Chapter 2
The Generation
of X-ray:
X-ray Tubes
The Generation of X-ray
•X-rays are produced
whenever electrons,
traveling at high
speeds, collide with
matter in any form.
The Generation of X-ray
• There are three essentials that must be
fulfilled before x-rays can be produced. They
are:
• 1. A source of electrons
• 2. A means of accelerating & controlling
their movement (via a difference in
potential)
• 3. A place to stop them with great
suddenness (a point of impact)
The Generation of X-ray
• The Law of Conservation of Energy energy can neither be created or
destroyed.
• Binding Energies - The forces that hold
electrons in orbit around the nucleus.
The nucleus (+) attracts the electron (-),
but the spin of the electron keeps them
from collapsing toward the nucleus.
The Generation of X-ray
•Ground State - Inner shells have
more energy available due to
their proximity to the nucleus
(stronger interaction). Outer
shells have less energy available
because they have weaker
interaction with the nucleus.
The Coolidge Hot-Cathode Tube
In modern x-ray tube the glass bulb is
exhausted to as complete a vacuum as
is possible to attain. The cathode (-) is
composed of a small spiral filament of
tungsten wire, about 1 cm long & 0.2 to
0.3 cm in diameter, which is housed in a
focusing cup. The anode (+) is a solid
rod of copper & molybdenum in the
opposite end of the tube.
The Coolidge Hot-Cathode Tube
The surface of the anode (+)
facing the cathode (-) filament is
beveled at between7 and 17
degrees& has a block of
tungsten set into it; it is only a
few centimeters from the
filament.
How The Three Essentials are Fulfilled in
the Coolidge Hot-Cathode Tube
• The source of electrons in modern
x-ray tubes is the tungsten
filament. This is connected to a
step-down transformer & heated to
incandescence by current from it.
The heating of the wire alters its
atomic stability in that the electrons
are less firmly combined with the
nucleus of the atom.
How The Three Essentials are Fulfilled in
the Coolidge Hot-Cathode Tube
• These loosely bound electrons
hover about the cathode like a
cloud. The greater the heat to the
filament, the more electrons
available at the face of the cathode.
• This is called Thermionic Emission
or Boiling Off of Electrons (the
source of electrons)
How The Three Essentials are Fulfilled in
the Coolidge Hot-Cathode Tube
• If a high voltage current is applied
to the tube (negative to the
cathode) these electrons will be
repelled from the cathode (like
charges repelling) towards the
anode (unlike charges attracting)
with one-third to one-half the
speed of light (a means of
acceleration).
How The Three Essentials are Fulfilled in
the Coolidge Hot-Cathode Tube
• These electrons will strike the anode
with great force (a point of impact) & be
converted into x-rays & heat.
• The energy of the speeding electrons is
converted into two type of energy:
• Greater than 99% Heat
• Less than 1% X-Ray
(exothermic reaction - heat > energy)
The Focal Spot of the X-Ray Tube
• Most of the electrons bombard the
target over a small area near its
center. This is the actual focal spot
of the tube. The actual focal spot
has an area nearly equal in size to
the overall dimensions of the
filament.
The Focal Spot of the X-Ray Tube
• The smaller the focal spot, the greater is
the detail produced on the radiograph,
but the smaller is the capacity of the
tube to produce x-rays. This is
explained by the fact that all the energy
is expended at the focal spot .The
smaller the focal spot, the more intense
will be the heat developed. Longer
exposure time is necessary with such
tubes.
The Focal Spot of the X-Ray Tube
•To control the size of the focal
spot a sleeve of molybdenum
can be placed around the
filament & given a negative
charge which repels the
electrons from all directions,
this will form a more narrow
stream.
The Focal Spot of the X-Ray Tube
• The principle of line focus is a method
used to give a smaller focal spot with a
larger target area. The actual focal spot
is a rectangle approximately 3 times as
long as it is wide.
The Focal Spot of the X-Ray Tube
• When the beveled anode (+) face is
viewed from the patients point of view,
the focal spot appears to be nearly
square. This is the effective projected
focal spot. It serves to bring the x-ray
source closer to being one point while
still maintaining the larger area for
impact.
Double Focus Tubes
•Are those tubes having 2 focal
spots - one fine (0.3 mm) focus
for maximum detail & one large
(2.0 mm) for heavier exposures.
A mechanical switch includes
the focus of choice in the circuit.
Most x-ray machines have this
built in.
Double Focus Tubes
• Penumbra - blurring of the edge of
an organ or bone due to the size of
the focal spot.
• Today the large focal spots are 0.8
mm - 1.0 mm, and are actually
smaller than the old small focal
spots. This is accomplished via new
ways to cool the tubes.
Double Focus Tubes
Summary
• Large Filament = larger effective
focal spot - used for larger body
parts - looses some detail.
• Small Filament = smaller effective
focal spot - used for small body
parts such as extremity fractures
(hair line) and fine detail.(this
causes incredible heat build up)
Methods of Cooling the Anode
• Sufficient heat is generated in the
operation of an x-ray tube to melt the
tungsten target (3370 C) that methods
had to be developed to dissipate the
heat & protect the tube.
• Construction of the anode with two
metals - one with a very high melting
point (tungsten) & the other with a high
conductivity for heat (copper) - was an
important first step.
Methods of Cooling the Anode
• The main cooling methods that are
used or have been used are as
follows:
• Natural Radiation - In this method
heat is lost via the glass tube into
the air. Low capacity tubes may be
used for short periods without any
means of cooling.
Methods of Cooling the Anode
• Air Cooling by Radiation - A
radiator (series of metal discs) may
be attached to the extreme end of
the anode to increase the surface
area which can give off heat into
the air.
• Water Cooling - Obsolete today water was carried away through a
hollowed out anode stem.
Methods of Cooling the Anode
• Oil Cooling - Almost all x-ray tubes in
use today are surrounded by oil. The oil
insulates as well as cools. oil & air
cooling may be combined.
• The Rotating Anode Tube - As the
name implies the anode target rotates
during the exposure. This allows us to
increase the exposure because of the
tremendous ability to dissipate heat.
The Rotating Anode
• 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 Rotating Anode
• 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.
The Rotating Anode
•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
Tube Capacity
•The tube capacity or their
ability to produce x-rays is
affected by the rotating anode.
•Tubes are rated in terms of:
•Kilovoltage (kV)
•Miliamperage (MA)
•Time of exposure (S)
Tube Capacity
• These factors are dependent on the
rectification system, cooling method and
focal spot size.
• kV - capacity is determined by the distance
from the filament to the target.
• MA - capacity is determined by size of the
focal spot & the rectification system used.
• S - capacity is determined by the anode &
tube cooling rates.
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.
The Heel Effect
• We can use this to our advantage in
radiography if we remember to position the
tube with the anode end of the tube towards
the more easily penetrated body part.
• Originally it was thought that the heel effect
was due to the angulation of the anode &
was based upon the theory of refraction.
The Heel Effect
• Today we believe that the electrons which are
traveling at high speeds, bombard the target and
xray is produced in 360 degrees of direction. 180
degrees of this x ray produced is absorbed by
the anode itself.
• Therefore, only 180 degrees of x-rays leave the
anode.
• Of those, some are absorbed by the lead shutters
of the collimator, allowing only those travelling
in the desired direction to exit the tube.
The Heel Effect
• 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.
The Heel Effect
•For example, in taking a
radiograph of the cervicalthoracic area, the neck area
should receive the rays from the
anode portion of the beam as
the neck is thinner than the
thoracic region.