X-Ray Tube Components and Housing

• X-ray imaging equipment employs an X-ray tube, which is essential.

X-Ray Tube Components

• The X-ray circuit includes an exposure timer, step-up transformer, rectifiers, circuit breaker, kVp meter, rheostat, autotransformer, filament circuit, and a step-down transformer.
• Key components include housing, collimator, mirror, light source, shutters, and a plastic bottom covering with cross hairs.

General Tube Construction: Housing

• The X-ray tube is housed in a protective structure that's lead-lined metal.
• The housing acts as an electrical insulator and thermal cushion.
• It incorporates an oil bath and cooling fans to dissipate heat.
• The housing absorbs unwanted photons, known as leakage radiation.
• It provides mechanical support to the tube.

General Tube Construction: X-Ray Tube

• The X-ray tube consists of an anode, cathode, and induction motor encased in a glass or metal envelope.
• The envelope maintains a vacuum within the tube.
• Glass envelopes are made of borosilicate glass for heat resistance.
• Vaporized tungsten deposition from the filament inside the glass leads to "sun tanning."
• Metal envelopes avoid arcing and extend tube life.
• Glass and metal envelopes feature a target window for X-ray exit.
• The target window is a thinned section on the enclosure.
• Target window is usually about 5 cm².

General Tube Construction: Anode

• The anode serves as the target for electron interaction, producing X-rays.
• It is designed as an electrical and thermal conductor.
• Some electrons interact with the target to produce X-rays, while the remainder continue as current.
• A tremendous amount of heat generates during X-ray production.
• The anode is designed to dissipate heat.

General Tube Construction: Anode Designs

• Stationary anodes are an older design with limitations.
• Stationary anodes feature a tungsten button embedded in a copper rod.
• Tungsten (W, Z = 74) facilitates heat dissipation because it's a good heat conductor with a high fusion temperature (~3400°C).
• A primary disadvantage of stationary anodes is rapid heat buildup, potentially damaging the tube and limiting exposure factors.
• Stationary anodes are compact, cost less, and found in dental X-ray sets with limited output.
• Rotating-anode designs are used in general-purpose tubes.
• Rotating anodes consist of a rotating disc of molybdenum with a tungsten coating mounted on a copper shaft.
• Copper is used as part of the shaft for its thermal and electrical conductive properties.
• Molybdenum is used as the disc base and core because it's strong, light, and has a low thermal conductivity.
• The target material (coating) is made of tungsten-rhenium alloy.
• Rotating-anode uses an induction motor for rotation within a vacuum at speeds up to 3400 rpm for general-purpose tubes.
• The rotating anode spreads heat over a larger surface area.
• Electrons strike a small part of the total anode surface area at any one time changing that area and creating a focal track versus a single spot.

General Tube Construction: Line-Focus Principle of the Anode

• The face of the target is angled to maintain a large actual focal spot size while creating a small effective focal spot size.

• The actual focal spot is the area bombarded by filament electrons; it is influenced by filament size, and heat is generated.

• The effective focal spot is the beam's origin from the patient's perspective.

• A smaller area of origin results in a sharper image.

• When the target face angle is less than 45 degrees, the effective focal spot will be smaller than the actual focal spot.

• Target angles generally range from 7 to 18 degrees, with 12 degrees most common.

• Smaller anode angles lead to smaller effective focal spots, while maintaining a large actual focal spot area.

• The line-focus principle balances heat area and projected focal spot.

• When the target angle is too small, the X-ray beam area may not fully expose a 35x43 cm image receptor at a 100 cm SID.

• Another disadvantage of the line-focus principle is an angle causing intensity variation in the X-ray beam because the "heel" of the target is in the path of the beam also known as the anode heel effect.

• Intensity reduction on the anode side makes the image "lighter," and this is mitigated in digital technology due to wider gray shades.

General Tube Construction: Cathode

• The cathode is the source of electrons and has filaments and a focusing cup.
• General-purpose tubes have two filaments called dual-focus tubes.
• Filaments are coils of wire typically 7 to 15 mm long and 1 to 2 mm wide, composed of tungsten with 1-2% thorium.

X-Ray Production Process

• Recall the principles of X-ray circuit operation.
• A cloud of electrons is created by the filament circuit by heating the filament.
• Kilovoltage applied to the X-ray tube generates a positive charge on the anode and a negative charge on the cathode (focusing cup).
• Positive charge attracts electrons boiled off the filament giving them kinetic energy.
• Negative charge on the cathode keeps the electrons crowded together.
• The electrons travel across to the anode where they interact to produce x-rays until the timer circuit terminates the process.

Quality Control and Extending Tube Life

• Several factors can shorten X-ray tube life or cause damage related to the thermal characteristics of X-ray production.
• Factors within a radiographer's control that can shorten tube life include:
  • Frequent use of very high or maximum exposure factors
  • Use of lower but very long exposure factors
  • Overloading the filament
• Three heat transfer processes are Conduction, radiation, and convection.
• Protection circuits prevent use of unsafe exposure techniques and heat overloads.
• Radiographer must understand anode thermal capacity and keep in mind that X-ray production is an inefficient process.
• The radiographer can extend tube life by avoiding holding the rotor switch unnecessarily as this increases the filament's thermionic emission to exposure levels, deposits vaporized electrons on tube surfaces, decreases the tube vacuum and causes tube failure.
  • Switches should be completely depressed in one motion.
• Lower-mA settings should be used to limit filament thermionic emissions.
• Lower-speed rotor should be used where possible since higher settings increase rotor wearing.
• Avoid repeated exposures near tube loading limits, since total heat units may approach anode or housing loading limits.
• Avoid rapidly rotating the tube housing which can damage the rotor and tube.
• Check for loud rotor bearings, which can cause a wobbling anode disk and result in tube failure.

Overview of X-ray tube components, including the housing, collimator, and internal parts like the anode and cathode. The housing provides electrical insulation, thermal cushioning, and absorbs leakage radiation. The X-ray tube itself relies on a vacuum environment maintained by a glass or metal envelope.