What is an x-ray tube?
The x-ray tube is a component of x-ray imaging system. It is evacuated tube which are produced x-ray by energy conversion when a fast moving stream of electrons suddenly decelerated in the target of anode. The x-ray is made of Pyrex glass which contains two electrode. The elections are produced at cathode side and they strike at anode side and the x-ray are produced.
There are mainly two types of components are.
1) Internal components.
2) External components.
1) Internal components.
The internal components are given below.
A) Cathode
B) Anode .
A) Cathode:- The negative terminal of the x-ray tube is called cathode. The cathode has following parts.
a) Filament
b) focusing cup.
c) Connecting wires.
a) Filament:- The source of electrons which is called filament. The filament is made of thoriated tungsten helical wire. The filament is 0.2 mm in diameter that coiled to vertical spiral about 0.2 cm in diameter and 1 cm in length. The number of x-ray production depends upon the number of electrons the flow from the filament and production of electron depends upon the current apply.
Normally modern x-ray tube supplied double filament. One smaller and other is larger. But only one filament is worked a particular exposure. Three filament x-ray tubes are present. Usual filament current used in 3- 5 Amp. And 10 V.
When the current apply in tungsten wire, it becomes heated. When a filament is heated it’s and produces electrons from filament. This phenomenon is called thermionic emission. A pure tungsten filament must be heated to a temperature of at least 2200°C to emit a useful number of electrons.
b) Focusing cup:- The filament is surrounded by a metal which is control the electrons at cathode side is called focusing cup. The Focusing cup has negative charged so that it electrostatically confines the electrons beam to a small area of the anode. The Focusing cup made up of nickel. Modern x-ray tube consists of single or double Filament which consists of a spiral wire. The both Filaments are mounted side by side, one above the other and one is longer then the other.
The selection of focusing is determined by its size and shape, it’s charge, the filament size, shape and position of the filament in the Focusing cup.
c) Connecting wires:- The connecting wires which supply the source of voltage. When the voltage apply in the x-ray tube, that heat the filament and metallic focussing cup. The range of voltage about 10 Volts and 3-5 amperes.
B) Anode:- The anode is positive side of the x-ray tube. There are two types of anode:-
a) Stationary anode.
b) Rotating anode.
a) Stationary anode:- The stationary anode is made of Tungsten and copper. The copper has very high heat storage capacity and conductivity for efficient dissipation of heat. The anode 2 to 3 mm thick and the anode angle is 15° to 20°. The tungsten plates plate is square or rectangular shape. The tungsten is choosing in target material because :-
– Tungsten has high atomic number (Z=74) .
– Tungsten has very high melting point (3370°C).
– Tungsten has high heat and thermal conductivity.
– Tungsten has low vapor pressure at high temperature to prevent the evaporation of target material.
– Tungsten has high conversation efficiency for electrons into x-ray, hence metal of high atomic number are favoured.
The stationary anode x-ray tube are used in dental x-ray machine system, some portable imaging system, and other special purpose units in which high tube current and power are not required.
b) Rotating anode:- The rotating anode x-ray tube allows the electrons beam to interact with large target area. So the heating of anode is not confined to one small spot as like a stationary anode tube.
The Rotating anode made of pure tungsten but most of modern rotating anode made of 5 % to 6% of Rhenium is mixed with tungsten. The Rhenium has high thermal capacity towards anode of pure tungsten and it produce to anode that is more resistance to surface roughness and thus it prevents to development of crackers in the anode. The molybdenum and graphite are also used in rotating anode. The molybdenum is used in the base layer of tungsten – Rhenium is coated. Molybdenum and graphite have lower mass density then the tungsten which makes anode easier to rotate.
The Rotating anode is strip mounted on disc 7.5 to 12.5 cm in diameter. The Rotating anode rotate at high speed 3000 rpm to 9000 rpm. The Rotating anode are constructed on the line focus principle which projected towards the film is smaller than is it’s actual area on the target. The anode angle are 7°, 10°, 12° or 17°.
The Rotating anode is consists of anode stem, Bearing, stator, motor system and focal spot.
– The anode stem is made up of molybdenum. It has high melting point
.- The silver is used as lubricant in bearing.
– The stator is located outside of the glass envelope and it’s consists of a series of electromagnets.
– Rotor is located inside the glass envelope.
C) Focal spot:- The area of target with in which the electrons are absorbed and x-ray are generated is called focal spot area or target area.
2) External components:- The x-ray tube and the housing assembly are very heavy so they require support.
The external support of x-ray system are consists of three components.
A) Support system.
B) Protective housing.
C) Glass or metal enclosure.
A) Support system:- The three methods of x-ray tube support.
a) Ceiling support:- The ceiling support system is manly used in x-ray system. It consist of two perpendicular set of ceiling mounted rails. This rails allowed for both longitudinal and transverse travel of the x-ray tube. A one column attaches the x-ray tube housing to the rails allowing for variable source to image receptor distance.
b) Floor to ceiling support system:– The ceiling support system has a single column roller at each end. One attached to a ceiling mounted rails and the other attached to a floor mounted rails. The x-ray tube slide up and down the column to help of column rotates.
c) C- Arm support system:- C- Arm systems are ceiling mounted and provides for very flexible x-ray tube positioning .The image receptor is attached to the other end of C- Arm from the x-ray tube.
B) Protective housing:- When the x-rays are escape from the protective housing are called leakage radiation. The leakage radiation have not any contribution of image formation but it increases the patient as well as radiographer/ Radiotechnologist radiation dose. The protective housing design to reduced the level of leakage radiation to less than 100mR / hrs.
The protective housing special design high voltage receptacles to protect against accidental electric shock. The protective housing made of aluminum with lead lining. Around the housing some x-ray tubes contains oil. The oil protect from insulator again electric shock and therm cushion to dissipate heat. Some housing of x-ray tube has cooling fan to cool the tube housing.
C) Glass or metal enclosure:- An x-ray tube is a vacuum tube with components contains with a glass or metal enclosure. It is large, 30-50 cm long and 20 cm in diameter.
The glass enclosure is made of Pyrex glass. The glass enclosure maintain a vacuum inside the tube. The glass enclosure tubes ages some tungsten vaporizes and coats the inside of the glass enclosure. Metal enclosure tube maintain electric potential between the electron of the tube current and the enclosure. So they have a longer life and are less like fail. The high capacity x-ray tubes now use metal enclosures.