7: xray tube

What are the three types of tube support for an X-ray tube?

Ceiling Support, Floor to Ceiling Support, C-Arm Support

What are the external components of the xray tube

Support structure, protective housing, glass or metal envelope

What is the primary function of the protective housing of an X-ray tube?

To guard against excessive radiation exposure and electrical shock.

Reduce leakage radiation

X-rays are produced

isotropically

What is the purpose of the glass envelope in an X-ray tube?

To maintain a vacuum inside the tube for efficient X-ray production.

What is the main disadvantage of glass enclosure tubes

As tungsten vaporizes it coats the inside of glass altering electric, properties of the tube causing pitting and arcing

Gas in tube decreases xray production and increases heat

Gassy tube

What is the cathode in an X-ray tube?

The negative side of the tube, consisting of a filament and focusing cup.

What materials are used in the filament of the cathode?

Thoriated tungsten for higher durability.

Advantage of metal enclosure

Maintains constant electrical potential between electrons of the tube current and the enclosure ( long tube life)

What is the most common cause for tube failure

Tungsten vaporization with deposition on the inside of the glass enclosure

What is the filament made from

thoriated tungsten

What is thermionic emission in the context of an X-ray tube?

The process where the filament heats up and ejects electrons.

What is the role of the focusing cup in an X-ray tube?

To confine the electron beam to a smaller area on the target.

What charge does the focusing cup have ?

negative charge

Relationship between tube current and filament depend on

Tube voltage

What is the significance of the space-charge effect in an X-ray tube?

It describes the cloud of electrons around the filament that can limit tube current.

What may cause space charge limit

Low kvp and high mA

What are the two types of anodes in X-ray tubes?

Stationary Anode and Rotating Anode.

What are the disadvantages of a stationary anode?

Heat builds up quickly in one spot, leading to damage over time.

What is the purpose of the target in an X-ray tube?

It is the area of the anode struck by electrons from the cathode.

How does a rotating anode improve X-ray tube performance?

It spreads heat build-up evenly, allowing for longer exposure times.

What is the typical composition of the anode target material?

Tungsten due to its high atomic number, good thermal conductivity, and high melting point.

What is the function of the cooling fan in the protective housing?

To cool the tube and/or oil to prevent overheating.

What are the benefits of a rotating anode

High tube currents w/ short exposure time

What is the purpose of high voltage receptacles in the protective housing?

To protect against accidental electric shock.

What is the maximum leakage radiation allowed from protective housing?

Less than 1 mGya/hr at 1 meter.

What happens to the X-ray production when gas enters the tube?

It decreases X-ray production and increases heat.

What is the melting point of thoriated tungsten used in the filament?

3410°C.

What is the effect of increasing kVp on tube current?

It raises the tube current until saturation current is reached.

What is the difference between small and large filaments in the cathode?

Small filaments provide better spatial resolution; large filaments are used for larger body parts.

What is the typical diameter of glass enclosures in X-ray tubes?

20 cm.

What is the purpose of the dual filament in the cathode?

To allow selection of focal spot size based on imaging needs.

What is the primary function of the anode in an X-ray tube?

To conduct electricity, radiate heat, and contain the target.

What is the relationship between tube current and filament current?

It depends on tube voltage and can be limited by space-charge effects.

What is the purpose of the anode in an X-ray tube?

The anode serves as the target for the electron beam, where X-rays are produced.

What materials are used for the anode and why?

Molybdenum alloy is used for the base of the target due to its low heat conductivity, while copper is used to conduct heat away from the anode.

What is the typical rotation speed of an anode in X-ray tubes?

Typical units rotate at 3400 rpm, while high-capacity units can reach up to 10,000 rpm.

What happens when the rotor of an anode malfunctions?

It can cause the anode to overheat, leading to pitting or cracking and resulting in tube failure.

How does an induction motor function in relation to the anode?

The induction motor rotates the anode using power administered from outside the enclosure, with a stator and rotor as its two main parts.

What is the Line Focus Principle?

relationship between the actual focal spot and the effective focal spot,

What is the effect of decreasing the target angle on the focal spot size?

As the target angle decreases, the effective focal spot size also decreases, resulting in greater resolution.

Place on target where the electrons bombard the target

Actual focal spot

Same area as viewed from directly below the target or projected onto the patient

Effective focal spot

Smaller focal spot greater _

Resolution

What is the Anode Heel Effect?

It refers to the phenomenon where greater intensity occurs on the cathode side of the beam due to photons being absorbed by the heel of the target.

Reduction in target angle also reduces

Effective focal spot size

How much does the target angle varies ?

10-20

Effective focal spot is smaller than

actual focal spot

How does the Anode Heel Effect benefit certain radiographic exams?

It is advantageous when performing exams of the T-spine by positioning the patient's head at the anode end to take advantage of the increased intensity on the cathode side.

As the target angle decreases, the effective focal spot …

Decreases

What is Extra Focal Radiation?

Also known as 'Off Focus' radiation, it occurs when some electrons bounce off the focal spot and produce X-rays outside of the intended focal spot.

What is the anode heel effect?

Variation in radiation intensity across the length of the radiation field- more rays absorbed at the heel (therefore less intense).

Why is the anode heel effected caused

Photons are absorbed at the heel of the target and loss intensity

which side of the anode heel is stronger?

cathode

Percentage of intensity variance of the anode heel effect

75-120

What are the three methods for heat dissipation in X-ray tubes?

The three methods are radiation, conduction, and convection.

What is the purpose of Tube Rating Charts?

They determine safe exposure levels for X-ray tubes by providing scales of time and kVp.

What does the Anode Cooling Chart indicate?

It shows the heat capacity of the anode and how long it will take to cool completely after maximum heat load.

How is thermal energy measured in an X-ray tube?

Thermal energy is measured in Heat Units (HU).

What is the significance of the focal spot size in X-ray imaging?

A smaller focal spot provides better image sharpness but also concentrates heat, which can lead to tube failure.

What is the ideal target angle for balancing focal spot size and heat dispersion?

The ideal angle is a compromise between making the focal spot small and spreading it out to disperse heat.

What is the typical shape of a focal spot in X-ray tubes?

The common shape is referred to as 'double banana.'

What is the effect of filament design on X-ray intensity?

Differences in filament design and the voltage on the focusing cup control the intensity of X-rays across the focal spot.

What is the impact of excessive exposure times on tube performance?

Frequent excessive exposure times can lead to overheating and potential tube failure.

What is the purpose of a diaphragm in the X-ray tube housing?

It helps to extract and conduct away reflected electrons to minimize extra focal radiation.

What does the Cathode side the xray tube consist of

Filament

Focusing cup

Filament current

3 functions of the anode

Electrical conductor

Mechanical support

Thermal dissipater

Single phase formula for anode cooling chart

HU= kvp X ma X s

Three phase anode cooling chart

HU= 1.4 X kvp X ma X s