grid Cabinet or Table
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Chiropractic X-ray Room
• Floor mounted x-ray tube stand.
• Wall grid cabinet or Bucky
• Mobile Table with grid cabinet (optional)
• Non-Bucky Film Holder (optional)
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Chiropractic X-ray Room
• Control Booth should contain the operator console and technique charts and space to store cassettes.
• The wall between the booth and X-ray unit is shielded.
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Chiropractic X-ray Room
• High Voltage Section or Generator used to change incoming power to levels needed to produce x-rays.
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The X-Ray Tube Development
• Dr. Roentgen used a Crookes-Hittorf tube to make the first x-ray image.
• There was no shielding so x-rays were emitted in all directions.
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The X-Ray Tube Development
• The Coolidge Hot cathode tube was a major advancement in tube Design. The radiator at the end of the anode cool the anode.
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The X-Ray Tube Development
• This is the variety of tube designs available in 1948.
• The Coolidge tube was still available.
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The X-Ray Tube Development
• Two major hazards plagued early radiography.
• Excessive radiation exposure
• Electric Shock
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The X-Ray Tube Development
• This is a modern rotating anode x-ray tube. It is encased completely in a metal protective housing.
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The X-Ray Modern X-ray Tube
• There are two principle parts:
• The rotating anode• The cathode• Any tube that has two
electrodes is called a diode.
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The X-Ray Modern X-ray Tube
• There are two principle parts:
• The rotating anode• The cathode• Any tube that has two
electrodes is called a diode.
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Part of the X-ray Tube
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Protective Housing
• The tube is housed in a lead lines metal protective housing.
• The x-ray photons are generated isotropically or in all directions.
• The housing is designed to limit the beam to window.
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Protective Housing
• The tube can not have more than 100 mR at 1 m (26 µ C/kg) / Hour when operated at it maximum output.
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Protective Housing
• The housing also provide mechanical support and protection from damage.
• On some tubes, the housing also contains oil that provides more insulation and a thermal cushion.
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Protective Housing
• Never hold the tube during an exposure.
• Never use the cables or terminals as handles.
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Protective Housing
• The housing incorporates specially designed high voltage receptacles to protect against electrical shock.
• Some housing have a fan for cooling.
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The X-Ray Tube Glass Envelope
• The glass envelope is made of Pyrex to withstand the tremendous heat produced during x-ray.
• The window is a 5 cm square with a thin section of glass where the useful beam is emitted.
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The Cathode
• The cathode is the negative side of the tube and contains two primary parts:
• The filaments• The focusing cup
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The Filaments
• Most tube have two filaments which provide a choice of quick exposures or high resolution.
• The filaments are made of thoriated tungsten.
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The Filaments
• Tungsten is used in x-ray tube because of it’s high melting point of 3410°C.
• X-rays are produced by thermionic emission when a 4 A or higher current is applied.
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Focusing Cup
• The focusing cup has a negative charge so that it can condense the electron beam to a small area of the anode.
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Filament Current
• When the x-ray machine is turned on, a low current flows through the the filament to warm it and prepare it for the big thermal jolt necessary for x-ray production.
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Filament Current
• The filament is not hot enough for thermionic emission. Once the current is high enough for thermionic emission a small rise in filament current will result in a large rise in tube current.
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Filament Current & Tube Current
• The x-ray tube current is adjusted by controlling the filament current.
• The relationship between tube and filament current is dependent upon the tube voltage.
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Space Charge
• When emitted by the filament, the electrons form a cloud near the filament momentarily before being accelerated to the anode. This is called a space charge.
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Saturation Current
• When very high mA and very low kVp, the thermionic emission can be space charge limited.
• With high mA the cloud makes it difficult for subsequent electrons to be emitted.
• Above 1000 mA space charge limited exposure can be a major problem.
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The Anode
• The anode is the positive side of the tube.
• X-ray tubes are classified by the type of anode:– Stationary ( top)– Rotating (bottom)
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The Stationary Anode
• Stationary anodes are used in dental x-ray and some portable x-ray machine where high tube current and power are not required.
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The Rotating Anode
• The rotating anode allows the electron beam to interact with a much larger target area.
• The heat is not confined to a small area.
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The Rotating Anode
• The anode serves three functions:– Receives the electrons
emitted from the cathode.
– It is a electrical conductor.
– Mechanical support for the target.
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The Rotating Anode
• The Anode must also be a good thermal conductor.
• When the electron beam strikes the anode more than 99% of the kinetic energy is converted to heat.
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The Rotating Anode
• Tungsten-rhenium is used as the target for the electron beam.
• Tungsten is used for three reasons– High atomic number– Heat conductivity– High melting point..
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The Rotating Anode
• The rotor is an electromagnetic induction motor.
• It spins at 3400 rpm.• High speed anodes
spin at 10,000 rpm.
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The Rotating Anode
• Even with the anode rotating, some melting occurs. The heat must be rapidly dissipated.
• Molybdenum and copper are used to rapidly transfer the heat from the target.
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The Rotating Anode
• When the exposure button is depressed, current is applied to the tube that produces a magnetic field that starts the rotation of the anode.
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The Rotating Anode
• When the anode is spinning at the correct speed, the exposure can be made.
• After the exposure is completed, it slows by reversing the motor.
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Tube cooling
• The x-ray tube uses all three forms of cooling.– Radiation– Conduction– Convection
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Focal Tracks
• With a rotating anode, the electrons strike a moving target forming focal tracks on the tube.
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Line-Focus Principle
• The focal spot is the area of the anode from which the x-rays are emitted.
• The focal spot impacts the geometric resolution of the x-ray image.
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Line-Focus Principle
• By angling the anode target, one makes the the effective focal spot much smaller than the actual area of interaction.
• The angling of the target is know as the line focus principle.
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Line-Focus Principle
• The Effective Focal Spot is the beam projected onto the patient.
• As the anode angle decreases, the effective focal spot decreases.
• Diagnostic tube target angles range from 5 to 15°.
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Line-Focus Principle
• The advantage of Line focus is it provides the sharpness of the small focal spot with the heat capacity of the large large focal spot.
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Line-Focus Principle
• Smaller target angles will produce smaller effective focal spots and sharper images.
• To cover a 17” the angle must be 12°
• To cover 36” the angle must be 14°
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Off Focal Radiation
• Remember that x-rays are produced in all directions. The electrons can rebound and interact with other areas of the anode.
• This is called Off-Focal Radiation.
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Control of Off Focal Radiation
• A diaphragm is placed between the tube and the collimator to reduce off focus rays.
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Anode Heel Effect
• One unfortunate consequence of the line-focus principle is that the radiation intensity on the cathode side of the x-ray tube is higher than the anode side.
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Anode Heel Effect
• The x-rays are emitted isotropically or in all directions.
• Some of the beam is absorbed by the target resulting in a lower intensity.
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Anode Heel Effect
• The difference in the intensity can vary by as much as 45%.
• If the center is 100% the anode side of the beam can as low as 75% and the cathode as much as 120%.
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Anode Heel Effect
• The heel effect should be considered when positioning areas of the body with different thickness or density.
• The cathode side should be over the area of greatest density.
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Anode Heel Effect
• As the angle of the anode decreases, the anode heel effect increases.
• This can result in incomplete coverage of the film with the beam.
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Anode Heel Effect on Resolution
• The sharpness of the image can be dependent upon which area of the beam coverage you are looking at.
• Similar to the shape distortion when the tube is not centered.
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Anode Heel Effect
• The anode should be up and the cathode down for the full spine x-ray.
• The patient is less dense at the c-spine and more dense at the pelvis.
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X-ray Tube Rating Charts
• With careful use, the x-ray tube can provide long periods of service.
• Inconsiderate or careless operation can lead to shortened life or abrupt failure.
• X-ray tubes are very expensive. Cost varies from $2,000 to $20,000.
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X-ray Tube Rating Charts
• Tube life is extended by :
• Use of minimum mAs & kVp appropriate for the exam.
• Use of faster images receptors require lower mAs and kVp. They extend tube life.
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Causes of X-ray Tube Failure
• All causes of tube failure relate to the thermal characteristics of the tube.
• When the temperature of the anode during a single exposure is excessive, localized melting and pitting occurs.
• These surface irregularities lead to variable and reduced radiation output.
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Causes of X-ray Tube Failure
• If the melting is severe, the tungsten vaporizes and can plate the port. This can cause added filtering or interference with the flow of electrons.
• If the temperature of the anode increases to rapidly,the anode can crack and then become unstable in rotation.
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Causes of X-ray Tube Failure
• Maximum radiographic techniques must never be applied to a cold anode.
• These images show damage to the anode,
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Causes of X-ray Tube Failure
• During long exposures (1 to 3 seconds) the anode may actually glow like a light bulb.
• The heat may cause a failure of the bearing for the anode or a crack in the glass envelope.
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Filament failure
• Because of the high heat of the filament, tungsten atoms are slowly vaporized and plate the inside of the glass envelope. This will eventually lead to arcing and tube failure.
• Continuous high mA radiography will actually lead to the filament breakage.
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Tube Warm-up Procedures
• By warming the anode through a series of exposures and increasing kVp settings, the anode will build up heat that is needed to avoid fracture of the anode.
• This process takes a little over one minute put will add to the life of the tube.
• Close shutters of collimator.
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Tube Warm-up Procedures
• Make exposure of 12 mAs @ 70 kVp
• Wait 15 seconds
• Make exposure of 12 mAs @ 80 kVp
• Wait 15 seconds
• Make exposure of 12 mAs @ 90 kVp
• Tube warm up is now complete.
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X-ray Tube Rating Charts
• It is essential for the x-ray operator to understand how to use tube rating charts.
• There are three types of charts:– Radiographic Rating Chart– Anode Cooling Chart– Tube Housing Cooling Chart
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Radiographic Rating Charts
• A tube may be used in many ways with many variables. Such as:– Large or Small Focal
spot– 10,000 RPM or 3,400
RPM Rotor Speed– Single-phase or high
frequency power.
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Radiographic Rating Charts
• Even the angle of the anode is important.
• Always look at the correct chart.
• The x-axis and y-axis are graduated in kVp and time.
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Radiographic Rating Charts
• The mA is graphed as a curved line.
• Any combination of kVp and Time below the line should be safe for a single exposure.
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Radiographic Rating Charts
• Most machine has built-in protection to help you avoid tube overload.
• Microprocessor controlled generator way display the percent of tube load.
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Anode Cooling Chart
• The anode has a limited capacity for storing heat.
• Heat is continuously dissipated to the oil bath and tube housing by conduction and radiation.
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Anode Cooling Chart
• It’s possible through prolonged use of multiple exposures to exceed the heat storage capacity of the anode.
• Thermal energy in x-ray is measured in Heat Units (HU)
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Anode Cooling Chart
• HU= kVp x mA x time (s)• This chart is not dependent
upon the filament size or speed of anode rotation
• The cooling is rapid at first but slows as the anode cools. It is not uncommon for it to take 15 minutes to cool the tube.
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Housing Cooling Charts
• The tube housing cooling chart is very similar to the anode cooling chart.
• The tube housing will generally have a capacity of about 1 to 1.5 million HU.
• Complete cooling may take 1 to 2 hours.
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