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Automatic Contrast for Digital Subtraction of Radiographic Images

IP.com Disclosure Number: IPCOM000078784D
Original Publication Date: 1973-Mar-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Chow, CK: AUTHOR [+2]

Abstract

The value of the contrast parameter which customarily is employed in the digital computation of subtracted radiographic images is automatically varied according to the present technique, in order to compensate for variations in the response of the photographic film to different radiation intensity levels, thereby enhancing the final image that is obtained.

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Automatic Contrast for Digital Subtraction of Radiographic Images

The value of the contrast parameter which customarily is employed in the digital computation of subtracted radiographic images is automatically varied according to the present technique, in order to compensate for variations in the response of the photographic film to different radiation intensity levels, thereby enhancing the final image that is obtained.

Radiographic investigation of an area of interest (e.g., part of the human anatomy) commonly is conducted by comparing two images produced, respectively, before and after a dye or other suitable contrast material has been introduced into the area under study. Subtraction of the "before" image from the "after" image removes the effect of the background so that it does not obscure the desired image. Such subtraction can readily be accomplished by digital computation. A suitable expression for computing the transmittance in the resultant subtraction image is: T(d)(x,y) = K[E(b)(x,y)/E(a)(x,y)]/Gamma(c)/ where

x,y = coordinates of each pixel

K = proportionality constant

E(b)(x,y = relative exposure function for "before" image

E(a)(x,y = relative exposure function for "after" image

Gamma(c) = contrast parameter.

It is assumed in the above that the exposure functions are for geometrically aligned images (post-registered coordinates). The contrast parameter is taken to be the slope (gamma) of the H-D curve, i.e., curve of optical density v. log of exposure time, for the film on which the resultant image is to be recorded under digital control.

Customarily...