Patent ID: 6438197
Filing Date: 2002-08-20
Classification: A61B,G01N,Y10S

Abstract:
X-ray computed tomography apparatus comprisingan X-ray radiator having a focus from which X-rays, in a beam path, are emitted, at least said focus being adapted for rotation around a subject for irradiating said subject with said X-rays from a number of different projections; a radiation filter arrangement disposed in said beam path; a detector arrangement that detects X-rays in said beam path that have passed through said subject and emits a set of measured intensity values for each projection, each of said measured intensity values being representative of the intensity of the detected X-rays in a respective projection sub-region of the projection; and an electronic evaluation unit that is programmed to determine an overall attenuation value g for each of the measured intensity values that is representative of an actual overall attenuation of the X-radiation in the respective projection sub-region effected by the radiation filter arrangement and the subject and that determines a patient attenuation value p corrected for beam hardening for each of the overall attenuation values g that is representative of a theoretical, linear attenuation of the X-rays by the subject in the respective projection sub-region, said evaluation unit having data about a correction function k(z) dependent on a variable z stored therein, and the evaluation unit being programmed to determine a respective patient attenuation value for each overall attenuation value according to the equation: p=gâˆ’fâˆ’k(&agr;g+&bgr;f) wherein p is the patient attenuation value to be respectively determined, g is the respective overall attenuation value, f is a filter attenuation value that is representative of a theoretical linear attenuation of the X-rays by the radiation filter arrangement in the respective projection sub-region, k(&agr;g+&bgr;f) is the value of the correction function at the location z=&agr;g+&bgr;f, and &agr; and &bgr; are constants, and wherein the evaluation unit is programmed to determine correction function k(z) by:determining a set of reference overall attenuation values for a combination of a material of the radiation filter arrangement and a reference material, said reference overall attenuation values being representative of the overall attenuation of the X-radiation effected by said combination for different thicknesses of the filter material and of the reference material, at least taking the beam hardening into consideration; determining an appertaining attenuation error value for each of the reference overall attenuation values according to the equation: e(dw, dt)=gâ€²(dw, dt)âˆ’w(dw)âˆ’t(dt) wherein e(dw, dt) is the attenuation error value given a thickness dw of the reference material and a thickness dt of the filter material, gâ€²(dw, dt) is the reference overall attenuation value given the thickness dw of the reference material and the thickness dt of the filter material, w(dw) is a first individual attenuation value that is representative of the theoretical linear attenuation of the X-radiation by the reference material given the thickness dw of the reference material, and t(dt) is a second individual attenuation values that is representative of the theoretical linear attenuation of the X-radiation by the filter material given the thickness dt of the filter material; defining the constants &agr; and &bgr; such that a value of a variablex=&agr;gâ€²(dw, dt)+&bgr;t(dt) â€ƒchanges in a direction transverse to lines of constant attenuation error in an attention error characteristics field (gâ€²(dw, dt), t(dt)) determining data about an error function u(x) dependent on the variable x, said error function u(x) representing a curve of the attenuation error along a reference curve v(x) placed into the attenuation error characteristics field; and determining the correction function k(z) according to the equation: k(z)=u(x=z).