A MODIFIED X-RAY IMAGE INTENSIFIER WITH CONTINUOUSLY VARIABLE FIELD-OF-VIEW - RESOLUTION CONSIDERATIONS

A conventional x-ray image intensifier (XRII) has been modified to ena ble the field of view (FOV) to be varied continuously, by adjusting th e potentials at the focusing electrodes. The benefit, to system resolu tion, from decreasing the FOV has been characterized by measuring the modulation transfer function (MTF) of the XRII coupled to a high-resol ution photo-diode array (PDA), at a number of different FOVs achieved either by electronic or optical zooming. Electronic zooming of the XRI I from FOV = 24 cm to FOV = 10 cm led to an increase in f0.1 (the freq uency at which MTF = 0.1) from 1.41 to 3.05 mm-1, while optical zoomin g increased f0.1 from 1.41 mm-1 only to 1.88 mm-1. It is proposed that the advantage, with respect to resolution gain, of electronic zooming over optical zooming was realized only when the XRII limits system re solution. The MTF of the XRII coupled to a video camera, with lower re solving power than the PDA, was measured at different FOVs to show tha t using electronic zooming is only marginally beneficial when the opti cal detector and the XRII contribute equally to the resolution degrada tion. However, when a higher-resolution optical detector is used, elec tronic zooming always yields a greater gain in resolution.