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Podcast
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Questions and Answers
Questions and Answers
What is the primary function of the oil contained within the housing of some x-ray tubes?
What is the primary function of the oil contained within the housing of some x-ray tubes?
- To filter out low-energy x-ray photons.
- To enhance the production of x-ray photons.
- To lubricate the moving parts of the x-ray tube.
- To provide additional insulation and act as a thermal cushion. (correct)
Why is the glass envelope of an x-ray tube made of Pyrex?
Why is the glass envelope of an x-ray tube made of Pyrex?
- To provide maximum electrical insulation.
- To minimize x-ray absorption.
- To allow for easy visual inspection of the tube components.
- To withstand the tremendous heat produced during x-ray production. (correct)
What is the purpose of maintaining a vacuum within the x-ray tube?
What is the purpose of maintaining a vacuum within the x-ray tube?
- To reduce the risk of electrical arcing.
- To increase the speed of the anode's rotation.
- To filter out unwanted x-ray frequencies.
- To allow for more efficient x-ray production and prolong tube life. (correct)
What is the impact of using a small focal spot size in an x-ray tube?
What is the impact of using a small focal spot size in an x-ray tube?
Why is tungsten commonly used for the filament in an x-ray tube?
Why is tungsten commonly used for the filament in an x-ray tube?
What is the effect of a 'space charge' in an x-ray tube?
What is the effect of a 'space charge' in an x-ray tube?
What defines the 'saturation current' in an x-ray tube?
What defines the 'saturation current' in an x-ray tube?
Which of the following is a key advantage of using a rotating anode in an x-ray tube?
Which of the following is a key advantage of using a rotating anode in an x-ray tube?
Why is the anode typically made of tungsten-rhenium alloy?
Why is the anode typically made of tungsten-rhenium alloy?
How does the 'line-focus principle' affect the effective focal spot size in an x-ray tube?
How does the 'line-focus principle' affect the effective focal spot size in an x-ray tube?
Questions and Answers
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Flashcards
Flashcards
What are the two main parts of an X-ray tube?
What are the two main parts of an X-ray tube?
The X-ray tube has a cathode and an anode.
What is the purpose of a protective housing?
What is the purpose of a protective housing?
A protective housing limits the X-ray beam and minimizes leakage radiation, preventing unnecessary exposure.
What is the inside of an X-ray tube?
What is the inside of an X-ray tube?
X-ray tube is an electronic vacuum tube.
What is the cathode?
What is the cathode?
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What is a filament?
What is a filament?
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What is thermionic emission?
What is thermionic emission?
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What is space charge?
What is space charge?
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What is a dual-focus tube?
What is a dual-focus tube?
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What are the types of anodes?
What are the types of anodes?
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Why is Tungsten used in anodes?
Why is Tungsten used in anodes?
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Flashcards
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Study Notes
Study Notes
X-Ray Machine Principal Parts
- Every x-ray machine consists of an x-ray tube, a high voltage section (generator), and a control console.
- Dental and portable x-ray machines house these components compactly.
Room Design
- Most x-ray rooms have the x-ray tube head in one room and the control console in an adjoining room.
- There is typically a protective barrier with a window included.
- The high-voltage generator is often housed in a 3ft cubicle container in the corner or ceiled above the room in newer designs.
X-Ray Tube
- Radiologic technologists rarely see the x-ray tube itself, due to protective housing.
- The two main parts of an x-ray tube are the cathode and the anode.
- The x-ray tube is a type of diode because it has both an anode and a cathode.
- Anode and Cathode referred to as an electrode
Protective Housing
- The x-ray tube is housed in a lead-lined metal protective housing.
- X-ray photons are generated isotropically, radiating in all directions.
- The housing is designed to limit the x-ray beam to a window.
- X-rays emitted through the window are useful x-rays.
- Radiation that penetrates the housing constitutes leakage radiation.
- Leakage radiation results in unnecessary exposure for both patients and technologists.
- The protective housing provides mechanical support and protection from damage.
- Some tubes have housing that contains oil to provide additional insulation and thermal cushioning.
- Some tubes have a cooling fan to cool the tube or the oil in which the tube is bathed.
- It's crucial to never hold the tube during exposure or use cables/terminals as handles.
X-Ray Tube Glass Envelope
- The x-ray tube functions as an electronic vacuum tube encased in a glass envelope.
- The glass envelope is made of Pyrex to endure the extreme heat generated during x-ray production.
- A vacuum environment is essential for efficient x-ray production and extending tube life.
- If the tube were filled with gas, the electron flow from cathode to anode would be hindered, decreasing x-ray production and generating excessive heat.
- The window is a 5 cm square area with a thin glass section, allowing the useful x-ray beam to be emitted.
- This design facilitates maximum x-ray emission while minimizing absorption in the glass envelope.
The Cathode
- The cathode is the negative side of the x-ray tube
- The cathode contains two primary parts: filaments and a focusing cup.
The Filaments
- A filament is a coil of wire, typically 2mm in diameter and 1 to 2 cm long, that emits electrons.
- When a filament current intensifies sufficiently it causes outer shell electrons to be boiled off and ejected in a process called thermionic emission.
- Most tubes are equipped with two filaments, allowing selection between quick exposures or high-resolution imaging.
- Filaments are made of tungsten because of the high melting point of 3410°C.
- X-rays are produced via thermionic emission when a current of 4 A or higher is applied.
- Tungsten does not vaporize easily; if it did, it could coat the internal parts, leading to tube failure.
- The addition of 1-2% thorium increases the efficiency of thermionic emission and prolongs tube life.
Focusing Cup
- A filament is embedded in a metallic focusing cup.
- The negative beam tends to spread out.
- The focusing cup concentrates the electron beam onto a small area of the anode because it has a negative charge.
- The effectiveness of the focusing cup is determined by several factors: its size and shape, its charge, the filament size and shape, and the position of the filament within the cup.
Filament Current
- A low current flows through the filament to warm up the x-ray machine in preparation for the high thermal requirements necessary for x-ray production.
- Low filament current isn't hot enough for thermionic emission.
- Once the filament current is high enough, a small increase leads to a significant rise in tube current.
Filament Current & Tube Current
- X-ray tube current is adjusted by controlling the filament current.
- The relationship between tube current and filament current is dependent upon the tube voltage.
Space Charge
- Electrons that are emitted by the filament momentarily form a cloud near the filament before being accelerated to the anode.
- The cloud of electrons is called a space charge.
- The space charge effect is due to the electrostatic repulsion between electrons, making it difficult for the filament to emit.
Saturation Current
- With very high mA and very low kVp, thermionic emission can be space charge limited.
- With high mA, the electron cloud makes it harder for subsequent electrons to be emitted.
- Space charge limited exposure above 1000 mA can be a major problem.
Dual-Focus Tube
- Most diagnostic tubes have two focal spots achieved with two filaments.
- A large focal spot (1 to 2.5 mm) uses a large filament for techniques producing high heat.
- A small focal spot (0.3-1 mm) uses a small filament when fine details are needed.
- Selection of the focal spot is made using the mA selector on the control console.
The Anode
- There are two types of anodes, stationary and rotating.
- Stationary anodes are used in portable x-ray machines where only low mA and power are needed.
- Rotating anodes produce high-intensity x-ray beams in a short time.
- The rotating anode allows the electron beam to interact with a much larger target area.
- This method ensures the heat is not confined to a small area.
Rotating Anode Functions
- The anode serves three primary functions: receiving electrons from the cathode, conducting electricity, and providing mechanical support for the target.
- It is an electrical conductor, channeling electrons from the tube to cables and back to the high-voltage section of the x-ray machine.
Rotating Anode Thermal Properties
- The anode must be a good thermal conductor because more than 99% of the kinetic energy of the electron beam is converted to heat when it strikes the anode.
- The heat must be conducted away quickly to prevent melting which makes Copper the most common anode material.
Anode Target Properties
- The target of the anode is the area struck by electrons and is a rotating disc in rotating anodes for more surface area.
- Tungsten-rhenium is used as the target for the electron beam.
- Tungsten is used because it has a high atomic number, high conductivity and high melting point of 2000°C during exposure.
- Using tungsten results in high efficiency x-ray production and energy.
Rotating Anode Mechanics
- The rotor is an electromagnetic induction motor that spins at 3400 rpm, but high-speed anodes spin at 10,000 rpm.
- Even with the anode rotating some melting can still occur because the heat must be rapidly dissipated.
- Molybdenum and copper are used to rapidly transfer the heat from the target.
- The exposure button applies current to the tube, which produces a magnetic field by the stator outside the glass envelope.
- The wait time before the rotating anode is spinning at the correct speed allows it to accelerate to its designed revolution per minute.
- After the exposure, the anode slows by reversing the motor, preventing it from melting.
Target Area
- Target, focus, focal point, and focal spot all refer to the same area where high-voltage electrons hit the anode.
- The actual focal spot is the physical area of the focal track that is impacted.
- The effective focal spot is the area projected out towards the object being radiographed.
Line-Focus Principle
- The focal spot is the area of the anode from which the x-rays are emitted, and it has a direct impact on the geometric resolution of the x-ray image.
- When size decreases it causes that heat to concentrate to a smaller area.
- By angling the anode target, the effective focal spot is smaller than the actual area of interaction.
- Angling the target is the line focus principle.
- The angle of the focal spot is a primary means of controlling the size of the effective focal spot.
- The larger the angle, the larger the effective focal spot.
- The anode target angle and the actual focal spot size control the effective focal-spot size.
- The effective focal spot's vertical dimension is stated as the focal-spot size.
- The effective focal spot is the beam as it projects onto the patient.
- The effective focal spot decreases as the anode angle decreases.
- Diagnostic tube target angles usually range from 5 to 15°.
- Line focus preserves sharpness of the small focal spot and enhances heat capacity.
- Smaller target angles create smaller effective focal spots and sharper images.
- To cover a 17" area, the angle must be 12°, and to cover a 36" area, the angle must be 14°.
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