* Ch.7 X-ray Tube *

What are the external components of the x-ray tube?

Support system (floor-to-ceiling, ceiling, C-arm), protective housing, and enclosure (glass or metal)

What are the internal components of the x-ray tube?

Cathode (filaments, focusing cup) and anode (rotating or stationary)

What does the cathode contain?

Filament(s) and focusing cup

What does the anode contain?

Rotor and stator assemblies

What is the function of the anode induction motor?

It spins the anode using electromagnetic induction; stators outside the glass envelope interact with the rotor inside to cause rotation

Where are the stators and rotor located in relation to the glass envelope?

Stators are outside the glass envelope; rotor is inside

What are the functions of the support systems of the x-ray tube?

Provide mechanical stability and positioning—types include floor-to-ceiling, ceiling, and C-arm systems

What is the function of the protective housing of the x-ray tube?

Controls leakage and scatter radiation, isolates high voltage, and provides cooling

What is the leakage radiation limit for x-ray tube housing compliance?

Less than 100 milliroentgens per hour (mR/hr) at 1 meter from the source

What helps cool the x-ray tube within the housing?

Oil surrounding the tube dissipates heat

What material are glass enclosures usually made of and why?

Pyrex glass—because it is strong and can withstand high temperatures without melting

What is the purpose of the enclosure in the x-ray tube?

Maintains the vacuum and provides a constant electrical potential between tube current and enclosure

What are the two main parts of the cathode?

Filament and focusing cup

What charge does the cathode have?

Negative

What is thermionic emission?

The process of heating the filament until electrons are "boiled off"

What is the average diagnostic x-ray tube life?

Approximately 6 to 9 hours (or 10,000-20,000 exposures)

Why are larger focal spots more tolerant to heat load?

They distribute heat over a larger area, allowing higher exposures without damage

What is the space charge?

The cloud of electrons surrounding the filament before acceleration to the anode

What is the space charge effect?

Electrostatic repulsion between electrons in the cloud that makes it harder for new electrons to be emitted

What is saturation current?

The maximum tube current for a given filament current, beyond which increasing kVp won't increase mA

What happens when exposure is made after thermionic emission?

Cathode becomes highly negative, anode becomes positive, and electrons accelerate toward the anode

What is the function of the focusing cup?

It is negatively charged to focus the electron stream into a small area on the anode

What are dual focus tubes?

Tubes with two filaments, providing small and large focal spots

When is a small focal spot used?

When better spatial resolution is required (e.g., for bone work)

When is a large focal spot used?

For large body parts or high technique settings that generate more heat

What are the typical sizes for small and large focal spots?

Small: about 0.1 mm; Large: 0.3-2 mm

What are the three functions of the anode?

Electrical conductor, mechanical support for the target, and thermal dissipater

What are the two types of anodes?

Stationary and rotating

What materials are anodes made from and why?

Tungsten (for high atomic number and heat capacity), Rhenium (for strength), Molybdenum and Graphite (for lighter weight and thermal properties)

Why is tungsten used in the anode target?

It has a high atomic number for better x-ray production and excellent thermal conductivity and melting point

What is the anode rotation speed?

Typically 3,400-10,000 revolutions per minute (rpm)

What percentage of kinetic energy of electrons striking the target is converted to heat?

Approximately 99%

What is the temperature range the anode can reach during exposure?

Around 1,000-2,000°C

Where is tungsten located in the anode?

On the focal track where electrons strike

Where is molybdenum found in the anode?

In the back or base of the anode disk (for support and weight reduction)

What is the line-focus principle?

Angling the anode target to provide a large actual focal area for heat dissipation while maintaining a small effective focal spot for resolution

What is the actual focal spot?

The area on the anode where electrons physically strike

What is the effective focal spot?

The area of the x-ray beam projected toward the patient and image receptor

How does the anode angle affect the effective focal spot?

A smaller anode angle reduces the effective focal spot size and increases the heel effect

What is the anode heel effect?

A decrease in x-ray beam intensity on the anode side due to x-ray absorption within the target

How does the anode angle affect the heel effect?

The smaller the anode angle, the greater the heel effect

How can the anode heel effect be used to advantage?

Positioning thicker anatomy (e.g., chest, abdomen, foot) toward the cathode side for more uniform exposure

What is extrafocal radiation?

X-rays produced outside the focal spot due to rebounding electrons

What are the effects of extrafocal radiation?

Increases skin dose, reduces image contrast, and enlarges the effective focal spot size

What is the most common cause of abrupt tube failure?

Electron arcing within the tube

How can radiographers prolong tube life?

By using minimal exposure factors (mA, kVp, and time) and avoiding excessive heat

What are the three types of x-ray tube charts?

Radiographic rating chart, anode cooling chart, and housing cooling chart

What does the radiographic rating chart show?

The safe exposure combinations of kVp, mA, and time without overloading the tube

What does the anode cooling chart show?

How long the anode takes to cool after exposures and its total heat capacity

What does the housing cooling chart show?

Cooling characteristics of the x-ray tube housing and oil

What are heat units (HU)?

A measure of the thermal energy produced in the x-ray tube during exposure

How are heat units calculated for single-phase generators?

HU = kVp × mA × time (seconds)

How are heat units calculated for three-phase or high-frequency generators?

HU = 1.4 × kVp × mA × time (seconds)

Example: What are the heat units for 90 kVp at 130 mAs in single-phase?

HU = 90 × 130 = 11,700 HU

Example: What are the heat units for the same technique in a three-phase/high-frequency unit?

HU = 1.4 × 90 × 130 = 16,380 HU

What are the three types of questions related to anode or housing cooling charts?

1. What is the total heat capacity 2. How long to cool completely from a given load 3. How long to wait before adding another exposure

What happens if heat storage capacity is exceeded?

Tube damage, cracking, or pitting of the anode surface

What is the main cause of reduced tube life?

Excessive heat or thermal stress from repeated exposures

Why is the x-ray tube considered a vacuum?

To allow free movement of electrons from cathode to anode without gas interference

What happens if the vacuum is lost in the x-ray tube?

Arcing, oxidation, and tube failure

What is the advantage of metal enclosures over glass?

They maintain a constant electric potential and prevent tungsten vaporization buildup