DEVELOPMENTS IN THE X-RAY TUBE
Bansal S.C, Gupta Lalit K and Khandelwal. N
Department of Radio-diagnosis & Imaging,
Postgraduate Institute of Medical Education and Research , Chandigarh.
INTRODUCTION
Since the discovery of X-rays more than 100 years ago, significant developments have taken place in the X-ray tube to meet various requirements may be shorter exposure time, multiple repetitive exposures, capacity to accept heavy load or enhanced tube life etc. This has become possible by having multiple focal sports, faster rotating anodes, better anode disc materials or may be by replacing glass envelope with metal etc, i.e lot of physical changes have taken place in the x-ray tube design, whereas , principle of production of x-rays remains the same and problem of high heat production remains still unresolved.
Important Dates in the Developments of x-ray tubes
1895 : Roentgen discovered x-rays (using crooks type of tube)
1913 : The Coolidge hot cathode x-ray tube.
1915 : Hooded anode tube
1920 : Oil Cooled tube
1925 : Grenz ray tube
1929 : Rotating anode tube
1932 : Grid controlled stationary anode tube
1937 : Grid Controlled rotating anode tube
1959 : High speed tube
1962 : Rhenium alloyed tungsten composite anode tube
1967 : First dedicated mammography unit with molybdenum anode
1971 : Glass metal tube with molybdenum anode
1973 : Three layer anode (W-Re) + Mo or (W-Re) +W +(W-Zr-Mo)
1979 : Metal Ceramic tube
1981 : Three focus tube
1989 : Direct anode cooling with noiseless rotor
X-Rays were discovered by Wilhelm Conrad. Rontgen on Friday Nov. 8, 1895 IN Germany
When he passed current through Crooks type tube covered with card board shield to stop any light arising from the fluorescence of glass walls in a dark room, he was surprised to note small glow coming from a nearby placed sheet of paper painted with Barium Platino Cyanide. He confirmed these emanations of rays coming from the tube and named these x-rays or unknown rays.
The Gas Tube (ion Tube)
Early x-ray tube was called a gas tube because its action depended upon
The presence of small residual amount of gas present in it. And the radiographer used to keep a selection of tubes of various mAs values on a rack, usually in the dark room. When a change of mAs was required the radiographer disconnected the tube and replaced it with a different one from the rack.
Limitations of gas tube:
Relatively low intensity of x-ray (not more than 5mA)
Was unreliable and unstable as x-ray production depended upon the gas content which was a very variable factor
The Coolidge Tube (Hot Cathode X-Ray Tube)
Also called electron tubes were first introduced by W.D Coolidge in 1913 employing the principle of thermionic emission i.e. on heating metal element emit electrons.
Electrons liberated from heated filament were repelled by negative charge of filament and accelerated towards positively charged anode in completely evacuated glass tube, resulted into production of x-rays after striking the target Coolidge tube is the proto type for modern x-ray tubes being used today
What is an X-Ray Tube
X-ray tube is a device in which energy conversion takes place i.e. kinetic energy of fast moving electrons is converted into heat (99%) and 1% X-ray energy. X-rays are produced when fast moving electrons are decelerated in matter (high speed electrons strike with a target)
Apparatus designed for generation of x-rays require :
- A Source of electrons i.e. filament with source of heating
- A system capable of accelerating electrons across a space where there is nothing to impede them i.e. an evacuated glass enclosure with H.T. transformer.
- A structure usually called the target where the electrons strike Anode
MODERN X-RAY TUBES
With few technical modifications modern tubes are similar to Coolidge tube
Main components of an x-ray tube :
Cathode assembly -filament (tungsten)
- Cathodecup (Molybdenum)
Anode assembly with target (tungsten) - Fixed anode or Rotating anode
Glass envelope (Borosilicate or Pyrex high heat resistant) with vacuum level 10-7 to 10 -8 mm of Hg
Electric circuit to provide current and voltage for
filament heating (4-6 Amp, 8-12 volt) and high tension circuit for acceleration of electrons towards anode
ADVANTAGES OF STATIONARY ANODE X-RAY TUBE
It has Compact Unit and has less cost.
But Limitations are
Since area covered by electrons beam on the target i.e. x-ray source and the area over which heat is spread are the same so we cannot use higher electrical loads or high mAs
Its Applications include
Dental x-ray sets, small portable and mobile x-ray units with limited output
ROTATING ANODE TUBES
Limitations of stationary anode tube were overcome by rotating anode tubes which were introduced in 1936
Based on the principle of removal of target from the electron beam before it reaches too, high a temp and by replacing it by another cooler target i.e. target form the face of rotating disk or the end of a rotating cylinder
MAIN FEATURES OF ROTATING ANODE TUBE
The shape of glass envelope is modified to accommodate different styles of electrodes, rotor and stator of an induction motor
Cathode cup and filament are offset opposite the target track near the periphery of anode disc.
Anode disc made of tungsten has its periphery bevelled at an angle be 10o -200
Diameter of disc determines the length of target (diameter between 75-125mm)
Disc is connected to rotor (made of copper) with molybdenum stem
Rotor rotates with speed of 3000RPM on ball bearings (made of steel) with dry lubricant (silver or lead coating)
Rotation is produced by stator coils which surround the neck of x-ray tube outside the glass envelope
ADVANTAGES of ROTATING ANODE TUBE
It permits selection of higher electrical load (exposure factors) without risk of over heating
APPLICATIONS
Almost universal use in radiography
ADVANCES IN ROTATING ANODE TUBE
New anode materials
Reduced target angle
Increased speed of anode rotation
Grid Controlled x-ray tube
Metal / ceramic x-ray tube
REDUCED TARGET ANGLE
Target angle was reduced to 6o from 15o to 20o
The steeper the slope of anode face, the smaller is the apparent focus for a given size of actual focus
Steeper slope will produce sharper images than the other one while taking the same
electrical loading.
However limit is placed that by small target angle the useful x-ray beam will cover a smaller area at given tube to film distance
Another development in the use of reduced target angle is by having two different target angles ( biangular tubes)
BIANGULAR TUBES
The anode of a biangular tube has two focal tracks (Inner for fine focus) and outer for broad focus with cathode having two filaments arranged one above the other
The surface of the anode disc is beveled at two angles
Thus fine focus used for radiographic examination requiring more details, where
as large for routine radiography purpose.
New Anode Materials
Now a days anode discs are not made of solid tungsten but of combination of metals; Therefore an alloy of rehnium & tungsten (10% Rhenium, 90% tungsten) is used to face the anode disc as it retain its smoothness better than tungsten as the tube ages. This resists the roughness process much better and the target surface does not deteriorate quality so higher electrical loads can be applied to it during its working life. Another material now being used for anode disc is a combination of tungesten and molybdenum. Molybednum has the advantage being not so dense as tungsten and of ability to accept a given amount of heat with less rise in temperature and can accept high electrical loads. So what is used in practice is a molybdenum disc with a coating of 10% rhenium and 90% tungsten over the target back.
The molybdenum disc may be alloyed walk titanium and zirconium. Another material which has been and as a base for anode dise is shafhite,applied as in thick layer behind molybdeness grafite (carbon)as a higher melting point than tengusten(3510o compared with 3370oc )and has a 10 time higher specifiec heat than that of tungsten . So disc with grafite as a higher heat storage capacity which is much grater than that of an equal volume of tungusten.Improved his storage his capacity his thuys gained to the use of grafivte without pauying the price of an increase in anode volume and weight of the dise. Grafite is use as a heat sink behind a reheniun -tungesten -molybedenum differences in there thermal expension and to the lower thermal conductivity of grafite. Earlier it was Tungsten alone Rhenium 10% + Tungsten 90% (higher tube life)
Rhenium and Tungsten +Molybdenum
(RTM anode) Rhenium and Tungsten + (Titanium + Zirconium +Molybdenum - TZM base) (Rhenium +Tungsten ) + (Molybdenum) + Graphite as base
(Rhenium +Tungsten )+CVD Graphite
Advantage : higher heat storage capacity permitting higher tube rating with prolonged tube life
INCREASED SPEED OF ANODE ROTATION
At normal speed the anode rotates with 3000 RPM whereas at high speed it rotates at 9000-10000 RPM with a 3 phase supply
At high speed quicker rate of heat dissipation will be there, allowing greater input load (higher rating capacity) but at short exposures
Advantage :
Allow use of high mA and shorter exposure time
Allow possible use of smaller focal spots
Disadvantages:
Initial cost of equipment
Longer prepare time
Greater wear on the bearings
Require braking system
GRID CONTROLLED X-RAY TUBE
A third electrode called grid is used in place of wire mesh
Focussing cup surrounding the filament can be used as third electrode to control the flow of electrons
A negative vias voltage (around 1500v) applied to the cup relative to filament to punch off the flow of electrons
Thus cup acts as a exposure switch to turn the current on or off as when required
Applications in cine fluorography and angiography
Metal/ceramic X-Ray Tube
In this glass envelope has been replaced with metal casing and ceramic is used as insulator for high voltage cables.
e.g. Super Rolatix ceramic x-ray tube by Philips
Features :
Anode rotates on an axle with bearings at each end providing greater stability and reduced stress on shaft This permits use of massive anode (approx. 2000gm in place of
700 gm in conventional tubes)
Metal envelope is grounded thus relatively positive with respect to electrons
Ceramic insulators (Al. Oxide) are used to insulate high voltage parts of x-ray tube form the metal envelope thus allowing more
compact tube design Metal used is an alloy of Chromium and iron
ADVANTAGES OF METAL\CERAMIC TUBE
Higher Tube Loading
Allows higher tube currents to be used because of larger heat storage capacity of anode
Longer Tube Life
Deposition of tungsten on the glass wall acts as electrode causing arcing bet. Glass and filament shortening tube life. When metal enclosure is grounded, this deposition will not alter grounding thus increasing its life
Reduced off Focus Radiation
Electrons back scattered from the anode may strike anode again producing x-rays from areas other than focal spot. The metal enclosure decreases off focus radiation by attracting off focus electrons to the grounded metal wall relatively Positive as compared to electrons. Low atomic no. of metal may produce few and low energy x-rays.
ROTATING SEGMENTED ANODE TUBES
(Developed by Machlett Laboratories incorporated in Dynamax 100 tube)
The cup shaped anode is made of graphite
Several hundred independent tungsten rhenium segment line the inside of cup
When segmented anode rotates, the tungsten segments are forced against graphite cup by centrifugal force
Advantage
Segmented anode tolerates the head of instantaneous exposures better than conventional anode apart from greater cumulative heat loads.
The breaks between segments behave like expansion joints in a concrete side walk, thus protecting anode from thermal stress
Different types of X-ray Tubes depending upon applications :
Radiotherapy X-ray Tube (Deep Therapy).
Stereographic X-ray Tube
Mammography X-ray Tube
C.T. X-ray Tube
Field Emission X-ray Tube
RADIOTHERAPY TUBES
Tubes for deep therapy have KVp in the range of 200-300 Kv and usually work at 15-20mA
Single focus tubes size (6-8mm) with hooded anode and target angle of about 35o
The glass envelope is about 60cm in length to prevent external arcing due to high voltage
These are oil cooled stationary anode tubes
To allow emission of required primary x-ray beam a hole is cut in the hood below the target. But now a days these are not being used.
STEREOGRAPHIC X-RAY TUBES
These are Similar to conventional rotating anode x-ray except Rotating anode is bombarded simultaneously by two beams of electrons from two independent cathode assemblies
These are used for stereoradiographic and stereofluoroscopic x-ray examinations
MAMMOGRAPHY X-RAY TUBE
For maximum visualization of soft tissues of the breast having similar ability to absorb x-rays a beam of soft radiation (longer wavelength ) is required
Longer wavelength can be produced by selecting x-ray tube which operate at low KVp (20-40)
Features of a Mammography Tube
Use of target made of molybdenum
Closer spacing of cathode and anode
Beryllium window (thinned glass window)
Use of molybdenum filter in place of aluminum filter
TUBES FOR COMPUTED TOMOGRAPHY
CT require longer exposure time at higher KV than needed for general radiography
Early generation CT Scanners used stationary anode, oil cooled tube as those use in radiotherapy
But these have been charged with heavy duty rotating anode tube with higher thermal capacity and smaller focal spot (upto 0.6mm). These tubes are air cooled with current value upto 600mA
Some CT tubes are grid controlled for pulsed radiations to reduce rad. Dose
CT tubes are oriented in the gantry with long axis perpendicular to the fan beam to avoid heel effect
TUBES FOR COMPUTED TOMOGRAPHY
FIELD EMISSION X-RAY TUBES (Also called cold cathode tubes)
In place of electron emission by thermionic effect the electrons are extracted from cathode by a high potential difference
Since electrons which can be emitted are less in number these tubes can be used only for neonatal radiography
If higher voltage (upto 350Kv) is applied these tubes can be used for high Kv chest radiography. But not useful for general purpose radiography
Cross section of commercially available field emission tube
A conical anode surrounded by a cylindrical cathode containing facing rows of needle (actually more than two)
The X-ray beam passes through a window in the end of the tube with less intensity in the centre of field
Generator used is generally capacitor discharge type
Needle tip diameter is about 1΅.
Electrons are extracted form the cathode by an intense electric field rather than by thermionic emission
Choice of an X-Ray Tube:
| Purpose | Heat Storage Capacity | Target Angle | Focal Spot Size |
| Neuroradiography | 3,00,000 H..U. | 7o | 0.3, 6.0 mm |
| Angiography | 4,00,000 H..U. to 1.35 MHU | 12o | 0.6, 1.2 mm |
| Mammography | 3,00,000 H..U. | 10o | 0.6 mm |
| General Radiography | 3,00,000 H..U. | 12o | 0.3, 0.6, 1.2 mm |
| General Radiography | 3,00,000 H..U. | 15o | 0.6, 0.1, 2.0 mm |
| CT Conventional | 1 MHU - 2 MHU | 12o | 0.6mm |
| CT Spiral | 3.5 - 6.3 MHU | 12o | 0.5mm X 0.7mm & 1.0 X 1.0mm |
References :- X-Ray equipment for student radiography, MO Chesney 1987 3rd edition - 209-293
- An introduction to the physics of Diagnostic Radiology Christensen, Curry, Dowdey, 2nd edition 1978, page 5-20
- Principles of Radiographic Imaging An Art and A Science, 3rd Edition by Carlton and Adler pages 116-127
- Computed Tomography Physical principle of clinical applications and quality control; Euclid seeram 2nd edition page 84-86
- X-Rays their original dosage and practical application W.E. Schall, 8th edition 1961 page 105-112
- Medical Radiation Physics : Williams R Handee 1970, Page 74-80.
