Xray tube

Xray tube

the same basic function and the same basic components.

Cathode

serves as the source of electrons needed for X-ray production

Cathode

Produces thermionic cloud (electron cloud)

Filament

coil of wire that is the source of projectile electrons needed for X-ray production

Filament

It is made up of thoriated tungsten.

Tungsten

resists thermal damage and

provides consistent electron emission over time

thorium-doped tungsten,

which enhances electron emission

efficiency and extends filament life. It does not vaporize easily

THERMIONIC EMISSION PROCESS IN THE X-RAY TUBE

1. Heating the filament

2. Electron release

3. Electron acceleration

4. Xray production

DUAL FOCUS TUBE

dual-focused X-ray tube has two filaments of different sizes within the cathode, allowing selection between small and large focal spots on the anode. This design provides flexibility for different imaging needs.

Two Filaments (Small & Large

Produces different electron beam sizes

Focusing Cup

Directs the electrons toward the anode.

Anode Target

Receives electrons and produces X-rays.

High Voltage (kVp)

Accelerates electrons toward the target.

Small Focal Spot (from the smaller filament)

Produces sharper images with higher spatial resolution.

Used for detailed imaging like extremities.

Large Focal Spot (from the larger filament)

Handles higher mA settings, producing more X-rays.

Used for imaging larger body parts where heat management is critical.

Focusing cup

Surrounds the filament on its back and sides, leaving the front open and facing the target.

Made of nickel, stainless steel, or molybdenum— materials with a high melting point and poor thermionic emission properties.

Focusing cup

receives a strong negative charge from the secondary circuit, which helps concentrate the electrons into a cloud as they are boiled off the filament (thermionic emission).

Focusing cup

It functions using electrostatic repulsion— since like charges repel, the negative charge of the focusing cup is stronger than the individual electrons’ negative charges, forcing them into a narrow, controlled beam directed toward the anode target.

Anode

serves as the target for electrons emitted by the cathode.

Anode

consists of a tungsten target embedded in a copper or molybdenum base

deceleration

bremsstrahlung and characteristic radiation.

When high- speed electrons strike the target, their sudden,

produces X-rays through

X-ray Production

Serves as the target for electron interactions that generate X-rays.

Electrical Conduction

Conducts high voltage from the cathode back into the circuitry.

Heat Dissipation

Acts as a thermal conductor to manage the significant heat generated during X-ray production.

Stationary anode

target remains fixed during exposure. Common in dental and small portable X-ray machines. Disadvantage: Limited heat dissipation, leading to potential overheating.

Rotating anode

The anode target spins during exposure to distribute heat over a larger surface area. Used in general radiography and fluoroscopy to handle higher exposure techniques.

Advantage: Greater heat dissipation allows for higher tube current and longer exposure times.

Target

the portion in an X-ray tube anode, where high voltage electron stream strokes to produce X-rays.

Target

typically made of tungsten with rhenium, ensuring durability and high X-ray production efficiency.

Stationary anode

the target is a focal spot embedded in copper.

rotating anode,

the target is a focal track mounted on a molybdenum disc, improving heat dissipation.

Tungsten

primary choice due to its high melting point (3410°C), good thermal conductivity, and high atomic number

Rhenium

added to increase tensile strength and prevent cracking at high temperatures.

Molybdenum

used as the target material in mammography to produce lower-energy characteristic X-rays suitable for imaging soft tissues.

beveled design

of the target influences the line focus principle and contributes to the anode heel effect.

Parts of stationary anode

Tungsten target (focal spot)

Copper rod

Tungsten target (fs)

Serves as the area where electrons from the cathode strike, producing X-rays.

Copper rod

Supports the tungsten target, conducts heat away to prevent overheating, and acts as an electrical conductor to return high voltage to the circuit.

PARTS OF THE ROTATING ANODE

• Molybdenum disc

• Molybdenum disc

• Induction motor

Moly disc

Supports the tungsten target. Molybdenum has a high melting point (2,620°C) and can store twice the heat of tungsten. It is also lightweight, reducing wear on bearings and making rotation easier.

Molybdenum stem

Connects the anode disc to the rotor. It has low heat conductivity, preventing heat from reaching and damaging the rotor.

Induction motor

which rotates the anode at the required speed that ranges from 3,600 to 10,000 rpm to help dissipate heat.

motor consists of two main parts

Rotor

Stator

Rotor

(inside the tube envelope): Made of iron bars embedded in a copper shaft, it spins the anode.

Stator

(outside the tube envelope): Made of electromagnets arranged in pairs, it generates a rotating magnetic field that induces movement in the rotor.

Glass envelope

serves as a protective housing that maintains a vacuum inside the tube. This vacuum prevents electron collisions with air molecules, allowing efficient electron travel from the cathode to the anode.

borosilicate glass

envelope is made of heat-resistant…

to withstand high temperatures and minimize X-ray absorption. It also provides electrical insulation and houses a thin exit window, allowing X-rays to pass through with minimal attenuation.

serves as a protective casing that surrounds the glass envelope. I

Tube housing

Tube housing

made of metal and lined with lead to shield against stray radiation and reduce radiation leakage. The housing also contains oil or a cooling system to dissipate heat generated during X-ray production. Additionally, it provides mechanical support and electrical insulation, ensuring the safe operation of the X-ray tube

Radiation shielding

Made of lead-lined metal, it

prevents unnecessary radiation leakage, protecting

both the patient and radiologic personne

Mechanical Protection

Provides structural support and protection to the fragile glass or metal X-ray tube inside.

Electrical Insulation

Prevents electrical hazards by isolating high-voltage components.

Glass or Metal X-ray Tube

Where X-rays are produced.

Cooling Oil & Heat Exchange System

Maintains optimal operating temperature

Lead Shielding

Reduces radiation leakage to regulatory limits.

Filtration

involves placing metal sheets in the X-ray beam to remove low- energy (soft) X-ray photons. These photons do not contribute to image quality but increase patient dose and scatter.

Types of Filtration:

Inherent f

Added f

Inherent filtration

Comes from the X-ray tube itself

Added filtration

Extra metal sheets

Total filtration

sum of inherent and added filtration. U.S. regulations require a minimum of 2.5 mm Al HVL for X-ray tubes operating above 70 kVp.

Aluminum and copper

Common in general X-ray imaging.

Molybdenum and rhodium

Used in mammography for low-energy beams.

collimator

device in X-ray machines used to shape and limit the size of the X-ray beam. It improves image quality by reducing scatter radiation and unnecessary exposure to surrounding tissues.

Lead shutter

Adjustable plates inside the collimator that shape the beam.

Light Source & Mirror

Helps in positioning by projecting the X-ray field onto the patient’s body

Aluminum Filter

Further refines the beam byremoving low-energy photons.

Radiographic scale

Marks for accurate beam adjustment.

LINE-FOCUS PRINCIPLE

Angling the anode target allows for a large actual focal spot (to manage heat) while creating a smaller effective focal spot (for better image sharpness).

Typical anode angles range from 7° to 20°, with 12° being the most common.

A smaller anode angle results in a smaller effective focal spot but increases the anode heel effect

(variation in X-ray intensity across the beam).

Actual focal spot:

The area where electrons bombard the anode. A larger actual focal spot

helps with heat dissipation.

Effective focal spot:

The projected X-ray beam area towards the patient. A smaller effective focal spot improves image sharpness.

THE ANODE-HEEL EFFECT

The anode angle causes the intensity of the X-ray beam to be less on the anode side than on the cathode side.

This is because the heel of the target is in the path of the beam and therefore absorbs some X-rays on that side.