x-ray tube

MODULE ON RADIOLOGICAL EQUIPMENT
CHAPTER ONE
X-RAY TUBES & X-RAY PRODUCTION
Objectives;
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
4 April 2016 By Marema Jebessa 1

• 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.

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”

COMPONENTS OF X-RAY TUBES
 protective house
Cathode
Anode

4 April 2016 9

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.

Cathode
Is 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.

Why 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

Anode
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

focal 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

Line-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 effect
The 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.

Anode 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.

 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.

Cooling 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.

Heat 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.

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.

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 PRODUCTION
2 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.

Recommendations 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

x-ray tube

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