Two common artifacts in image-intensified fluoroscopy and photospot imaging arise from the use of the image intensifier because of geometrical issues and internal light scatter. The first artifact, “pincushion distortion,” is caused by the spherical input phosphor structure and photocathode electron image mapped onto the planar output phosphor structure. Curvature of the input phosphor / photocathode causes a more severe distortion and greater distance between equidistant points at the periphery of the image compared to the center (similar to the way that pins placed in a pincushion are parallel in the center of the cushion but diverge at the periphery). This distortion is more pronounced with the use of the large FOV, where the curvature of the input phosphor surface is the greatest at the edges of the image. Typical pincushion distortion at the image periphery causes distortions on the order of +5% to +15%. Geometric distortion is less at the center of the image.A corollary to pincushion distortion is known as “S” distortion, caused by the influence of external magnetic fields on the electron trajectories from the input phosphor to the output phosphor. This can result in a time-varying non-linear warping of the image as the II system is rotated; the distortion can be reduced by shielding the II with “mu-metal” magnetic field attenuators.
An image of a uniform phantom of small spherical attenuators placed with equal 1-cm spacing in the matrix is shown in Figure A. Note the geometric distortion at the periphery of the image, with increased distance and non-linear mapping of the sphere locations. One advantage of flat-panel detectors manufactured for fluoroscopy is the planar geometry of the scintillator, which eliminates the pincushion distortion artifacts as illustrated with the same phantom in Figure B.
Figure A. Left: Image intensifier-TV system for fluoroscopy applications. Right: Image of pincushion distortion of regularly-spaced phantom in the circularly collimated image..
Figure B. Left: Flat-panel fluoroscopy/cineradiography system. Right: Image of regularly-spaced phantom, showing good geometric linearity of the rectangularly-shaped image..
The second artifact/degradation is chiefly related to internal light scattering at the output phosphor, known as veiling glare, which results in a significant loss of image contrast particularly in image areas representing high x-ray attenuation in the object. Light from bright image areas spread into dark image areas, which significantly reduces image contrast. The “contrast ratio” is theS measurement of the image signal of a uniformly exposed II with no attenuators to the signal under a centered, highly attenuating lead disk of diameter equal to 10% of the input phosphor diameter. Ideally, the contrast ratio would be infinite under these conditions, but image intensifiers typically have values that range from 50 to 500, depending on many variables including output phosphor structure, optical coupling technology, age, etc. Vignetting is another effect caused by light scatter within the active image area, and describes the increased intensity of a uniformly exposed image relative to the periphery. As light scatter is essentially isotropic (equal scatter in all directions) light scatter from the periphery contributes to the signal in the center, but there is no light scatter outside of the active image area contributing to the signal at the periphery.