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addition, all devices have the possibility of interrupting this control for certain situations (automatic stop) before continuing to work either with the last controlled operating values (kV and mA) or in manual mode. In the so-called manual mode, tube voltage can be adjusted by the user.

The techniques involved in automatic dose control, automatic stop and manual mode are highly significant for situations where instruments or implants of low radiation transparency, e.g. metallic materials, are placed in the effective ray path. In these cases, the automatic function adjusts the voltage and current values in such a way that sufficient radiation passes through these objects, because the automatic dose control does not differentiate between body tissue and foreign bodies or other materials. As a result, the radiation passes over the parts of the body which are more transparent to radiation so that the picture on the monitor is too bright with insufficient contrast for the body regions. In these situations, use must be made of the automatic stop or manual setting just before such materials are introduced into the effective ray path. In the manual setting, the operating values for tube voltage can then be varied by the user at the controller so that the tissues or parts of the body concerned are shown with an optimum picture on the monitor.

In the case of surgical procedures where the parts of the body concerned are scanned initially for guidance without any implants or metallic instruments, the user should use the automatic stop button already during this initial step so that the operating parameters adjusted for an optimum X-ray picture can then remain unchanged for all further stages of the operation.

The following list summarises the most important radiation protection rules when using surgical image inten-sifiers:

4 reduce the radiation times as far as possible, 4 gate the effective radiation field well, 4 keep the greatest possible distance between staff and the effective radiation field and the patient's body, 4 use optimum radiation protection clothing for the users (doctors and assistants), 4 when using radiation for the head and extremities, cover the patient's trunk with radiation protection aprons,

4 position the image receiver system as close as possible to the patient's body, 4 do not start the scanning equipment until the emitter and image receiver system are correctly positioned, 4 use the interrupt switch and perhaps the possibility of intermittent scanning (pulsed scanning), 4 use the high-level mode carefully and for the shortest possible time (with an incident dose of >0.087 Gy/min at a distance of 30 cm to the image intensifier input side of the C-arm unit)

4 use the parts of the body being examined when repositioning emitter and image receiver system, not the image on the monitor, 4 use the automatic stop button or manual setting when metallic instruments or implants have to be brought into the radiation path, 4 keep records about the X-ray times, exposed parts of the body and the value of the dose surface product (or in machines without this feature, the operating parameters image intensifier input format, automatic dose control curve type or level) together with tube voltage (kV), the mAs product or current (mA), shutter times and the radiation field size and position for X-ray pictures produced in the direct method in the operating suite for documentation purposes; these records are then kept with the patient's records.

4.2 Surgical image intensifier systems

Volker Böttcher

After the discovery of the X-ray by Wilhelm Conrad Röntgen in 1895, another 50 years passed before this technique for supporting surgical procedures made an impact on the operating theatre.

During the 1950s, the development from luminescent screen to image intensifier tube and the rapid progress in camera and monitor technology made it possible to work without having to darken the room. The generator was a 1- or 2-pulse generator, the image intensifier had a lens coverage of 15 cm diameter, and the picture taken by the camera was only visible on the monitor during radiation.

The key components of surgical image intensifier (also called C-arm because of its shape) were therefore already present:

4 generator (usually single-tyke generator, X-ray tube and high-voltage generator in one housing), 4 image intensifier, 4 camera, 4 monitor.

Together with their diagnostic use, X-rays also have a harmful effect so that rules and laws for radiation protection were issued at a very early stage. The most important set of regulations on this topic is the X-ray Ordinance. The aim of this ordinance is to reduce the dose for patient and medical staff as far as possible. To this end, technical minimum requirements were stipulated for the equipment which were regularly adjusted to technical progress.

These minimum requirements (O Table 4.4) and competition between manufacturers of surgical image inten-

D Table 4.4. Minimum requirements for surgical image intensifiers (SV-RL dated 27 August 2003)

Focal spot rating

< 1.8 mm

Rating of the shortest cycle time

< 100 ms

Limit dose for direct radiography

< 5 |jGy

Limit dose for digital radiography (with 23 cm image intensifier BV)

< 2 |jGy

Limit dose for X-ray radiation (with 23 cm image intensifier)

< 0.6 |jGy/s

Limit resolution (including memory image with 23 cm image intensifier 23 cm BV)

< 1.0 Lp/mm

sifiers resulted in huge progress in technology and above all in radiation protection over the next few years.

The generator. It was developed into a high-frequency generator (almost direct current) with a clear increase in radiation hygiene.

The image intensifier. The luminous layers at input and output and the intensification were considerably improved. The diameter of the lens coverage was increased to 23 and 31 cm, removing the need for elaborate positioning.

The camera. Highly sensitive, non-ageing CCD cameras with high photosensitivity have replaced the tube camera.

Image memory. Image memories brought an essential reduction in radiation dose. Following a short radiation pulse, the picture is »frozen« on the monitor. The surgeon can now assess the picture without any time pressure.

The monitor. High-resolution, high-contrast monitors make it easier to assess the picture. Introduction of a second monitor allows for the direct comparison of two pictures.

Automatic dose control (ADR). This automatically adjusts the optimum dose for the corresponding object.

Image processing. Filter techniques have improved the image quality in spite of a lower dose. Subsequent picture processing allows for visual improvement of the picture. Cinema memories make it possible to access and repeat dynamic procedures from the image memory without having to repeat the radiation.

Documentation. Direct radiography has been extensively replaced by digital radiography and indirect systems such as video printers.

4.2.1 Expert inspection

A system of inspection and monitoring was developed for compliance with these regulations. After completion, every surgical image intensifier undergoes acceptance testing by the manufacturer. A corresponding report must be drawn up. During initial commissioning, the acceptance is checked by an independent expert. The owner of the machine must perform constancy tests at regular intervals. An independent expert checks the machine again every 5 years. This guarantees that all machines comply with the statutory regulations at all times.

4.2.2 X-ray radiation

When using X-rays on a patient, a differentiation is made between effective radiation and scattered radiation. The effective radiation passes through the patient and is absorbed to a differing extent by the body, depending on the density of the organs. The radiation leaving the patient's body thus forms a so-called radiation relief on the image receiver input screen which is used to produce the pictures.

Part of the effective radiation is scattered by the patient's body and leaves the body as lower-energy scatter radiation in all directions. The user is essentially exposed to this scattered radiation.

4.2.3 Radiation protection

The first commandment is to protect the user and the parts of the patient's body not being examined from this scattered radiation. The following rules should be observed:

4 Prevention of scattered radiation

— Keep the radiation times as short as possible. Memorising technology today makes it possible to »freeze« a top quality picture on the monitor after a short X-ray pulse.

— Use pulse techniques for procedures with movement.

— As far as possible, always work with the program with the lowest dose (half-dose program).

— Use the slot or iris diaphragm for gating because the amount of scattered radiation is directly related to the p atient volume through which radiation has passed.

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