Radiographic Instrumentation I - Lesson 4: X-Ray Tube 1

Practice flashcards covering x-ray tube components, cathode and anode assembly functions, target principles, and housing specifications based on the Radiographic Instrumentation I lecture.

What is the core component of the x-ray machine responsible for producing x-rays?

The x-ray tube, which converts electrical energy into x-ray photons through the interaction of high-speed electrons with a metal target.

What are the four main components of an x-ray tube?

1. Cathode assembly, 2. Anode assembly, 3. Envelope, 4. Housing.

What are the functions of the cathode assembly?

To produce a thermionic cloud, conduct high voltage to the gap between cathode and anode, and focus electrons toward the anode.

What material is used for the filament and what is its melting point?

Tungsten is the material of choice because of its high melting point of $3,370^\text{o}C$ and its difficulty to vaporize.

What are the standard dimensions of a filament wire and coil?

The wire is about $0.1-0.2\text{ mm}$ thick, and the coil is $1-2\text{ mm}$ wide by $7-15\text{ mm}$ long.

What is the purpose of utilizing dual filaments in an x-ray tube?

To permit different focal spot sizes, specifically large and small focal spots.

Define thermionic emission.

The emission or ejection of electrons from the surface of the wire due to increased heat, resulting in an electron cloud surrounding the filament.

What is the focusing cup and what is its material?

A shallow depression in the cathode designed to house the filament, typically made of nickel.

What are the primary functions of the anode assembly?

It serves as the target surface for electrons (becoming the source of x-ray photons), conducts high voltage back to the generator circuit, and serves as the primary thermal conductor.

Identify the three components of a rotating anode assembly.

The Anode, the Stator, and the Rotor.

Why is Tungsten the metal of choice for the anode target surface?

Due to its high atomic number, high melting point, and heat-conducting ability.

Distinguish between a stationary anode and a rotating anode.

Stationary anodes are limited to low-power, have an angle of $45^\text{o}$, and a static target area. Rotating anodes turn during exposure, present a larger target area, have greater heat loading capacities, and retain fine focus.

What is the difference between the actual focal spot and the effective focal spot?

The actual focal spot is the physical area of the focal track that is impacted by electrons, while the effective focal spot is the area projected out of the tube toward the object being radiographed.

Explain the Line-Focus principle.

It involves tilting the anode target so that the effective focal spot size is smaller than the actual area struck by electrons, improving image sharpness while maintaining a large area for heat dissipation.

According to the Line-Focus principle, what happens as the anode target angle decreases below $45^\text{o}$?

The effective focal spot size decreases.

What is the Anode Heel Effect?

The geometry of the angled anode target causes radiation intensity to be greater on the cathode side (more photons) than on the anode side (less photons).

How should the x-ray tube be positioned relative to the patient due to the Anode Heel Effect?

The cathode end, which has a more intense beam, should be positioned toward the thicker or denser part of the body to obtain uniform density.

What is the function of the stator?

Located outside the vacuum, it uses induction-motor electromagnets to turn the rotating anode and accelerate/decelerate it to working speed.

How are the ball bearings in the rotor lubricated?

They use silver plating as a high-temperature lubricant because liquids would produce gas and reduce the vacuum in the tube.

What is the function of the vacuum-sealed envelope?

To maintain a vacuum by removing air, allowing electrons to flow from cathode to anode without colliding with gas atoms, thereby increasing efficiency and tube life.

What roles does the tube housing play?

It controls leakage and scatter radiation, isolates high voltages, contains oil for insulation and cooling, provides mechanical support, and protects the tube from damage.

Why is dielectric oil used within the tube housing?

To fill the space between the envelope and housing to absorb heat produced during x-ray production and provide electrical insulation.