X-Ray Machine: Components and Room Design

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°.