x-ray tube

May 1, 2023 By Marema Jebessa 1

MODULE ON RADIOLOGICAL EQUIPMENT

CHAPTER ONEX-RAY TUBES & X-RAY PRODUCTIONObjectives;

Upon successful completion of this chapter students will be able to:

Identify components of x-ray tube & describe their functions

Describe line-focus principle for x-ray tubes

Describe basic process of x-ray production


• ASSESSMENT QUIZZES


X-RAY TUBE ION& X-RAY PRODUCTION

INTRODUCTION

• X-ray tube:-is a relatively simple

electrical device w/c converts energy

from one form into another.

May 1, 2023 4

X-ray tube & x-ray production

Function:-receives electrical energy & converts

into: heat and x-radiation.

NB; x-rays are produced when a fast moving

electrons suddenly decelerated in the target

“ANODE”By Marema Jebessa

May 1, 2023 7

X-ray tube & x-ray productionCOMPONENTS OF X-RAY TUBES

protective house

Cathode

Anode

By Marema Jebessa

May 1, 2023 9


By Marema Jebessa



Protective housing

A metal lead-lined

Designed to enclose & support the components

Control excessive radiation exposure &

electrical shock.



The space between the housing and insert is

filled with oil, which provides electrical insulation

and transfers heat from the insert to the housing

surface.



X-rays are emitted isotropically.

useful beam through the window.

leakage radiation-through protective

housing.



A properly designed protective housing

reduces the level of leakage to less than

100mr/hr at 1m when operated at

maximum conditions.



CathodeIs the negative terminal of the tube.The primary parts:

The filament &

a focusing cup.



FILAMENT-

Is a small-coiled wire

Commonly made from TUNGSTEN.

Emits electrons when the current through the filament is

sufficiently intense…called thermionic emission..

Normally powered by an alternating current that is

supplied to it by a separate transformer.


X-ray tube & x-ray productionWhy Tungsten is chosen for filament ?because;

Has high melting point(3410°c)

Can be formed into helical=will have larger area

High resistance=the passed current produce high

heat



Focusing Cup

Usually made from nickel or molybdenum

Contains a negative charge.

Designed to repel electrons.

condense electron beam to small area on focal

track.


…

The focusing cup is negatively charged so that

when the electrons are emitted by the filament

they will congregate and surge across to the

anode when the exposure begins.



The effectiveness of the focusing cup is

determined by;

1)its size, shape

2)its charge

3)the filament size, shape

4)the position of the filament in the

focusing cup


X-ray tube & x-ray productionAnode

It is a positive electrode.

receives the electrons which bombard it.

It serves three important functions:

1-It provides a complete circuit for accelerating the electrons.

2-It houses the target material.

3-It helps to cool the tube.



The metal used is tungsten &is chosen for

1)its higher melting point

2)has higher atomic number

3)Is a good conductor of heat

4)doesn’t evaporate easily

5)can be worked and made smooth



Focal spot

The piece of tungsten within the x-ray tube which

contains the area of electron bombardment is

known as the target.

The target is rectangular or circular in shape.



Most x-ray tubes have two focal spot sizes

(small and large), which can be selected

by the operator according to the imaging

procedure.



The dimensions of focal spots usually range from 0.1 mm to 2 mm.

X-ray tubes are designed to have specific focal spot sizes.

small focal spots produce less blurring and better visibility of detail.

large focal spots have a greater heat-dissipating capacity.



Tubes with small focal spots are used when high image

visibility of detail is essential and the amount of radiation

needed is relatively low because of small and thin body

regions as in mammography.



The radiation is produced in a very small area on

the surface of the anode known as the focal spot.

In most x-ray tubes, the focal spot is approximately

rectangular.

The dimensions of the focal spot are determined by

the dimensions of the electron beam arriving from

the cathode.


Small focal spot used when fine image is required.

0.3-1mm

associated with small filament.


Large focal spot

used when techniques that produces

high heat are required.

1-2.5mm

Associated with large filament


X-ray tube & x-ray productionfocal spot size

The size of this area is determined by the features that are

would expect.

By the size and shape of the filament.

By the dimension of the focusing slot and by the depth of the

filament in it.



By the characteristics of the electric field

associated with the focusing slot.

By the spacing of the two electrodes-that is,

how far apart the cathode and the anode are.



The larger this area is, the more heat it will take

on a short exposure before the temperature of

focal spot rises to the melting of tungsten.

Langer focal spot permits higher milliamperage

to be used without damage the x-ray tube.



Higher MA mean greater intensity of

radiation out put and this all a small amount

of x-radiation is produced & a great amount

of heat is spread over the focal spot.



The anode and image sharpness

The rectangle is the area bombarded by electrons and it is

called the actual focus of x-ray tube.

The larger this area is the larger is the area receiving heat

from the electron bombardment.



On a short exposure a larger area can take more heat

than smaller areas before temperature rises to the

melting point of tungsten.

This means that an x-ray tube with large focus can be

operated at higher milliamperage while decreasing image

sharpness.



The sloping anode is able to reconcile these

two conflicts

a large area to take the electron bombardment

a small x-ray source



Apparent focus

It is the square projection of the actual

focus of x-ray tube.

It is known as effective or apparent focus.

It determines unsharpness.



It is the size of the x-ray source as it

appears to be when viewed from the film.

It determines the amount of geometric

unsharpness present in the image.



Actual focus

Is the true size of x-ray source

It determines the electrical load which the tube

will take.



The angle of the anode

The anode angle is defined as the angle of the target

surface to the central axis of the X-ray tube.

The anode of x-ray tube has a sloping face.

This allows x-rays leave the tube sideways, and the RT

uses a beam which is at right angle to the long axis of the

x-ray tube


X-ray tube & x-ray productionLine-Focus Principle

Angling of the target

Used to reduce the effective area of the focal spot.

The effective focal-spot size is controlled by the size of the actual focal

spot and the anode target angle.

The effective focal spot's vertical dimension is the one that is stated as

the focal-spot size.



The steeper the slope of the anode face,

the smaller is the apparent focus for a given

size an actual focus.

The steeper slope anode will produce

sharper image than the other while taking

the same electrical load.

a steep slope means a narrow useful beam.



The heel effectThe variation in intensity along the long axis of the x-

ray tube is known as the heel effect

Heel effect is the term applied to the fact that x-ray

radiation does not exit the long axis of the tube in

uniform intensities.


The intensity of the beam is equal to the number of

rays & diminishes fairly rapidly from the central ray

to the anode side of the patient, while increasing

slightly toward the cathode side of the patient.


X-ray tube & x-ray productionAnode Heel Effect

Is due to the geometry of the angled anode target, the

radiation intensity is greater on the cathode side.

As the figure below indicates the intensity of the x-ray beam

is greater towards the cathode (filament) end of the tube.



More x-rays will be able to get out of the anode

on the right side (the angle side) than on the left

side because of all the material the x-rays must

pass through to get out of the anode.


X-ray tube & x-ray production The magnitude of influence of the heel effect on

the image depends on factors such as :

– anode angle

– size of film

– focus to film distance



X-ray is not uniform along the film surface.

Therefore the effects of the heel effect are seen toward the edges of the

film.

To decrease the heel effect, increase the collimation & decrease the film

size.

With increased collimation you get less heel effect.


X-ray tube & x-ray productionCooling the x-ray tube

Almost all the electrical energy put into x-ray tube is

converted into heat.

While small proportion of it gives rise to x-radiation.

The amount of heat is related to the product

Hu=Kilovoltage x milliampere x duration of exposure



Example 1

Calculate the heat units generated for the

following exposures.

Single-phase, rectified unit: 250 mA, 0.7

seconds, and 200 kVp.



Example 2



Three-phase, six pulse, rectified unit: 300 mA, 0.5

seconds, and 110 kVp.



Example 3



High Frequency unit: 500 mA, 0.9 seconds,

and 300 kVp.

May 1, 2023 61

X-ray tube & x-ray productionHeat dissipation

If dissipation of heat were not taking place, the melting point of tungsten

would soon be reached.

The process by which heat is dissipated are:

conduction – which takes place in solids

Conduction- which takes place in liquids & gases

Radiation-which can occur in a vacuum.

By Marema Jebessa



Conduction is for the heat to be removed

from the target

From tungsten wire to copper block.

The target also conveys a small part of its

heat by radiation across the vacuum of x-ray

tube to the glass envelope.


Heat is conveyed or setup in the oil and the

heat is conveyed to the metal casing.

The metal casing then losses heat by

convention and by radiation to the air surrounding

the x-ray tube.


X-ray tube & x-ray production The rotating anode x-ray tube

The anode in the tube is a beveled tungsten disc attached

to a rotor that revolves when the tube is on.

The cathode filament is offset to one side so that the

electron stream hits near the edge of the revolving disc.

The rotating anode continually presents a different area on

the target to the electron stream.



The focal spot remains fixed in space while the

circular anode rotates during the exposure to

provide a cooler surface for the electron stream

to strike.



The heat is distributed over a broad band, thus

maintaining the temperature rise well within safe limits.

As the capacity of the tube to withstand heat is

increased, the capacity of the tube to produce x-rays is

increased.



It also permits manufacturers to produce tubes with smaller effective focal spots.The disadvantages are:

The tube is very delicate

Special lubricants are necessary for the

motor which will not produce volatile gases



X-RAY PRODUCTION2 kinds of X-Rays are produced:

–“Bremsstrahlung” –“Characteristic”

Bremsstrahlung X-Rays • Electron hits atom and slows down, losing kinetic energy Energy emitted as photon



Bremsstrahlung

Bremsstrahlung = breaking radiation (in German), white light.

An electron may have one or more bremsstrahlung interactions.

The bremsstrahlung photon may have any energy up to the initial energy of the electron.



Electron hitting atom makes many photons (X-

Rays),all with different energy.

– Many different wavelengths



If all of electron’s energy is lost to a single

photon, photon has maximum energy

(minimum wavelength).



Characteristic X-Rays

Electron knocks one of the two K shell

(ground state) electrons out of an atom.

L (n=2) or higher shell electron falls down to K

shell (ground state) and x-ray photon is

emitted



Electron knocks one of the two K shell (ground

state) electrons out of an atom.

L (n=2) or higher shell electron falls down to K

shell (ground state) and x-ray photon is emitted

Bremsstrahlung X-Rays and Characteristic X-

Rays both occur at the same time.


X-ray tube & x-ray productionRecommendations for Extending Tube Life

1. Warm up the anode following manufacturer's recommendations.

2. Do not hold the rotor switch unnecessarily. Double-press switches should be completely depressed in one motion. Dual switches should have exposure switch depressed first, followed by the rotor switch.

3. Use lower mA stations when possible.


4. Use lower-speed rotor when possible.

5. Do not make repeated exposures near tube loading limits.

6. Do not rotate the tube housing rapidly from one position to another.

7. Do not use a tube when you can hear loud rotor bearing


THANK U!!

x-ray tube

Health & Medicine

Transcript of x-ray tube