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IAEA

Good practices in fluoroscopy

» Does the kV value that I select for fluoroscopy have an effect on the absorbed dose to tissues in the patient?

Yes, in general, increasing the kV reduces the exposure of the patient and especially to the skin exposed by the beam. This is because the higher kV produces radiation beams with increased penetration through the patient’s body and less radiation is required at the entrance surface (*) to produce the necessary exposure to the image receptor. The other factor that must be considered in selecting the appropriate kV value is the effect on image contrast. In general, lower kV values produce increased image contrast. This can be especially significant in fluoroscopy when using iodine contrast media.

(*) The number of photons and the total energy carried by these photons per unit area. For a more rigorous definition and detailed discussion (energy fluence) see Conversion Coefficients for use in Radiological Protection against External Radiation and Dosimetry in Diagnostic Radiology: An International Code of Practice.

» Does using the automatic brightness control (ABC) ensure that I am delivering the lowest exposure to my patients?

Not in all cases.

While the ABC is useful for adjusting the exposure to produce a good image for different patient thicknesses and densities, the actual exposure depends on the exposure level that has been set by the manufacturer or the engineer who maintains the equipment. The optimum setting of the ABC exposure level is one that delivers only the exposure to the image receptor that is required to produce the necessary image quality in terms of the visual noise.

» Does changing the field of view, or magnification mode, have an effect on the exposure to the patient?

Yes with regard to the absorbed dose and no with regards to the energy imparted.

Changing from a large field of view to an increased magnification increases the exposure required by the image intensifier tube. Therefore, the absorbed dose to tissues within the beam is also increased. Decreasing the field of view by a factor of two increases the dose rate by a factor of four. Attention has to be paid to magnification.

Example:

Field of view, diameter 25 cm Dose rate= 0.3 mGy/s
Field of view, diameter 17 cm Dose rate = 0.6 mGy/s
Field or view, diameter 12 cm Dose rate = 1.23 mGy/s.

However, since the X ray beam is covering a smaller area, the total energy imparted is about the same as with the larger field of view that produces a lower dose rate.

» Does moving the X ray beam to different areas of the patient’s body during a procedure have an effect on the exposure to the patient?

Yes.

Since the absorbed dose to a specific tissue is influenced by the number of photons impinging on the same area of the skin, moving the beam spreads the radiation over more of the patient’s body and reduces the absorbed dose to any one area of the skin.

The highest absorbed dose to a specific tissue occurs when the X ray beam is not moved but remains at the same location on the patient’s body during a procedure.

Special attention should be paid to avoiding overlapping image areas combined with projections through the body at relatively low angles of the X ray beam (obliquity, e.g. cranio caudal or caudio cranial projections).

In summary, moving the beam can help avoid radiation injuries to the skin. The PKA, and the total energy imparted, is not changed by moving the beam during a procedure. The total energy imparted to a given region of the body is related to the probability of radiation induced cancer.