Even though careful considerations on the functioning of the equipment will give the possibility to perform procedures with low patient doses, the main factor deciding the patient dose is the methodology used by the operator. Important factors in this respect are the fluoroscopy time, restriction of the radiation field and positioning of the patient.
Use of last-image-hold (LIH), which enables the last live image to be displayed continuously when the radiation is terminated, could reduce the fluoroscopy time to half compared to when it is not used. It enables the operator to examine the image as long as necessary without the use of radiation. Many types of equipment also have the possibility to see, on the LIH image, the effect of adjustment of the collimators on the image field. This further decreases the beam-on time. It has been shown that equally large dose savings can be obtained if appropriate restriction of the radiation field is employed [3
]. Reduction of a circular radiation field size from 20 cm to 16 cm will reduce the amount of radiation to the patient by about 40 percent.
Images needed for documentation should preferably be exposed using the fluoroscopy system since the absorbed dose needed is less compared to radiography.
For procedures employing cine runs it is equally important to limit the number of frames to what is essential for the examination. Short cine loops viewed repeatedly usually provide adequate information [3
]. It is not uncommon for the length of the cine runs to increase when shifting from film to digital techniques [15
]. This is probably because long runs no longer present a handling problem for the personnel.
There is a possibility to increase detail resolution by using magnification mode of the image intensifier. This will however decrease the brightness gain of the intensifier and the generator will compensate for this by increasing the exposure by the square of the magnification factor. Magnification mode should therefore not be used unless it is necessary to perform the procedure.
Positioning of the patient with respect to the X-ray tube and the detector is very important not only for the possibility of visualising the anatomy but also for the image quality and to restrict the radiation dose to the patient. Tube angulations influence the exposure significantly due to the large effect on the projected path through the patient. Orientations giving rise to high dose rates should not be used more than absolutely necessary [3
]. Integral to good practice is to position the patient as close as possible to the detector. ESD increases dramatically as the patient is moved towards the X-ray tube. If combined with thick patients, a short distance between X-ray tube and the patient will lead to very high dose rates (). This could lead to infliction of radiation injury to the patient even with modest fluoroscopy time. In extensive interventional procedures it is advisable to reposition the equipment with respect to the patient during some occasions to avoid irradiation of the same part of the skin.
Figure 3 Entrance skin dose level dependent on patient thickness, tube voltage and focus-skin distance. For tube voltage 70 kV and focus-skin distance 70 cm (♦), 100kV, 70cm (■), 70 kV,40cm (▲). This figure gives an example. The actual (more ...)
Transmission of the radiation through the patient can be increased if additional filtration of the beam is used. This has been applied for several types of examination [14
] and substantial dose reductions (about 50%) have been reported. Usually, about 0.2 mm Cu is added to the original filtration of the radiation beam. Another way of reducing the patient dose is to remove the antiscatter grid. The grid not only removes scattered radiation but also a part of the primary radiation. The dose rate has to be increased by approximately a factor 2 when the grid is used. In small-sized patients such as small children, the amount of scattered radiation is also small and no grid is needed. It is therefore important that the grid is easily removable in equipments used for paediatric examinations. For medium-sized objects, an air gap could be used for scatter rejection instead of a grid. The reduction of radiation dose is also large for air gap technique but care has to be taken to avoid small distance between patient and X-ray tube [18
]. For large-sized patients a grid is necessary to avoid deterioration of image contrast.