The X-Ray Tube

The voltage to the tube is supplied by a circuit composing of mains electricity and a step up transformer. A high voltage is needed to produce the kinetic energy required of the electrons to and a relatively lower one is used for the filament cathode. This is achieved by a potential divider circuit.

Electrons are produced by thermionic emission in the cathode. This is heated by a relatively low voltage supply.

At a cathode current of 100 mA, for example, 6 x 1017 electrons will travel from the cathode to the anode of the X-ray tube every second.They are accelerated from the cathode to anode across an alternating high voltage - they will therefore only be attracted in half of the cycle. As the kinetic energy of the electrons increases, both the intensity (number of x-rays) and the energy (their ability to penetrate) of the X-rays produced are increased.

When these electrons bombard on the heavy metal atoms of the target, they interact with these atoms and transfer their kinetic energy to the target. These interactions occur within a very small depth of penetration into the target. As they occur, the electrons slow down (brake!) and finally come nearly to rest, at which time they can be conducted through the x-ray anode assembly and out into the associated electronic circuitry.

The interactions result in the conversion of kinetic energy into thermal energy and electromagnetic energy in the form of X-rays.

Most of the the kinetic energy is converted into heat. The electrons interact with the outer-shell electrons of the target atoms but do not transfer sufficient energy to these outer-shell electrons to ionize them. Rather, the outer-shell electrons are simply raised to an excited, or higher, energy level. The outer-shell electrons immediately drop back to their normal energy state with the emission of infrared radiation. The constant excitation and restabilization of outer-shell electrons is responsible for the heat generated in the anodes of X-ray tubes.

Generally, more than 99% of the kinetic energy of projectile electrons is converted to thermal energy, leaving less than 1% available for the production of X-radiation. In this sense,the X-ray machine is a very inefficient apparatus.

The production of heat in the anode increases directly with increasing tube current. Doubling the tube current doubles the quantity of heat produced.

Heat production also varies almost directly with varying the high tension voltage too.

The efficiency of X-ray production is independent of the tube current. Regardless of what mA is selected, the efficiency of X-ray production remains constant. The efficiency of X-ray production increases with increasing projectile-electron energy. At 60 keV, only 0.5% of the electron kinetic energy is converted to X-rays; at 120 MeV, it is 70%.

Target material

The anode is made to rotate at steady speed so the point of impact continually changes to prevent overheating. But it stillneeds to have:

- a high Z (proton number) so that transitions of high enough energy to emit X-ray radiation are possible

- a high melting point because so much heat energy is produced.

Tungsten is ideal (Molybdenum for softer X-rays needed for breast X-rays)

Focal Spot

The area of the anode from which X-rays are emitted is referred to as the focal spot. This must be as small as possible otherwise features in the image would be blurred instead of being sharp. The anode surface is at an angle of about 70° to the electron beam so that the X-rays effectively originate from a much smaller area than the impact area of the beam.

The Options....

Graph of Intensity against X-Ray photon energy
Clarity of image
Increasing the tube voltage Increasing the high p.d. that is used to accelerate the electrons will give the average electron more energy when it hits the target

Shape of spectrum spreads out to encompass higher energies

range is increased

Characteristics in the same place (natch!!)

area under the curve increases

Too high an energy of X-ray will penetrate too well to give good definition - if they all get through - no shadow - picture!

60-125 kV is usually employed - giving energy of about 30 keV

AC/DC voltage

(AC necessary to get higher voltages - can use transformers! DC acquired by electronic rectification and 'smoothing' circuitry)

Electrons produced by thermionic emission only accelerated across half of the time!

graphs for both are the same except the DC one is double the intensity throughout (only accelerated across to target on half of the wave).

Increasing the tube current (low voltage one!) Increases the rate of thermionic emission - more electrons hit the target - more X-rays produced.

Shape of spectrum remains the same

range is the same

Characteristics in the same place (natch!!)

area under the curve increases

Overall increase of exposure of film

but bigger dose to patient!

more heating of the target

Increasing exposure time

Overall increase of exposure of film

but bigger dose to patient!

more heating of the target

risk of blur due to movement of patient - big problem with organs that cannot be constrained.

Changing Target Material

An increase in Z (proton number) will increase the probability of electron interactions of enough energy to produce X-rays - so more X-rays will be produced.

The Characteristic peak positions will change - Ks will shift towards higher energies (these depend on the target material!).

range is the same

area under the curve increases

allows choice of X-ray energies that give best difference in attenuation for the part to viewed.

soft X-rays are needed for soft tissue - harder ones for bone.

Using a filter (material placed in the X-ray beam path) Absorbs mainly lower energy X-rays - and produces a 'harder' more penetrating beam)

area under the curve is smaller (as some of the X-rays have been absorbed).

Shape changes as mainly X-rays are reduced from the lower energy values.

range is smaller - but high energy the same.

Characteristics in the same place (natch!!)

reduces unwanted X-rays and therefore the scatter due to them - better contrast


Reducing beam size

less scatter - better contrast - especially if a collimator is used (lead grid that only allows X-rays in a particular direction to get through.

Focal spot size

Small focal spot produces sharp images

BUT also intense heating of target

Artificial Contrast Media See Barium Meal and Enema

Clearly outlines the inner surface of internal bodily organs by coating them in a radio-opaque material - barium sulphate.

Intensifying Screens Decreases the required exposure time.
  • Make image clearer with a lower X-ray dose
Detectors photographic film