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filter (fil´ter)

1. A porous substance through which a liquid or gas is passed in order to separate it from contained particulate matter or impurities.filtrum; 2. To use or to subject to the action of a f. 3. In diagnostic or therapeutic radiology, a plate made of one or more metals such as aluminum and copper that, placed in the x- or gamma-ray beam, permits passage of a greater proportion of higher energy radiation and attenuation of lower and less desirable energy radiation, raising the average energy or hardening the beam. 4. A device used in spectrophotometric analysis to isolate a segment of the spectrum. 5. A mathematical algorithm applied to image data for the purpose of enhancing image quality, usually by suppression of high spatial frequency noise. [Mediev. L. filtro, pp. -atus, to strain through felt, fr. filtrum, felt]
bandpass f. a device that allows a limited range of frequencies to pass.
Berkefeld f. a bacterial f. used in 1891, made of earth known as Kieselguhr taken from the name of the mine in Hanover, Germany, from which the earth was found. Ground water at this mine had a clear blue color suggesting the use of the earth as a filter. [Berkefield, name of owner of the mine from which the material to make the filter was taken]
bird's nest f. a wire mesh vena cava f.
Greenfield f. a multistrutted spring-styled filter usually placed in the inferior vena cava to prevent venous emboli from reaching the pulmonary circulation from the lower extremity.
high-pass f. a device or material that allows high frequency signals to pass while attenuating other signals.
low-pass f. a device or material with the opposite effect from a high-pass f.; most tissues act as low-pass f.'s of ultrasound signals.
nitinol f. a vena cava f. made with a metal that springs into shape when warmed to body heat by the blood after catheter insertion.
vena cava f. a f. used for interruption of IVC to prevent pulmonary embolism; e.g., Greenfield f.venocaval f;
venocaval f. vena cava f


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