X-Ray Tube Lecture Notes

These flashcards cover key concepts and details about the X-Ray Tube, its components, processes, and operational principles.

What is the primary material used for the filament in X-ray tubes?

Tungsten alloyed with thorium (thoriated tungsten).

What process describes the emission of electrons from the heated filament?

Thermionic emission.

What is the typical temperature that the filament reaches during operation?

About 3700 degrees Fahrenheit.

What forms a cloud of electrons around the filament in an X-ray tube?

Space charge.

What is the role of the anode in an X-ray tube?

To decelerate electrons and serve as a target for x-ray production.

What percentage of energy from incoming electrons is turned into x-ray photons?

Approximately 0.5%.

What is the key feature of a grid controlled tube?

It prevents electrons from escaping by using oblique repulsion from a negatively charged mesh.

What is the function of the focusing cup in the cathode?

To help direct and focus the electrons towards the anode.

What occurs to electrons when they bombard the target?

X-rays and heat are created.

What happens to the anode temperature during X-ray production?

It becomes very hot due to electron bombardment.

Why is it important for the anode to rotate in certain X-ray systems?

To reduce heat concentration and extend anode life.

What is the function of the mA in X-ray machines?

Controls the filament current and the rate of thermionic emission.

How does the focal spot size affect spatial resolution?

A smaller focal spot results in higher spatial resolution.

What is the recommended range for the anode rotation speed in regular X-ray tubes?

3400 to 3700 revolutions per minute (RPM).

What is the composition of the anode typically made from?

Molybdenum with a tungsten-rhenium alloy focal tract.

What does the line focus principle affect?

The effective focal spot size projected onto the image receptor.

What kind of failure is the most common in X-ray tubes?

Tube arcing due to electrical arcing from vaporized tungsten accumulation.

What causes heat damage to X-ray tubes?

Heat conducted down the rotor shaft to the ball bearings.

How can tube life be extended?

By performing a tube warm-up procedure before usage.

What happens to electrons during excitation?

They are raised to higher energy states but are not knocked out.

What are heat units (HU) used to measure in X-ray systems?

The product of kVp, mA, and exposure time (s).

What effect does using three-phase and high-frequency power have on heat generation?

It generates more heat, about 1.4 times stronger.

What defines the effective focal spot in an X-ray tube?

The area projected downward through the patient onto the image receptor.

What is the significance of filament size in determining focal spot?

Smaller filaments lead to smaller focal spots and higher heat.

What is the melting point of tungsten?

3410 degrees Celsius.

What happens to the X-ray tube if air molecules are present?

They would disrupt the electron flow and could lead to tube failure.

What are the typical melting points of Molybdenum and Rhenium?

Molybdenum: 2617 °C, Rhenium: 3180 °C.

How does the focusing cup affect electron behavior?

It causes electrons to focus into a smaller area, forming a focal spot.

What is the typical size of the focal spot when using a large filament?

1.0 - 1.2 mm wide.

What is the purpose of the glass envelope in X-ray tubes?

To maintain a vacuum around the cathode and anode.

How does heat loss in X-ray tubes relate to filament aging?

The filament loses tungsten over time and can eventually burn out.

Why is a smaller focal spot limited in its use?

Because it heats up more quickly and the mA is restricted.

What is a disadvantage of the inefficient process of X-ray production?

99.5% of energy is converted to heat, not useful radiation.

What determines the rate at which thermionic emission occurs?

The mA setting on the tube.

What is the purpose of the rotor in an X-ray tube?

To facilitate the rotation of the anode.

What can happen if a filament burns out in an X-ray tube?

The tube may stop functioning properly.

What leads to the formation of a 'cloud' of electrons?

The heated filament during operation.

What effect does the atomic number of tungsten have on X-ray production?

It allows tungsten to effectively 'catch' electrons for interaction.

What does HU stand for in the context of X-ray systems?

Heat units.