02 X-ray Tube

Rotating Anode Tube

Rotating anode tube is similar in many respects to the stationary anode X-ray tube, plus a number of additional features (Figure).

Figure: Rotating anode tube and casing

Rotating anode X-ray tube produce higher intensities of X-ray beams than the stationary anode tube. This is due to 2 factors: The heat deposited on the anode is spread over a much larger area. The cooling characteristics of the rotating anode are superior.

Figure: Rotating anode tube

Construction of the rotating anode X-ray tube Construction of the rotating anode X-ray tube. Cathode has the same basic construction except that it is off-set from the central axis of the tube in order that electrons emitted from it strike the bevelled surface of the anode.

Anode Assembly Anode is made of a tungsten or molybdenum disk with an accurately bevelled edge.

The anode has a central hole through which it is connected to a beryllium anode stem and hence to the rotor of the induction motor (Figure).

The diagrammatic representation of the anode and rotor assembly in figure below illustrates the arrangement. Figure: X-ray tube insert and stator

Insert tiub sinar-x dan stator

The stator is composed of 2 windings set into a circular iron core (figures below).

Figure: Stator core and windings

Figure: Photograph of stator core

Rotor, stem and anode disk are accurately balanced so that no wobbling occurs when the whole assembly rotates.

In the more modern rotating anode tubes, the anode is made of a solid block of molybdenum with a thin coating of an alloy of tungsten and rhenium on the surface.

Molybdenum has double the specific heat capacity of tungsten and so produces an anode of much higher heat storage capacity in terms of Heat units.

Rhenium has an atomic number of 75 (tungsten, 74) and so has a good conversion of electron energy to X-rays. It also has the advantage of slowing down the ageing process on the anode surface caused by the inevitable pitting of the surface during exposures.

Induction Motor Induction motor is used to drive the rotor. The motor work on the principles of electromagnetic induction, particularly Lenz’s law. The principle of this type of motor is depicted in Figure below.

Figure below shows more efficient system, where more magnets are used and a higher flux linkage is obtained. The copper drum (the rotor) follows the direction of rotation of the magnets.

The magnetic field from the solenoids penetrates the glass envelope of the X-ray tube and interacts with the copper rotor on the anode assembly (Figures below).

Electron Focusing During conduction, the anode of the X-ray tube is positively charged and the cathode is negatively charged (Figure).

The electron space charge emitted by the heated filament is thus repelled from the cathode and attracted towards the anode, since electrons are themselves negatively charged.

In the case of a rotating anode tube, the heat caused by conversion of their energy is spread over a much larger area, the disc’s focal track. For a stationary anode, electrons bombard a small, rectangular area.

Figure: A conventional rotating anode disc. (a) Face view; (b) In profile.

If both the anode and cathode were flat plates, the electric field consists of parallel lines starting from the anode and finishing on the cathode (Figure). Figure: Focusing of the electrons in an X-ray tubes. The concave focusing cup directs the electrons from thermionic emitter F toward the central axis, so that they strike the anode over a small area.

The plume of electrons from the thermionic emitter F, being charged negatively, travel against the direction of the electric field, striking the anode over a width, W (W>F). The situation  produces an unacceptably large focal area on the anode.

In the case of focussing-cup cathode, the thermionic electrons from F now experience a force which is always towards the central axis (as well as towards the anode) owing to the shape of the electric lines of force produced by the focusing cup (Figure).

The plume of electrons is  spread over a much smaller width, w, on the anode (w<F) and the electron beam is said to have been focused. A line-focus is produced. Figure: The focusing cup directs the electrons from thermionic emitter F toward the central axis, so that they strike the anode over a small area.

Anode Heel Effect Figure below shows crazing or roughening of the target surface. Such crazing results in a loss of radiation output from the tube. Photons of radiation being absorbed to a greater extent as they must now penetrate a greater thickness of metal.

Roughening of the target surface, due to cumulative effects of heat stress, creates pitting, the formation of small crevices into which electrons from the filament may enter.

X-ray photons so produced have a greater thickness of anode substance to penetrate before they emerge from its surface. This reduces the beam intensity, compared with the output from a new, smooth target surface (Figure).

Figure: Effects of target surface pitting

Figure (a) Effect of anode crazing. (b) Photographs of crazed anodes.

Care of the X-ray tube An X-ray tube is an expensive and vulnerable piece of equipment. If damaged, it is unlikely to be repairable.

Tube damage may be either physical or thermal Tube damage may be either physical or thermal. Physical damage is avoided simply by ensuring that the tube is never allowed to collide with any other object, and that all movement is reasonably smooth and steady. Thermal damage is minimized by:

Keeping the rate of heat production within the safe limits, determined by the tube manufacturer. Ensuring, before making each exposure, that the anode can safely accept the extra heat which the exposure will bring.

X-Ray Tube Rating

X-ray Tube Rating Definition The rating of an X-ray unit is that combination of exposure settings which the unit can just withstand without incurring unacceptable damage.

Any exposure gives rise to some ‘damage’ to the X-ray tube, since the anode becomes slightly more pitted and the filament becomes slightly thinner. However, in this context unacceptable damage means that amount of damage which will seriously impair the performance of the unit for further exposures or even to make the unit completely inoperative.

Single Exposure Factors The exposure factors which are under the control of the operator are: Selectable Factors Non-selectable factors

Selectable Factors kVp, mA, exposure time/mAs, focal spot size, single / multiple exposures, radiography / screening.

Non-selectable factors Stationary Anode Tube Rectification, thermal capacity of anode/shield, efficiency of heat loss from anode/shield, anode angle, filtration, rating of high-tension cables and transformers. Rotating Anode Tube As above, plus: anode diameter, anode rotation speed.

Rating Chart Stationary Anode Tubes Rating chart shows the effect of varying the quantities in the selectable factors list, i.e. those which may be altered.

For any particular X-ray unit, the quantities in the non-selectable factors list is unalterable, and a rating chart is used which is applicable only to that unit.