X-Ray Tube Components CH5

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Questions and Answers

What is one of the primary uses of a small focal spot in x-ray imaging?

To enhance electrical conductivity

To produce high heat during imaging

To image larger body parts

To achieve better spatial resolution (correct)

Which material is commonly used for anodes in x-ray tubes due to its thermal conductivity?

Titanium

Copper (correct)

Lead

Aluminum

Which of the following correctly describes a stationary anode?

It is designed for high tube current applications.

It is primarily used in high-power imaging systems.

It can rotate to disperse heat effectively.

It is used in some portable imaging systems. (correct)

What happens to most of the kinetic energy of projectile electrons when they interact with the anode?

More than 99% is converted into heat. Signup and view all the answers

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

To conduct electrons emitted by the cathode Signup and view all the answers

What is the purpose of the protective housing of the X-ray tube?

To confine and control the direction of emitted x-rays Signup and view all the answers

Which support system allows for both longitudinal and transverse movement of the x-ray tube?

Ceiling support system Signup and view all the answers

Which of the following statements about isotropic emission of x-rays is correct?

X-rays are emitted with equal intensity in all directions Signup and view all the answers

What is a unique feature of the C-arm support system?

It is shaped like a C and allows flexible positioning Signup and view all the answers

Which support mechanism has a column with rollers at each end?

Floor-to-ceiling support system Signup and view all the answers

What is NOT one of the three main types of x-ray tube support systems mentioned?

Wall-mounted support system Signup and view all the answers

What is the primary role of the support mechanisms for the x-ray tube?

To facilitate positioning of the x-ray tube Signup and view all the answers

What are the x-rays that contribute nothing diagnostically and result in unnecessary exposure called?

Leakage radiation Signup and view all the answers

What is one primary function of the protective housing in an x-ray tube?

To reduce leakage radiation to less than 1 mGya/h Signup and view all the answers

Which x-ray tube component is inaccessible due to its protective housing?

The rotating anode Signup and view all the answers

How does the glass enclosure of an x-ray tube improve performance?

By maintaining a vacuum inside Signup and view all the answers

What happens to x-ray production if a little gas enters the tube enclosure?

It falls, increasing heat generation Signup and view all the answers

What is the purpose of oil contained in some protective housings?

To act as an insulator and thermal cushion Signup and view all the answers

Which type of enclosure is most commonly used in high-capacity x-ray tubes?

Metal enclosure Signup and view all the answers

What is the primary role of the dual-filament cathode in an x-ray tube?

To supply separate electron beams for two focal spots Signup and view all the answers

What is the size of the x-ray tube window through which the useful beam is emitted?

5 cm2 Signup and view all the answers

What is the primary function of the focusing cup in an x-ray tube?

To confine the electron beam Signup and view all the answers

Which material is commonly used for filaments in x-ray tubes due to its efficient thermionic emission?

Thoriated tungsten Signup and view all the answers

What happens to the x-ray tube current when the filament current is increased significantly beyond the thermionic emission threshold?

It increases to a limit Signup and view all the answers

At what point is an x-ray tube said to be emission limited?

When operated at saturation current Signup and view all the answers

How does tungsten compare to other metals in terms of thermionic emission?

It provides higher thermionic emission Signup and view all the answers

What is the role of the low current that initially passes through the filament when the x-ray imaging system is turned on?

To warm the filament for thermionic emission Signup and view all the answers

What occurs when the x-ray tube current reaches saturation?

The maximum available electrons are used Signup and view all the answers

Which of the following factors does NOT affect the effectiveness of the focusing cup?

Color of the filament Signup and view all the answers

Study Notes

X-Ray Tube

The X-ray tube is a component of the X-ray imaging system, often not directly seen by radiologic technologists.
It's contained within a protective housing, making it inaccessible.
Figure 6-1 is a schematic diagram of a rotating anode diagnostic X-ray tube.

Internal Components

Cathode: The negative side of the X-ray tube. It consists of a filament and focusing cup.
Filament: A coil of wire, similar to a toaster's heating element, but much smaller—typically 2mm in diameter and 1-2cm long. The filament heats up to emit electrons. Tungsten is a common filament material. Tributes to higher thermionic emission than other metals.
Focusing Cup: A metal shroud that surrounds the filament and is negatively charged to electrostatically focus the electron beam, preventing it from spreading.
Anode: The positive side of the X-ray tube, with two types: stationary and rotating.
Stationary Anode: Used in applications with lower tube current and power requirements, like dental X-ray machines.
Rotating Anode: Common for most diagnostic X-ray machines, enabling efficient dissipation of heat from the large amounts of electron bombardment. Composed of copper, molybdenum, or graphite, these metals efficiently dissipate heat.

External Components

The X-ray tube and housing are quite heavy, requiring a support mechanism for positioning by radiologic technologists.
Figure 6-2 illustrates two common X-ray tube support methods: ceiling support and floor-to-ceiling support.

Ceiling Support System

The most common support system, employing two perpendicular sets of ceiling-mounted rails for longitudinal and transverse adjustment of the tube.

Floor-to-Ceiling Support System

A single column with rollers on both ends connects to ceiling and floor mounted rails, for vertical and rotational adjustment of the tube. Variations may exist with the column on a single floor support.

C-Arm Support System

Typically part of interventional radiology suites; shaped like the letter "C". Allows for flexible positioning. The image receptor attaches to the opposite end of the "C" shaped support. Variations such as L-arm or U-arm support are common.

Protective Housing

X-rays are emitted isotropically (with equal intensity in all directions).
Only X-rays passing through the window are used. These are called the "useful beam".
Leakage radiation (X-rays escaping the housing) contributes no diagnostic information and increases unnecessary patient and technologist exposure.
Protective housing reduces leakage radiation to below 1 mGy/h at 1 meter, and it also provides mechanical support, protecting the tube from damage.
Some protective housings include oil to act as both electrical and thermal insulation to dissipate heat, and fans to further enhance cooling.

Glass or Metal Enclosure

The X-ray tube is a vacuum tube with components within a glass or metal enclosure.
Pyrex glass is frequently used for the enclosure, due to its heat resistance properties.
The enclosure maintains a vacuum to enhance X-ray production and extend tube life. Reduced gas inside the enclosure obstructs electron flow resulting in less X-ray production and more heat.

Focal Spot

Most diagnostic X-ray tubes have two focal spots (one large, one small).
The small focal spot is used for higher spatial resolution.
The large focal spot is used for large body parts or when techniques requiring significant heat are used. A dual filament cathode enables the use of both a large and small focal spot. The smaller spot is associated with the smaller filament, while the larger is associated with the larger filament.
Appropriate filament is heated and the electric current is directed to the appropriate filament.

Anode

There are two main types of anodes: stationary and rotating.
The anode is the positive side of the X-ray tube, acting as a conductor of electricity and a means of dissipating heat generated during operation. This heat dissipation is critical to maintain tube life.

Current and Voltage Relationship

Filament current and X-ray tube current are related but depend on the tube's voltage.
A small change in filament current will result in a large change in X-ray tube current at higher voltages; this is known as thermionic emission.
At a certain X-ray tube current, saturation is achieved at high voltage; further tube voltage increases result in no change in X-ray current. This is because all available electrons have already been used.

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Description

Explore the essential components of the X-ray tube, including the cathode, filament, focusing cup, and anode. This quiz covers the roles and functions of these components in the X-ray imaging system. Perfect for radiologic technologists and students in radiology courses.