Unit 3 - X-ray tube

What are the three main components of the x-ray tube?

Cathode, anode, and envelope

What is the negative side of the x-ray tube called?

Cathode

What is the positive side of the x-ray tube called?

Anode

What is the purpose of the X-ray tube envelope?

Maintains the environment necessary for X-ray production.

What two main components are enclosed within the X-ray tube envelope?

cathode and anode

What encases the entire X-ray tube?

The protective housing.

What three things do the X-ray tube components provide that are necessary for X-ray production?

An electron source, appropriate target material, and a vacuum.

Why are the electron source, target material, and vacuum important?

They are necessary for the electrical production of X-rays.

What is the filament made of and what is its shape?

A small coil of thin thoriated tungsten wire.

Where is the filament located?

Within the cathode assembly.

What is the purpose of the filament?

To provide sufficient resistance to electron flow to produce heat, causing thermionic emission.

What happens during thermionic emission?

Electrons are released from the filament’s surface, forming a thermionic cloud.

What happens to the electrons when high voltage is applied during an exposure?

They are driven toward the anode target, where X-rays are produced.

What is the focusing cup and what does it contain?

A shallow depression in the cathode assembly that houses the filament.

What is the focusing cup made of and what charge does it have?

Nickel with a negative charge.

What is the purpose of the focusing cup?

To narrow and focus the thermionic cloud into a smaller electron stream toward the anode.

How does the focusing cup cause electrons to converge?

Its negative charge repels the negatively charged electrons, forcing them toward one another in a converging pattern.

Why is Tungsten used as the primary filament material?

high melting point of 3,370°C and low vapor pressure at high temperatures

Why is tungsten’s high melting point important for the filament?

It allows the filament to withstand the high temperatures required for thermionic emission.

Why is tungsten’s low vapor pressure important for the filament?

It makes tungsten difficult to vaporize, reducing tungsten deposits and helping maintain the tube’s vacuum.

What are the three main functions of the anode?

1. Serves as the target for high-voltage electrons, producing X-ray photons.

2. ELECTRICAL CONDUCTOR!!! Conducts high voltage from the cathode back into the X-ray generator circuitry.

3. Serves as the primary thermal conductor, carrying heat away from the target area.

What are two advantages of a rotating anode?

Increased heat capacity and the ability to use larger exposure techniques.

How does a rotating anode increase heat capacity?

Rotation allows the electron beam to strike a larger area of the focal track, spreading heat over a larger surface area instead of concentrating it in one spot.

Why are rotating anodes suitable for general radiography?

Their greater heat capacity allows them to withstand larger exposure techniques.

What are the 4 conditions required for X-ray production?

1. Source of electrons - cathode is filament

2. Acceleration of electrons - High potential difference from x-ray circuit

3. Focusing of electrons - focusing cup

4. Declaration of electrons - anode target area

(CQ) What is the purpose of the focusing cup?

Focus electrons towards the target

(CQ) What cathode component is responsible for thermionic emission?

Filament

Which part of the x-ray circuit is responsible for heating the filament?

Filament circuit

What device in the circuit is specifically responsible for increasing the current in the filament for thermionic emission to occur?

Step-down transformer

(CQ) Why is tungsten used as the primary filament material?

High melting point

What are the three reasons tungsten is used as the target material for X-ray production?

→ High atomic number (74) – enhances X-ray photon production.
→ High melting point (~3,370°C) – withstands extremely high temperatures.
→ Retains strength at high temperatures – allows the target to withstand the intense heat of electron interactions.

Which of the following is not a function of the anode?

1. Provide target surface

2. thermionic emission

3. Electrical emission

4. Thermal emission

Thermionic emission

What is the primary reason to use a rotating anode in comparison to a stationary anode?

Dissipates heat better which means can use a higher tube current

What are the three major components of the induction motor assembly?

Stators, rotor, and bearings.

What are the stators and what is their function?

Electromagnets that surround the rotor and are energized in series to induce rotation of the rotor without direct contact.

What is the function of the rotor?

It rotates and has a shaft attached to the anode disk, causing the anode disk to rotate.

What are the bearings and what is their function?

Low-friction spheres that allow the rotor to rotate freely and smoothly.

What sound can be heard during the prep phase of an X-ray exposure?

The rotor turning on the bearings.

(CQ) hen the filament is heated, what occurs?

Electrons are emitted from the filament

(CQ) A technologist wants the best recorded detail possible an selects the small filament for every exam, why is this not always appropriate?

The small filament cannot handle high heat loads required for some exams

(CQ) Why is the anode rotated during an exposure?

To spread heat over a larger surface area

What is the primary purpose of the x-ray tube envelope?

To maintain a vacuum between the cathode and anode.

What pressure is maintained inside the x-ray tube envelope?

Less than 10⁻⁵ mm Hg.

Why is air removed from the x-ray tube?

So electrons can travel from the cathode to the anode without interacting with gas atoms.

Why is the vacuum important for x-ray production?

It allows electrons to flow efficiently from the cathode to the anode.

What could happen if gas atoms remained inside the x-ray tube?

Electrons could interact with and ionize the gas atoms.

How would gas atoms affect electron flow?

They would interfere with the flow of electrons from the cathode to the anode.

(CQ) What metal is added to the filament to increase thermionic emission and extend tube life?

Thorium

What happens to the tube after the air is removed?

It is permanently sealed.

(CQ) Which filament would typically be selected for a finger radiograph requiring maximum detail?

Small filament

What are the 3 materials of the anode?

Tungsten, molybdenum, and rhenium

Where are Molybdenum, Tungsten, and Rhenium located on the anode?

Molybdenum - Disk

Tungsten - focal track/target area

Rhenium - added to tungsten at focal track

Why are Molybdenum, Tungsten, and Rhenium used in the anode?

M - Makes up anode neck and disk, doesn’t have good thermal conduct properties, is light, and you don’t want the anode heat to go to the induction motor

T - high melting point and high heat capacity

R - Allows metal to expand more easily so it doesn’t crack the anode

Why is Graphite sometimes used on the anode disk?

Sometimes backs the anode disk

What are the three purposes of the protective housing?

Control leakage/scatter radiation, isolate high voltages, and provide cooling.

(CQ) The anode disc and stem are made of:

Molybdenum

How does the protective housing control leakage and scatter radiation?

It absorbs most unwanted photons and limits radiation escaping from the tube.

What is the maximum permitted leakage radiation from the protective housing?

Less than 100 mR/hr at 1 meter.

(CQ) What material is added to the Tungsten target?

Rhenium

How does the protective housing isolate high voltages?

Dielectric oil electrically insulates the high-voltage components from the housing.

(CQ) X-ray photons are created at what area in the x-ray tube?

Anode target

(CQ) The anode is responsible for:

Deceleration of electrons

What is the purpose of the dielectric oil?

It provides electrical insulation and helps cool the x-ray tube.

How does the protective housing help cool the x-ray tube?

The dielectric oil absorbs heat and transfers it away from the tube.

What does tungsten do in terms of x-ray photons?

Creates x-ray photons, high atomic number and high melting point

What is the purpose of the line focus principle?

To create a large actual focal spot and a small effective focal spot.

Which focal spot is larger in the line focus principle?

The actual focal spot.

Which focal spot is smaller in the line focus principle?

The effective focal spot.

Why is a small effective focal spot desirable?

It improves spatial resolution and image detail.

Will a smaller or larger effective focal spot create greater spatial resolution/detail?

A smaller effective focal spot.

How does a smaller effective focal spot affect penumbra?

It decreases penumbra.

How does a larger effective focal spot affect penumbra?

It increases penumbra.

What is penumbra?

Blurriness along the edges of objects in the image.

How does a smaller effective focal spot affect spatial resolution?

It increases spatial resolution, producing sharper detail.

How does a larger effective focal spot affect spatial resolution?

It decreases spatial resolution.

Why does the line-focus principle use a large actual focal spot?

To provide greater heat capacity.

Why does the line-focus principle use a small effective focal spot?

To provide greater spatial resolution/detail.

Where are the rotors located?

Inside tube

Where are stators located, and what are their purpose?

Electromagnets that are outside glass envelope and maintain vacuum

Explain how the induction motor works.

Stator gets alternating current, alternates the magnetic field and produces. Rotor is inside the glass envelope, current is induced from within motor, magnetic properties work against eachother to rotate the anode constantly. MUTUAL INDUCTION!!!!

(CQ) Which part of the induction motor is located outside the envelope?

Stator

(CQ) Which part of the induction motor is delivered with AC?

Stator

The main purpose of the envelope is to:

Provide a vacuum

Which is the following is not a purpose of the protective housing?
A. produces photons
B. controls leakage radiation
C. high voltage isolation
D. tube cooling?

A. produces photons