X-ray computed tomography apparatus

Claim 1

An X-ray computed tomography apparatus comprising: a radiator/detector arrangement that supplies respective sets of measured intensity values for X-ray projections of a body slice of a patient, each measured intensity value of each set representing an intensity of the X-rays after penetration of the body slice in a partial projection region of an overall projection region; and an electronic evaluation and reconstruction unit connected to the radiator/detector arrangement for a) determining an overall attenuation value for each of the measured intensity values, said overall attenuation value representing an actual overall attenuation of the X-rays in the body slice in the appertaining partial projection region; b) reconstructing an overall image of the body slice proceeding from the overall attenuation values; c) extracting a first partially image of a first substance of the body slice from said overall image; d) determining a respective first attenuation partial value from said first partial image for each partial region of the respective X-ray projection, each first attenuation partial value being a criterion for attenuation of the X-rays in the appertaining projection partial region in the first substance of the body slice, and allocating said first attenuation partial value to the overall attenuation value of the corresponding partial region; e) extracting a second partial image from said overall image of a second substance in the body slice; f) determining a respective second attenuation partial value from said second partial image for each partial region of the respective X-ray projection, each second attenuation partial value being a criterion for attenuation of the X-rays in the appertaining projection partial region in the second substance of the body slice, and allocating said second attenuation partial value to the overall attenuation value of the corresponding partial region; g) determining a correction value for each overall attenuation value from the first and second attenuation partial values from predetermined beam-hardening correction information stored in the evaluation and reconstruction unit; h) determining an overall attenuation value corrected for beam hardening for each overall attenuation value according to the equation: g corr =g+k ( t 1 , t 2 ) wherein g corr is the overall attenuation value corrected for beam hardening, g is the overall attenuation value, t 1 is the first attenuation partial value, t 2 is the second attenuation partial value and k(t 1 , t 2 ) is the correction value determined from t 1 and t 2 , and for determining the correction value k(t 1 , t 2 ), said evaluation and reconstruction unit i) determining a set of reference overall attenuation values g ref (s 1 , s 2 ) for a material sequence composed of a first reference material and a second reference material different therefrom for different thickness combinations, each reference overall attenuation value g ref of the set representing the overall attenuation of the X-rays for a specific thickness combination of the material sequence; j) calculating a theoretical linear attenuation of the X-rays for each thickness of the first reference material from the thickness combinations of the material sequence represented by a first individual attenuation value s 1 ; k) calculating a theoretical linear attenuation of the X-rays for each thickness of the second reference material from the thickness combinations of the material sequence represented by a second individual attenuation value s 2 ; and l) determining the correction value for each thickness combination of the material sequence according to the equation k ( t 1 , t 2 )= t 1 +t 2 −g ref ( s 1 =t 1 , s 2 =t 2 ), wherein g ref (s 1 =t 1 , s 2 =t 2 ) is the reference overall attenuation value for a thickness combination of the material sequence for which s 1 =t 1 and s 2 =t 2 .