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 enable the field of view (FOV) to be varied continuously, by adjusting the potentials at the focusing electrodes. The benefit, to system resolution, from decreasing the FOV has been characterized by measuring the modulation transfer function (MTF) of the XRII coupled to a high-resolution photo-diode array (PDA), at a number of different FOVs achieved either by electronic or optical zooming. Electronic zooming of the XRII from FOV = 24 cm to FOV = 10 cm led to an increase in f0.1 (the frequency at which MTF = 0.1) from 1.41 to 3.05 mm-1, while optical zooming 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 resolution. The MTF of the XRII coupled to a video camera, with lower resolving power than the PDA, was measured at different FOVs to show that using electronic zooming is only marginally beneficial when the optical detector and the XRII contribute equally to the resolution degradation. However, when a higher-resolution optical detector is used, electronic zooming always yields a greater gain in resolution.

A light source for testing radiological television cameras.

A light source for testing and setting up radiological television systems has been designed and built using green light emitting diodes and an optical integrating sphere. It is shown to have an optical spectrum similar to the green output phosphors used in x-ray image intensifiers (XRII's). It can be made uniform in intensity across the output port and does not show the small scale granularity characteristic of phosphors. It has a Lambertian angular distribution which, by the addition of neutral density filters, can be made to mimic the angular distribution at the output of an XRII. It can easily and accurately be modulated or switched and a luminance comparable to that of an XRII output phosphor can be achieved.