Technical Note: Characterization of x‐ray beam profiles for a fluoroscopic system incorporating copper filtration

Abstract

Purpose The goal of this study was to investigate x‐ray beam profiles at various water depths to characterize the two‐dimensional x‐ray dose distribution, allowing for off‐axis and out‐of‐field radiation dose estimation for a wide range of x‐ray beam spectra commonly encountered in fluoroscopically guided interventional procedures. Methods A Siemens Artis interventional fluoroscope was operated in a service mode to generate a continuous x‐ray beam at fixed x‐ray beam spectra, defined by their kVp and the thickness of additional copper filtration. A PTW scanning water tank with a diode detector was used to measure the x‐ray beam profiles at several depths in water at various fields of view and x‐ray beam spectra, both parallel and perpendicular to the anode‐cathode axis direction. Results X‐ray beam profiles, including out‐of‐field tails, were characterized for a wide range of beam qualities. The anode heel effect was pronounced even at depth, resulting in large dose variations across the x‐ray field; this effect was even more definite at large fields of view, at higher kVps, and in the absence of additional copper filtration. Conclusions This study investigated and characterized 2D radiation dose deposition in water from x‐ray beam spectra commonly used by modern fluoroscopes in interventional procedures. This knowledge can be applied to manual dosimetry calculations or can be used to refine the accuracy of automated dose mapping tools or Monte Carlo simulations of the radiation dose to soft tissue within the x‐ray field and to tissue adjacent to the primary beam. Additionally, this study illustrates a substantial reduction of the anode heel effect by using moderate amounts of additional copper filtration to harden the x‐ray beam spectrum.

The American College of Radiology Fluoroscopy Dose Index Registry Pilot: Technical Considerations and Dosimetric Performance of the Interventional Fluoroscopes

“…This additional uncertainty was estimated in a previous study (7) to be 5% for beam spectra commonly encountered during adult fluoroscopic imaging, but this uncertainty may vary by as much as 12% for beam spectra commonly encountered during pediatric imaging when using a correction factor derived from the American Association of Physicists in Medicine Task Group 190-recommended beam spectra (90-100 kV without a prescribed amount of additional filtration). Second, there are substantial variations in X-ray beam quality and intensity across the X-ray field because of the anode heel effect; with dependencies on X-ray beam quality and field size, variations at or near the skin entrance can exceed ±20% from central axis measurements (15,16). Application of these uncertainties and specific approaches to estimating PSD are beyond the scope of this work; however, valuable discussions regarding approaches to PSD calculations have been published previously (12,13).…”

Estimation of patient skin dose in fluoroscopy: summary of a joint report by AAPM TG357 and EFOMP

“…Furthermore, the same work reported that 20% of the primary beam intensity was found just outside the edge of the beam, with a decline to 3% at a position 6 cm from the edge. X‐ray beam intensity variations, due to the heel effect, field size, tube voltage and spectral filtration, are important for backscatter corrections in PSD estimation in FGI procedures with overlapping fields 41,42 …”

“…X-ray beam intensity variations, due to the heel effect, field size, tube voltage and spectral filtration, are important for backscatter corrections in PSD estimation in FGI procedures with overlapping fields. 41,42 To illustrate the variability and dependencies of backscatter factors, Fig. 6 shows the ratio of exposure from a 6 cm 2 × 6 cm 2 X-ray beam measured with an ionization chamber on the entrance surface of a number of different phantoms, compared to that of a 30 cm 3 × 30 cm 3 × 20 cm 3 block of solid water.…”

“…The uncertainty imposed by the heel effect, depending on the X-ray beam quality and area striking the patient, may be up to 30%-40%. 41,42…”

“…There is a knowledge gap identified by AAPM Task Group 357 and EFOMP in the literature on the magnitude of many of these uncertainties. More work should be done in general to investigate uncertainties in applied radiation dosimetry and further expand the possibilities of corrections to improve accuracy, for example, as in recent studies on collimator scatter and X‐ray beam inhomogenities 42,75 . Furthermore, it is recommended that patient skin dose estimates should be reported together with the associated uncertainty, for example, a 95% confidence interval.…”