Production of diagnostic X-ray
Diagnostic radiation can be produced in two ways:
Use of radioactive isotopes for diagnostic purposes with exposure
facilities to pump them in appropriate chamber to expose the
radiographic film.
When high-speed electrons collide with a positively charged target.
The kinetic energy of these electrons is partially converted into Xradiation.
The X-ray machine
The X-ray tube and its power supply.
The arm.
A control panel
X-ray tube
borosilicate glass vacuum tube. The component parts of the X-ray
tube are leaded-glass housing,
a negative electrode as a cathode,
and a positive electrode as an anode.
The central area of the X-ray tube has an opening (window) that
permits The X-ray beam to exit
Evacuation of the glass tube is done:
to prevent the loss of kinetic energy of the electrons by colliding with
the gas molecules and also
to prevent the oxidation burnout of the filament.
The cathode consists of:
The filament: is the source of electrons within the X-ray tube, which is a coil
of tungsten wire about 2mm in diameter and 1cm or less in length. It is
mounted on two stiff wires that support it and carry the electric current.
These two mounting wires lead through the glass envelope and connect
to both high-and low-voltage electrical sources. The filament is heated to
incandescence by the flow of current from the low-voltage source and
emits electrons at a rate proportional to the temperature of the filament.
The focusing cup: is a negatively charged concave reflector made of molybdenum
which the filament lays in it. The focusing cup electrostically focuses the electrons
emitted by the incandescent filament into a narrow beam directed a small
rectangular area on the anode called the focal spot.
The anode consists of:
a thin tungsten target plate embedded in a solid copper stem block. Tungsten is
used as a target material because of its high atomic number (74) because it is
most efficient in producing X-ray, high melting point (3370oC) because heat is
generated at the anode, high thermal conductivity thus dissipating heat into
the copper stem, and low vapor pressure of tungsten at high temperatures
also helps maintain the vacuum in the tube at high operating temperature.
The target converts the kinetic energy of the electrons into X-rays photons.
The tungsten target is embedded in a large block of copper to dissipate heat.
Copper is good thermal conductor, dissipates heat from the tungsten, thus
reducing the risk of the target melting.
The sharpness of the radiographic image improves if the focal spot in the
target is small. The heat generated per unit target area, however,
becomes greater as the focal spot decreases in size. The target is placed
at an angle of 20o with respect to the central beam to achieve a small
focal spot and to effectively distribute the striking electrons for better heat
dissipation.
The projection of the focal spot perpendicular to the electron
beam called the effective focal spot is smaller than the
actual size of the focal spot about 1*1mm as opposed to
the actual focal spot, which is about 1*3mm. This principle
is called as the line focus principle.
Heel effect refers to the loss of intensity of X-ray beam in the
peripheral region. The cathode side of the beam is more intense
than anode side due to self-absorption of some of the
bremsstrahlung photons by the target.
When the kinetic energy of the electrons is directed at the target, about
99% of it is converted into heat and only 1% is used for production of Xrays. The copper block is used to dissipate the excessive heat produced.
The other methods of dissipating the heat are:
Use of rotating anode usually used in tomographic or cephalometric
unites.
Insulating oil around the tube.
Angulating the target.
Air-conditioning of the X-ray room.
Power supply
The primary functions of power supply of an X-ray machine are to:
Provide a low-voltage current to heat the X-ray tube filament by
use of a step-down transformer
Generate a high potential difference between the anode and
cathode by use of a high-voltage transformer.
Electricity/electric current and circuits
The milliamperage (mA) is the measurement of the number of electrons
moving through the filament. The number of electrons passing through
the cathode filament can be controlled by mA adjustment on the control
panel of the X-ray machine.
The voltage of the X-ray tube current, the current of electrons passing
from the cathode to the anode is controlled by kilovoltage peak (kVp)
adjustment on the control panel.
The two circuits used in the production of X-rays are:
The low-voltage filament circuit: The filament circuit uses 7 to 10 volts
and is controlled by the milliamperage setting.
The high-voltage circuit uses 65 to 70 kVp and provides high voltage
required to accelerate the electrons and to generate X-rays in the X-ray
tube and is controlled by the kilovoltage setting.
Transformer
A transformer either increases or decreases the voltage in an electrical circuit
A step-down transformer is used to decrease the voltage from the incoming
110or 220 line voltages to 7 to 10 volts. The step-down transformer has
more wire coils in the primary coil or input coil than in the secondary coil or
output coil.
A step-up transformer is used to increase the voltage from the
incoming 110 or 20 line voltages to 60 to 100 kVp. It has more wire coils
in the secondary coil than in the primary coil.
An autotransformer serves as a voltage compensator for correcting any
minor fluctuation in the current.
Because the line current is AC, which usually 60 cycles per second, the
polarity of the X-ray tube alternates at the same frequency. When the polarity
of the voltage applied across the tube causes the target anode to be positive
and the filament to be negative, the electrons around the filament accelerate
toward the positive target and current flows through the tube so the electrons
strike the focal spot of the target; some of their energy convert to X-ray
photons.
self-rectified or half-wave rectified
This half of the cycle is called inverse voltage or reverse bias so that no Xrays are generated. Therefore when an X-ray tube is powered with 60-cycle
AC, 60 pulses of X-rays are generated each second, each having duration of
1/120 second. This type of power supply circuitry, in which the alternating high
voltage is applied directly across the X-ray tube, limits X-ray production to half
the AC cycle
In full-wave rectification or full wave rectified circuit, even the negative half of
the cycle is used for the production of X-ray. In these machines, X-rays are
not produced in pulses but as a stream of X-rays. A full wave rectified X-ray
machine produces 120 bursts of X-ray photons per second.
In full-wave rectification or full wave rectified circuit, even the negative half of
the cycle is used for the production of X-ray. In these machines, X-rays are not
produced in pulses but as a stream of X-rays. A full wave rectified X-ray
machine produces 120 bursts of X-ray photons per second.
Timer
A timer is built into the high-voltage circuit to control the duration of the X-ray
exposure. The timer controls the length of time that high voltage is applied to
the tube and therefore the time during which tube current flows and X-rays are
produced.
Some X-ray machine timers are calibrated in fractions and whole numbers of
seconds. The time intervals on other timers are expressed as numbers of
impulse per exposure (e,g,, 3,6,9,15). Such numbers represent the number of
impulses of radiation emitted during the exposure; thus the number of
impulses divided by 60 the frequency of the power source gives the exposure
time in seconds. Therefore a setting of 30 impulses means that there will be
30 impulses of radiation and is equivalent to a half-second exposure.