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Four inverted gray scale EPID images of the gantry‐angle phantom acquired at gantry angles 170°, 150°, 130°, and 90° (in the International Electrotechnical Commission (IEC) standard). These images illustrate how the intersection points uniquely depend on the gantry angle. The axial (vertical line) and helical (casting a sinusoidal shadow when imaged) tungsten wires are observed.
