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A computer model of an image intensifier system working under automatic brightness control

Department of Medical Physics, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK

View larger version (16K): [in a new window]   Figure 1. Geometry used for a simple example of the functionality of the model. The entrance surface dose rate is calculated for different depths of water.

View larger version (38K): [in a new window]   Figure 2. Flowchart outlining the procedure the model would follow to calculate dose rates in an ion chamber of geometry shown in Figure 1. ABC, automatic brightness control.

View larger version (10K): [in a new window]   Figure 3. Comparison between the mass-energy absorption coefficients of air and caesium iodide (CsI). It can be seen that the K absorption properties of CsI (—) are observed between 33 keV and 36 keV whilst air (– – –) has no significant K-edge. (Data from [14].)

View larger version (15K): [in a new window]   Figure 4. Siremobil 2000 automatic brightness control curves determined experimentally. , 5 mA anti-isowatt; , 3 mA anti-isowatt; , 3 mA high contrast; +, 5 mA high contrast; x , 3 mA low dose.

View larger version (18K): [in a new window]   Figure 5. Illustration of the binary chop algorithm finding the best operating point for an arbitrary assortment of attenuators in the beam. The x-axis represents successive points along the entire automatic brightness control (ABC) curve data set whilst the y-axis shows the image intensifier air kerma rate for the tube potential/tube current combination of the ith point on the curve. Each arrow shows the operating point chosen by successive iterations of the algorithm and the spot represents the tube potential/tube current point that results in the air kerma rate being closest to the desired value.

View larger version (16K): [in a new window]   Figure 6. Geometry used in MCNP calculation of backscatter factors.

View larger version (11K): [in a new window]   Figure 7. Backscatter factors calculated by MCNP for water () and PMMA () phantoms using a field of diameter 20 cm, an accelerating potential of 110 kVp and the geometry in Figure 6.

View larger version (14K): [in a new window]   Figure 8. Results of validation experiments, using (a) PMMA and (b) water phantoms. Points indicate experimental measurements and lines represent predictions of model.

View larger version (12K): [in a new window]   Figure 9. Calculated air kerma rate at surface of image intensifier for different thicknesses of PMMA (....) and water (—) phantoms. The 5 mA anti-isowatt curve was used in the automatic brightness control algorithm with the target intensifier dose rate set nominally at 0.13 µGy s-1.

View larger version (10K): [in a new window]   Figure 10. Calculated absorbed dose rate in the caesium iodide (CsI) input phosphor for different thicknesses of PMMA phantom. The automatic brightness control algorithm was set to provide the input phosphor with an air kerma rate of 0.13 µGy s-1 using the 5 mA anti-isowatt curve.

View larger version (16K): [in a new window]   Figure 11. Comparison between the operating points chosen to maintain a constant air kerma rate (0.13 µGy s-1) or caesium iodide (CsI) dose rate (17 µGy s-1) at the surface of the image intensifier for different thicknesses of PMMA phantom. ---, model based on air kerma rates; —, model governed by CsI dose rates. (Each operating point is a unique combination of tube potential and tube current, and automatic brightness control (ABC) curves are designed so that the dose rate at the intensifier, in the absence of attenuating materials, monotonically increases from one operating point to the next. The points are numbered incrementally with increasing dose.) ABC curves: (a) 5 mA anti-isowatt; (b) 3 mA anti-isowatt; (c) 5 mA high contrast; (d) 3 mA high contrast; (e) low dose/paediatric.