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Table 4.3. Measurable effects of the interaction between X-ray radiation and material

Effects

X-ray radiation capability

Table 4.3. Measurable effects of the interaction between X-ray radiation and material

Effects

X-ray radiation capability

Attenuation effect

to penetrate material while being attenuated

Luminescence effect

to make certain substances luminant

Ionisation effect

to ionise certain substances, i.e. make them electrically conductive

Photographic effect

to make photographic films black

Semiconductor effect

to change the electrical conductivity of semiconductors

Biological effect

to cause changes to the tissue of living creatures

the heated cathode (electron current or also called tube current defined in the physical unit amperes [A] or milliamperes [mA]) within the X-ray tube to such an extent that this is stopped at high speed at the atomic shell of the anode material and generates X-rays in this process (O Fig. 4.10).

The higher the electric voltage - between 25 kV and 150 kV (kV, kilovolt; 1 kV, 1000 V) depending on the application - the higher the speed of the electrons on their way to the anode and the higher the energy in the resulting X-rays. When the tube high voltage is switched off, the electron bombardment of the anode is interrupted so that no more radiation can be generated.

The efficiency of this conversion of electrical energy into radiation energy amounts to about 1%, i.e. about 99% of the electrical energy is converted into heat energy. This efficiency balance illustrates the huge thermal load on X-ray tubes and their anodes. The heat problems have also resulted in the use of X-ray tubes with rotating anodes for X-ray emitters in surgical image intensifiers.

During the interaction between X-ray and material, the effects featured in O Table 4.3 can be observed and measured in technical terms.

Technically constrained bundling of the electrons on their way to the anode means that X-rays are generated in a punctiform part of the anode, the so-called focal spot, from where they spread as a divergent radiation field strictly limited by the emitter diaphragm, so that the decrease in radiation intensity depends on the distance from the focal spot in accordance with the distance square law (O Fig. 4.8): »The intensity of the radiation from a divergent radiation field of punctiform origin decreases with the square of increasing distance from its point of origin.« This law is also important for practical radiation protection in terms of image generation.

Tube protective housing with X-ray tube. For several reasons, the X-ray tube is installed in a tube protection housing (O Fig. 4.11) which offers protection from tube high voltage (contact protection and insulation protection), thermal loads (heat dissipation by laying the tubes for example in insulating oil), emitted X-ray radiation outside the effective radiation bundle and mechanical loads.

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Fig. 4.11. Tube protection housing and X-ray tube

D Fig. 4.12. Diagram to show image receiver system in scanning units

D Fig. 4.12. Diagram to show image receiver system in scanning units

In addition, the tube protective housing also offers structural possibilities for fitting diaphragms to limit or variably restrict the effective radiation field.

4.1.4 The image receiver system for surgical image intensifiers

The image receiver system for surgical image intensifiers consists of the X-ray image intensifier(RBV),a downstream TV camera and a monitor (D Fig. 4.12).

This system is also referred to as image intensifier/TV chain. Together with the TV camera and a monitor, other recording and image storing cameras are also used, such as a film camera (e.g. 35 mm camera for »cinema films«, 100 mm camera for single pictures of the image intensifier output screen), storing on magnetic tape, digital image storing and image processing together with video imager technology, i.e. photographing the stored monitor picture from a special monitor. The rapid development and production of high-performance electronic storage media has resulted in digital image generation, image storage and image reworking possibilities substituting the conventional imaging procedures. This development is also considered in the amended X-ray Ordinance from 2002 in § 28, which states that digitally documented records and X-ray pictures must be made available in a suitable form to a doctor sharing in the treatment or responsible for follow-up treatment, together with the Medical Departments. The records and X-ray pictures must coincide in terms of images and contents with the original data records and be suitable for evaluation of the findings. When data are transmitted by electronic means, it must be certain that no information will be lost during remote data transfer.

4.1.5 The main components in surgical image intensifiers

The main components in surgical image intensifiers are:

4 the radiation emitting system with its components high voltage generator and controller, together with tube protection housing with X-ray tube and radiation apertures,

4 the image receiver system or depiction system with the X-ray image intensifier (RBV), TV camera, monitor and the downstream digital image storage devices.

4.1.6 Technical minimum requirements for examinations with surgical image intensifiers

According to the above mentioned »Guideline for the technical testing of X-ray equipment and interference emitters subject to permission« (expert test guideline -SV-RL) [5], certain technical minimum requirements have to be fulfilled for examinations with image intensifiers (O Table 4.4).

The technical minimum requirements for the application must be used as the basic appraisal standards for tests during initial commissioning (§ 3 and § 4 of the X-ray Ordinance) and for repeat tests according to § 187 of the X-ray Ordinance.

4.1.7 Application-related radiation protection in the operating suite

The effective radiation hits the patient's body and penetrates it in part, so that it then arrives as a so-called radiation relief at the input of the image receiver system where it is used for imaging. Another part of the effective radiation is absorbed by the patient's body and therefore cannot contribute to imaging.

Yet another part of the effective radiation is scattered in the patient's body (»Compton scattering«) and leaves the patient's body again on all sides as so-called scattered lower energy radiation. Radiation protection for the user refers essentially to protection from this scattered radiation. The share of scattered radiation is far lower for radio

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