□ Fig. 4.13 a, b. Leakage radiation in surgical image intensifiers with small and larger scanned patient volume, shown using isodoses, i.e. curves with the same dose

graphy of smaller patient volumes than in the case of larger volumes (□ Fig. 4.13).

Together with scattered radiation from the patient volume, notable radiation exposure can also be caused by leakage radiation through the housing of the X-ray emitter, if the medical activities which the users have to perform entails them staying close to the X-ray emitter (e.g. at a distance of less than 20 cm).

When using surgical image intensifiers in the operating theatre, it is apparently not possible to rule out the risk of the hands and lower arms of the users being in the effective radiation bundle or in the area of intensive interference radiation during their surgical activities, at least occasionally and usually only for a short period. To protect the hands and lower arms during such activities, DIN EN 61331-3 [11] recommends radiation protection gloves (five finger gloves) and surgical radiation protection gloves (mittens with open hand surface) with an attenuation equivalent of at least 0.25 mm Pb.

An important rule for practical radiation protection for patient and user can be derived from all this:

Gate the effective radiation field well!

A well gated effective radiation field improves not only radiation protection for patient and user but also the »physical image quality« of the monitor image and the image documentation systems downstream from the imaging process.

Depending on the situation, the effective radiation field can be gated using the iris diaphragm or the parallel diaphragm from the control desk. The diaphragms then appear in the monitor picture and stay in position even after the scan has been briefly interrupted so that repeated gating will not be necessary. In some new machines, the position of the effective radiation diaphragms is also illustrated for the user on the monitor when there is no radiation exposure, so that the effective radiation diaphragms can be brought into a suitable, favourable position for the application, already during the preparatory phase. Many new machines offer enlargement possibilities for special situations (sector enlargement or zoom formats) for the specific part of the body by changing over to a smaller image intensifier input format. The effective radiation diaphragms on the X-ray emitter then automatically adjust to a smaller image intensifier input format when the system changes over.

Another important aspect of radiation protection,which unfortunately is still not given sufficient attention, is:

Keeping radiation times as short as possible!

All newer machines today offer technical support to help implement this radiation protection aspect in the form of an interrupt setting, with compulsory interruption of scanning after a certain interval, which can be preset in some cases. The user then has to trigger a further scanning interval again.

A similar possibility for reducing the radiation times consists of pulsed, intermittent scanning when the ON-switch is held down. In this case, the X-rays are emitted in adjustable time intervals (pulse frequency) for an adjustable period of time, with considerable reductions in radiation exposure for patient and user.

All new surgical image intensifies are equipped with digital image memories as another contribution to reducing scanning times. After only a short scanning time, the user has a saved monitor image which allows him to evaluate the current situation. For documentation purposes, as far as possible indirect scanning (video imager technology or multi-format camera) should be used, or digital image documentation. Interim results can be produced with video printers, but this is no substitute for documentation on an X-ray picture generated by direct and indirect scanning.

4.1.8 Correct positioning of the image receiver system

Another radiation protection measure resulting from equipment handling consists of positioning the image receiver part of the surgical image intensifier as close as

□ Fig. 4.14. Influence of the focus/object distance on the incident dose and object enlargement possible to the patient's body. This not only improves the physical image quality but also considerably reduces radiation exposure for the patient. Misalignment of the emitter/image receiver system becomes apparent to the user when during the scan, the examined part of the body or organs are extremely enlarged on the monitor.

□ Figure 4.14 shows the normal patient incident dose (main dose) for various focus/object distances when using surgical image intensifiers. For very small focus/object distances, the patient incident dose increases out of all proportion. The figure also shows the object enlargements resulting from misalignment for the listed focus/object distances.

4.1.9 Correct use of the automatic dose control (ADR)

According to the X-ray Ordinance, all scanning equipment for examining the human body and therefore also all surgical image intensifiers must be equipped with automatic dose control (ADR) or at least an equivalent device and a device for electronic image intensifying with TV chain. In

Patient incident dose in pGy/s

Was this article helpful?

0 0

Post a comment