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 X-

radiation.

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 X-

rays. 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 X-

rays 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.