Method and apparatus for automatically correcting subtraction image density

Claim 2

In temporal subtraction processing for radiation images including the steps of exposing two or more stimulable phosphor sheets respectively to radiation passing through an object injected with contrast media into a specific structure and radiation passing through said object without injection of contrast media to thereby store radiation images of said object in said stimulable phosphor sheets, the image information from said specific structure being different between radiation images, scanning said stimulable phosphor sheets by stimulating rays which cause said stimulable phosphor sheets to emit light in proportion to the stored radiation energy, photoelectrically detecting and converting the amounts of emitted light into digital image signals, obtaining a difference signal for forming an image of said specific structure by carrying out subtraction processing of said digital signals between corresponding picture elements of said radiation difference signal on the basis of a predetermined gradation conversion table, a method of automatically correcting the density of a subtraction image, which comprises the steps of: obtaining a histogram of said difference signal, calculating a signal corresponding to a maximum frequency point of said histogram from said histogram, calculating the difference Δ x between a standard background density of a subtraction image formed by said difference signal and said signal corresponding to a maximum frequency point of said histogram, and carrying out correction by adding said difference Δ x to said difference signal prior to said image gradation processing.

Claim 7

In temporal subtraction processing for radiation images including the steps of exposing two or more stimulable phosphor sheets respectively to radiation passing through an object injected with contrast media into a specific structure and radiation passing through said object without injection of contrast media to thereby store radiation image of said object in said stimulable phosphor sheets, the image information from said specific structure being different between radiation images, scanning said stimulable phosphor sheets by stimulating rays which cause said stimulable phosphor sheets to emit light in proportion to the stored radiation energy, photoelectrically detecting and converting the amounts of emitted light into digital image signals, obtaining a difference signal for forming an image of said specific structure by carrying out subtraction processing of said digital signal between corresponding picture elements of said radiation images, and conducting gradation processing of said difference signal on the basis of a predetermined gradation conversion table, a method of automatically correcting the density of a subtraction image, which comprises the steps of: obtaining a histogram of said difference signal, calculating a signal corresponding to a maximum frequency point of said histogram from said histogram, calculating the difference Δ x between a standard background density signal representing the standard background density of a subtraction image formed by said difference signal and said signal corresponding to a maximum frequency point of said histogram, and carrying out correction by shifting said gradation table used for said gradation processing by the value of -Δ x, which has the same absolute value as that of said difference signal Δ x and the sign reverse thereof, along the input signal coordinate axis.

Claim 8

In temporal subtraction processing for radiation images including the steps of exposing two or more stimulable phosphor sheets respectively to radiation passing through an object injected with contrast media into a specific structure and radiation passing through said object without injection of contrast media to thereby store radiation image of said object in said stimulable phosphor sheets, the image information from said specific structure being different between radiation images, scanning said stimulable phosphor sheets by stimulating rays which cause said stimulable phosphor sheets to emit light in proportion to the stored radiation energy, photoelectrically detecting and converting the amounts of emitted light into digital image signals, obtaining a difference signal for forming an image of said specific structure by carrying out subtraction processing of said digital signal between corresponding picture elements of said radiation images, and conducting gradation processing of said difference signal on the basis of a predetermined gradation conversion table, a method of automatically correcting the density of a subtraction image, which comprises the steps of: obtaining a histogram of said difference signal, calculating a signal corresponding to a maximum frequency point of said histogram from said histogram, calculating the difference Δ x between a standard background density signal representing the standard background density of a subtraction image formed by said difference signal and said signal corresponding to a maximum frequency point of said histogram, and carrying out correction by shifting said gradation table used for said gradation processing by the value of -Δ x, which has the same absolute value as that of said difference signal Δ x and the sign reverse thereof, along the input signal coordinate axis.