Infographics / Image Quality

Image intensifier / fluoroscopy image quality

A comparison of four image-quality problems in a fluoroscopy image intensifier. Pincushion distortion: a square field warps with concave sides and enlarged corners because magnification is greatest at the edges, caused by the curved input phosphor and electron optics, not the output-screen curvature. S-distortion: vertical grid lines bend into S shapes because an external magnetic field deflects the electron paths; reduced by magnetic shielding. Vignetting: a brightness profile that is high in the centre and falls off at both edges, so the centre is brighter than the periphery; reduced by curving the input phosphor. Veiling glare: an ideal high-contrast signal profile compared with a glare profile that has a raised uniform fog floor and reduced amplitude, lowering contrast; the fog comes from scatter at both the input x-ray and output light stages and is worse in larger intensifiers. A footer notes that gain raises brightness but amplifies signal and noise equally, so signal-to-noise ratio is unchanged and improves only with more detected quanta or higher detector DQE.