Laser Light Diffraction, Spatial Filtering, and Reconstruction of Medical Radiographic Images: A Pilot Study

Using a laser light source, an optical bench, transform lenses, and spatial filters in a system called LaserScan,1 we are studying laser light diffraction, spatial filtering, and reconstruction of medical radiographic images, with the help of GTS Corporation, whose laboratories house the laser computer. Medical radiographs are minified to a 35 mm second-generation transparency, then transilluminated by monochromatic, coherent, collimated light from a helium-neon gas laser. A first transform lens produces a Fourier Transform of the minified image in the form of a diffraction pattern that is photographed on Polaroid film, and/or viewed with a CCTV system. Spatial filtering is accomplished in the plane of the diffraction pattern by using various opaque elements of appropriate geometric form to filter undesired spatial frequencies. Spatial filters take the form of ``wires,' ``sector-wedges,' and circular apertures, as well as other empirically determined shapes. A second lens performs an inverse Fourier transform on the diffraction pattern producing a reconstructed, filtered, real image of the minified film. This reconstructed image is also photographed on Polaroid film or viewed on the CCTV screen. One of our goals is to remove certain unnecessary pattern detail from particular radiographic images so that characteristic patterns of diagnostic picture detail might be more readily identified and diagnosed. Experimental results, including photographs of diffraction patterns and some filtered reconstructed images, are presented.