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Choice of Phantom Material and Test Protocols to Determine Radiation Exposure Rates for Fluoroscopy¹

¹ From the Department of Radiology, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Blvd, Dallas, TX 75235-9071. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received April 27, 1999; revision requested July 13 and received August 17; accepted August 23. Address correspondence to J.A.A. (e-mail: jon.anderson@email.swmed.edu).

View larger version (33K): [in a new window]   Figure 1.   Graph depicts the function that converts incident photon flux to exposure. Exposure is shown in terms of the number of roentgens that will be indicated when an ideal ion chamber is illuminated with an x-ray flux of one photon per square centimeter in the energy range of 20-100 keV.

View larger version (29K): [in a new window]   Figure 2.   Energy response function for the input faceplate of an image intensifier tube. The amount of light generated by the image intensifier at the output faceplate is assumed to be proportional to the energy deposited in the input faceplate. The image intensifier is most sensitive to x rays with energies greater than 30 keV, but the ion chamber is more sensitive to x rays with lower energies.

View larger version (33K): [in a new window]   Figure 3.   X-ray tube current versus potential for two C-arm fluoroscopes from different vendors. These curves define the algorithm under which the system operates to maintain adequate exposure at the input face of the image intensifier as attenuation in the patient increases. System CB is in a cardiac catheterization laboratory. System TB is in an interventional neuroradiology room.

View larger version (33K): [in a new window]   Figure 4.   Schematic illustrates measurement geometries for determining the ESER and IIEER. Left: In the standard configuration for determining ESER, an ion chamber is placed 30 cm in front of the grid faceplate. The phantom is located between the ion chamber and the grid. For the measurements of IIEER in this study, the ion chamber was placed either behind the grid (position i) or in front of the grid (position ii). Right: The phantom was placed on the collimator exit window to determine IIEER.

View larger version (34K): [in a new window]   Figure 5.   Examples of the energy spectra of x-ray beams as determined with the model of Boone and Siebert (5). The tube was assumed to have adequate filtration to produce a beam with a half-value layer of 3.4 mm of aluminum when the tube was operated at 80 kVp. The generator was assumed to have no ripple.

View larger version (37K): [in a new window]   Figure 6.   Beam hardening of an 80-kVp x-ray beam filtered by phantoms that produce the same exposure attenuation as 20 cm of water. Water and Plexiglas (acrylic) produce similar beams, but aluminum and copper shift the average energy of the beam upward. This hardening effect causes the AERC in a fluoroscope to track differently with phantom thickness than with exposure, as measured with an ion chamber.

View larger version (37K): [in a new window]   Figure 7.   Exposure equivalency of aluminum and water. The thickness of aluminum required to produce the same exposure attenuation as a given thickness of water is shown for the x-ray beams illustrated in Figure 5.

View larger version (35K): [in a new window]   Figure 8.   Exposure equivalency of copper and water. The thickness of copper required to produce the same exposure attenuation as a given thickness of water is shown for the x-ray beams illustrated in Figure 5.

View larger version (31K): [in a new window]   Figure 9.   Exposure equivalency of acrylic and water. The thickness of acrylic plastic required to produce the same exposure attenuation as a given thickness of water is shown for the x-ray beams illustrated in Figure 5.

View larger version (35K): [in a new window]   Figure 10.   Variation of the IIEER (II Response) as a function of equivalent phantom thickness for several materials. The AERC was assumed to work perfectly to maintain the light output of the image intensifier. For this family of curves, the IIEER was normalized to a value of 1.0 for no phantom in the beam, and the x-ray tube potential was assumed to be fixed at 80 kVp.

View larger version (31K): [in a new window]   Figure 11.   Variation in the IIEER (II Response) as a function of x-ray tube potential for several phantom materials. The phantom was assumed to be equivalent to 20 cm of water for an 80-kVp x-ray beam. These curves are normalized to the same point as are the data in Figure 10. The two sets of curves can be considered orthogonal cuts—intersecting at 80 kVp and 20 cm of water equivalent—through the set of three-dimensional surfaces describing the variation in the IIEER with tube potential and equivalent phantom thickness for the four phantom materials.

View larger version (36K): [in a new window]   Figure 12a.   Measured IIEER for system CB (a) and system TB (b). Both machines have an IIEER of about 3.5 µR (9.03 x 10-10 C/kg) per frame when measured with thick aluminum or copper phantoms. (a) For phantoms equivalent to 15 cm of water, the IIEER values measured with copper or aluminum are about 30% smaller than those measured with acrylic. The falloff of IIEER as the phantom thickness is increased from zero to about 10 cm is a phenomenon in agreement with the model predictions of Figure 10. (b) This system shows the same general characteristics, but for very thin phantoms it does not reduce the tube output properly and produces bright, washed out images. Al = aluminum, Cu = copper, Plexi = Plexiglas.

View larger version (38K): [in a new window]   Figure 12b.   Measured IIEER for system CB (a) and system TB (b). Both machines have an IIEER of about 3.5 µR (9.03 x 10-10 C/kg) per frame when measured with thick aluminum or copper phantoms. (a) For phantoms equivalent to 15 cm of water, the IIEER values measured with copper or aluminum are about 30% smaller than those measured with acrylic. The falloff of IIEER as the phantom thickness is increased from zero to about 10 cm is a phenomenon in agreement with the model predictions of Figure 10. (b) This system shows the same general characteristics, but for very thin phantoms it does not reduce the tube output properly and produces bright, washed out images. Al = aluminum, Cu = copper, Plexi = Plexiglas.

View larger version (35K): [in a new window]   Figure 13a.   Ratios of IIEER measurements made in front of the grid (pregrid) to those made behind the grid (postgrid) for system CB (a) and system TB (b). The ratios obtained for thicker aluminum and copper phantoms correspond to that expected on the basis of grid transmissions of about 60%. (a) Data were obtained with the phantom entrance face about 20 cm (28 cm for the acrylic phantom) from the grid. (b) Data were obtained with the phantoms placed on the collimator. The dotted line shows the effect of moving the acrylic phantom closer to the grid. This demonstrates that the increase in IIEER with thickness for the acrylic phantom is due to scatter entering the ion chamber.

View larger version (42K): [in a new window]   Figure 13b.   Ratios of IIEER measurements made in front of the grid (pregrid) to those made behind the grid (postgrid) for system CB (a) and system TB (b). The ratios obtained for thicker aluminum and copper phantoms correspond to that expected on the basis of grid transmissions of about 60%. (a) Data were obtained with the phantom entrance face about 20 cm (28 cm for the acrylic phantom) from the grid. (b) Data were obtained with the phantoms placed on the collimator. The dotted line shows the effect of moving the acrylic phantom closer to the grid. This demonstrates that the increase in IIEER with thickness for the acrylic phantom is due to scatter entering the ion chamber.

View larger version (29K): [in a new window]   Figure 14a.   Effects of the choice of phantom material on the ESER. Results are expressed in roentgens per minute and correspond to measurement 30 cm in front of the entrance face of the image intensifier assembly. (a) Theoretic results are for a fluoroscope with perfect AERC operating at a constant 80-kVp potential. The curves have been normalized to the experimental data by matching calculated and measured values for the copper phantom with an equivalent thickness of 16 cm of water. The acrylic phantom produces ESER values about 20% greater than those for the metal phantoms at this thickness. (b) ESERs for system CB. Use of the plastic phantom increased the ESER values by about 40%.

View larger version (30K): [in a new window]   Figure 14b.   Effects of the choice of phantom material on the ESER. Results are expressed in roentgens per minute and correspond to measurement 30 cm in front of the entrance face of the image intensifier assembly. (a) Theoretic results are for a fluoroscope with perfect AERC operating at a constant 80-kVp potential. The curves have been normalized to the experimental data by matching calculated and measured values for the copper phantom with an equivalent thickness of 16 cm of water. The acrylic phantom produces ESER values about 20% greater than those for the metal phantoms at this thickness. (b) ESERs for system CB. Use of the plastic phantom increased the ESER values by about 40%.